Alkali-activated Materials Research Articles

Developments in biochar incorporated geopolymers and alkali activated materials: A systematic literature review

Annually, industries related to agricultural products generate substantial volumes of waste biomass, presenting critical challenges for solid waste management due to its environmental and health implications. Pyrolysis emerges as a significant technique for converting such waste into biochar (BC), a material with notable internal microstructure, porosity, and chemical properties, contingent on the feedstock and processing conditions. BC is recognized for its exceptional water retention and alkalinity, primarily utilized for soil enhancement and fertilization. Its application across various fields, particularly in enhancement for sustainability, has gained momentum. This review meticulously explores BC's role in sustainable composites, focusing on geopolymers and alkali-activated binders through a comprehensive examination of existing literature, underscored by bibliometric analysis. This pioneering review, leveraging the PRISMA framework and VOSviewer for bibliometric insights, aims to bridge the research gap in this evolving domain, offering a critical assessment of BC's integration in sustainable construction materials, its challenges, and future research directions. Hence, this pioneering systematic review represents the first-ever exploration within the research domain of BC's applications in geopolymers and alkali-activated materials, marking a foundational step toward understanding its potential and challenges in sustainable engineering practices.

Mechanical Properties, Workability, and Experiments of Reinforced Composite Beams with Alternative Binder and Aggregate

Arguably the most important element in the sustainability of concrete development is the discovery of an optimal sustainable binder and substitution for the increasingly depleted reserves of natural aggregates. Considerable interest has been shown in alkali-activated materials, which possess good characteristics and could be considered environmentally friendly because of their use of secondary materials in production. The aim of this study was the determination of the mechanical properties of three different mixtures based on the same locally accessible raw materials. The reference mixture contained Portland cement, the second mix contained a finely ground granulated blast furnace slag instead of cement, and the third mixture contained a portion of light artificial aggregate. The experiments focused on the testing and mutual comparison of the processability of the fresh mixture and mechanical characteristics (like compressive and flexural strength, as well as resistance to high temperatures and surface layer tear strength tests). Reinforced concrete beams without shear reinforcement and with three levels of reinforcement were also tested with a three-point bend test. The results show that, overall, the mechanical properties of all the tested mixtures were similar, but each had its own disadvantages. For example, the blast furnace slag-based mixture had a more vulnerable surface layer or a debatable loss of bulk density in the light aggregate mix at the expense of the mechanical properties. One of the main results of the research is that it was possible to technologically produce beams from the alkali-activated concrete (AAC) mixture. Then, the performed beam experiments verified the mechanism of damage, collapse, and load capacity. The obtained results are essential because they present the use of AAC not only in laboratory conditions but also for building elements. In beams without shear reinforcement, the typical tensile cracks caused by bending and shear cracks appeared under loading, where their character was affected depending on the degree of beam reinforcement and loading.

Pseudo strain-hardening alkali-activated composites with up to 100 % rubber aggregate: Static mechanical properties analysis and constitutive model development

Strain-hardening alkali-activated materials (SHAAM) provide a sustainable alternative to conventional strain-hardening cementitious composites (SHCC), offering significant environmental and economic benefits. This study examines the effects of various rubber powder (RP) volume replacement ratios (0 %, 25 %, 50 %, 75 %, and 100 %) on the properties of Rubber Powder Modified Strain-Hardening Alkali-Activated Composite Material (R-SHAAM), assessing workability, uniaxial compressive behavior, and uniaxial tensile behavior. The findings reveal that R-SHAAM with 100 % RP aggregate significantly enhances durability, achieving a 50.7 % reduction in crack width. Optimal axial tensile performance is observed at a 25 % replacement ratio, where R-SHAAM shows an 8.1 % increase in tensile strength and a 4.1 % increase in ultimate tensile strain. The robust multi-dimensional performance of R-SHAAM across different RP replacement ratios suggests its potential for diverse applications including pavement, marine structures, and explosive engineering, thereby offering valuable insights into sustainable material design and the effective recycling of waste materials in construction.

