Multi-scale mechanistic evaluation of reactivity balance in metakaolin-nano-CaCO(3) systems for high-performance stabilization of problematic clays.

Multi-scale mechanistic evaluation of reactivity balance in metakaolin-nano-CaCO3 systems for high-performance stabilization of problematic clays - PMC Skip to main content An official website of the United States government Here's how you know Here's how you know Search Log in Primary site navigation Search Log in Search PMC Full-Text Archive Search in PMC Journal List User Guide As a library, NLM provides access to scientific literature. Inclusion in an NLM database does not imply endorsement of, or agreement with, the contents by NLM or the National Institutes of Health. Learn more: PMC Disclaimer | PMC Copyright Notice Sci Rep . 2026 Jun 2;16:16881. doi: 10.1038/s41598-026-44722-y Multi-scale mechanistic evaluation of reactivity balance in metakaolin-nano-CaCO 3 systems for high-performance stabilization of problematic clays Mahtab Hosseinzadeh 1, ✉ , Mohsen Abdollahi 2, ✉ Author information Article notes Copyright and License information 1 Department of Civil Engineering, Sharif University of Technology, Tehran, Iran 2 Department of Civil Engineering, University of Birjand, Birjand, Iran ✉ Corresponding author. Received 2025 Dec 30; Accepted 2026 Mar 13; Collection date 2026. © The Author(s) 2026 Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/ . PMC Copyright notice PMCID: PMC13230719  PMID: 42230673 Abstract This study introduces a multiscale framework for optimizing the metakaolin-nano-CaCO 3 (MK–NC) system for enhanced clay stabilization. While previous studies have typically investigated MK or nano-CaCO 3 independently and often without a systematic factorial matrix or integrated environmental assessment, the interactive mechanisms governing their coupled performance remain insufficiently clarified. In particular, the absence of full compositional mapping and multi-scale correlation between geotechnical behavior, microstructure, and sustainability metrics has limited mechanistic understanding of synergy domains. Using a comprehensive 4 × 4 factorial design with varying MK (1–10 wt%) and NC (0.1-1.0 wt%) concentrations, we evaluated the effects on plasticity, compaction, unconfined compressive strength (UCS), and consolidation, alongside microstructural and mineralogical analysis via SEM/EDS and XRD. The findings reveal that an optimal MK-NC formulation (about 4 wt% MK + 0.1 wt% NC) significantly improves soil properties, with a 30% reduction in plasticity index, a 2–3% increase in optimum moisture content, and a 250% enhancement in 28-day strength. Consolidation characteristics also improve, indicating a denser, faster-draining microstructure. Furthermore, a cradle-to-gate Life Cycle Assessment (functional unit: 1 kg of material), incorporating the modeled energy requirements for metakaolin calcination and nano-CaCO₃ processing, indicates that the selected C-N1-M40 binder achieves an estimated 68% reduction in Global Warming Potential (GWP) relative to OPC under the defined system boundaries. These results position the MK-NC system as a mechanistically optimized and environmentally promising alternative for high-performance clay stabilization. Keywords: Clay fabric, Pozzolanic reaction, Microstructure, nucleation, Consolidation Subject terms: Engineering, Environmental sciences, Materials science Introduction Highly plastic and compressible clay soils present persistent challenges in geotechnical engineering due to their strong water affinity, high volume change potential, low shear strength, and sensitivity to physicochemical interactions at the particle scale 1 – 3 . The dominance of clay-sized fractions, the formation of diffuse double layers, and the presence of weakly bonded platelet structures contribute to excessive settlement, reduced stability, and long-term deformation problems in earthworks and infrastructure systems 4 , 5 . Conventional stabilizers such as cement and lime have historically been employed to overcome these limitations 6 – 8 ; however, their use is increasingly scrutinized due to high embodied energy, significant CO 2 emissions 9 , 10 , potential durability issues under cyclic moisture exposure, and the incompatibility of traditional chemical stabilization with modern sustainability goals 11 , 12 . These constraints have intensified interest in environmentally responsible alternatives that enhance engineering performance through microstructural and mineralogical modification rather than heavy chemical cementation 13 , 14 . Among emerging low-carbon stabilizers, metakaolin (MK) has gained prominence as a highly reactive aluminosilicate capable of forming C-S-H and C-A-S-H gels through pozzolanic reactions 15 , 16 . Its amorphous structure facilitates dissolution of silica and alumina species and contributes to the development of refined pore networks, aggregated clay platelets, and enhanced interparticle bonding 17 – 19 . Complementing MK, nano-calcium carbonate (Nano-CaCO 3 ) has recently attracted attention due to its multifunctional role in soil modification 20 – 22 . At the nanoscale, CaCO 3 particles improve fabric arrangement through filler effects, act as nucleation sites for cementitious reaction products, modulate double-layer thickness, and promote controlled flocculation 23 . By refining pore structure and strengthening interplatelet links, nanoparticles can contribute to mechanical improvement even at very low dosages, consistent with particle-shape-controlled packing and fluidity responses observed in granular systems 24 , 25 . Despite this promise, the combined application of MK and Nano-CaCO 3 in clay stabilization remains insufficiently explored, especially concerning their interactive mechanisms across geotechnical, physicochemical, mineralogical, and microstructural domains 26 . Although previous studies have individually examined MK 27 or nano-CaCO 3 28 in soil improvement, their synergistic behavior balancing pozzolanic reactivity and nanoparticle-driven nucleation has not been systematically evaluated 29 . Critical aspects remain poorly understood, including how MK-induced water demand interacts with nanoparticle dispersion, how compaction behavior evolves under simultaneous fines modification and pore refinement, and how nonlinear or overshoot behaviors emerge at higher nanoparticle dosages due to agglomeration or disruption of packing density, consistent with packing-fraction-controlled deformation transitions reported in granular systems under dynamic loading 30 – 33 . Likewise, the literature rarely integrates Atterberg limits, compaction, UCS, consolidation parameters (Cc, Cs, Cv, mv), SEM morphology, and XRD mineralogy into a unified multi-scale interpretation capable of explaining how chemical and microstructural evolution governs engineering-scale response 34 – 36 . The absence of studies employing a full factorial MK-Nano-CaCO 3 matrix, such as the one implemented in this manuscript, further underscores the gaps in understanding the mechanistic interactions between these two sustainable additives 37 , 38 . Despite increasing interest in combining reactive aluminosilicates with nano-scale calcium sources, the fundamental interaction mechanisms remain debated. Existing studies report competing interpretations regarding whether nano-CaCO 3 primarily accelerates aluminosilicate dissolution or acts mainly as a heterogeneous nucleation substrate for C-(A)-S-H formation 39 . Furthermore, the balance between Ca 2+ availability, MK dissolution kinetics, and nanoparticle dispersion stability introduces a dissolution-nucleation competition that may either enhance gel connectivity or induce agglomeration-driven heterogeneity at higher dosages. The absence of systematic compositional mapping has limited the ability to distinguish beneficial synergy from mechanistic overshoot behavior. Clarifying these competing processes requires a factorially controlled framework capable of simultaneously linking mineralogical evolution, microstructural development, and engineering performance an approach adopted in the present study. This study addresses these knowledge gaps by presenting one of the first multi-scale, factorially controlled investigations of MK-Nano-CaCO 3 stabilization in an active clay 40 , 41 . Sixteen mixtures spanning four MK dosages and four Nano-CaCO 3 dosages were tested to systematically map the interaction domain. The results reveal clear synergy windows in which MK reactivity and Nano-CaCO 3 nucleation produce optimal outcomes, including substantial reductions in plasticity, improved packing efficiency, peak unconfined compressive strength, reduced compressibility, accelerated consolidation, and formation of continuous cementitious gel networks. Outside these windows, non-linear declines in performance occur due to excessive water demand, nanoparticle agglomeration, or incomplete polymerization of reaction products, highlighting mechanistic overshoot behaviors that the literature has rarely documented 42 . A coherent chain of interactions linking plasticity, compaction, UCS, consolidation, SEM/XRD, and LCA is established, providing a comprehensive framework for interpreting how microstructural and mineralogical evolution influences engineering behavior in modified clays. The relevance of this work extends beyond performance optimization: it directly supports global sustainability initiatives