The effect of metakaolin on the properties of MSWI fly ash-slag binder: Compressive strength and chloride ions immobilization.

The use of a multi-component binder (MCB), consisting of Ordinary Portland Cement (OPC) combined with one or more supplementary cementitious materials (SCMs), has gained prominence for enhancing sustainability and improving the performance of cementitious systems. This study provides an integrated approach to optimize both binder composition and aggregate gradation through advanced mixture design and particle packing techniques. The MCB system consists of OPC partially replaced with SCMs such as fly ash (FA), Ground Granulated Blast Furnace Slag (GGBFS), metakaolin (MK), and silica fume (SF), with particle sizes ranging from micron to sub-micron scale. The D-optimal mixture design (DOD) method is used to determine the optimal material proportions by evaluating the relation between binder composition and wet packing density measured through the wet packing method (WPM). To further enhance packing efficiency, the Modified Toufar Model (MTM) is employed to optimize fine aggregate gradation. The maximum packing density is considered the primary criterion for identifying the optimal mix design, as it reflects the minimum void ratio and the most efficient particle size distribution. The optimized mortar mixes are evaluated for mechanical strength, pozzolanic reactivity, capillary water sorptivity, and drying shrinkage. Results indicate that the optimized MCB and optimized fine aggregate gradation improve the packing density and pozzolanic activity, significantly enhancing strength and durability performance. The incorporation of SCMs offers an effective strategy to improve performance while mitigating carbon emissions. Compared with C100, CFGMS-based systems achieved energy reductions of 35-40% and CO2 emission reductions of 34-48%.

The rising environmental burden of Portland cement production has intensified the demand for eco-friendly binders that support sustainable construction. This study investigates the development and performance of eco-friendly self-compacting geopolymer concrete (SCGC) produced from industrial by-products, including fly ash (FA), ground granulated blast furnace slag (GGBFS), silica fume (SF), metakaolin (MK), and glass waste powder (GWP). Twenty-one binder formulations were evaluated for fresh-state workability, mechanical performance, durability, and microstructural characteristics under different curing regimes. Fresh properties were assessed using slump flow, V-funnel, L-box, and J-ring tests, while hardened-state evaluations included compressive and flexural strength, Young’s modulus, and water absorption. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis were performed on selected mixes to examine microstructural features and crystalline phase development. Results highlight a strong dependency of SCGC performance on binder composition and curing conditions. Mixes rich in GGBFS and SF demonstrated superior mechanical and durability performance, achieving compressive strengths of up to 102.4 MPa under water curing and 107.6 MPa under heat curing, along with negligible water absorption, reflecting a dense and well-developed gel matrix. SEM micrographs confirmed homogeneous, compact microstructures in high-performing mixes, while XRD analysis revealed broad amorphous humps indicative of well-formed N-A-S-H and C-A-S-H gel phases with minimal crystalline residues. In contrast, FA-dominant mixes displayed delayed strength development, and MK-GWP-rich systems exhibited higher porosity and reduced strength. This study underscores the significance of precursor synergy, optimized curing strategies, and microstructural refinement in tailoring SCGC for high-performance, durable, and low-carbon applications in sustainable construction with values ranged from 38.64 GPa (Mix 21) to 25.04 GPa (Mix 19) at 28 days. Stiffer mixes corresponded to denser matrices containing GGBFS and silica fume, whereas lower values were linked to weaker bonding and higher porosity.