Promoting low carbon construction using alkali-activated materials: A modeling study for strength prediction and feature interaction

Abstract In place of Portland cement concrete, alkali-activated materials (AAMs) are becoming more popular because of their widespread use and low environmental effects. Unfortunately, reliable property predictions have been impeded by the restrictions of conventional materials science methods and the large compositional variability of AAMs. A support vector machine (SVM), a bagging regressor (BR), and a random forest regressor (RFR) were among the machine learning models developed in this study to assess the compressive strength (CS) of AAMs in an effort to gain an answer to this topic. Improving predictions in this crucial area was the goal of this study, which used a large dataset with 381 points and eight input factors. Also, the relevance of contributing components was assessed using a shapley additive explanations (SHAP) approach. In terms of predicting AAMs CS, RFR outperformed BR and SVM. Compared to the RFR model’s 0.96 R 2, the SVM and BR models’ R 2-values were 0.89 and 0.93, respectively. In addition, the RFR model’s greater accuracy was indicated by an average absolute error value of 4.08 MPa compared to the SVM’s 6.80 MPa and the BR’s 5.83 MPa, which provided further proof of their validity. According to the outcomes of the SHAP research, the two factors that contributed the most beneficially to the strength were aggregate volumetric ratio and reactivity. The factors that contributed the most negatively were specific surface area, silicate modulus, and sodium hydroxide concentration. Using the produced models to find the CS of AAMs for various input parameter values can help cut down on costly and time-consuming laboratory testing. In order to find the best amounts of raw materials for AAMs, academics and industries could find this SHAP study useful.

Rainfall-induced wind erosion in soils stabilized with alkali-activated waste materials

This study evaluates the efficacy of sustainable erosion control using slag-based alkali-activated cement crusts under varying rainfall and wind conditions. The rainfall intensities ranged from 30 mm/h to 120 mm/h, with durations ranging from 15 min to 90 min, and crust slopes of ∼2° (gentle) and 30° (steep). Wind tunnel experiments were conducted at wind velocities of 14 m/s, 21 m/s, and 28 m/s to investigate post-rainfall wind erodibility, along with changes in crust strength and microstructure analysis. The findings show the development of hydrated cementitious phases in alkali-activated material, which form around and between the particles during the alkaline activation process. Alkali-activated cement crusts significantly reduced erosion caused by rainfall and subsequent wind by several orders of magnitude. At the highest rainfall intensity of 120 mm/h, rainfall erosion was measured to be 1654.81 kg/m2 for untreated samples and 0.89 kg/m2 for treated samples, demonstrating a substantial 99.95% reduction in erosion due to the treatment. Similarly, at the highest wind speed tested, wind erosion was 122.75 kg/m2 for untreated samples and 0.095 kg/m2 for treated samples, indicating a significant 99.92% reduction in erosion due to the formation of an alkali-activated cement crust on the soil surface. However, exposure of the samples to 120 mm/h rainfall for 90 min resulted in a 5.2-fold increase in wind erosion compared to pre-rainfall conditions. Similarly, penetrometer results indicated a 37%–54% reduction in post-rainfall surface strength.

Developing green slag/bentonite-based geopolymers modified with meso-porous tungsten oxide: Zeolitic phases, mechanical performance and gamma-radiation mitigation

In an attempt to maintain the sustainable development goals in the construction sector via reducing the raw-materials/energy consumption and greenhouse-gas emissions related to cement production, a green nano-modified slag/bentonite-based alkali-activated material was developed. Firstly, the green composites were prepared by mixing slag and bentonite with a ratio of 2:1. Several factors like NaOH-concentration (6, 8, 10 wt%); thermal treatment of bentonite (as-received “RB” and thermally treated at 650 °C “TB”); curing conditions (normal-curing for 3 and 28-days as well as hydrothermal-curing at 3, 6, 9, 15 bar for 4 h) and meso-porous tungsten oxide nano-particles “WO3-NPs” inclusion (0.25, 0.5 and 1 wt%) were studied to assign the optimum conditions for fabricating composites with adequate mechanical properties and radiation-shielding ability. The mechanical performance and radiation shielding were evaluated by measuring the compressive-strengths and linear attenuation coefficient “μ”/ half value layer “HVL” using 137Cs, respectively. The results reveal the feasibility of using 8 wt% NaOH, TB, hydrothermal-curing at 3 bar/4 h and 0.5 wt% WO3-NPs in fabricating low-cost/pre-cast/environmentally friendly building material with superior compressive-strength (53.6 MPa). Also, the radiation shielding results substantiate that this developed composite achieved adequate μ and HVL values, referring to its efficiency as a radiation-blocker. The synergistic impact of alkali-hydrothermal-activation, the high pozzolanicity of TB and the nucleation-site/potential-seeds effect of WO3-NPs are the main reasons behind forming strength-giving-phases from stratlingite, hydrogarnet, analcime and pentasil zeolitic phase (ZSM-5), as proved by X-ray diffraction (XRD), thermogravimetric analysis (TGA/DTG) and scanning electron microscopy (SEM). The presence of such phases reinforced the microstructure, thus improving the mechanical performance and radiation shielding capability.