in geotechnical engineering. MK offers a low-carbon alternative to cement, with significantly reduced CO 2 footprint and lower energy requirements associated with production 43 , 44 . Nano-CaCO 3 , even at very small dosages, contributes to densification and bond development without the environmental burden of traditional cementitious stabilizers 45 , 46 . By reducing reliance on high-embodied-energy binders and enabling high-performance stabilization at low additive contents, the combined MK-Nano-CaCO 3 approach aligns with current strategies emphasizing carbon mitigation, resource efficiency, and long-term durability in ground improvement 47 , 48 . Furthermore, the complexity of soil-additive interactions necessitates advanced interpretative frameworks to bridge the gap between micro-scale phenomena and macro-scale engineering responses. Recent advancements emphasize the role of multi-scale modeling, sensitivity analysis of geotechnical parameters, and the evaluation of nonlinear time-history behaviors in ensuring the structural integrity of stabilized systems, while analytical developments in saturated porous media further demonstrate the importance of fluid-solid coupling and nonlinear interaction mechanisms in complex geomaterial responses 49 – 59 . Integrating LCA with rheological and mechanical characterization has further refined the selection criteria for sustainable binders, highlighting the importance of upcycling industrial wastes into high-performance geopolymers 60 – 69 . Moreover, the evolution of pore networks and mineralogical phases is increasingly analyzed through the lens of kinetics and molecular dynamics, providing deeper insights into how nanoparticle-driven nucleation and aluminosilicate dissolution govern the long-term stability and fracture mechanisms of modified geomaterials 70 – 79 . These multidisciplinary perspectives underscore the necessity of employing robust numerical methods and predictive models to capture the synergistic effects of multi-component stabilizers, ensuring that laboratory-scale findings can be reliably translated into field-scale geotechnical applications 80 – 89 . This work integrates, for the first time in a unified mechanistic framework, engineering-scale tests (plasticity, compaction, UCS, consolidation) with physicochemical and mineralogical techniques (SEM, EDS, XRD) to directly link microstructural evolution to macro-scale geotechnical performance in MK-Nano-CaCO 3 stabilized clays. The results reveal how C-S-H and C-A-S-H gel formation, aluminosilicate dissolution, nanoparticle-driven nucleation, and double-layer compression collectively refine pore structure, reorient clay platelets, and strengthen the soil skeleton, establishing multi-scale correlations that have been largely absent from prior studies. The central novelty of this work lies in its systematic identification of the optimum MK-Nano-CaCO 3 synergy region and its mechanistic explanation of the nonlinear behaviors and performance “windows” that emerge only when pozzolanic reactivity and nanoparticle nucleation are precisely balanced, a phenomenon not previously mapped or interpreted in a full factorial design. Accordingly, the primary objective is to (i) define the optimal synergy zone between MK and Nano-CaCO 3 , (ii) elucidate the governing multi-scale mechanisms responsible for improved engineering behavior, (iii) establish coherent mechanistic links between plasticity reduction, compaction response, strength development, consolidation characteristics, and microstructural densification, and (iv) contribute to the development of next-generation sustainable, high-performance stabilizers for problematic clays. By fusing geotechnical testing with physicochemical interpretation and microstructural evidence into a single, mechanistically consistent model, this study provides a distinctly novel contribution, advancing current understanding of MK-Nano-CaCO 3 interactions and offering a scientifically grounded pathway for designing environmentally responsible and performance-optimized soil stabilization systems. Materials and methods Materials Clay soil The natural clay used in this study is a fine-grained cohesive soil exhibiting high plasticity, low shear strength, and considerable compressibility properties typical of problematic clays encountered in geotechnical engineering. The bulk soil was air-dried under ambient laboratory conditions until moisture equilibrium was achieved, gently disaggregated to prevent alteration of mineral structure, and screened through a 4.75-mm sieve to obtain the fine fraction used in all mixtures. Fundamental engineering properties, including particle size distribution, Atterberg limits, specific gravity, compaction characteristics, and baseline mechanical parameters were determined following the corresponding ASTM standards and used as a reference for evaluating the performance of the stabilized mixtures. Metakaolin (MK) Metakaolin, a highly reactive calcined aluminosilicate pozzolan, served as the primary stabilizing agent. Its amorphous structure and high content of reactive silica (SiO 2 ) and alumina (Al 2 O 3 ) promote the formation of cementitious reaction products such as C-S-H and C-A-S-H gels. These products contribute to matrix densification, improved interparticle bonding, and enhanced mechanical behavior. Metakaolin was incorporated into the clay at four dosage levels of 10%, 20%, 30%, and 40% expressed as percentages of the dry soil mass. Nano-calcium carbonate (Nano-CaCO 3 ) Nano-CaCO 3 , with particle sizes in the sub-100-nm range, was used as a supplementary additive to enhance filler action, provide nucleation sites for cementitious gel formation, and improve packing density within the clay matrix. Its nanoscale dimension enables effective pore refinement, enhanced uniformity, and improved microstructural stability. Four dosages of 0.25%, 0.50%, 1.00%, and 1.50% of the dry soil mass were utilized in the experimental program. The nano-CaCO₃ used in this study had an average particle size below 100 nm and a specific surface area of approximately 42 m²/g (BET). The relatively high surface area enhances its filler efficiency and provides abundant heterogeneous nucleation sites for the development of cementitious reaction products within the clay matrix. Mix design and sample preparation A systematic factorial mix design was established to investigate both the isolated and synergistic effects of metakaolin and nano-CaCO 3 on the engineering and microstructural behavior of the clay. Sixteen distinct mixtures were produced using combinations of four metakaolin contents (10%, 20%, 30%, 40%) and four nano-CaCO 3 contents (0.25%, 0.50%, 1.00%, 1.50%), as summarized in Table 1 . In all mixtures, the percentages of metakaolin and nano-CaCO 3 were calculated with respect to the dry mass of the untreated soil, ensuring a consistent reference basis for all formulations. Each blend is denoted as C-N-(x)-M-(y), where x and y represent the nano-CaCO 3 and metakaolin percentages, respectively. The matrix presented in Table 1 provides a comprehensive framework for evaluating the interaction mechanisms and performance trends across all binder combinations. For each mixture, specimen preparation followed a strictly controlled laboratory protocol designed to ensure reproducibility, eliminate segregation, and preserve the structural integrity of the mixtures. The air-dried, sieved clay was weighed accurately, and the required amounts of metakaolin and nano-CaCO 3 were measured using high-precision digital balances. Dry blending was performed in a mechanical mixer: the clay and metakaolin were mixed initially until a uniform texture and color indicated complete dispersion, after which nano-CaCO 3 was added slowly to prevent nanoparticle agglomeration and was incorporated thoroughly by extended low-speed mixing. Table 1. Mix design matrix showing the combinations of metakaolin (MK) and nano-CaCO 3 contents used in the sixteen experimental mixtures. Row Mix ID Nano-CaCO 3 (wt%) MK (wt%) 1 C-N-0.1-M-1 0.1 1 2 C-N-0.1-M-4 0.1 4 3 C-N-0.1-M-7 0.1 7 4 C-N-0.1-M-10 0.1 10 5 C-N-0.4-M-1 0.4 1 6 C-N-0.4-M-4 0.4 4 7 C-N-0.4-M-7 0.4 7 8 C-N-0.4-M-10 0.4 10 9 C-N-0.7-M-1 0.7 1 10 C-N-0.7-M-4 0.7 4 11 C-N-0.7-M-7 0.7 7 12 C-N-0.7-M10 0.7 10 13 C-N-1-M1 1 1 14 C-N-1-M4 1 4 15 C-N-1-M7 1 7 16 C-N-1-M10 1 10 Open in a new tab After achieving a fully homogeneous dry blend, distilled water was added gradually while continuous mechanical mixing ensured uniform hydration throughout the mass. Moisture levels were selected based on the compaction characteristics of the untreated clay, with adjustments made to account for the increased surface area and water demand associated with metakaolin. After wet mixing, the material was sealed briefly to facilitate moisture equilibration. Specimens were compacted using the Standard Proctor energy level (ASTM D698). The mixtures were placed into molds in thin layers, with consistent compaction effort applied to each layer to eliminate density gradients and lamination. Cylindrical specimens intended for unconfined compressive strength testing were extruded carefully from split molds to preserve structural integrity, while specimens required for consolidation testing were trimmed precisely from compacted blocks using specialized cutting tools to achieve smooth, uniform surfaces and accurate dimensions. All specimens were immediately sealed in