This work supports the circular economy and sustainable material by facilitating the creation of low-carbon materials with enhanced elimination of nutrients from wastewater, thereby assisting in preventing eutrophication. Porous geopolymers, owing to their distinctive pore structure and numerous superior properties, including noise reduction and thermal insulation, have a wide range of potential applications in the building sector, chemical industry, and water treatment. Developing low-carbon-footprint porous geopolymer materials is an important step toward creating multipurpose lightweight materials that can serve as structural materials and, at the same time, as adsorbents. In this study, it was revealed that the porous material created during the hydrothermal synthesis of (lime–Portland cement-based aerated composition), by replacement of sand with wood biomass bottom ash (WBA), can be used as porous aggregates (PA) for adsorbent development. PA was produced with an apparent porosity of 65%, a density of 610 kg/m3, and a compressive strength of 2.0 MPa. The effectiveness of employing an air-entraining additive (AEA) and creating PA in geopolymers was tested. A different-molarity activator was used, and wood biomass fly ash (WFA) and metakaolin (MK) waste were used as precursors for the synthesis of porous geopolymers. Using an air-entraining admixture in geopolymers allows for the production of lightweight geopolymers with densities up to 1400 kg/m3, compressive strengths up to 8.0 Mpa, and apparent porosities up to 38.4%. Such properties, together with their low cost, offer good prospects for geopolymers in the construction industry. By utilizing PA in the geopolymer composition, a lightweight geopolymer (GPA) with a density of 985 kg/m3 and a compressive strength of 3.9 Mpa, with 42.0% apparent porosity, was obtained. The materials effectively removed phosphorus from biologically treated wastewater: PA had an efficiency of up to 82.5%, the geopolymer with AEA had an efficiency of up to 88.4%, and GPA had an efficiency of up to 97%. The created GPA enhances the adsorbent’s sorption capacity, resulting in extremely high phosphorus uptake efficiency.

The growing demand for eco-efficient cementitious materials has increased the use of high levels of pozzolanic additions, which, despite their environmental benefits, may adversely affect durability, particularly resistance to carbonation. This study investigates the influence of hydrated lime (HL) on the performance of pozzolanic cementitious mortars, with emphasis on carbonation resistance. HL was incorporated into the mortar composition and into the curing solution. A total of 45 mixtures combining cement, fly ash (FA), metakaolin (MK) and HL were produced with different water-to-binder (W/B) ratios. Workability, compressive strength and resistance to accelerated carbonation were experimentally assessed. The results show that workability is primarily governed by the W/B ratio and decreases at high HL contents. Although FA and MK improve mechanical performance, they increase carbonation susceptibility due to reduced alkaline reserve. For the mixtures investigated, moderate HL incorporation into the mortar composition mitigates carbonation, reducing carbonation depth by up to 30–50% relative to the reference mixture. Curing in lime-saturated water does not provide additional benefits under the conditions investigated when compared with conventional water curing.

To address the issues of lengthy test cycles and singular evaluation methods for the durability of cement-based materials in sulfate environments, this study conducted accelerated sulfate erosion experiments on 3D-printed cement-based materials incorporating various supplementary cementitious materials, such as fly ash (FA), silica fume (SF), and metakaolin (MK), by applying a controlled external electric field. Electrochemical impedance spectroscopy (EIS) was employed to characterize the extent of material degradation. By regulating electric field intensity and sulfate solution conditions, and integrating multiple analytical techniques including electrochemical impedance spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TG), this research revealed that the applied electric field accelerates both cement hydration and sulfate-induced degradation through enhanced ion migration. Furthermore, the impedance response characterized by electrochemical impedance spectroscopy showed a positive correlation with macroscopic mechanical properties. Comparative evaluation of sulfate resistance among cement composite systems, FA–cement composites, SF–cement composites, and MK-cement composites indicated that cement-based material with 10% FA addition exhibited the strongest corrosion resistance. This study analyzes the changes in electrochemical impedance spectroscopy characteristic parameters before and after the application of an electric field, systematically elucidating the evolution of the material’s microstructure and the patterns of durability degradation. It provides key mechanisms for understanding the durability evolution of cement-based materials exposed to sulfate environments under electric-field assistance, and lays both experimental and theoretical foundations for developing methods to enhance durability.