Acid resistance of alkali-activated binders based on clays from phosphate mining by-products

This paper outlines the results of studying the resistance of alkali-activated materials (AAM), based on calcined yellow clay from phosphate mine by-products as a precursor, to an acid attack (with sulfuric and phosphoric acids, 5 %, pH ≈ 0,8–2). Calcined yellow clay (YC850) is used as the source of aluminosilicates, containing montmorillonite, dolomite, and quartz. The alkali-activation of such materials allows the determination of an adequate valorization way and evaluating their durability leads to defining the range of application. The residual compressive strengths were measured after 1, 2, and 4 weeks of immersion in those acids. The deterioration suffered by the paste specimens was examined using different techniques.The degree of deterioration differs from one acid to another; in sulfuric acid, the strengths decrease from 47 MPa to 10.6 MPa presenting 77.4 %. While in phosphoric acid, the decrease was about 36.2 % (30 MPa) after 28 days of attack. This detriment in the strengths is associated with changes not only in the microstructure but also with the alteration of the reaction products, mainly the binding gels C-A-S-H and (C,N)-A-S-H, which undergoes dealumination and decalcification. What is more, the chemical attack involves several reactions; calcium coming from the precursor reacts with the sulfates (in the H2SO4 attack) inducing the precipitation of gypsum, which is responsible for the significant strength loss. In addition, the dissolution of crystalline minerals, such as periclase and gehlenite (present in the precursor) generates higher porosity explaining the obtained results.

Effects of silica fume/ultrafine fly ash on the rheology and hardening of alkali-activated slag-waste ceramic powder paste

Fast setting and high viscosity are the two main obstacles to obtain desired rheological properties of alkali-activated materials (AAMs). This work aims to remove these obstacles by using a simple yet effective approach via introducing ultra fines with different characteristics. Here in the systems of alkali-activated slag-waste ceramic powder, the mechanism behind the effect of various ultra fines on rheological and hardened properties is studied. Results show that the introduction of ultrafine fly ash (UFFA) is beneficial to maintain good rheological properties and control setting, yet adding silica fume (SF) harms fresh performance due to the extremely low liquid film thickness. By 1H NMR and interstitial solution analysis, the large water demand for wetting SF rather than its dissolution, has been demonstrated to cause the insufficient dissolution of slag, which leads to lower Al3+ and Ca2+ concentrations and less precipitation of early gels. However, once gels start to precipitate, shorter particle spacing favors the rapid formation of percolation network in SF-systems, thus resulting in higher yield stress, viscosity and setting rate, as well as a faster decrease in mobile water. Furthermore, the incorporation of 5 %-10 % UFFA provides superior fresh performance and hardened properties. The results and relevant mechanisms of this study are essential towards better development of high-performance AAMs.

Ethylene Vinyl Acetate Copolymer Emulsion-Modified Alkali-Activated Slag Repair Material: Mechanical Strength and Durability Linked to Microstructural Properties

Ethylene Vinyl Acetate Copolymer Emulsion-Modified Alkali-Activated Slag Repair Material: Mechanical Strength and Durability Linked to Microstructural Properties

Application of Alkali-Activated Cementitious Materials Made of All Solid Wastes in Waterproof Curtains

Application of Alkali-Activated Cementitious Materials Made of All Solid Wastes in Waterproof Curtains

