Alkali Activation Reaction Research Articles

Experimental Investigation on Ambient-Cured One-Part Alkali-Activated Binders Using Combined High-Calcium Fly Ash (HCFA) and Ground Granulated Blast Furnace Slag (GGBS).

The challenges of handling user-hostile alkaline solutions in the conventional alkali-activated binders (AAB) have initiated the development of “just add water” or one-part solid-based AAB systems. This paper aims to present a preliminary investigation on the development of one-part ambient-cured alkali-activated binders produced by synthesising high-calcium fly ash (HCFA) and ground granulated blast furnace slag (GGBS) using sodium metasilicate anhydrous. Three test series were conducted in this study to investigate the effects of GGBS/binder, activator/binder and water/binder ratios on the fresh and hardened properties of the one-part synthesis AAB system. It was found that the SiO2/Al2O3 molar ratio plays an important role in the attainment of compressive strength and limits the amounts of solid activators effective in contributing to the alkali-activation reaction process. The optimum SiO2/Al2O3 molar ratio was found between 3.20 and 3.30. The test results revealed that the optimum proportion between HCFA and GGBS was discovered at a GGBS/binder ratio of 0.50. The optimum activator/binder ratio was between 0.08 and 0.12, and it is recommended that the water/binder ratio should not exceed 0.50. This study demonstrated the potential of the one-part synthesis method in the production of alkali-activated binder for practical structural applications.

Impact of Na/Al Ratio on the Extent of Alkali-Activation Reaction: Non-linearity and Diminishing Returns

To address the high CO2 footprint associated with cement production, many alternative, sustainable binders are now gaining worldwide attention–including alkali-activated materials. The alkali-activation reaction of metakaolin is a fairly complex process involving transformation of one amorphous reactant (precursor metakaolin) into another amorphous product or products (N-A-S-H gel and/or disordered zeolite type phases). In spite of this complexity, researchers in the past 2 decades have gained significant knowledge on the nature of this reaction at multiple scales. Understanding and developing a clear relationship between the alkalinity of the mix and the extent of reaction is of high interest for practical applications. However, detailed and thorough investigations on this important relationship are limited. Here, in this study, we address this gap by systematically investigating a series of alkali-activated materials samples with a wide range of Na/Al ratios (0.5–1.8) using seven different yet complementary analytical techniques (isothermal calorimetry, FTIR, XRD, TGA, NMR, and Raman imaging). Applied in tandem, these tools reveal a clear but non-linear relationship between the Na/Al ratio and the extent of alkali-activation reaction indicating diminishing returns at higher Na/Al ratios, where higher Na/Al ratios cause an increase in the degree of reaction until a certain point at which the increase in Na/Al ratio does not significantly affect the reaction kinetics, but may affect the gel polymerization. These findings could potentially aid decision making for commercial applications of AAMs where alkalinity of the mix is an important parameter for performance as well as safety.

Effects of fly and coal bottom ash ratio on backfill material performance

Fly ash (FA) and coal bottom ash (CBA) were selected to study the effects of their ratios on backfill materials. The rheological properties, stability, mechanical properties, crystal structure, and microstructure of backfill materials were investigated at different FA/CBA ratios. Increased FA/CBA values improved the slump, rheological performance, setting time, mechanical strength, and elastic modulus. However, the excellent water retention of CBA reduced the bleeding capacity, and the alkali-activated reaction by CBA generated a large amount of ettringite (AFt) that produced early strength. In the late period, the FA/CBA values affected the morphology and compactness of the calcium-silicate-hydrate gels. Furthermore, the performance of backfill materials with 0.5% superplasticizer and 30%-50% FA/CBA ratio could satisfy the requirements, and this formula has been applied to the filling mining project.

Synergistic Excitation Mechanism of CaO-SiO2-Al2O3-SO3 Quaternary Active Cementitious System

In this study, the influence of steel slag (SS) content on the strength of the cementitious materials was investigated. The quaternary active cementitious material (CaO-SiO2-Al2O3-SO3) was prepared using various proportions of steel slag (SS), granulated blast furnace slag (BFS), and desulfurized gypsum (DG). The mechanism of synergistic excitation hydration of the cementitious materials was examined using various techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectrometry (FTIR). The strength of the mortar test block was initially increased and decreased later with the increase of the SS content. Mortar test block with 20% steel slag, 65% granulated blast furnace slag, and 15% desulfurized gypsum with 0.35 water-binder ratio showed the highest compressive strength of 57.3 MPa on 28 days. The free calcium oxide (f-CaO) in the SS reacted with water and produced calcium hydroxide (Ca(OH)2) which created an alkaline environment. Under the alkaline environment, the alkali-activated reaction occurred with BFS. In the early stage of hydration reaction, calcium silicate hydrate (C-S-H) gel and fibrous hydration product ettringite (AFt) crystals were formed, which provided early strength to the cementitious materials. As the hydration reaction progressed, the interlocked growth of C-S-H gel and AFt crystals continued and promoted the increase of the strength of the cementitious system.

