Alkali-activated Fly Ash/slag Research Articles

Study on triaxial compression performance and damage characteristics of fiber-reinforced ecological matrix cementing gangue gypsum fill material.

Polypropylene fiber was equally mixed into alkali-activated slag fly ash geopolymer in order to ensure the filling effect of mine goaf and improve the stability of cemented gangue paste filling material with ecological matrix. Triaxial compression tests were then conducted under various conditions. The mechanical properties and damage characteristics of composite paste filling materials are studied, and the damage evolution model of paste filling materials under triaxial compression is established, based on the deviatoric stress-strain curve generated by the progressive failure behavior of samples. Internal physical and chemical mechanisms of the evolution of structure and characteristics are elucidated and comprehended via the use of SEM-EDS and XRD micro-techniques. The results show that the fiber can effectively improve the ultimate strength and the corresponding effective stress strength index of the sample within the scope of the experimental study. The best strengthening effect is achieved when the amount of NaOH is 3% of the mass of the solid material, the amount of fiber is 5‰ of the mass of the solid material, and the length of the fiber is about 12 mm. The action mode of the fiber in the sample is mainly divided into single-grip anchoring and three-dimensional mesh traction. As the crack initiates and develops, connection occurs in the matrix, where the fiber has an obvious interference and retardation effect on the crack propagation, thereby transforming the brittle failure into a ductile failure and consequently improving the fracture properties of the ecological cementitious coal gangue matrix. The theoretical damage evolution model of a segmented filling body is constructed by taking the initial compaction stage end point as the critical point, and the curve of the damage evolution model of the specimen under different conditions is obtained. The theoretical model is verified by the results from the triaxial compression test. We concluded that the experimental curve is in good agreement with the theoretical curve. Therefore, the established theoretical model has a certain reference value for the analysis and evaluation of the mechanical properties of paste filling materials. The research results can improve the utilization rate of solid waste resources.

Durability of alkali-activated fly ash-slag concrete- state of art

India ranks among the foremost global producers and consumers of cement, and the cement industry contributes significantly to carbon emissions. Alkali-activated materials have gained significant attention as a sustainable alternative to Portland cement, offering the potential to mitigate carbon dioxide emissions and promote effective recycling of waste materials. Fly ash (FA) and Ground granulated blast furnace slag (GGBS) are preferred raw materials for Alkali-activated concrete (AAC) owing to their effective repurposing of waste, widespread accessibility, advantageous chemical composition, and performance attributes. This review provides a comprehensive analysis of the current state-of-the-art on the durability aspects of fly ash/slag-based AAC. The paper explores the unique characteristics of FA/GGBS-based AAC, emphasizing their potential to enhance the durability of concrete structures. Insights into the material behaviour under various environmental exposures, including aggressive chemical environments and freeze–thaw cycles, are presented. Furthermore, the article addresses both the obstacles and prospects associated with implementing fly ash/slag-based AAC as a potential construction material suitable for large-scale infrastructure projects. This overview is designed to direct future research efforts and provide practitioners with insights into the potential of FA/GGBS-based AAC for ensuring the prolonged durability of concrete structures.

Microstructure and Phase Characterization of Alkali-Activated Slag–Fly Ash Materials with Tetrasodium of 1-Hydroxy Ethylidene-1, 1-Diphosphonic Acid (HEDP·4Na)

The effect of tetrasodium of 1-hydroxy ethylidene-1, 1-diphosphonic acid (HEDP·4Na) on the microstructure and phase characterization of alkali-activated fly ash–slag (AAFS) materials is not clear or well documented. In this study, XRD, DTG, TAM-air, and SEM analyses of AAFS were used to identify the microstructural changes in AAFS made with HEDP·4Na. Meanwhile, the workability and compressive strength of AAFS were evaluated. The results demonstrated that the early-age alkaline-activated reactions were retarded due to the addition of HEDP·4Na in the AAFS mixture. However, the degree of gel formation was relatively increased at a later age in the AAFS made with HEDP·4Na compared to the plain AAFS mixture. Additionally, in comparison to the control group, the incorporation of HEDP·4Na in AAFS specimens resulted in improved flowability, with increments of 5%, 15%, and 24% for concentrations of 0.1%, 0.2%, and 0.3%, respectively. The initial and final setting times were prolonged by 5% to 50%, indicating a beneficial impact on the rheological properties of the AAFS fresh mixture. Furthermore, the addition of HEDP·4Na led to an improvement in compressive strength in the AAFS mixtures, with enhancements ranging from 13% to 16% at 28 days compared to the control group. With the presence of HEDP·4Na, the increase in the degree of reactions shifted to the formation of gel phases, like C-S-H, through the combined measurement of TGA, XRD, and SEM, resulting in a denser microstructure in the AAFS matrix. This study presents novel insights into the intricate compatibility between the properties of AAFS mixtures and HEDP·4Na, facilitating a more profound comprehension of the potential improvements in the sustainable development of AAFS systems.