This study investigates the synthesis and characterization of metakaolin-based geopolymers for the immobilization of simulated aluminum-containing radioactive liquid waste. Two kaolin precursors with different Si/Al ratios and purities were calcined between 700 °C and 900 °C. Geopolymers were prepared using a sodium silicate–NaOH activating solution (10 M NaOH) with and without sand, and cured at 60 °C. The effects of curing time and simulated liquid waste incorporation (10–40 wt%) on mechanical strength and microstructural development were evaluated through compressive strength tests, XRD, and SEM analyses. The results showed that curing time influenced strength development. Incorporation of simulated liquid waste generally reduced compressive strength, probably due to increased porosity and decreased metakaolin (MK) dissolution; however acceptable performance was achieved at a 20 wt% addition for MKSR-based geopolymers. XRD analyses confirmed the formation of an amorphous band between 25° and 35° typical of geopolymer structures. In contrast, MKS-based geopolymers exhibited lower mechanical strength and incomplete gel formation under the tested conditions. These findings demonstrate the potential of local precursor MKSR metakaolin-based geopolymers as promising matrices for the immobilization of aluminum-bearing radioactive liquid waste.

Using Metakaolin as an Enhancing Component in Supersulfated Cement: A Comprehensive Evaluation.

This study systematically compares the effects of low-alkalinity metakaolin (MK) and high-alkalinity ground granulated blast furnace slag (GBFS) on the hydration, K-struvite stability, and long-term mechanical performance of magnesium potassium phosphate cement (MKPC), with the aim of addressing the gap in existing research that lacks a clear correlation between mineral admixture alkalinity and MKPC durability. Specifically, the low-alkalinity MK acts mainly as inert fillers during early hydration, maintaining the pH conditions required for hydration and promoting K-struvite crystallization via nucleation sites from fine particles. At later ages, MK enhances K-struvite stability and refines the microstructure through limited reactivity, leading to improved long-term compressive strength and volume stability of MKPC. In contrast, the high-alkalinity GBFS rapidly raises the system pH during early hydration, hindering MgO dissolution and K-struvite formation. At later ages, its high alkalinity accelerates K-struvite decomposition and induces excessive amorphous phases, ultimately resulting in strength degradation and volume expansion.

To meet 2050 climate targets, the construction sector must reduce CO2 emissions and transition toward circular material flows. Recycled aggregates (RA) derived from construction and demolition waste (CDW) and industrial byproducts such as oil shale ash (OSA) show potential for use in concrete, although their application remains limited by standardisation and performance limitations, particularly in structural uses. This study aims to develop and evaluate low-strength, resource-efficient concrete mixtures with full replacement of natural aggregates (NA) by CDW-derived aggregates, and partial or full replacement of cement CEM II by OSA–metakaolin (MK) binder, targeting non-structural 3D-printing applications. Mechanical performance, printability, cradle-to-gate life cycle assessment, eco-intensity index, and transport-distance sensitivity for RA were assessed to quantify the trade-offs between structural performance and global warming potential (GWP) reduction. Replacing NA with RA reduced compressive strength by ~11–13% in cement-based mixes, while the aggregate type had a negligible effect in cement-free mixtures. In contrast, full cement replacement by OSA-MK binder nearly halved compressive strength. Despite the strength reductions associated with the use of waste-derived materials, RA-based cement-free 3D-printed specimens achieved ~30 MPa in compression and ~5 MPa in flexure. Replacing CEM II with OSA-MK and NA with RA lowered GWP by up to 48%, with trade-offs in the air-emission, toxicity, water and resource categories driven by the OSA supply chain. The cement-free RA mix achieved the lowest GWP and best eco-intensity, whereas the CEM II mix with RA offered the most balanced multi-impact profile. The results show that regionally available OSA and RA can enable eco-efficient, structurally adequate 3D-printed concrete for construction applications.

