The main problem in expansive soil treatment with steel slag (SS) is the relatively slow hydration reaction that occurs during the initial period. To circumvent this, SS-treated expansive soil activated by metakaolin (MK) under an alkaline environment was investigated in this study. Based on a series of tests on the engineering properties of the treated soil, it can be reported that SS could enhance the strength and compressibility of expansive soil, with strength increasing by approximately 108 % for SS contents exceeding 10 % compared to 3 % lime-treated soil, and the compression index reducing by 20 %. Further addition of MK plays a dual role, enhancing strength for higher SS content while excessive MK leads to strength reduction due to insufficient pozzolanic reactions and hydration product transformation. Expansive and shrinkage behaviors are notably improved, with a 5 % increase in SS content reducing the free swelling ratio by 0.66 %-5.9 %, and the combination of 15 % SS and 6 % MK achieving a nearly 300 % reduction in the linear shrinkage ratio. Microstructural analysis confirms the formation of hydration gels, densification of the soil structure, and reduced macropores, validating the enhanced mechanical and shrinkage resistance properties of the SS-MK-treated expansive soil. Additionally, to develop predictive models for mechanical and the content of hardening agents (SS and MK), the experimental data are processed utilizing a backpropagation neural network (BPNN). The results of BPNN modeling predict the mechanical properties perfectly, and the correlation coefficient (R) approaches up to 0.98.

Due to the detrimental ecological impacts and the exorbitant expenses associated with the cement industry, researchers have sought to find natural, sustainable, low-carbon alternatives to Portland cement for weak soil stabilization. This research used geopolymer based on metakaolin (MK), a natural pozzolanic material with different activator concentrations (NaOH and Na2SiO3), to stabilize loose poorly graded sand soils. The research investigated the effect of different amounts of addition MK (5, 10, and 15 %) on the soil's mechanical properties. Furthermore, the effect of parameters such as the type and concentration of the alkaline solution and curing time (1, 3, and 7 days) on the unconfined compressive strength, failure strain, Young's modulus, California bearing ratio, and direct shear test were evaluated. This research also aims to measure the sub- grade reaction modulus (Ks) by developing and manufacturing a laboratory testing apparatus and steel mold to simulate the natural conditions of sandy subgrade soil obtained from performing nonrepetitive static plate load tests. Additionally, scanning electron microscopy images (SEM) and X-ray diffraction analysis (XRD) were also used to study the microstructural changes and the chemical composition of the stabilized soil samples. The results indicate that the soil samples that were stabilized with MK 10 % and NaOH had notably higher compressive strength (2936 kPa), indicating a denser and less porous structure (improved stiffness stabilized soil) in comparison to the soil samples stabilized with MK 10 % and Na2SiO3 which was (447 kPa). Ultimately, Microstructural analysis showed that, due to the addition of 10 % MK, stabilized soils have a denser and more homogeneous structure.

In cold regions' engineering applications, cement stabilized soils are susceptible to strength degradation under freeze-thaw (F-T) cycles, posing significant challenges to infrastructure durability. While metakaolin (MK) modification has shown potential in enhancing static mechanical properties, its dynamic response under simultaneous F-T cycling and impact loading remains poorly understood. This study investigates the dynamic mechanical behavior of cement-MK stabilized soil through split Hopkinson pressure bar (SHPB) tests under varying F-T cycles. The effects of strain rate and F-T cycles on the dynamic failure process and mechanical properties of cement-MK stabilized soil were investigated. Pore characteristics were analyzed using a nuclear magnetic resonance (NMR) system, providing an experimental basis for revealing the degradation mechanism of F-T cycles on the strength of cement-MK stabilized soil. Based on the Lemaitre's strain equivalence principle, a composite damage variable was derived to comprehensively characterize the coupled effects of F-T cycles and strain rate. A dynamic constitutive model is established based on damage mechanics theory and the Z-W-T model. The results indicate that under the effect of F-T cycles induce progressive porosity increase and aggravated specimen damage. At varying strain rates, the strength of cement-MK stabilized soil decreases with increasing F-T cycles, while the rate of strength reduction gradually diminishes. Under impact loading, both strain rate and the number of F-T cycles significantly reduce the average fragment size of fractured specimens. The modified Z-W-T model effectively predicts the stress-strain relationship of the cement-MK stabilized soil under impact loading.