Utilization of Copper–Molybdenum Tailings to Enhance the Compressive Strength of Alkali-Activated Slag-Fly Ash System

Utilizing a variety of solid wastes to prepare alkali-activated cementitious materials is one of the principal trends in the development of cementitious materials. Commonly used alkali activation precursors such as granulated blast furnace slag (GBFS) and fly ash (FA) will be less available due to resource pressures. Supply limitation is an important reason to research alternative precursors. To realize the high value-added utilization of copper–molybdenum tailings (CMTs), this study adopted the modified sodium silicate solution as an alkaline activator to activate GBFS-FA-CMTs cementitious system to prepare alkali-activated cementitious materials. The influence of CMTs content on the compressive strength of GBFS-FA-CMTs cementitious system was analyzed, and the mechanism of GBFS-FA-CMTs cementitious system was also analyzed through hydration product types, physical phase composition, and microscopic morphology. The results indicated that a paste with the incorporation of CMTs, S50F30C20 (50% GBFS, 30% FA, 20% CMTs), achieved the highest compressive strength of 79.14 MPa, which was due to the filling effect of the CMTs and the degree of participation in the reaction. Pastes with different contents of CMTs, while maintaining a constant CBFS content, exhibited similar strength development. Excessive amounts of CMTs could result in reduced compressive strength. Microstructural analysis revealed that the hydration products were structurally altered by the addition of CMTs. In addition to ettringite, quartz, C(-N)-S-H gel, and calcite, gaylussite was also formed; moreover, the mass of chemically bound water increased, and the microstructure of reaction products became denser. An excess of CMTs may restrict the growth of the hydration gel, leading to more microstructural defects. The study suggests that CMTs could enhance the compressive strength of hardened paste within an alkali-activated slag-fly ash system, possibly due to a filling effect and participation in the chemical reaction. This research confirms the feasibility of using CMTs in alkali-activated cementitious materials.

Piezoresistivity of carbon fiber-reinforced alkali-activated materials: Effect of fly ash microspheres and quartz sands

Alkali-activated materials (AAMs) have gained increasing attention as potential candidates for conductive and self-sensing concretes. This study investigated the effect of fly ash microspheres and quartz sands on the piezoelectric properties of carbon fiber-reinforced alkali-activated fly ash/slag (AAFS) mortars. The rheological property, mechanical property, resistivity, and piezoelectric properties of different AAFS mortars were investigated. Our findings demonstrated that the use of fly ash microspheres (FAM) significantly reduced the yield stress and plastic viscosity of the mortars due to the "ball bearings" effect. In addition, the finer particles of FAM led to a dense gel pore structure of AAFS. The use of FAM improved carbon fiber dispersion and formed a stable conductive network. Therefore, AAFS mortars with FAM exhibited superior conductivity and piezoelectricity. The use of quartz sands exhibited adverse effects on the self-sensing properties of AAFS mortars. The incorporation of quartz sands predominantly hindered the conductive network formation of carbon fibers, resulting in a significantly higher resistivity and piezoresistivity with low strain sensitivity.