Mechanical, microstructure and reaction process of calcium carbide slag-waste red brick powder based alkali-activated materials (CWAAMs)

The paper attends to the utilization and performance improvement of waste red brick powder activated by calcium carbide slag (CCR). Therefore, the improvement of mechanical properties and microstructure of calcium carbide slag-waste red brick powder based alkali-activated materials (CWAAMs) by silica fume, composite activator and curing temperature was explored. Compressive strength tests, reaction process and microstructural analysis were performed including Fourier transform infrared, X-ray diffraction, scanning electron microscope-energy dispersive X-ray spectroscopy and thermogravimetry. The results showed that the compressive strength of CWAAMs increases by 8 times that of CWAAMs without silica fume, resulting from that silica fume increased the stacking speed and yield of C-S-H gel, making the structure of CWAAMs compact. Through the microscopic analysis of the reaction products in the reaction process, it is found that high alkali concentration and high-temperature curing conditions can promote the alkali-activated reaction to proceed faster. Furthermore, the 28d compressive strength of the CWAAMs reached 46.70 MPa curing at 100 °C for 1 d. Because the high concentration of Ca2+ and Na+ ions of the composite activator (CCR and NaOH solution) may produce synergistic effect curing at high temperature, forming massive C-S-H and (C, N)-A-S-H gel to fill the space between the particles, thus forming a more compact paste structure.

Systematic Review on Alkali-Activated Binders Blended with Rice Husk Ash

Production of alkali-activated binders is a developing research field that utilizes industrial/agricultural by-products and solid waste for the development of sustainable concrete. This paper comprehensively reviews the literature relating to rice husk ash (RHA)–based alkali-activated binders incorporating fly ash (low/high calcium) and blast furnace slag. The literature demonstrates that the properties of raw material significantly influence the formation of the alkali-activated gel matrix. Every precursor (low/high calcium fly ash and slag) that is used to develop alkali-activated binders with RHA have their own reaction mechanism dependant on their specific chemical composition. Hence the incorporation of RHA influences each binder in a unique way depending upon the alkali activation process and reaction mechanisms. The incorporation of RHA, in the range 5%–15%, with blended slag alkali activated binders, yields better compressive strength, when compared with RHA blended with fly ash (low/high calcium). The review presented in this paper is very useful to understand the behavior of alkali-activated binders incorporating RHA and in advancing the research into the successful application of RHA as a binder for alkali-activated materials.

Cu- and Zn-doped alkali activated mortar – Properties and durability in (bio)chemically aggressive wastewater environments

Metakaolin-based alkali activated mortars (AAM) - with and without CuSO4·5H2O and ZnO addition (mass ratio Mn+/solid binder 0.08% to 1.7%) - were casted and exposed within an extensive long-term field campaign over the period of 20 months to a sewer basin, strongly affected by biogenic acid corrosion. (Un-)exposed AAM were tested regarding their physicochemical and microstructural properties, bioreceptivity and overall durability. Metal addition led to a retarding effect during alkali-activation reaction, as well as to an increase in open porosity of up to 3.0% and corresponding lower compressive strength of up to 10.9%. Reduced microbial colonization and diversity were observed on AAM with Cu, while Zn addition led to increased biodiversity. We propose that the observed higher durability of Cu-doped AAM is due to antibacterial effects and associated reduction of biogenic acid production, superseding overall negative effects of metal-dosage on physical material properties. Observed lower durability of Zn-doped AAM was related to combined negative physicochemical and microbial effects.

CO2 capture behavior and chemical structure of the alkali zirconate-silicate hybrid sorbent from ZrSiO4 by alkali activation method

The zirconate-silicate hybrid as CO2 capture sorbent at high temperature were decomposed from ZrSiO4 by NaOH and KOH. The chemical structure and CO2 adsorption/desorption dynamics of zirconate-silicate hybrid were studied by the Fourier transform infrared, X-ray photoelectron spectroscopy and the thermogravimetric measurements. The results show that alkali divides ZrSiO4 into ZrO2 and Si-OH groups, then Si-OH groups diffuse into molten alkali and condensate to form a silicate shell with CO2 capture capacity on the surface of alkali activation products. The electron-donating ability of the molten alkali determines the chemical structure and the CO2 storage capacity of sorbents. When the ratio of NaOH to ZrSiO4 is 6:1, the sorbent with the optimal chemical structure for CO2 capture can be acquired after the alkali activation reaction. The CO2 capture process is kinetically controlled by metal cation diffusion after the carbonate-oxide external shell is formed, and the diffusion process is more dependent on the chemical structure of silicates in the zirconate-silicate hybrid than CO2 direct chemisorption reaction.