Acoustic emission monitoring for engineered barriers in nuclear waste disposal

To safely dispose of nuclear waste in underground facilities, engineered barrier systems are needed to seal shafts and galleries. The material used in these barriers must be adapted to the host rock parameters. Shrinking and cracking must be avoided to provide a barrier with almost zero permeability. For repositories in salt rock environments, several types of salt concrete (SC) are used. Within the project SealWasteSafe, we compared the performance of an innovative alkali-activated material (AAM) with standard SC in their hydration and hardening phase. To monitor the microstructural changes within the two materials SC and AAM, acoustic emission (AE) signals have been recorded for up to ~250 days on 340-liter-cubic specimens. The phenomenon of AE is defined as the emission of elastic waves in materials due to the release of localized internal energy. Such energy release can be caused by the nucleation of micro-fracture, e.g. in concrete while curing or when exposed to load. The occurrence of AE events gives first rough indications of microstructural changes and potentially occurring cracking and thus, provides insights for structural health monitoring (SHM). The results show, that for the first 28 days after casting, less AE activity was detected in the AAM compared to SC. After 61 days, in the AAM material, the number of AE events exceeded those observed in the SC. However, the majority of the AE detected in AAM was related to surface effects, and not to microstructural changes within the matrix. Additionally, the source location analysis indicated, that despite lower activity in SC, we observed some clustering of the events. In contrast, in AAM, the activity inside the specimen is randomly distributed over the whole volume. The monitoring results help to estimate the material’s sealing properties which are crucial to assess their applicability as sealing material for engineered barriers.

The effect of NaOH and KOH on the strength-mechanical properties of alkali-activated composites based on granulated blast-furnace slag

Abstract Alkali-activated systems are a more sustainable alternative to Portland cement concrete. The activation of latently hydraulic and pozzolanic raw materials in these composites is one of the many investigated factors, where the price ratio and the ability to optimally activate the mentioned precursors with the given activator play a major role. The subject of the presented work is a comparison of the influence of NaOH and KOH on the development of the strength-mechanical properties of alkali-activated materials based on granulated blast furnace slag - the secondary raw material of metallurgical processes.

Valorization of municipal solid waste incineration fly ash in low-carbon alkali-activated materials

Municipal solid waste incineration fly ash (MSWIFA) is a hazardous waste with high alkalinity and is rich in heavy metals (HMs) and chlorine. To facilitate resource disposal, this paper proposes recycle MSWIFA as an alkali activator to develop low-carbon and cement-free alkali-activated materials. The designed MSWIFA-activated slag exhibited impressive compressive strength (>70 MPa). The microstructure of the MSWIFA-activated slags was tightly filled with various reaction products, including ettringite, calcium aluminosilicate hydrate (C-A-S-H), and Friedel’s salt. The leaching behaviors of the HMs and chlorine were significantly restricted owing to the chemical fixation of the reaction products and the dense microstructure of the MSWIFA-activated slags. The immobilization of chlorine primarily depended on the formation of Friedel’s salt, and the remaining chlorine could be captured by the positively charged sites of C-A-S-H. The reaction mechanism of the designed MSWIFA-activated slags could be divided into three stages: ettringite precipitation, C-A-S-H formation, and Friedel’s salt generation. This paper provides constructive insights into the resource utilization of hazardous waste and contributes to the development of low-carbon alkali-activated materials.

The cementitious properties of alkali-activated municipal solid waste incineration fly ash-phosphorus slag-secondary aluminum dross matrix composites and the mechanism of solidification of heavy metals

Based on the purpose of safe and resourceful disposal of hazardous waste, this paper proposes a new method for the combined treatment of municipal solid waste incineration fly ash (MSWIFA), phosphorus slag (PS), and secondary aluminum dross (SAD). A novel alkali-activated binder was synthesized by orthogonal experimental design, and the strength formation mechanism of the binder and the solidification and stabilization (S/S) mechanism of heavy metals were revealed. The results show that the maximum strength of the sample at 60 d is 22.4 MPa. The addition of SAD will affect the development of the strength of the solidified body and the types of hydration products of the gel. The hydration products of the solidified body are mainly calcium silicate hydrate (C-S-H), calcium aluminosilicate hydrate (C-A-S-H), and calcium alumina (AFt). The solidified body of alkali-activated cementitious material has a good S/S effect on heavy metals, and its solidification efficiency is Zn > Cd > Pb > Cu > Ni > As > Cr. The maximum solidification rates of Zn, Pb, and Cd were 99.97 %, 97.90 %, and 99.68 %, respectively. At the same time, the leaching concentration of various heavy metals meets the threshold requirements stipulated by relevant international norms and has good environmental safety.