airtight containers to prevent moisture loss and were handled carefully to avoid structural disturbance prior to testing. After compaction, all specimens were immediately sealed in airtight containers to prevent moisture exchange and were cured under controlled laboratory conditions at a constant temperature of (23 ± 2 °C). The specimens were stored in a stable indoor environment without exposure to direct airflow or temperature fluctuation for the designated curing period (28 days for UCS and microstructural testing). This controlled curing regime ensured consistent hydration and pozzolanic reaction development prior to testing. Experimental methods Particle size distribution Particle size characteristics were determined using combined sieve and hydrometer analyses (ASTM D6913 and ASTM D422). These tests were used to evaluate changes in fines distribution, aggregation, and grading behavior induced by the stabilizers. Atterberg limits Liquid limit, plastic limit, and plasticity index were measured following ASTM D4318. The changes in plasticity indices reflect alterations in soil fabric, clay platelet interaction, and physico-chemical behavior resulting from the additives. Unconfined compressive strength (ASTM D2166) The unconfined compressive strength of each mixture was determined using cylindrical specimens molded at controlled moisture and compaction conditions. Axial loading was applied under displacement control at a constant strain rate, and the stress-strain response was recorded continuously from initial loading to post-peak behavior. Peak axial stress, stiffness characteristics, ductility or brittleness, and failure mode were documented to quantify the extent of mechanical improvement produced by metakaolin-nano-CaCO 3 stabilization. Consolidation test One-dimensional consolidation characteristics were evaluated according to ASTM D2435. Parameters obtained included compression index (Cc), recompression index (Cr), coefficient of consolidation (Cv), preconsolidation stress, and void ratio evolution. These results were used to assess microstructural densification and the reduction of compressibility. Microstructural analysis Scanning Electron Microscopy (SEM) was used to observe surface morphology, pore refinement, particle bonding, and the development of cementitious reaction products. Representative fragments were gold-coated prior to imaging. X-ray Diffraction (XRD) analysis was conducted to identify mineralogical changes, crystalline phase alterations, and structural modifications associated with the pozzolanic and filler effects of the additives. XRD analysis was performed using Cu-Kα radiation (λ = 1.5406 Å) operated at 40 kV and 30 mA. Diffraction patterns were collected over a 2θ range of 5–70° with a step size of 0.02° and a scanning rate of 1°/min. These parameters were selected to enable identification of both crystalline phases and changes in amorphous content. Sustainability analysis The environmental sustainability assessment is structured according to the ISO 14,040/14,044 standard for Life Cycle Assessment (LCA) 90 . This established international framework defines a systematic, four-stage methodology encompassing (1) Goal and Scope Definition, (2) Life Cycle Inventory (LCI), (3) Life Cycle Impact Assessment (LCIA), and (4) Interpretation 91 . Goal and scope This study aims to conduct a comparative life cycle assessment of a binder material versus Ordinary Portland Cement (OPC). The novel material assessed is C-N1-M40, identified as the optimal formulation from a screening of 16 candidate detailed earlier in this work. The primary objective is to quantify and compare the impacts associated with the production of each material, thereby identifying potential sustainability advantages or trade-offs at the point of manufacture. The analysis uses a functional unit of 1 kg of C-N1-M40 vs. 1 kg of OPC. This establishes an equitable basis for assessing their respective production impacts. It is important to note that the present comparison is conducted at the material production level (1 kg basis) and does not represent a fully performance-normalized functional equivalence between OPC and the soil-binder composite system. The analysis is intended as an early-stage cradle-to-gate environmental screening of binder production impacts. A performance-based functional unit (e.g., per unit strength gain or per volume of stabilized soil achieving a target strength) would require additional field-scale assumptions and is suggested for future investigation. The assessment follows a cradle-to-gate system boundary. Included processes are: (i) the extraction and upstream production of all constituent raw materials, (ii) the transportation of these materials to the manufacturing site, and (iii) all industrial processing and compounding steps required to produce the final 1 kg of material. The use phase and end-of-life management are excluded from this analysis, as the focus is on the environmental burdens inherent to material production. This boundary is appropriate for a comparative screening assessment targeting production-related environmental hotspot. LCI The LCI shown in Table 2 was compiled using the Ecoinvent 3 database as the primary source for background system data. However, as the standard database lacks specific unit processes for the production of nano-sized calcium carbonate (nano-CaCO 3 ) and metakaolin, a customized inventory was developed to accurately model the C-N1-M40. Where applicable, existing datasets for similar materials (e.g., conventional CaCO 3 , kaolin) were used as proxies and modified to reflect the specific energy and material flows of the specialized processing steps, including nano grinding and calcination. Transportation of raw materials to the manufacturing site was included within the cradle-to-gate system boundary. Standard regional freight transport datasets from Ecoinvent 3 were adopted to represent upstream logistics, and consistent transport distance assumptions were applied to both OPC and C-N1-M40 scenarios to maintain comparability. No additional long-distance or international transport stages were assumed beyond the representative regional supply chain conditions. Table 2. LCI for OPC and C-N1-M40. Inventory C-N1-M40 OPC Data source OPC – 1 kg Ecoinvent dataset Clay 0.709 kg – Ecoinvent dataset nano-CaCO 3 0.007 kg – Custom process Metakaolin 0.284 kg – Custom process Open in a new tab The custom foreground processes developed for nano-CaCO₃ and metakaolin production were constructed by adapting proxy datasets from Ecoinvent 3 and supplementing them with literature-based industrial energy benchmarks. The nano-grinding stage was modeled using representative electrical energy consumption ranges reported for high-energy milling systems, while metakaolin production incorporated thermal energy demand consistent with industrial calcination of kaolin (typically 650–800 °C). Standard regional freight transport distances were assumed within the cradle-to-gate system boundary. These parameters were applied consistently to ensure internal comparability between scenarios. Results and discussion Grain size and plasticity behavior This section is presented with the objective of clarifying the interaction between particle size distribution and plasticity behavior in the clay soil under the influence of varying amounts of metakaolin and nano-calcium carbonate, in order to determine how these two additives, through modifying the proportion of active fine particles, adjusting the thickness of the diffuse double layer, and regulating adsorbed water, affect the liquid limit (LL), plastic limit (PL), and plasticity index (PI). The focus is not merely on describing the data, but rather on elucidating the mechanisms by which changes in grading structure and surface reactivity lead to reduced susceptibility to plastic deformation and improved engineering behavior mechanisms that form the basis of subsequent analyses of compaction, mechanical response, and microstructure, and that are essential for identifying the optimal MK-Nano-CaCO 3 range for soil stabilization. Analysis of the particle size distribution indicates that the natural soil exhibits a uniform distribution of fine particles, with the percent passing remaining nearly constant and above 97% in the range of 4.75 to 0.3 mm, while a gradual yet significant reduction in passing percentage occurs below 0.15 mm. According to Fig. 1 , the notable decrease from 96.5% passing at 0.15 mm to 40% at 0.001 mm clearly reflects the high proportion of active clay-sized particles, which govern the soil’s plastic behavior. Such a curve, characterized by a steep slope in the region below 0.02 mm, is typically indicative of soils with high laminar compaction tendency, substantial water adsorption capacity, and pronounced sensitivity to chemical modifications and stabilizers 21 , 92 . The presence of these fines, which predominantly control double-layer formation and interparticle forces in clay soils, provides the basis for interpreting the plasticity behavior and its changes following the addition of MK and Nano-CaCO 3 . Fig. 1. Open in a new tab Particle size distribution of the natural clay, showing a high proportion of fine particles and a sharp drop in percent passing below 0.02 mm, indicating the dominance of clay-sized fractions. Examination of the Atterberg limits results shows that the incorporation of MK and nano-calcium