The thixotropy, shootability, and mechanical properties of wet-mix shotcrete (WMS) mixtures with high replacement rates of recycled fine aggregate (RFA) of 75% and 100% are examined in this study. Four different alternatives including reduced free mixing water and addition of metakaolin (MK), thixotropy enhancing agent (TEA), and polypropylene fibres (PPF) are evaluated in order to improve the WMS properties. Test results demonstrated that the build-up thickness improved by 1.3- to 2-folds with the incorporation of MK, PPF, and TEA, reflecting their suitability to overcome the inferior shootability at high RFA additions. The TEA significantly increased the development of thixotropy, which was ascribed to the chemical reactions that create a gel and dense network structure. The use of 1% TEA in the mix containing 75% RFA increased by 64% the thixotropic initial shear stress. Nevertheless, this was accompanied with the highest drop in strength, requiring proper tailoring of the dosage rate and mortar composition. Mixtures prepared with reduced water-to-binder ratio from 0.45 to 0.4 compensated the drop in strength due to high RFA rates. Yet, this alternative is not an efficient to counterbalance the decline in shootability, given the increased superplasticizer demand that promotes bleeding and sagging despite the high thixotropy.

Metakaolin (MK) is commonly added to enhance the mechanical and durability properties of concrete. The pozzolanic activity of MK can be improved by calcining approximately ≈ 1 wt% ZnO with kaolin. However, the role of ZnO in enhancing the pozzolanic activity of MK remains unclear. The aim of this work is to investigate the surface modification of MK caused by ZnO that could enhance the pozzolanic activity of MK. ZnO-modified metakaolin (MMK) was prepared by calcination of kaolin powder (90μm) with zinc carbonate basic equivalent to 1% by weight of ZnO, at 850°C, and was analyzed by FTIR, XRD, and SEM techniques. The strength activity index according to ASTM C618, and the physico-chemical properties of blended cement mortars were measured at 28 days. Cement mortar samples were analyzed by FTIR, XRD, and SEM techniques. The XRD and FTIR results of MMK did not detect products of the interaction of ZnO and MK due to the detection limits. The SEM results illustrate the formation of uniform, non-aggregated (MMK) particles. The physico-chemical properties, strength activity index, FTIR, and XRD results of MK blended cement mortars indicated the higher pozzolanic activity of MMK. Whereas the SEM imaging shows the dispersion of cement particles coated with intense honeycomb-like C-S-H without being agglomerated in the case of MMK blended cement mortar. It was concluded that ZnO improves the pozzolanic activity by modifying the surface properties of MK during the calcination process as well as during the hydration process. The proposed mechanisms of surface modification of MK by ZnO were discussed. The addressed mechanism for visualizing the surface chemistry and microstructure of MMK paves the way for future studies on improving the pozzolanic activity of MK and the sustainability of cementitious-pozzolanic compositions.

The use of waste concrete powder (WCP) in geopolymer synthesis offers a sustainable method for recycling construction and demolition waste (CDW). However, its low reactivity and high calcium content frequently result in geopolymers with inadequate mechanical strength. To overcome this challenge, this study proposes a novel method that involves the synergistic addition of metakaolin (MK) to improve the performance of WCP‐based geopolymers. The study systematically examines the effects of NaOH concentration (6, 10, 14 M) and MK content (0%–100%) on the unconfined compressive strength (UCS), microstructure, and reaction products. The findings indicate that the inclusion of MK significantly improves the UCS. An optimal blend of 75% MK and 25% WCP, activated by a 14 M NaOH solution, achieves a peak strength of 46.84 MPa, which is considerably higher than that of geopolymers made from either precursor independently. Microstructural analyses (X‐ray diffraction [XRD], scanning electron microscopy/energy‐dispersive X‐ray spectroscopy [SEM/EDS], and Fourier‐transform infrared spectroscopy [FTIR]) confirm that the strength improvement is due to the formation of a denser and more uniform matrix. This is characterized by the cohesive coexistence of sodium aluminosilicate hydrate (N–A–S–H) gel from MK and calcium (aluminate) silicate hydrate (C–A–S–H) gel from WCP. The innovation of this study lies in demonstrating the effective synergy between a highly reactive aluminosilicate source (MK) and a low‐reactivity waste material (WCP), transforming a potential performance inhibitor (calcium) into a strength‐enhancing element. This study presents a practical and efficient strategy for converting waste concrete into valuable geopolymer materials, offering significant environmental advantages and fostering sustainable development within the construction sector.