Reaction mechanisms of alkali-activated glass powder-ggbs-CAC composites

This paper discusses the alkali activation reaction and the alkali-silica reaction (ASR) of the alkali activated cement (AAC) using glass powder (GP) as the major precursor and glass cullet (GC) as the fine aggregate. For the AAC, the proportion of GP was fixed at 75% by weight and different dosages of ground granulated blast furnace slag (GGBS) and calcium aluminate cement (CAC) were incorporated. The main reaction product of the alkali-activated glass powder and slag was an amorphous gel N-(C)-A-S-H according to scanning electron microscopy (SEM)-back scattered electron (BSE) result. New crystalline phases including zeolite and katoite were detected by X-ray diffraction (XRD) in the AAC pastes after the incorporation of CAC in addition to the N-(C)-A-S-H gel. The compressive strength of the AAC mortar was increased with the increase of CAC dosage when the CAC content was lower than 10%. But further increase of the CAC content decreased the strength of the alkali-activated mortar due to the formation of zeolite and katoite loosening the microstructures. The alkali-activated glass/slag cement suffered from severe expansions due to ASR and the expansion exceeded 1000 micro strain after 14 days of alkaline immersion. The addition of CAC significantly decreased the ASR expansion.

Volcanic Ash as a Sustainable Binder Material: An Extensive Review.

The construction industry is affected by the constant growth in the populations of urban areas. The demand for cement production has an increasing environmental impact, and there are urgent demands for alternative sustainable solutions. Volcanic ash (VA) is an abundant low-cost material that, because of its chemical composition and amorphous atomic structure, has been considered as a suitable material to replace Portland cement clinker for use as a binder in cement production. In the last decade, there has been interest in using alkali-activated VA material as an alternative material to replace ordinary Portland cement. In this way, a valuable product may be derived from a currently under-utilized material. Additionally, alkali-activated VA-based materials may be suitable for building applications because of their good densification behaviour, mechanical properties and low porosity. This article describes the most relevant findings from researchers around the world on the role of the chemical composition and mineral contents of VA on reactivity during the alkali-activation reaction; the effect of synthesis factors, which include the concentration of the alkaline activator, the solution-to-binder ratio and the curing conditions, on the properties of alkali-activated VA-based materials; and the mechanical performance and durability properties of these materials.

Portlandite consumption by red ceramic waste due to alkali activation reaction

Abstract Red ceramics, due to the low compressive strength and high porosity, make it difficult to use it as an artificial aggregate in the production of mortars and concretes. However, since it has a silicon-rich composition, the ceramic material from blocks and tiles has been studied as a possible supplementary cementitious material in concrete and also as a raw material in alkali-activated binders. This paper aims to evaluate the content of calcium hydroxide (portlandite) fixed in the alkali activation reaction and the microstructure of red clay waste from construction and demolition waste (CDW) and hydrated lime mixtures, varying the atomic ratio between the silicon and the calcium. The results indicated that higher availability of lime is directly related to the content of hydrated compounds and its porosity. The increase in the silicon/calcium ratio resulted in a reduction of available lime content by 40% and an increase, in volume, of micropores by 7%.

Reaction mechanisms of alkali-activated materials

abstract: The Alkali-Activated Materials (AAM) are defined as materials obtained through the reaction between precursors and activators, and are separated into two classes depending on the products formed in the reaction, those rich in calcium, as the blast furnace slag, whose Ca/(Si+Al) ratio is higher than 1; and poor in calcium, which is the geopolymers subclass. In this review article, some bibliographical aspects were discussed regarding the discovery of these materials, through research conducted by Victor Glukhovsky and through the characterization of historical monuments by Davidovits, which began in the 50s and 60s and persist to the present day. The main products obtained in the alkaline activation reaction were also addressed, using the definition of polysialates and zeolites, in the case of geopolymers, and the tobermorite structure, in the case of materials rich in calcium. The main steps of the alkali-activated reaction, such as dissolution, condensation, polycondensation, crystallization, and hardening, were discussed. Some techniques for characterizing the AA reaction products were also examined, such as X-ray diffraction (XRD), nuclear magnetic resonance spectrometry (NMR), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). Finally, the main factors that interfere in the kinetics of AA reactions were explored, in which the type of cure and the activating solution used in the alkali-activated materials production stands out.