Effect of curing conditions on the alkali-activated blends: Microstructure, performance and economic assessment

To enhance the application of alkali-activated materials in engineering, this study investigates the influence of cast-in-place and prefabrication by steam curing and autoclaved curing on alkali-activated materials with different slag-fly ash mass ratios. The selection of the three curing methods is based on current construction techniques: onsite construction (ambient curing), production of prefabricated cement-based components (steam curing), and production of aerated concrete blocks (autoclaved curing). In this paper, the dissolution ability of CaO, SiO2 and Al2O3 in fly ash and slag was first evaluated. Subsequently, the hydration products of the samples were analyzed using X-ray diffraction, thermogravimetric analysis, and a scanning electron microscope. Additionally, the degree of hydration of the samples was quantitatively analyzed using the selective dissolution method combined with 29Si NMR. The results showed that compared to ambient curing, autoclaved curing, which provides a high-temperature and high-pressure environment, enhances the reactivity of slag and fly ash. This enhancement is reflected in the increase in their solubility and the degree of reaction from 28.3% to 14.6%–69.9% and 63.7% respectively. The pore structure, compressive strength, and shrinkage of the samples were also evaluated. The results demonstrate that autoclaved curing enhances the formation of hydration products, reduces porosity, and increases compressive strength. Compared to ambient curing, steam curing can reduce the shrinkage rate from 64.8% to 68.4%, while autoclaved curing can decrease it from 87.9% to 95.1%. Based on the economic analysis, autoclaved curing can offset the total costs by reducing the usage of slag and lowering raw material expenses, demonstrating its cost-effectiveness. The obtained results can provide a theoretical foundation for the large-scale application of alkali-activated slag-fly ash materials in civil engineering construction.

Effect of basalt fibers and silica fume on the mechanical properties, stress-strain behavior, and durability of alkali-activated slag-fly ash concrete

Previous investigations have shown that there is limited exploration into the combined mechanism of silica fume and basalt fibers of various lengths on the performance of alkali-activated concrete. Based on this, the primary focus of this study is to elucidate the current knowledge gaps concerning the mechanical properties and durability of basalt fiber-reinforced alkali-activated concrete, with a particular emphasis on its stress-strain behavior. The effects of basalt fiber content (i.e., 0.2%, 0.4%, and 0.6% Vf), basalt fiber length (i.e., 6 and 12 mm), mixing proportions of 6 mm and 12 mm basalt fibers (i.e., 75:25, 50:50, and 25:75) and silica fume content (i.e., 3%, 6%, and 9%) on the workability, mechanical properties, stress-strain behavior, and durability of alkali-activated concrete were investigated. Based on experimental results, predictive models were proposed for the splitting tensile strength, flexural tensile strength, elastic modulus, peak strain, and stress-strain relationship of basalt fiber-reinforced alkali-activated concrete, showing excellent predictive accuracy. Additionally, the durability properties of all samples, including shrinkage, carbonation depth, and chloride ion permeability, were also tested. The results indicate that the combined effect of fibers and silica fume can enhance the mechanical properties and durability of the concrete. Finally, the reinforcement mechanism of the synergistic action of fibers and silica fume is discussed.

Microstructure, strength development mechanism, and CO2 emission analyses of alkali-activated fly ash-slag mortars

A comprehensive investigation was conducted to examine the influence of fly ash/slag mass ratio on the strength and microstructure of alkali-activated fly ash-slag (AAFS) mortars. NaOH and Na2SiO3 were employed as activators for different fly ash/slag ratios. The results revealed that AAFS mortar with a fly ash/slag ratio of 25/75 exhibited the highest strength, reaching 104 MPa. The porosity and gel content of AAFS showed a weak correlation with compressive strength, emphasizing the significant impact of the generated gel type on AAFS strength. The comprehensive evaluation of AAFS strength incorporates factors related to the structure and chemical composition of reaction products. A strength index derived from AAFS mortar is proposed and validated against seven additional literature datasets from different systems, demonstrating a robust correlation between the calculated index and the compressive strength. Furthermore, carbon emission analysis reveals that, compared to OPC, FA100 exhibits 7.9%–21.2% lower carbon emissions, while FA25BF75 shows 0.1%–97.5% lower CO2 intensity than OPC, owing to the higher strength of AAFS. In general, AAFS exhibits improved properties and environmental benefits, indicating significant potential for practical applications as a complete replacement for OPC.