A series of geopolymers were synthesized by employing phosphoric acid (PA) as activator to activate low-calcium fly ash (FA) and metakaolin(MK), and geopolymer mortar was prepared using PA-activated FA-MK geopolymer and dredged soil. The PA-activated low-calcium FA geopolymer typically exhibited low compressive strength. Incorporating MK introduced reactive aluminum, which enhanced the compressive strength of the geopolymer. This strength improvement was further amplified as the M:F ratio (MK:FA ratio) increased. Under a certain M:F ratio, there existed an optimum H3PO4/Al2O3 molar ratio that maximized the compressive strength of geopolymer. A positive correlation was observed between the M:F ratio and the optimum H3PO4/Al2O3 molar ratio, with the latter exhibiting a gradual increase from 0.56 (M:F ratio = 0:1) to 0.64 (M:F ratio = 0.4:0.6) and ultimately 0.86 (M:F ratio= 1:0). The compressive and flexural strengths of the geopolymer mortar were significantly affected by the geopolymer/soil ratio and the PA concentration. When the actual PA concentration in geopolymer mortar approached the optimum PA concentration for the geopolymer paste, the mortar achieved its best mechanical properties. The stabilization of dredged soil using PA-activated geopolymer demonstrates significant sustainability benefits, while their cost-effectiveness and mechanical performance require further optimization. This research provides new approaches and data support for the reuse of low-calcium FA and dredged soil.

Soilcrete is an innovative construction material made by combining naturally occurring earth materials with cement. It can be effectively used in areas where other construction materials are not readily available due to financial or environmental reasons since soilcrete is made from readily available natural clay. It can also help to cut down the greenhouse gas emissions from the construction industry by encouraging the use of resources that are locally available. Thus, it is imperative to reliably predict different properties of soilcrete since the accurate determination of these properties is crucial for the widespread use of soilcrete materials. However, the laboratory determination of these properties is subjected to significant time and resource constraints. As a result, this research was undertaken to provide empirical prediction models for the density, shrinkage, and strain of soilcrete mixes using two machine learning algorithms: Gene Expression Programming (GEP) and Extreme Gradient Boosting (XGB). The analysis revealed that XGB-based predictions correlated more with real-life values than GEP having training R2=0.999\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{R}}{2}=0.999$$\end{document} for both density and shrinkage prediction and R2=0.944\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{R}}{2}=0.944$$\end{document} for strain prediction. Moreover, several explanatory analyses including individual conditional expectation (ICE) analysis and shapely analysis were done on the XGB model which showed that water-to-binder ratio, metakaolin content, and modulus of elasticity are some of the most important variables for forecasting soilcrete materials properties. Furthermore, an interactive graphical user interface (GUI) has been developed for effective utilization in civil engineering industry to forecast these properties of soilcrete materials.

Geopolymers made from simulated Martian regolith would suffer from exhibit poor engineering properties, rendering them unsuitable as base materials. To improve the mechanical properties of geopolymers, this study prepared a new Martian soil simulant named DH-1. DH-1- based geopolymers were synthesized using Al2O3 (Al group) and metakaolin (MK group) as modifying agents. The durability of geopolymers with ultraviolet (UV) radiation was assessed under simulated Martian atmospheric conditions. The results indicated that the UCS and FS of both the Al and MK groups increased with curing time, with maximum UCS and FS of 55.27 MPa and 15.16 MPa, respectively. The UCS and FS of the Al and MK groups exhibited a two-phase change. The inflection points for rapid to slow in the UCS and FS occurred on day 28 for the Al group and day 14 for the MK group. The addition of Al2O3 and metakaolin promoted the replacement of silicon atoms by aluminum atoms in the silicaoxygen group, producing more gels product. After UV irradiation, the UCS and FS of the geopolymer decreased by 13 % and 44 %, respectively. Furthermore, the geopolymers underwent carbonation, with new cracks forming due to the combined effects of UV exposure, causing strength reduction.

This study aims to predict compressive strength (CS) and modulus of elasticity (E) of soilcrete mixes to foster their widespread use in the industry. Soilcfigrete has the potential to promote sustainable construction practices by making use of locally available raw materials. However, the accurate determination of mechanical properties of soilcrete mixes is inevitable to foster their widespread use. Thus, this study employs different machine learning algorithms including Extreme Gradient Boosting (XGB), Gene Expression Programming (GEP), AdaBoost, and Multi Expression Programming (MEP) for this purpose. The XGB and AdaBoost algorithms were implemented using python programming language while MEP and GEP were implemented using specialized softwares. The data used for model development was obtained from previously published literature containing five input parameters and two output parameters. This data was split into two sets named training and testing sets for training and testing of the algorithms respectively. The developed models for CS and E prediction were validated using several error metrices and residual comparison. The objective function value which should be closer to zero for an accurate model is the least for XGB model for prediction of both variables (0.0036 for CS and 0.00315 for E). Moreover, shapley analysis was carried out using XGB model to get insights into the underlying model framework. The results highlighted that water-to-binder ratio (W/B), metakaolin (MK), and ultrasonic pulse velocity (UV) are the most significant variables for predicting E and CS of soilcrete materials. These insights can be used practically to optimize the mixture composition of soilcrete mixes according to different site requirements.