carbonate, both individually and synergistically, has induced meaningful alterations in the physicochemical structure of the soil. As evident in Fig. 2 , for a nano-calcium carbonate content of 0.1 wt%, increasing MK from 1% to 10% reduces the LL from 49% to 44% and decreases the PI from 31% to 22%, a trend indicating that MK, by providing a highly reactive surface and consuming part of the adsorbed water through initial pozzolanic reactions, contributes to double-layer compression and reduced cohesion between clay platelets 31 . Simultaneously, Nano-CaCO 3 in this mixture, through its filler effect and the provision of nucleation sites for C-A-S-H-like products, promotes mild flocculation and reduces the soil’s plastic deformability 21 , 93 . Fig. 2. Open in a new tab Changes in LL, PL, and PI of clay stabilized with different MK and Nano-CaCO 3 contents, illustrating the combined effects of pozzolanic activity and nanoparticle filler action on plasticity reduction. Increasing the nano-calcium carbonate content to 0.4 wt% produces a relatively similar pattern, though with a more moderate slope of variation. For example, the PI across the MK range of 1% to 10% changes only slightly from 24% to 23%. This minimal reduction suggests that, at this dosage, the predominant role of the additive is uniform pore filling and restricting water mobility within interparticle spaces, rather than inducing major structural transformation in the clay fabric. Meanwhile, LL values in this range remain nearly constant at approximately 43–46%. Such stability in the LL typically occurs when the soil enters a stage of structural rearrangement in which pozzolanic reactions of MK have not yet progressed sufficiently 21 , 94 – 96 . At a nano-calcium carbonate content of 0.7 wt%, the dual effects of MK and Nano-CaCO 3 become more pronounced. The PI decreases from 32% to 28% at MK = 7%, but then increases again to 33% at MK = 10%. This nonlinear behavior, also visible in Fig. 2 section (c), is commonly known as overshooting in plasticity 97 , and arises when the number of nanoparticles becomes sufficiently high to promote agglomeration, which rather than reducing water adhesion leads to the formation of localized clusters with higher water affinity. Additionally, the reduction in PL from 18% to 16% at MK = 10%, concurrent with an increase in PI, reflects multifactorial changes in the soil structure, partially due to increased MK reactivity and partially due to nonuniform alterations in interparticle void spaces. At the highest nano-calcium carbonate dosage of 1 wt%, the soil exhibits a more stable behavior and approaches structural equilibrium. The PI decreases from 31% at MK = 1% to 26% at MK = 10%, without showing the secondary rise observed at 0.7 wt%. This indicates that, at 1 wt%, the nanoparticles are dispersed more uniformly within the clay matrix, enabling their role in reducing surface energy and constraining adsorbed water to reach an optimal level 98 . In this condition, the LL remains nearly constant within the range of 49 − 48%, consistent with literature reporting that, in MK-Nano-CaCO 3 systems, the LL is generally less sensitive than PL and PI, and its variations are more closely linked to mesoscale structural adjustments than microscale reactivity 99 . Comparison of the observed trends suggests that MK reduces the PI primarily by diminishing surface charge and altering interlayer spacing, whereas nano-calcium carbonate enhances structural stability by pore filling, moderating the diffuse double layer, and generating nucleation sites. The combined action of these additives, particularly within the range of 0.4-1 wt% Nano-CaCO 3 and 4–10 wt% MK leads to a substantial reduction in PI and the formation of a more stable, less plastic matrix, consistent with mechanisms reported in MK-based geopolymer-clay systems 20 . This concurrent decrease in LL and PI not only indicates improved geotechnical workability but also signifies the onset of early-stage pozzolanic reactions and the formation of more stable microstructural networks, which are further corroborated in the Microstructure section 100 . Compaction characteristics The compaction behavior of the mixtures reveals a clear interaction between metakaolin reactivity, the filler and nucleation effects of nano-CaCO 3 , and the intrinsic suction-controlled packing characteristics of the clay matrix. As demonstrated by the scattered measurements of moisture content and dry density across various MK and Nano-CaCO 3 levels, the transition from water-controlled to structure-controlled compaction is strongly dependent on the balance between fines activity and microstructural densification. According to Fig. 3 , increasing MK content from 1% to 10% generally leads to a systematic increase in moisture demand, particularly at higher Nano- CaCO 3 dosages, where the optimum moisture content (OMC) rises from approximately 19.8–20.3% at MK = 1% to values exceeding 22–23% at MK = 10%. This rising trend is attributed to the high surface area of MK, which enhances water adsorption and early aluminosilicate dissolution, thereby increasing the required moisture to achieve maximum rearrangement during compaction 101 . Simultaneously, the influence of Nano-CaCO 3 is dual in nature: while small amounts (0.1–0.4 wt%) act primarily as inert fillers promoting particle packing, larger dosages (0.7-1 wt%) alter the double-layer structure, increase localized water retention, and consequently shift the compaction curve toward higher OMC values 102 . Fig. 3. Open in a new tab Moisture content and dry density of MK-Nano-CaCO 3 mixtures at different MK levels, showing how nanoparticle dosage and MK content jointly affect moisture demand and compaction response. The dry density data exhibit a more nuanced pattern. As seen in Fig. 3 , the mixtures containing 0.1 wt% Nano-CaCO 3 show the highest maximum dry density (MDD) values, with peaks of 1.69–1.70 g·cm − 3 at MK = 1% and 4%, reflecting the optimal balance between flocculated clay clusters and effective coarse-fine packing. This condition supports the classical understanding that small quantities of reactive or semi-reactive fines can lubricate grain boundaries and promote denser packing under compactive effort 103 . However, increasing Nano-CaCO 3 beyond 0.4 wt% does not sustain this improvement. For example, at 0.7 wt% Nano-CaCO 3 , the MDD decreases to 1.64–1.66 g·cm − 3 despite increased MK availability. This reduction is linked to partial agglomeration of nanoparticles and the formation of rigid micro-domains that inhibit full rearrangement during compaction, a phenomenon previously reported for CaCO 3 -modified clays and lime-stabilized fines 13 , 21 . The lowest densities are observed at the interaction of high MK (7–10%) and intermediate Nano-CaCO 3 (0.7-1.0%), where the competition between water adsorption by MK and non-uniform filler distribution results in densities as low as 1.49–1.56 g·cm − 3 . A clearer perspective emerges when MDD and OMC trends are jointly examined. Figure 4 shows that OMC consistently increases with MK content for all Nano-CaCO 3 levels, rising from approximately 19.9–20.3% at MK = 1% to around 22–23% at MK = 10%. This behavior mirrors the moisture sensitivity of MK, whose amorphous aluminosilicate structure increases both the adsorptive water layer and the chemical-bound water fraction required for surface wetting 104 . In contrast, MDD exhibits a plateau rather than a monotonic trend: for Nano-CaCO 3 = 0.1%, density rises slightly with MK, while for 0.4% and 0.7% it remains nearly constant (1.65–1.66 g·cm − 3 ), and at 1% Nano-CaCO 3 it fluctuates but generally stays below the peak density of the 0.1% series. These patterns indicate that the compaction response is governed more by the microstructural packing efficiency than by water availability alone, reinforcing the notion that MK acts primarily as a water-reactive modifier, whereas Nano-CaCO 3 dictates the packing and rearrangement capacity through its filler-nucleation function 105 . Fig. 4. Open in a new tab MDD and OMC trends of the mixtures as functions of MK content for all Nano-CaCO 3 levels, highlighting the rise in OMC with MK and the nonlinear variation of MDD. To evaluate the net effect of each combination relative to the untreated soil, the ΔMDD heat map provides a mechanistic overview of compaction efficiency across the MK-Nano-CaCO 3 domain. As illustrated in Fig. 5 , the most favorable region is confined to low MK (1–4%) and low Nano-CaCO 3 (0.1–0.4%), where ΔMDD reaches values of + 0.02 to + 0.03 g·cm − 3 , confirming improved packing arising from fine particle lubrication and enhanced intergranular contacts. Conversely, negative ΔMDD values dominate the upper MK and Nano-CaCO 3 levels, with reductions as large as -0.04 g·cm − 3 occurring at MK = 7% and Nano-CaCO 3 = 1%. This substantial decrease is linked to the simultaneous thickening of the adsorbed water layer (due to MK) and creation of rigid nanoparticle-rich domains (due to Nano-CaCO 3 ), resulting in reduced compaction efficiency and a more open fabric 106 . These observations collectively demonstrate that the compaction response of MK-Nano-CaCO 3 stabilized clays is not simply additive; rather, it emerges from a complex balance between water-reactive, filler-driven, and microstructural reorganization mechanisms. Fig. 5. Open in a new tab Heat map of ΔMDD across the MK-Nano-CaCO 3 matrix, identifying