Efficient Compressive Strength Prediction of Alkali-Activated Waste Materials Using Machine Learning.

This study explores the integration of machine learning (ML) techniques to predict and optimize the compressive strength of alkali-activated materials (AAMs) sourced from four industrial waste streams: blast furnace slag, fly ash, reducing slag, and waste glass. Aimed at mitigating the labor-intensive trial-and-error method in AAM formulation, ML models can predict the compressive strength and then streamline the mixture compositions. By leveraging a dataset of only 42 samples, the Random Forest (RF) model underwent fivefold cross-validation to ensure reliability. Despite challenges posed by the limited datasets, meticulous data processing steps facilitated the identification of pivotal features that influence compressive strength. Substantial enhancement in predicting compressive strength was achieved with the RF model, improving the model accuracy from 0.05 to 0.62. Experimental validation further confirmed the ML model's efficacy, as the formulations ultimately achieved the desired strength threshold, with a significant 59.65% improvement over the initial experiments. Additionally, the fact that the recommended formulations using ML methods only required about 5 min underscores the transformative potential of ML in reshaping AAM design paradigms and expediting the development process.

Improving the stabilization/solidification performance of Pb-containing alkali-activated slag via incorporation of superabsorbent polymer

Alkali-activated materials (AAM) have garnered significant attention for the treatment of Pb-contaminated solid waste, attributed to their low carbon footprint and superior stabilization/solidification (S/S) capabilities. However, recent concerns arise regarding their long-term S/S performance, especially when exposed to rainwater environments and considering the inherent shrinkage and potential cracking of AAM. This research delves into the feasibility of enhancing the S/S efficacy of alkali-activated slag (AAS) through the integration of super absorbent polymers (SAP). Findings reveal a marked improvement in the S/S performance of Pb-laden AAS upon SAP addition. Specifically, incorporating 0.5 wt% and 1 wt% SAP curtailed Pb leaching in AAS samples (cured for 90 days) by 47 % and 75 %, respectively. Comprehensive analyses using XRD, TGA, MIP, BSEM, EDX, XPS and MAS NMR suggest that the bolstered S/S performance of SAP-augmented AAS is not rooted in alterations to the reaction degree, pore structure, or reaction products of AAS. Instead, it hinges on the innate capacity of SAP to absorb and retain Pb. Leaching tests reveal that, in AAS cured for 90 days, approximately 1.72 mg of Pb ions were retained per gram of SAP. This investigation furnishes a novel vantage point for refining and deploying alkali-activated materials (or other cementitious materials) as effective S/S agents.

Viability of hybrid and alkali-activated slag materials for thermal energy storage: Analysis of the evolution of mechanical and thermal properties

Technological progress is needed to develop renewable energies. Improvements should be especially focused on the energy storage element to help correct the mismatch between energy supply and demand. In concentrated solar power (CSP) plants, ordinary concrete made of Portland cement (PC) has been proven to be a good thermal energy storage (TES) medium. However, the substantial environmental impact of PC manufacturing makes it necessary to develop new materials. Thus, in this work, alternative alkali-activated mortars (AAM) and hybrid materials (HM) have been developed using blast furnace slag to replace PC. This has been done in order to study their stability at high temperature (up to 500 °C) and their viability to operate as TES, undergoing thermal cycles between 200 °C and 400 °C, as they would in CSP technologies. Studying the mechanical and thermal properties after the thermal treatments has revealed that the alternative materials offer improved mechanical properties as well as very good thermal conductivity and storage capacity values. In particular, the best results in terms of mechanical properties were achieved by the AAM system, where the compressive strength value is increased with respect to the reference PC sample by 195% after exposure to 500 °C and by almost 97% after 20 thermal cycles between 200 °C and 400 °C. Furthermore, in terms of thermal properties, the AAM system showed a 31% increase in thermal conductivity after thermal exposure compared to PC. In addition, both AAM and HM systems demonstrated substantial improvements in specific heat and thermal storage capacity, outperforming PC by up to 46% during CSP-like thermal cycles. These promising results open new doors to the study of these alternative materials, such as TES, as they are more suitable than PC from an operational point of view.