The objective of this study was to improve the physical and mechanical properties of adobes reinforced by cement-metakaolin mixtures. For this purpose, a raw clayey material from Burkina Faso consisting of kaolinite (62 wt%), quartz (30 wt%), and goethite (6 wt%) and belonging to the category of sandy-silty soils with medium plasticity has been used for adobe manufacturing. Metakaolin was produced by thermal activation of a local raw clayey material at 680 degrees C for 2 h. Adobes were first formulated with cement up to 12 wt%. It appears from this formulation that 10 and 12 wt% cement offer good mechanical strength elaborated adobes. Taking into account the high cost of cement in Burkina Faso, 10 wt% of cement was retained to be replaced by 2, 4, 6, 8, and 10 wt% metakaolin. The microstructural (by SEM-EDS), physical (apparent density, porosity, linear shrinkage, water absorption test by capillarity, spray test), and mechanical (compressive and flexural strengths) characteristics of formulated adobes were evaluated. The obtained results showed that this substitution improved adobe microstructure with pores reduction leading to composite densification. The presence of metakaolin slows down the phenomenon of capillary rise of water in adobes. Also, the metakaolin presence within the cementitious matrix improves the mechanical behavior and reduces rain erosion effect. The improvement of different properties was mainly due to formation of calcium silicate hydrates (CSH) resulting by cement hydration and metakaolin's pozzolanic reaction. Adobes reinforced with 6 wt% cement and 4 wt% metakaolin have suitable technological characteristics to be used as building materials for developing countries.

To foster the sustainability of green construction materials utilized in transport infrastructure and generally in soil stabilization for the same purpose, there have been continued efforts towards innovative results for consistent improvement of the mechanical properties of soils. Metakaolin (MK) has been in use as a supplementary material due to its pozzolanic properties. However, it has always produced a limit beyond which there is recorded decline in its ability to cement and strengthen soils in a stabilization protocol. In this research work, a new innovative cementitious material made from 1:1 NaCl + NaOH blend activator mixed with sawdust ash called Ashcrete (A) has been introduced. It is blended with MK in the lateritic soil stabilization procedure. Preliminary results showed that the lateritic soil (LS) has weak consistency with plasticity index above 17%, maximum dry density (MDD) of 1.77 g/cm3 and classified as A-7 soil on American Association State Highway and Transportation Officials (AASHTO) method. The MK and the Ashcrete (A) showed high compositions of aluminosilicates qualifying them as supplementary cements. The MK was used at the rate of 3, 6, and 9%, while the Ashcrete (A) was incorporated at the rate of 2, 4, 6, 8, and 10%. The results of the stabilization exercise showed that the California bearing ratio (CBR) and unconfined compressive strength (UCS) consistently increased with the addition of MK + A blend. This outcome was a shift from the previous work, which had used only MK and recorded 6% addition at which the MK-treated lateritic soil recorded its highest strength, and beyond this mark, there was a decline. The highest strength in this research work was recorded with the stabilization pattern of LS + 9%MK + 10A, which translates to that for a 200 g LS to be treated, 18 g of MK, and 20 g of A are needed to achieve the highest CBR and UCS recorded in this research paper. Finally, the recorded CBR (7-day soaked and unsoaked) and the UCS (7, 21, and 28 days) of the MK + A-treated LS fulfilled the requirements for the construction of a subgrade and subbase.

As a new type of backfill material, Self-compacting solidified soil (SCSS) takes the abandoned slurry of cast-in-place piles after dewatering and reduction as the main raw material, which brings a problem of coordinating the working performance with the mechanical property under the condition of high mobility. In this paper, hydroxypropyl methyl cellulose (HPMC) and metakaolin were introduced as additives to solve this problem. First, the workability and mechanical properties of SCSS were regulated and optimized by means of the water seepage rate test, the flowability test, and the unconfined compressive strength test. Second, this study also used X-ray diffraction (XRD) and scanning electron microscopy (SEM) to investigate the effects of HPMC and metakaolin on the physical phase and microstructure of SCSS. In this way, the results showed that there was a significant impact on the flowability of SCSS, that is, when the dosage reached 0.3%, the water seepage rate of SCSS was reduced to less than 1%, and the compressive strength at 7 days reached its peak. At the same time, HPMC weakened the strength growth of SCSS in the age period of 7 days to 14 days. However, the addition of metakaolin promoted its compressive strength. XRD analysis showed that the additives had no significant effects on the physical phases. And, from the SEM results, it can be seen that although the water-retaining effect of HPMC makes hydration of cement more exhaustive, more ettringite (AFt) can be observed in the microstructure. In addition, it can be observed that the addition of metakaolin can generate more hydrated calcium silicate (C-S-H) due to the strong surface energy possessed by metakaolin. As a result of the above factors, SCSS filled the voids between particles and improved the interface structure between particles, thus enhanced the compressive strength.