Behaviour of alkali-activated fly ash-slag paste at elevated temperatures: An experimental study

As an environmentally friendly material, alkali-activated fly ash-slag (AAFS) is considered as a promising alternative cement because of its excellent mechanical properties and thermal stability. Nevertheless, high temperature induced damage in AAFS can potentially bring serious risks for its practical applications. This paper presents a systematic experimental study on behaviour of AAFS paste subjected to elevated temperatures up to 800 °C in terms of microstructure as well as thermal and mechanical properties. Microstructural evolution was characterised by means of Rietveld-based quantitative X-ray diffraction (QXRD), backscattered scanning electron microscope (BSEM) and X-ray microcomputed tomography (XCT), while thermal and mechanical properties were evaluated from thermogravimetric analysis (TGA), thermal deformation and compressive and flexural strengths. Results indicate that the compressive strength of AAFS paste rises by 77.5 % at 200 °C, followed by a mitigation from 200 to 600 °C and a regain at 800 °C. Different phases in AAFS paste including unreacted particles, reaction products and pores took up 30 %, 67.7 % and 2.31 % at ambient temperature, and 4.95 %, 84.8 % and 10.3 % after exposure to 800 °C, respectively, suggesting the recrystallisation with the formation of nepheline and gehlenite. In addition, the relationships between microstructural characteristics including three-dimensional (3D) pore structure features and thermal and mechanical properties of AAFS paste at elevated temperatures were explored and discussed in depth to gain insights into the underlying degradation mechanisms including pore pressure build-up, thermal gradient and phase transformation.

Fresh and Mechanical Characteristics of Alkali Activated Fly Ash Slag Concrete Activated with Neutral Grade Liquid Glass

Abstract: The development and use of geopolymer concrete continues to evolve as researchers and engineers refine mix designs, develop standardized guidelines, and explore new applications. One of the key benefits of geopolymer concrete is its lower carbon footprint compared to traditional Portland cement concrete. This reduction in carbon emissions is due to the elimination of clinker production, which is energy intensive and emits significant amounts of carbon dioxide. This sustainable alternative to Portland cement concrete plays an important role in reducing the environmental impact of the construction industry. Many prior studies on fly ash-slag based geopolymers have primarily focused on the characteristics of concrete that has undergone heat curing. This approach is seen as a constraint when it comes to in-situ casting applications in ambient conditions. Ambient curing of geopolymer concrete is a more energy-efficient and environmentally friendly option compared to heat curing, as it doesn't require the use of specialized curing chambers or high temperatures. However, it may require longer curing times to achieve the desired strength and making it important to plan the construction schedule accordingly. An experimental study was carried out using neutral grade liquid glass with a silica modulus (SiO2/Na2O) of 2.92 coupled with combinations of fly ash and Ground Granulated Blast Furnace Slag (GGBS) in proportions of 30:70, 50:50. The primary aim is to explore how altering the ratios of fly ash and GGBS impacts the mechanical characteristics of the resultant concrete for different solution/ binder ratios (0.6, 0.65 and 0.7) with a fixed binder quantity of 400 and 500 Kg/m3 under ambient curing conditions. The experimental program employed the fractional factorial method of experimentation to minimize the number of mix variations. In general, the findings indicated that higher levels of neutral-grade sodium silicate led to improvements in all mechanical properties. Conversely, an increase in the alkaline solution-to- binder ratio and curing temperature had a detrimental impact on the alkaliactivated slag concrete. Optimal results for hardening of concrete for demolding were obtained after two days of casting. The implications of this research could lead to the development of more sustainable and environmentally friendly alternatives in the field of construction materials.

Application of cellulose nanocrystals in 3D printed alkali-activated cementitious composites

With the rapid development of concrete 3D printing for construction projects, it is crucial to produce sustainable 3D-printed cementitious composites that meet the required fresh and hardened properties. This study develops sustainable 3D-printed cementitious composites of ordinary portland cement and alkali-activated materials using cellulose nanocrystals (i.e., green natural nanomaterials). The extrudability and buildability of the alkali-activated slag-fly ash mixtures were improved and the extrusion pressure was reduced by ~ 35 % by increasing the cellulose nanocrystals content (up to 1 %) suggesting their viscosity-modifying properties in alkali-activated materials. The inclusion of cellulose nanocrystals improves the overall mechanical performance (8–20 % increase) and reduces the porosity of ordinary portland cement and heat-cured alkali-activated samples. Further, the addition of cellulose nanocrystals (up to 0.30 %) in sealed-cured alkali-activated samples improves their flexural strength by 20 %. The ordinary portland cement sample with cellulose nanocrystals densifies the microstructure and has an approximately 25 % increase in the degree of hydration at inner depths indicating cellulose nanocrystals' internal curing potential. The developed 3D-printable alkali-activated composites with cellulose nanocrystals can provide an overall reduction in the environmental impacts by eliminating/reducing the need for chemical admixtures to improve material consistency and stability, and replacing 100 % of portland cement.