Reverse osmosis reject water management by immobilization into alkali-activated materials

Water-intensive industries face challenges due to water scarcity and pollution. In the management of these challenges, membrane processes play an important role. However, they produce significant amounts of reject waters, in which the separated salts and pollutants are concentrated. This study aims to develop a novel management concept for reject waters using alkali activation to immobilize salts in a solid phase using metakaolin, blast furnace slag (BFS), or their mixture as precursors and to create alkali-activated materials with sufficient properties to be potentially used in construction applications. Seven different waters were used to prepare the NaOH-based alkali activator solution: deionized water, three simulated seawaters with increasing salinity, and three reverse osmosis (RO) reject waters from mining or pulp and paper industries. Overall, BFS-based samples had the highest immobilization efficiency, likely due to the formation of layered double hydroxide phases (hydrotalcite, with anion exchange capacity) and hydrocalumite (chloride-containing mineral). Moreover, high-salinity water enhanced the dissolution of precursors, prolonged the setting time, and increased the compressive strength compared with nonsaline water. Thus, the obtained materials could be used in construction applications, such as backfilling material at mines where RO concentrates are commonly produced.

The research on the mechanical properties, microstructure, environmental impacts of environmentally friendly Alkali-activated ultra high performance concrete (AAUHPC) matrix with varied design parameters

Alkali-activated material prepared by the silicon aluminum solid wastes are commonly considered as the low-carbon cementitious material, which has important significance for promoting the sustainable development of construction materials. The mechanical properties, reaction mechanisms, and microstructure evolutions of Alkali-activated Ultra High Performance Concrete (AAUHPC) matrix with 7 design parameters were comprehensively studied through testing the fluidity, 28 days compressive strength, 28 days flexural strength, hydration heat, and microstructures. The environmental impacts including carbon emissions and energy consumption are evaluated. Appropriately increasing Na2O dosage, silicate modulus (Ms), and granulated blast furnace slag (GBFS) to fly ash (FA) mass ratio (GBFS/FA) and decreasing silica fume (SF) content and water to binder ratio (W/B) are conducive to accelerating the early reaction of AAUHPC. High Na2O dosage, Ms, SF content, and W/B are not conducive to increasing the 3 days reaction degree of AAUHPC, while the moderate GBFS/FA is corresponding to the higher 3 days reaction degree of AAUHPC. The optimal Na2O dosage, Ms, GBFS/FA, SF content, sand to binder ratio (S/B), fine sands content in quartz sands, W/B are 8 %, 1.4, 4.5:1, 10 %, 1.0, 50 %, and 0.34 for the 28 days mechanical properties. The acquired AAUHPC matrix with fluidity of 218 mm and the lowest porosity has 28 days compressive strength of 114.8 MPa and 28 days flexural strength of 10.6 MPa, whose microstructure has more high polymerization degree C(N)-A-S-H gels with higher Al/Si, lower Na/Al, and lower Ca/Si molar ratios. Compared to the traditional Ultra High Performance Concrete (UHPC), the environmentally friendly AAUHPC can reduce the carbon emission by 42%–52 % and the energy consumption by 5%–18 %. The total CO2 emission of per unit compressive strength (1.0 MPa) of concrete per cubic meter (CI) is in range of 2.5–3.7 kg, which is significantly lower than the traditional UHPC (6.98 kg/MPa•m3).

Influence of interface agent and form on the bonding performance and impermeability of ordinary concrete repaired with alkali-activated slag cementitious material

Alkali-activated slag cementitious materials (AASCMs) exhibit fast hardening, early strength development, high strength, and low carbon content, making them suitable for repairing damaged concrete. However, the bonding performance of the repaired interface and the impermeability of the repaired specimens require further evaluation. Accordingly, this study performed splitting tensile, water penetration, chloride ion penetration, and microanalysis tests on ordinary concrete specimens repaired with AASCMs. The results demonstrated that epoxy resin and cement mortar improved the interfacial bonding strength, water permeability, and chloride ion permeability of the alkali-activated slag cementitious material-Portland cement (AASCM-PC) specimens. The rectangular mortise joint and S-shaped interface improved the interfacial bonding strength and water permeability of the AASCM-PC specimens but slightly reduced their chloride ion permeability. Compared with ordinary concrete, AASCM has fewer harmful voids and denser microstructure, improving the impermeability of the repaired specimens.