regions of density improvement and zones where compaction efficiency decreases. Ultimately, the combined analysis of the three figures confirms that optimum compaction behavior is realized only within a narrow zone characterized by low to moderate MK (1–4%) and low Nano-CaCO 3 (0.1–0.4%), where the mixture simultaneously benefits from enhanced packing, controlled flocculation, and a balanced moisture requirement. Outside this window, either excessive moisture demand or disrupted particle rearrangement limits compaction performance. These mechanistic insights are crucial for linking compaction behavior to the subsequent mechanical and microstructural improvements discussed later in the manuscript 107 . Uniaxial compressive strength (UCS) The 28-day uniaxial compressive strength results reveal a highly non-linear interaction between metakaolin reactivity, the filler-nucleation functions of nano-CaCO 3 , and the evolving microstructural densification within the stabilized clay matrix. As shown in Fig. 6 , the untreated soil exhibits a modest compressive strength of 1.65 MPa, reflecting its inherently weak fabric dominated by dispersed clay platelets and limited interparticle bonding. Incorporating MK and Nano-CaCO 3 significantly alters this baseline response, but the extent and direction of improvement strongly depend on the balance between pozzolanic activity and nanoparticle distribution. At 0.1 wt% Nano-CaCO 3 the most favorable regime the strength rises sharply, reaching 3.58 MPa at MK = 1% and peaking at 5.82 MPa at MK = 4%, corresponding to an enhancement of more than 250% relative to the untreated soil. This remarkable increase can be attributed to accelerated nucleation of C-A-S-H and C-S-H reaction products on CaCO 3 nanoparticle surfaces and the efficient consumption of calcium-bearing dissolved species by metakaolin, which together promote matrix densification and development of rigid load-bearing microstructures 108 . At this dosage, the nanoparticles remain sufficiently dispersed to act primarily as nucleation centers rather than forming agglomerated clusters, thereby maximizing reactive surface exposure. Fig. 6. Open in a new tab 28-day UCS of MK-Nano-CaCO 3 mixtures, showing peak strength at 0.1 wt% Nano-CaCO 3 with 4 wt% MK and lower strengths at higher nanoparticle contents. Beyond MK = 4%, however, the beneficial trend diminishes. Strength decreases to approximately 3.03–3.05 MPa at MK = 7–10%, illustrating that excessive metakaolin increases the consumption of free water for surface wetting and dissolutive activation, leading to reduced packing and incomplete cementitious phase continuity. Such reductions at higher pozzolan content have been similarly reported for MK-rich stabilized clays due to the limited availability of exchangeable cations and decreased Ca/Al ratios, which restrict long-range polymerization in the gel network 109 . At 0.4 wt% Nano-CaCO 3 , the UCS values exhibit a moderated response. Strength reaches 3.21 MPa at MK = 4%, followed by a decline to 2.17–2.23 MPa at MK = 7–10%. This indicates that increasing Nano-CaCO 3 weakens the packing efficiency and introduces localized heterogeneity, reducing the synergy otherwise observed at low nanoparticle content. While some nucleation benefit remains at MK = 4%, the partial agglomeration of nanoparticles reduces the effective reactive surface and disrupts the formation of a continuous cementitious skeleton 110 . At 0.7 wt% Nano-CaCO 3 , the balance shifts further toward performance loss. The UCS drops to as low as 0.84–1.19 MPa for MK = 1–7%, showing that excessive nanoparticle content creates rigid, poorly bonded domains that impair stress transfer paths. Only at MK = 10% does strength partially recover to 1.98 MPa, reflecting the delayed contribution of pozzolanic reactions compensating for a part of the microstructural disturbance. The same trend extends to 1 wt% Nano-CaCO 3 , where strength values remain generally low (1.30–1.68 MPa), with only minor recovery at MK = 10%. Such behavior is consistent with prior findings showing that CaCO 3 nanoparticles, when exceeding an optimal concentration, tend to agglomerate, increase local porosity, and hinder the formation of continuous reaction products 111 . Overall, the results confirm that UCS improvement is not controlled by individual additives alone but by their precise interaction domain. The highest 28-day strength develops only within a narrow window characterized by Nano-CaCO 3 = 0.1 wt% and MK = 4 wt%, where nucleation, filler action, and pozzolanic reactivity reach optimal balance. Outside this domain, either excessive nanoparticle clustering or water-demanding MK content disrupts bonding continuity and reduces the efficiency of the cementitious microstructure. This mechanistic understanding aligns with consolidation and compaction observations and reinforces the notion that strength development in MK-Nano-CaCO 3 stabilized clays emerges from a multi-scale synergy rather than additive behavior 22 , 112 . It should be noted that the MK-Nano-CaCO₃ matrix was designed to systematically explore compositional interactions from a mechanistic perspective rather than to develop a predictive statistical model. Although formal ANOVA or response-surface modeling was not performed, the non-parallel and composition-dependent response trends observed across MK and Nano-CaCO₃ levels clearly demonstrate interaction behavior, supporting the identification of a composition-sensitive synergy window. Consolidation behavior The consolidation response of the stabilized clay exhibits a strong dependency on the combined effects of metakaolin content, nano-CaCO 3 dosage, and the resulting evolution of clay microstructure under increasing effective stress. The variation of void ratio with applied pressure clearly demonstrates how MK and nano-CaCO 3 alter compressibility and structural rearrangement. As illustrated in Fig. 7 , all mixes display the expected reduction in void ratio with stress; however, the slope and curvature of the e-logσ profiles show significant material-dependent differences. For Nano-CaCO 3 = 0.1%, the mixtures with low MK (1–4%) exhibit the steepest reduction in e, with e decreasing from 0.82 − 0.62 at zero load to 0.56 − 0.47 at 16 MPa, indicating strong structural collapse and efficient particle reordering. This behavior corresponds to the formation of a denser packing skeleton in mixtures where MK provides sufficient reactive surface for flocculation and limited cementation, while nano-CaCO 3 serves primarily as an effective filler 113 . In contrast, increasing MK to 7–10% results in flatter e-logσ curves and higher final void ratios (0.50–0.52), implying an MK-dominated fabric where the adsorbed water layer thickens and inhibits full structural collapse during loading 114 . Fig. 7. Open in a new tab Void ratio-pressure (e-logσ) curves of the MK-Nano-CaCO 3 mixtures showing the reduction in compressibility with increasing stress and the distinct effects of MK content and nanoparticle dosage on structural collapse and fabric rearrangement. A similar trend is observed at Nano-CaCO 3 = 0.4%, where the 1–4% MK mixtures show relatively low compressibility (final e ≈ 0.45–0.48), whereas the 7–10% MK mixtures maintain higher void ratios, confirming a transition from particle-packing-dominated mechanics toward water-softened microstructures. The effect becomes more pronounced at Nano-CaCO 3 = 0.7% and 1.0%, where high nanoparticle content increases structural heterogeneity and suppresses uniform collapse during loading. The C-N-0.7 mixes retain void ratios as high as 0.55–0.57 at 16 MPa, especially for MK = 7–10%, reflecting nanoparticle agglomeration and incomplete dissipation of interlayer water that hinder consolidation efficiency 115 , 116 . The flattening of the unloading branches in Fig. 7 further supports reduced elastic rebound at high nanoparticle dosages, highlighting a shift toward stiffer but more porous fabrics. The extracted compression and swelling indices provide a quantitative assessment of these trends. Figure 8 shows that Cc values range from 0.12 to 0.25 across all mixes, while Cs varies from 0.02 to 0.06. For Nano-CaCO 3 = 0.1%, Cc decreases from 0.251 to 0.147 as MK increases from 1% to 4%, indicating reduced compressibility due to early-stage flocculation and denser initial packing. However, Cc increases again at MK = 10% (0.235), consistent with the re-dispersion of clay particles and increased water demand identified earlier in compaction analyses 117 . A similar nonlinear behavior is observed for Nano-CaCO 3 = 0.4% and 0.7%, where Cc peaks at MK = 7% (0.202 and 0.19, respectively), confirming that excessive MK disrupts uniform stress-induced structural collapse. The Cs values follow complementary trends; for instance, Cs at Nano-CaCO 3 = 0.4% increases from 0.025 to 0.059 with MK, suggesting stronger elastic recovery in more dispersed structures. Overall, the Cc-Cs relationships highlight a mechanistic balance between MK-induced reactivity and nano-CaCO 3 -controlled packing, consistent with prior findings on pozzolan-nanoparticle stabilized clays 21 , 118 . Fig. 8. Open in a new tab Compression index (Cc) and swelling index (Cs) of the stabilized mixtures as functions of MK and Nano-CaCO 3 content, highlighting the nonlinear changes in compressibility and elastic rebound across different additive combinations. The rate-dependent behavior of