Insights into factors influencing coal gangue-filled backfill cemented by self-consolidating alkali-activated slag grouts

The cemented backfilling is an effective measure to mitigate surface subsidence caused by mine goaf and to consume solid waste generated from mining activities. In this study, sodium hydroxide (SH) and sodium silicate (SS) were used as activators to respectively prepare self-consolidating alkali-activated slag grout (AASG) and alkali-activated fly ash-slag grout (AAFSG) for disposal of iron tailings (TS) and coal gangue (CG). These grouts were casted into a pre-placed CG skeleton to obtain coal gangue-filled backfill (CGFB). The factors influencing the mechanical properties of CGFB were investigated through fresh property tests (rheological properties, bleeding and segregation, setting time, and filling ability) and mechanical property tests of AASGs and AAFSGs. The micro-mechanical properties of the interfacial transition zone (ITZ) between grouts and CG aggregates were investigated by microhardness test. The microstructure of AASGs and AAFSGs was characterized using scanning electron microscope-energy dispersive spectrometer (SEM-EDS) and thermogravimetric analysis (TGA). The results showed that the filling ability of fresh AASGs and AAFSGs was dependent on the flowability and increased with the modulus (SiO2/Na2O molar ratio). The compressive strength of CGFBs was shown to be influenced by the filling index and mechanical strength of AASGs and AAFSGs, which were also related to the modulus of activators. The microhardness gradually increased with distance from ITZ and then tended to be stable, higher modulus led to enhanced microhardness around ITZ, which benefited the mechanical properties of CGFB by strengthening the weak ITZ.

Interactions between alkali-activated and cement-based mortars and biofilm and their influence on microbial induced mortar deterioration

Alkali-activated fly ash (AAF) and alkali-activated slag (AAS) mortars were exposed to the sewage environment with acidophilic sulfur oxidizing bacteria and high concentration H2S to study their interaction. It was found that when they suffered the biogenic sulfuric acid attack, both the AAF and calcium aluminate cement (CAC) mortars had a strong inhibition effect on the growth and activity of microorganisms. There are more aluminum ions dissolved from AAF mortar into sewage, and its structure was characterized by the increase in 100-1000 nm pores. Still, a hard high silicon skeleton was formed to keep the apparent intact. However, CAC mortar had a higher killing rate for the bacteria in the attached biofilm. For AAS mortar, the formation of excessive gypsum increased the macropores content and was more conducive to the growth of biofilm, thereby accelerating the penetration of corrosive medium and the deterioration of mortar.

Dry shrinkage and micro-structure of alkali-activated fly ash/slag pastes incorporated with silane coupling agent modified MWCNTs

In this study, functionalized Multi-Walled Carbon Nanotubes (MWCNTs) grafted with a silane coupling agent (MS) at concentrations of 0.1 %, 0.2 %, 0.3 %, and 0.4 % were used as additives to investigate their impact on the reaction kinetics, working performance, mechanical strength, and drying shrinkage of alkali-activated fly ash/slag (AAFS) pastes. Additionally, various micro-structure techniques, including X-ray Diffraction (XRD), Thermogravimetric Analysis (TG), Differential Scanning Calorimetry (DSC), Differential Thermal Analysis (DTA), nitrogen (N2) absorption, and Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS), were employed to examine the morphology, chemical composition, and mineralogy of AAFS pastes. The experimental findings indicate that all the modified AAFS pastes exhibit prolonged setting time and increased flowability compared to the control AAFS pastes, with the enhancement of these properties being proportional to the MS content. Specifically, with the addition of 0.3 wt% MS, the compressive strength and flexural strength of LGMS-0.3 % were 41.2 MPa and 6.4 MPa, respectively, at the 28-day curing age, representing a 15.7 % and 39.1 % increase compared to the control AAFS pastes. Simultaneously, significant reductions in mass loss and drying shrinkage of over 70 % and 90 % were observed, respectively, in comparison to the control AAFS pastes. Micro-structural analysis reveals that the inclusion of MS leads to an increase in the quantity of reaction products without affecting their type. Furthermore, the incorporation of MS decreases the total porosity and the proportion of capillary pores with diameters ranging from 2 nm to 50 nm, resulting in the refinement of the overall pore structure of AAFS pastes. Notably, the three-dimensional interpenetrating network formed by MWCNTs and the silane coupling agent positively influences the mitigation of drying shrinkage and the enhancement of the mechanical strength of AAFS pastes.