Red Mud (RM), a byproduct of the Bayer’s process, possess significant environmental challenges due to its limited applications beyond landfill disposal. This study investigates the feasibility of developing a novel geopolymer by incorporating Red Mud, Ground granulated blast furnace slag (GGBS) and Metakaolin (MK) mixtures using the simplex-centroid experimental design method and optimize the ternary binder mix with respect to setting time and compressive strength. The role of appropriate mineral proportions presents in all three binders activated with sodium hydroxides were studied to understand their influence on the synthesized geopolymers mixtures. Alkaline activators comprising NaOH and Na₂SiO₃ in a 1:2.5 ratio was used in this study. NaOH molarity was varied between 8, 10 and 12 to identify the optimal concentration. Seven mortar mixes for each molarity were prepared with Alkaline-to-Binder (Al/B) ratio of 0.5 and assessed for setting time and compressive strength under ambient conditions (7 and 28 days) and thermal curing (80 °C for 6 hours). Special cubic model equations were developed to quantify component interactions, with model validity confirmed by high predictive accuracy (R² > 0.95). Quantitative modeling demonstrates that the enhanced effect of the ternary systems compared to binary blends. The mixes with 10M NaOH exhibited better performance across the parameters evaluated. The optimal binder proportions lie within the range of 50–55% GGBS, 20–35% RM, and 20–30% MK. Microstructural studies through X-ray diffraction (XRD) and Fourier transform infrared (FTIR) analyses confirmed a higher degree of geopolymerization, demonstrating a viable alternative to conventional RM disposal as sustainable construction material.

Waste management in coal‐fired power plants presents a significant industrial challenge. Co‐disposal of flue gas desulfurization sludge and reject fly ash (RFA) requires solidification/stabilization before landfilling, due to high concentrations of toxic heavy metals and salts. Cementitious additives enhance the encapsulation capacity of brine‐RFA mixes by activating reactions and precipitation of toxic‐element binding phases. This study assesses chloride ion (Cl − ) immobilization capacity of binary, ternary, and alkali‐activated blends of Portland cement (PC), calcium aluminate cement (CAC), calcium sulfoaluminate cement (CSA), blast furnace slag (BFS), and metakaolin (MK) under NaCl exposure using thermodynamic modeling. Ternary blends of 45% CAC, 10% PC, and 45% MK, and 50% CSA, 10% PC, and 40% MK achieved the highest Cl − sequestration, while 58% CSA and 42% BFS, and 86% BFS with 14% NaAlO 2 achieved the highest Cl − binding capacity in their respective categories.

The present research is aimed at exploring formulations of concrete mixes that incorporate metakaolin (MK), derived from kaolin calcined at different temperatures (700°C and 800°C). The metakaolin was generated in a laboratory setting from pure kaolin extracted in the Magistral de Copala development area, Concordia, Sinaloa. The experimental design encompasses five variants of concrete mixes, including a reference standard and options with 15% and 20% cement replacement with metakaolin at 700°C, as well as mixes with the same substitution percentage using metakaolin at 800°C. The collected results indicate that fluctuations in the calcination temperature do not exert a substantial impact on the mechanical and deterioration resistance characteristics of concrete, as both temperatures promote the formation of metakaolins with high pozzolanic activity. The formulation highlighted for its superior performance in terms of mechanical strength (compression) and durability (microstructural parameters, electrical resistivity, chloride migration) is the one using MK calcined at 800°C, replacing 15% of the weight of the cement. These findings underscore the possibility of obtaining environmentally friendly mineral additions by substituting significant amounts of cement, thus contributing to reducing the carbon footprint associated with its manufacturing.

Circular binder design using CDW and conventional precursors: Engineering and environmental performance assessment.