consolidation, represented through Cv and mv contours in Fig. 9 , reveals important mechanistic insights. The three mixes shown C-N-0.1-M-4, C-N-0.4-M-4, and C-N-1-M-10 were deliberately selected because they represent the three characteristic consolidation regimes identified earlier: (1) Optimal-performance mix (C-N-0.1-M-4) with the best compaction, lowest compressibility (Cc = 0.147), and highest UCS; (2) Intermediate-performance mix (C-N-0.4-M-4) representing balanced pozzolanic-packing behavior; (3) Low-performance mix (C-N-1-M-10) exhibiting the weakest consolidation response and highest microstructural heterogeneity. Fig. 9. Open in a new tab Contours of Cv and mv for the representative mixes (C-N-0.1-M-4, C-N-0.4-M-4, and C-N-1-M-10), illustrating the transition from efficient consolidation at optimal dosages to reduced drainage and higher compressibility at elevated nanoparticle content. These three conditions map the full behavioral spectrum from optimum particle reorganization to dispersed and nanoparticle-dominated fabrics, making them representative for capturing the mechanistic extremes without unnecessary redundancy. Including all mixtures would introduce strong overlaps in Cv-mv fields and reduce interpretability; therefore, limiting Fig. 9 to the three mechanistic “signature mixes” is not only justified but methodologically superior for isolating the effects of MK and Nano-CaCO 3 on consolidation rate. For the optimal mix (C-N-0.1-M-4), Cv starts high at 0.0102–0.0113 cm 2 /sec at low stresses and gradually drops to 0.00155 cm 2 /sec at 16 MPa, indicating rapid dissipation of pore water pressure due to a dense yet well-connected microstructure. The corresponding m v values (from − 0.00425 to 0.0038 cm 2 /kg) remain low across all stress levels, confirming the low compressibility inferred from e-logσ and Cc. The negative sign associated with certain m v values reflects the adopted strain sign convention (compression recorded as negative) during data processing and does not indicate negative compressibility. For engineering interpretation, mv is considered in terms of its magnitude. In contrast, the intermediate mix (C-N-0.4-M-4) exhibits a more moderate Cv profile (0.0068 → 0.00674 cm 2 /sec), while m v increases to 0.0168 − 0.00462 cm 2 /kg at lower stresses, reflecting delayed consolidation from disrupted packing. The weakest mix (C-N-1-M-10) shows minimal Cv at high stress (0.000647 cm 2 /sec) and relatively high m v values (0.00966 − 0.00605 cm 2 /kg), consistent with nanoparticle agglomeration, reduced permeability, and inhibited fabric collapse. This transition from efficient consolidation to inhibited consolidation with increasing Nano-CaCO 3 is consistent with previous studies emphasizing the adverse effect of excessive nanoparticles on drainage pathways and skeletal continuity 119 . Taken together, the results confirm that consolidation behavior in MK-Nano-CaCO 3 stabilized clay emerges from the interplay of three dominant mechanisms: (i) MK-induced flocculation and pozzolanic bonding, which reduce compressibility at low to moderate MK levels; (ii) nano-CaCO 3 -driven densification, which improves structural collapse only at low nanoparticle dosage; and (iii) nanoparticle agglomeration and MK-driven water absorption at high dosages, which restrict drainage, increase compressibility, and markedly reduce Cv. The combined analysis of Figs. 7 and 8 , and 9 demonstrates that optimum consolidation performance low e, low Cc, high Cv, and low mv is achieved only within a narrow window centered around Nano-CaCO 3 = 0.1 wt% and MK = 4 wt%, matching the optimum domains independently identified in compaction and UCS analyses. These converging observations confirm a coherent multiscale mechanism in which MK and nano-CaCO 3 must coexist within a strict ratio to maximize structural improvement in stabilized clay 120 . Microstructural analysis (SEM & XRD) SEM The SEM observations provide direct microstructural evidence for the distinct roles of metakaolin and nano-CaCO 3 in governing the macroscopic behavior of the stabilized clays and strongly support the trends identified in compaction, consolidation, and UCS. As seen in Fig. 10 , the optimum mixture C-N-0.1-M-4 exhibits, at all magnifications, a dense and relatively homogeneous fabric in which clay platelets are largely aggregated into coalesced clusters with blurred interparticle boundaries and only a limited number of interconnected pores. At low magnification (75×), the surface appears massive and blocky, with few visible fissures, indicating that the initial flocculated network formed during compaction has been effectively locked by cementitious products. At 1000× and 10,000×, this mixture shows extensive coverage of the clay surfaces by gel-like phases that fill interplatelet voids and bridge adjacent particles, consistent with the formation of C-S-H/C-A-S-H-type reaction products produced by the pozzolanic interaction of MK with calcium-bearing species in the presence of finely dispersed nano-CaCO 3 121 . The nanoparticles in this mix evidently act as nucleation sites rather than discrete inclusions, leading to a refined pore structure and continuous bonding network that explains the low compressibility, high Cv, and superior 28-day UCS previously quantified for this formulation. Fig. 10. Open in a new tab SEM images showing the dense, well-bonded microstructure in the optimum mix (C-N-0.1-M-4) and the increasingly porous, heterogeneous fabric with particle agglomeration in higher Nano-CaCO 3 and MK dosages. In contrast, the microstructure of the intermediate mixture C-N-0.4-M-4 reveals a less compacted and more heterogeneous fabric. At 75×, although the general texture is still relatively dense, more surface undulations and micro-voids are visible, indicating incomplete infilling of interaggregate spaces. The 1000× images show clay platelets arranged in stacked and partially laminated configurations, with local patches of fine reaction products that appear as thin coatings rather than continuous bridges. At the highest magnification, discrete, sub-micrometric particles and small clusters can be distinguished, which are interpreted as partially agglomerated nano-CaCO 3 that did not fully participate as homogeneous nucleation centers. This morphology is consistent with the moderate mechanical performance and higher compression indices recorded for this mix: the presence of reaction products improves bonding relative to the untreated soil, yet the non-uniform distribution of gels and nanoparticle clusters leaves residual pores and weak zones that limit the extent of matrix densification 122 . Similar observations have been reported in MK-nano CaCO 3 blended systems where exceeding a threshold nano-CaCO 3 content causes a transition from beneficial filler/nucleation behavior to microstructural heterogeneity and incomplete percolation of the cementitious skeleton 123 . The poorest-performing mixture, C-N-1-M-10, displays a distinctly different microstructural signature that rationalizes its inferior compaction, consolidation, and strength behavior. At 75×, the surface appears rough and irregular, with pronounced cracks and interaggregate voids, suggesting a fabric that remained relatively open even after compaction and subsequent curing. The 1000× and 10,000× images exhibit large, loosely packed plate-like particles with limited gel coverage and numerous elongated or irregular pores extending between them. Filamentous or cluster-like features can be discerned, which may correspond to severely agglomerated nano-CaCO 3 and unreacted MK-rich domains. Such features indicate that the high combined dosage of MK (10%) and nano-CaCO 3 (1%) overwhelms the available calcium supply and water, leading to an over-saturated pozzolanic system in which only part of the MK participates in reaction, while excess nanoparticles aggregate and disrupt the continuity of the load-bearing skeleton 124 . . The resulting microstructure has higher effective porosity, lower connectivity of cementitious bridges, and more tortuous drainage paths, directly explaining the increased mv, reduced Cv, and depressed UCS measured for this mix. Taken together, the SEM evidence confirms that optimal performance is obtained only when MK and nano-CaCO 3 are dosed such that nanoparticles remain well dispersed and reaction products form a continuous, pore-refining network; beyond this window, agglomeration, incomplete reaction, and microstructural heterogeneity dominate, leading to diminished macroscopic improvements 125 . Furthermore, The observed gel-like interparticle bridges and pore-filling phases are morphologically consistent with the formation of C-(A)-S-H/C-S-H-type reaction products commonly reported in metakaolin-calcium systems, although direct quantitative phase determination was beyond the scope of the present study. XRD The XRD patterns clearly demonstrate that the incorporation of metakaolin and nano-CaCO 3 drives distinct mineralogical transformations that correlate strongly with the mechanical and microstructural behavior previously observed. As shown in Fig. 11 , the natural soil exhibits dominant diffraction peaks characteristic of kaolinite (d ≈ 7.15 Å; 2θ ≈ 12.3°), quartz (2θ ≈ 20.8°, 26.6°), and minor illite-mica reflections, forming the baseline crystalline fingerprint of the untreated clay. Upon stabilization, systematic changes occur in both peak intensity and