A comprehensive intelligent framework for interpreting and quantifying the microstructure of waste-derived alkali-activated materials: A case study of fly ash/slag pastes

The alkali-activated fly ash/slag (AAFS) pastes exhibit complex compositions and heterogeneous spatial distributions of phases, necessitating a comprehensive tool capable of quantifying phase assemblage, characterizing ion diffusion, and unveiling microstructure evolution. However, existing techniques have limitations in providing interpretability and visual analyses. This study introduces a versatile analysis methodology for energy dispersive X-ray spectroscopy (EDS) images, encompassing data preprocessing, data interpretation, and four downstream applications: phase inference, phase quantification, ion diffusion characterization, and quantification of phase spatial distributions. Through the analysis of five representative elements, this study successfully identifies and quantifies three components in precursors (slag, quartz, mullite) and four reaction products (slag-products, quartz-products, mullite-products, mixture products) in the AAFS pastes. Moreover, by characterizing ion diffusion and quantifying spatial distributions, insights into the interactions among various precursors and reaction products are elucidated. The proposed EDS image analysis methodology is expected to yield valuable understandings of the reaction mechanisms and phase evolution involved in utilizing solid waste as cementitious materials.

Influence of Blast Furnace Slag on Pore Structure and Transport Characteristics in Low-Calcium Fly-Ash-Based Geopolymer Concrete

Alkali-activated fly ash slag (AAFS) has emerged as a novel and environmentally sustainable construction material, garnering substantial attention due to its commendable mechanical attributes and minimal ecological footprint. This investigation delves into the influence of slag incorporation on the strength, pore structure, and transport characteristics of AAFS, encompassing various levels of fly ash replacement with slag. To assess the mechanical properties of AAFS concrete, unconfined compression and ultrasonic pulse velocity tests were conducted. Meanwhile, microstructural and mineralogical alterations were scrutinized through porosity, N2-adsorption/desorption, and SEM/EDX assessments. In addition, transport properties were gauged using electrical surface resistivity, water permeability, and water vapor permeability tests. According to the results, a remarkable refinement in the pore volume was found by increasing the slag content. The volume of the gel pores and surface area increased significantly associated with the increase in tortuosity. Accordingly, Ca inclusion in the cross-linked sodium aluminosilicate hydrate gel remarkably reduced the transport properties.

Optimization design for alkali-activated slag-fly ash geopolymer concrete based on artificial intelligence considering compressive strength, cost, and carbon emission

Traditional mix proportion design methods are difficult to consider the multiple objectives, while the design of economical and environmentally friendly alkali-activated slag-fly ash geopolymer concrete on the premise of guaranteeing compressive strength is an essential and meaningful task. The novelty of this paper is that the optimization design model for alkali-activated slag-fly ash geopolymer concrete considering 28 days compressive strength, cost, and carbon emission was developed based on machine learning and Particle Swarm Optimization (PSO) algorithm. The results show that Random Forest (RF), GB, and Back Propagation Neural Network (BPNN) can achieve a good prediction effect for compressive strength with R2 over 0.85 and 0.70 for training and testing set. Slag and sodium hydroxide (NaOH) contents have remarkable effects on the compressive strength of alkali-activated slag-fly ash geopolymer concrete. The addition of slag is beneficial to enhancement of compressive strength, and the relatively optimal coarse and fine aggregate contents are 1200 kg/m3 and 750 kg/m3. It is noted that alkali-activated geopolymer is suitable for preparation of high-strength concrete. Production cost is need to be paid more attention in the optimization process. The alkali-activated slag-fly ash geopolymer concretes with production cost reduction of 7.6%∼10.6% and carbon emission reduction of 77.3%∼80.7% at strength grade of C30, and with production cost reduction of 22.5%∼27.0% and carbon emission reduction of 76.9%∼81.3% at strength grade of C50 are achieved.