Replacing cement with supplementary cementitious materials (SCMs) is widely recognized as an effective strategy to reduce the environmental footprint of cement production. However, focusing solely on technical or environmental dimensions is insufficient for drawing comprehensive sustainability conclusions. A thorough evaluation requires integrating key sustainable development indicators across all life-cycle stages. Life Cycle Assessment (LCA) is a valuable tool that quantifies environmental impacts across all stages of a product's life, including green concretes, which are often assessed from cradle to gate. This study examined the technical, economic, and environmental performance of self-consolidating concretes (SCCs) incorporating various SCMs, including silica fume (SF), ground granulated blast furnace slag (GS), metakaolin (MK), and pumice (PU), across 13 mix designs. The technical evaluation included measures of slump flow, compressive strength, and chloride migration, along with a service life prediction model to estimate long-term environmental impacts associated with each mix. Furthermore, an expert questionnaire was developed to prioritize the importance of these performance indicators from construction professionals' perspectives. The results indicate that mixes containing 20% and 15% metakalolin or 15% silica fume demonstrated superior performance across combined technical, environmental, and economic criteria, as confirmed by expert evaluations. Compared to the control mixture, these mixes achieved CO₂ emission reductions of 94%, 93%, and 89% per cubic meter per service-year, respectively. Experts prioritized the highest weightage to the technical criterion (66%), followed by economic (21%) and environmental (13%) considerations.

ABSTRACT Durability of reinforced concrete in marine environments is primarily influenced by chloride-induced corrosion. This research evaluates the performance of concrete mixes containing metakaolin (MK) and fly ash (FA) as partial cement replacements (10% and 20%) under simulated saline exposure. Mechanical properties (compressive and split tensile strength) and durability parameters such as rapid chloride penetration test (RCPT), water absorption, sorptivity, half-cell potential, and accelerated corrosion behavior were studied. Results indicate that MK10FA10 exhibited the highest compressive strength (42.2 MPa), while MK20FA20 demonstrated the lowest chloride permeability (1300 C), lowest sorptivity, lowest water absorption, and maximum resistance to corrosion. The combined use of MK and FA significantly enhanced durability, reduced permeability, and delayed corrosion initiation. The study confirms that MK20FA20 offers superior corrosion resistance, making it suitable for marine and coastal construction applications. Keywords: Metakaolin, Fly Ash, Chloride Penetration, RCPT, Corrosion Resistance, Saline Conditions, Reinforced Concrete.

Concrete production has a drastic effect on the environment with ordinary portland cement (OPC) contributing approximately 6-8% to the world CO2 emissions. The best solutions to reduce these effects is the use of recycled concrete aggregate (RCA) and secondary cementitious materials (SCM) as an alternative to natural aggregates and OPC. Nevertheless, RCA based on low-strength parent concrete is normally characterized by high porosity, low-bond mortar and low mechanical performance which restricts the scope of its structural use. This paper examines the improvement of RCA-based concrete using metakaolin (MK) slurry treatment and addition of silica fume (SF) at different dosages (2.5-10%) with the replacement contents of RCA being 0, 50, 75, and 100%. It has been experimentally found that the addition of MK and SF can significantly enhance mechanical strength and durability and adequately address the intrinsic weaknesses of low-grade RCA. The statistical validation with one-way ANOVA showed that all the P-values were less than 0.05 and proved that the improvements due to the addition of SCM and the adjustment of RCA were significant. In addition, 96 experimental and 48 literature-based datasets were used to predict and optimize compressive strength with the use of machine learning (ML) models. K-fold cross-validation was used to fine-tune hyperparameters and the Grey Wolf Optimizer (GWO) was used to optimize them. Extreme Gradient Boosting (XGB) gave the best accuracy with the highest R2 of 0.949 (training) and 0.899 (testing) and low RMSE of 1.490 and 1.845 respectively. AdaBoost (ADB) also provided satisfactory results after XGB (R2 = 0.929 training, 0.878 testing). In general, the findings substantiate the claim that the ensemble learning frameworks especially XGB are quite effective to derive complex relationships in RCA-based concrete data. RCA, SCMs (MK and SF) and predictive ML modeling can provide a sustainable mix design optimization route and structural life enhancement of RCA concrete in rigid pavement applications.