sharpness, indicating modifications in crystallinity, interlayer stacking, and the formation of secondary reaction products. Fig. 11. Open in a new tab XRD patterns of the natural soil and selected MK-Nano-CaCO 3 mixtures (C-N-0.1-M-4, C-N-0.4-M-4, C-N-1-M-10), showing reductions in kaolinite intensity, the emergence of amorphous reaction products, and increased calcite crystallinity at higher nanoparticle dosages. In the optimal mixture C-N-0.1-M-4, the intensity of the kaolinite peak at 12–13° decreases markedly compared to the untreated soil, reflecting partial disruption of the kaolinite structure through MK-induced pozzolanic reactions and the dissolution of aluminosilicate layers 126 . Simultaneously, slight broadening of the quartz peaks most evident around 26.6° suggests surface deposition of amorphous C-A-S-H/C-S-H-type gels that coat crystalline grains, consistent with the dense gel-rich microstructure observed in SEM (Fig. 10 ). The emergence of a subtle hump in the region of 25–35° indicates an increase in the amorphous phase fraction, commonly associated with the development of pozzolanic reaction products 127 . These changes correspond precisely to the superior UCS, low compressibility, and high Cv measured for this mixture, confirming that moderate MK and low nano-CaCO 3 contents promote optimal gel nucleation and microstructural refinement. The reduction in kaolinite peak intensity together with the development of a broad hump in the 25–35° 2θ range indicates an increase in amorphous aluminosilicate reaction products consistent with C-(A)-S-H-type phases reported in MK-calcium systems. However, it is important to note that XRD has limited sensitivity to poorly crystalline or nanostructured gels, and therefore the phase assemblage is interpreted qualitatively rather than as a definitive quantitative identification. Complementary techniques such as TGA, FTIR, or solid-state NMR would be required for precise phase quantification, which was beyond the scope of the present study. At the intermediate formulation C-N-0.4-M-4, the mineralogical modifications are less pronounced. The kaolinite peak shows only moderate reduction, and the quartz peaks remain sharp, indicating that although some pozzolanic activity occurs, the formation of new amorphous phases is less extensive. This aligns with SEM observations showing discontinuous gel patches and partially laminated clay platelets. The attenuation of kaolinite peaks together with the development of a broad amorphous hump in the 25–35° range suggests the formation of amorphous aluminosilicate reaction products consistent with C-(A)-S-H-type gels, as widely documented in MK-based stabilization systems. The relatively unchanged crystallinity suggests that increasing nano-CaCO 3 to 0.4% begins to shift its role from uniformly dispersed nucleation sites toward localized filler domains, reducing the efficiency of MK dissolution and gel precipitation 128 . This mechanistic interpretation is fully consistent with the moderate mechanical improvement and intermediate consolidation response recorded for this mixture. The weakest-performing mixture, C-N-1-M-10, shows a markedly different XRD signature. While the quartz and kaolinite peaks remain clearly defined, their relative intensities change minimally compared to the untreated soil, indicating limited dissolution or structural alteration of the clay minerals. Instead, a dominant and sharply defined calcite peak at 2θ ≈ 29.4° becomes significantly more intense than in the other mixes, demonstrating the presence of substantial unreacted nano CaCO 3 . This strong calcite crystallinity validates the SEM evidence showing severe nanoparticle agglomeration and the absence of a continuous gel network. The minimal reduction in kaolinite intensity and the absence of a pronounced amorphous hump confirms that pozzolanic reactions are severely inhibited when both MK and nano-CaCO 3 are overdosed, likely due to insufficient available Ca 2+ for MK activation and excessive competition for water that limits dissolution processes 40 , 129 , 130 . This mineralogical stagnation directly explains the poor UCS, low Cv, and high mv previously reported for this formulation. Collectively, the XRD results reveal a clear mechanistic pathway: (i) At low nano-CaCO 3 (0.1%) and moderate MK (4%), enhanced nucleation promotes MK dissolution and extensive formation of amorphous C-A-S-H/C-S-H-like gels, reducing crystalline peak intensities, increasing amorphous content, and producing superior engineering performance. (ii) At moderate nano-CaCO 3 (0.4%), mineralogical alterations are weaker, reflecting partial inhibition of pozzolanic reactions due to non-uniform nanoparticle dispersion. (iii) At high nano-CaCO 3 (1%) and MK (10%), agglomeration dominates, calcite crystallinity increases, and pozzolanic products diminish, yielding a largely unreacted crystalline structure with minimal amorphous phases and poor engineering outcomes. These trends confirm that the development of amorphous reaction products not the persistence of crystalline minerals is the primary driver of mechanical enhancement in MK-nano-CaCO 3 stabilized clays, fully supporting the conclusions drawn from compaction, UCS, consolidation, and SEM results 131 , 132 . The observed improvements in strength and fabric densification under optimal MK-NC ratios are consistent with prior studies showing that Ca availability critically influences aluminosilicate gel structure, cross-linking density, and mechanical performance in geopolymer-stabilized clays 133 , 134 . In particular, Ca 2+ substitution and controlled CaO/Al 2 O 3 and SiO 2 /Al 2 O 3 ratios have been shown to regulate gel compactness and interparticle bonding, supporting the proposed reactivity-balance mechanism in which Nano-CaCO 3 supplies nucleation sites and Ca species that facilitate controlled gel evolution within the MK-modified clay matrix 135 , 136 . LCIA and interpretation Environmental assessment The LCIA was performed using SimaPro 9.1 software and the ReCiPe 2016 methodology, applying the Hierarchist (H) characterization perspective. This perspective provides a balanced consensus, integrating scientific plausibility with typical policy timeframes. The analysis was structured on two levels. First, a midpoint analysis quantified impacts across 18 specific categories, such as climate change, fine particulate matter formation and fossil resource scarcity, providing a detailed profile of environmental mechanisms. These midpoint results were then aggregated at the endpoint level into three areas of protection: Human Health, Ecosystems and Resources. Finally, these endpoint damages were combined into a single score, enabling a direct and comprehensive environmental comparison between the conventional concrete and the novel composite material. The full set of midpoint impact categories is detailed in Table 3 . The comparative results of the midpoint analysis are presented in Fig. 12 , the aggregated endpoint damages are shown in Fig. 13 , and the final single-score comparison is illustrated in Fig. 14 . Table 3. ReCipe 2016 midpoint impact category for C-N1-M40 vs. OPC. Impact category Unit C-N1-M40 OPC Global warming kg CO 2 eq 0.2779004 0.86592548 Stratospheric ozone depletion kg CFC11 eq 1.13E-07 9.41E-08 Ionizing radiation kBq Co-60 eq 0.03214167 0.008686382 Ozone formation, Human health kg NOx eq 0.000618671 0.001538635 Fine particulate matter formation kg PM2.5 eq 0.000596949 0.000614902 Ozone formation, Terrestrial ecosystems kg NOx eq 0.000624942 0.00155617 Terrestrial acidification kg SO 2 eq 0.001020622 0.001443961 Freshwater eutrophication kg P eq 0.0001397 8.58E-05 Marine eutrophication kg N eq 9.27E-06 5.45E-06 Terrestrial ecotoxicity kg 1,4-DCB 0.37814636 0.89535344 Freshwater ecotoxicity kg 1,4-DCB 0.006435459 0.005493753 Marine ecotoxicity kg 1,4-DCB 0.008871221 0.007956737 Human carcinogenic toxicity kg 1,4-DCB 0.011387118 0.008810728 Human non-carcinogenic toxicity kg 1,4-DCB 0.15889834 0.16773755 Land use m 2 a crop eq 0.004447034 0.00547379 Mineral resource scarcity kg Cu eq 0.007699705 0.00427268 Fossil resource scarcity kg oil eq 0.070846656 0.087653663 Water consumption m 3 0.002544641 0.001847584 Open in a new tab Fig. 12. Open in a new tab LCA results of ReCiPe 2016 midpoint impact category for C-N1-M40 vs. OPC. Fig. 13. Open in a new tab LCA results of ReCiPe 2016 endpoint damage assessment for C-N1-M40 vs. OPC. Fig. 14. Open in a new tab LCA results of ReCiPe 2016 endpoint single score for OPC vs. C-N1-M40. The comparative analysis at the midpoint level reveals distinct environmental profiles for the two binder systems. As detailed in Table 3 ; Fig. 12 , the proposed C-N1-M40 binder demonstrates a significant reduction in Global Warming Potential (GWP), releasing 0.278 kg CO 2 eq per kg compared to 0.866 kg CO 2 eq for OPC11. This represents a reduction of approximately 68% in carbon footprint. This substantial decarbonization is directly attributable to the exclusion of Portland cement clinker from the formulation. The production of clinker is inherently carbon-intensive due to the geogenic CO 2 released during the decarbonation of limestone (CaCO 3 = CaO + CO 2 ) and the high thermal energy required for the rotary kilns 137 . C-N1-M40 bypasses this fundamental chemical step. Beyond decarbonization, C-N1-M40 exhibits broad environmental superiority in categories typically driven by fossil