Multiscale study of microstructural evolution in alkali-activated fly ash-slag paste at elevated temperatures

To promote the high-temperature application of alkali-activated fly ash-slag (AAFS), it is vital to thoroughly understand the performance of AAFS at elevated temperatures. This paper presents a systematic study on the multiscale microstructural evolution in AAFS paste in terms of nanostructure, chemical composition and morphology changes characterised using 29Si nuclear magnetic resonance, X-ray diffraction and Fourier transform infrared spectroscopy, as well as the pore structure evolution by means of mercury intrusion porosimetry, backscattered electron microscopy and X-ray microcomputed tomography. Results indicate that the decomposition of C-A-S-H and N–C-A-S-H gels occurs while gel pores are filled at elevated temperatures up to 800 °C, along with the crack development, whereas micro-cracks are healed by melting and viscous sintering. This study provides an in-depth insight into the damage mechanisms of AAFS paste at elevated temperatures from a multiscale viewpoint, accounting for Level I: solid gel particles, Level II: gel matrix, and Level III: AAFS paste.

Effect of graphite on the self-sensing properties of cement and alkali-activated fly ash/slag based composite cementitious materials

This study investigated the self-sensing performances of cement and alkali-activated fly ash/slag (AAFS) systems with graphite powders as a conductive filler. Graphite powders must be evenly dispersed through the cement-based and AAFS matrices to obtain an adequate electrical response. The effects of graphite fineness on the fluidity, mechanical, rheological behavior, and cement hydration of cement paste were investigated. The finer graphite was found to have less effect on the mechanical property but seriously damaged the workability. Therefore, this research explored using four surfactants including sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), sodium salt of polynaphthalene sulphonic acid (NNO), and Triton X-100 (TX100) as dispersing agents for fine graphite in cement. The combination of NNO and TX100 was found to be the ideal admixture for improving the fluidity of graphite-containing cementitious materials. Furthermore, cement- and AAFS-based composites were produced with the graphite dispersions, and several tests such as electrical impedance spectroscopy (EIS), energy dispersive X-Ray (EDX) map analysis, mercury intrusion Porosimeter (MIP), X-ray diffraction (XRD), and thermogravimetric analysis (TGA) were used to investigate their intrinsic characteristic. The results concluded that AAFS had a better conductivity than cement due to the higher porosity in the range of very small pores (<6 nm) and the higher ion concentration contribution. Finally, the self-sensing behavior of the mortars was investigated under cyclic loading, and the self-sensing mechanism was proposed. The sensing properties of graphite composite mortars under compression load can be attributed to the combination of ionic, contact, and tunneling conduction.

Physical, hydrolytic, and mechanical stability of alkali-activated fly ash-slag foam concrete

Foam concrete is prone to instability during mixing, casting, and curing. Besides preformed foam stability characteristics and volume, several geopolymer mix parameters, viz., precursor composition, silica modulus, alkali content, and water content, influence the overall foamed concrete stability, the understanding of which is still limited. This study evaluates the effect of the above parameters on the physical, hydrolytic, and mechanical stability of fly ash and slag-based foamed geopolymer. Physical stability was assessed by the design of experiments using a central composite design (inscribed), while systematic laboratory experiments assessed hydrolytic and mechanical stability characteristics. The foamed geopolymer behavior is sensitive to silica modulus and solid alkali-to-binder ratio, showing maximum interaction effects for producing stable foamed geopolymer for i) target to actual fresh density difference and ii) settlement along the height. The water-to-binder ratio was the most significant factor affecting the homogeneity across the cross-section along the height.Further, the alkali and slag content significantly affected the hydrolytic and mechanical stability characteristics. An optimum proportion of slag content, silica modulus, solid alkali-to-binder ratio, and water-to-binder ratio is suggested for the maximum benefit of fly ash and slag-based foamed geopolymer. This study provides a framework for comprehensive stability assessment of foamed geopolymers and can aid in developing sustainable lightweight materials for industrial applications.