fuel combustion and heavy industrial emissions. The novel binder achieves substantial reductions in Ozone Formation (both Human Health and Terrestrial Ecosystems), Terrestrial Acidification, and Terrestrial Ecotoxicity, with impacts ranging from 28% to 60% lower than the reference OPC. Specifically, Terrestrial Ecotoxicity drops significantly from 0.895 to 0.378 kg 1,4-DCB. This performance improvement is linked to the reduced combustion of coal and petcoke typically utilized in cement kilns, which are major sources of nitrogen oxides (NO x ) and sulfur dioxide (SO 2 ) the primary precursors to acidification and photochemical smog 138 , 139 . Additionally, the lower Fossil Resource Scarcity (0.071 vs. 0.088 kg oil eq) confirms that the geopolymer production route is less dependent on primary hydrocarbon extraction than the conventional cement industry. The LCI identifies environmental trade-offs, leading to a burden shift with C-N1-M40 showing increased impacts in Ionizing Radiation (0.032 vs. 0.009 kBq Co-60 eq), Mineral Resource Scarcity (0.0077 vs. 0.0043 kg Cu eq), and Water Consumption (0.0025 vs. 0.0018m 3 ). These adverse effects, along with higher Freshwater and Marine Eutrophication scores, are primarily linked to the upstream processing of non OPC materials. Since the nano powder was produced via mechanical grinding of nano-CaCO 3 , the elevated Ionizing Radiation is attributed to the high specific electrical input required for the comminution process, acting as a proxy for the nuclear fraction of the grid electricity 140 . The higher Mineral Resource Scarcity and Water Consumption stem from the intensive resource concentration and beneficiation steps necessary for the metakaolin and nano-CaCO 3 production. To reconcile the conflicting indicators observed at the midpoint level, an endpoint damage assessment was conducted. Figure 13 aggregates the results into three areas of protection. Despite the specific trade-offs noted above, the C-N1-M40 binder demonstrates lower overall damage across all three aggregated categories compared to OPC. The most significant benefit is observed in Human Health, where the normalized impact of C-N1-M40 is roughly 56% of the impact generated by OPC. This suggests that the reduction in fine particulate matter and global warming emissions outweighs the increases in toxicity categories when weighted for actual damage to human life years. Similarly, the Ecosystems and Resources damage score for the novel binder is approximately 40% and 59% of the OPC baseline. Within the defined cradle-to-gate system boundary and foreground modeling assumptions, the C-N1-M40 system corresponds to an estimated reduction of approximately 68% in Global Warming Potential relative to OPC. The final Single Score comparison (Fig. 14 ) provides the definitive environmental verdict. The total environmental load of OPC is calculated at 23.16 points (summing Human Health, Ecosystems, and Resources), whereas C-N1-M40 totals 12.79 points. To reconcile the conflicting indicators observed at the midpoint level, the results were aggregated into the three endpoint damage areas of protection. Despite the necessary environmental trade-offs documented in categories like Ionizing Radiation and Mineral Resource Scarcity, the novel binder consistently demonstrates lower overall damage across Human Health, Ecosystems, and Resources. Overall, the analysis confirms the environmental viability of the novel material. This outcome confirms that the substantial reductions achieved in high-impact categories, particularly Global Warming Potential and emissions leading to acidification and ecotoxicity, successfully outweigh the burdens associated with the energy-intensive processing of the nano-components. This holistic assessment validates the C-N1-M40 formulation as a significant and effective technological shift for achieving environmental load reduction in the construction sector. It should be noted that the LCA results are inherently sensitive to assumptions regarding foreground energy intensity, transport distances, and regional electricity mix. While absolute impact magnitudes may vary under different industrial or geographic conditions, the elimination of clinker production remains the dominant factor driving the comparative GWP reduction observed in this modeled scenario. Eco-cost assessment To provide an economic valuation of environmental externalities, an eco-costs analysis was conducted, utilizing the same LCI data. The eco-costs method represents a sophisticated monetization approach, calculating society’s Willingness to Pay (WTP) to prevent environmental pollution and resource depletion, expressed in euros per unit of emission. These values reflect the welfare loss associated with releasing one additional kilogram of a pollutant, thereby quantifying external costs absent from conventional market prices. This conversion of the LCI into a unified monetary metric offers a direct, supplemental comparison of the total environmental cost between OPC and C-N1-M40. Figure 15 shows the results of eco-cost assessment. Fig. 15. Open in a new tab LCA results of eco-cost assessment for OPC vs. C-N1-M40. The total eco-cost for C-N1-M40 is approximately 0.065 €/kg, which is about 40% lower than the 0.109 €/kg calculated for OPC. This significant reduction underscores the economic benefit of the novel binder when external environmental costs are accounted for. The primary driver of this advantage is the drastically lower cost associated with Climate Change (0.0155 € vs. 0.0488 € for OPC), contributing the largest single saving. Substantial savings are also evident in Particulate Matter Formation and Terrestrial Acidification, consistent with the avoidance of fossil fuel combustion in cement kilns. The analysis confirms specific trade-offs identified in the LCIA. C-N1-M40 shows a higher eco-cost in Ionising Radiation due to the electricity-intensive calcination of metakaolin. A moderately higher cost in Human Toxicity is also observed. Despite these specific increases, their magnitude is overshadowed by the major savings in climate and air pollution impacts. The eco-cost valuation quantitatively supports the conclusion from the eco-cost assessment; C-N1-M40 offers a more sustainable profile not only in environmental terms but also by reducing the hidden societal costs of production. Conclusions This study introduced a multiscale stabilization approach for active clay soils by coupling metakaolin with nano-calcium carbonate, identifying the optimal pozzolanic-nanoparticle interaction window and establishing a direct link between microstructural evolution and macroscopic engineering behavior. A comprehensive experimental program, including plasticity, compaction, consolidation, UCS, SEM, XRD, and LCA was used to capture the multiscale mechanisms governing soil response. The key findings are summarized as follows: A narrow interaction domain (4 wt% MK + 0.1 wt% Nano-CaCO₃) produces optimal mechanical and volumetric behavior, confirming that strength and densification are governed by compositional balance rather than additive content alone. Strength enhancement (250% UCS increase) and reduced compressibility arise from the formation of a continuous, well-dispersed C-(A)-S-H-type gel network that promotes interparticle bridging, pore refinement, and efficient stress transfer. Excess MK or nanoparticle dosage induces agglomeration, incomplete polymerization, and microstructural heterogeneity, leading to performance decline. Compaction and consolidation responses corroborate the microstructural findings, demonstrating that optimum performance emerges only when pozzolanic reactivity and nanoparticle-driven nucleation are precisely balanced, thereby minimizing void ratio and enhancing drainage efficiency. Within the defined cradle-to-gate system boundary (1 kg functional unit), the optimized binder formulation demonstrates a substantial reduction in Global Warming Potential relative to OPC, supporting its potential as a sustainable alternative for high-performance clay stabilization. Although tested on a specific active clay, the observed multiscale mechanisms are generalizable to fine-grained soils with similar mineralogical characteristics. Long-term durability, chemical resistance, and field-scale behavior remain open questions. Future work should investigate aging processes, cyclic wetting-drying, ion-exchange effects, and field-compaction conditions to extend the applicability of the MK-Nano-CaCO 3 synergy window. Author contributions M.H.: Methodology, Validation, Writing–original draft, Writing–review & editing, Conceptualization, Investigation, Resources.M.A.: Methodology, Validation, Writing–original draft, Writing–review & editing, Investigation, Resources, Supervision, Conceptualization. Funding This research received no external funding. Data availability All data generated or analyzed during this study are included in this published article. Declarations Competing interests The authors declare no competing interests. Footnotes Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Contributor Information Mahtab Hosseinzadeh, Email: mahtab.hosseinzadeh@alum.sharif.edu, Email: m.hosseinzadeh.geo@gmail.com. Mohsen Abdollahi, Email: mohsenabdolahi101010@gmail.com. References 1. Gaspar, T. A. V. et al. The mechanical properties of a high plasticity expansive clay. Eng. 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