Elaboration of alkali activated materials using a non-calcined red clay from phosphate mines amended with fly ash or slag: A structural study

This study aims to explore the feasibility of using waste red clay from a sedimentary, interlayered phosphate deposit to produce a geopolymer mortar without any pre-treatment. Alkali activated mortars were prepared by mixing fly ash or slag with red clay and sand, using different fly ash or slag/red clay ratios. The effects of sodium silicate to sodium hydroxide ratio (SS/SH) and curing time on the mortar's properties were studied. The results showed that compared to fly ash-based alkali activated materials, an increase of the slag content resulted in the formation of dense mortars while in the case of fly ash based-samples more porous mortars were formed when the fly ash amount was increased. The mineralogical investigations indicated a partial contribution of montmorillonite and palygorskite from the clay to the alkali activation reaction. The mechanical properties of the materials were promising with a maximum compressive strength of up to 39 MPa obtained for the mixture of fly ash and red clay at a fly ash/red clay ratio of 2 and a SS/SH of 1.

Thermal stability and microstructure of metakaolin-based geopolymer blended with rice husk ash

Thermal stability of metakaolin -based geopolymer blended with rice husk ash with replacement levels of 5–20% by weight of MK was investigated. The phase composition, thermal stability and microstructure of geopolymer before and after high-temperature exposure were explored by using X-ray diffraction(XRD), Thermogravimetric analysis (TGA), Fourier Transform Infrared Spectrometer (FTIR), BET specific surface area test (BET) and Scanning electron microscope (SEM). The optimum chemical activity of rice husk ash can be obtained when calcined at 600 °C for 4 h. The addition of rice husk ash increases the compressive strength of geopolymer by accelerating the alkali-activated reaction process. The compressive strength at 28 days of curing increases by 24% with 15 wt% rice husk ash. The porous rice husk ash accelerates the discharge of water vapor from geopolymer and protects geopolymer from cracking caused by air pressure. The high chemical activity of rice husk ash accelerates the high-temperature geopolymerization process and contributes to the healing of microcracks. Rice husk ash reduces the melting sintering temperature of geopolymer from 870 °C to 780 °C, improves the formation of compact ceramic protective layer and prevents the high temperature spalling of geopolymer.

Low-Carbon Concrete Based on Binary Biomass Ash-Silica Fume Binder to Produce Eco-Friendly Paving Blocks.

The civil construction industry consumes huge amounts of raw materials and energy, especially infrastructure. Thus, the use of eco-friendly materials is indispensable to promote sustainable development. In this context, the present work investigated low-carbon concrete to produce eco-friendly paving blocks. The binder was defined according to two approaches. In the first, a binary binder developed with eucalyptus biomass ash (EBA) and silica fume (SF) was used, in total replacement for Portland cement. In the second, the mixture of residues was used as a precursor in alkali-activation reactions, forming alkali-activated binder. The experimental approach was carried out using five different mixtures, obtained by varying the amount of water or sodium hydroxide solution. The characterization of this new material was carried out using compressive strength, expandability, water absorption, deep abrasion, microstructural investigation, and organic matter degradation potential. The results showed that the EBA-SF system has a performance compatible with Portland cement when used as an alternative binder, in addition to functioning as a precursor to alkali-activated concrete. The blocks produced degraded organic matter, and this degradation is more intense with the incidence of UV. In this way, the EBA-SF binder can be successfully used for the manufacture of ecological paving blocks with low carbon emissions.

Use of calorimetry and thermal analysis to assess the heat of supplementary cementitious materials during the hydration of composite cementitious binders

The present paper focuses on the suitability, variability and versatility of thermal analysis and calorimetry methods in the study of cement hydration as physical and chemical process including pozzolanicity and hydraulicity of supplementary cementitious materials. Isothermal calorimeter TAM AIR and simultaneous TGA/DSC were used. Not only activation energy of system comprising SCMs, but also heat generated through alkali-activated reaction of metakaolin and ground granulated blast furnace slag (BFS) can be successfully determined by using conduction calorimetry. The incremental heat flow and incremental cumulative heat of the alkali-activated reaction of ground granulated BFS and metakaolin (MK) were determined and found dependent on temperature and mass ratio between cement and SCMs. The incremental heat flow presents the same characteristics as parent heat flow with different peaks, denoting the formation of C–S–H, ettringite and C–A–S–H trough alkali-activated reaction. While BFS and MK influenced moderately the formation of C–S–H, their influence on the formation of C–A– $${\bar{\text{S}}}$$ –H (ettringite and monosulphate) and C–A–S–H is significant as evidenced by peak position and intensity. The method of calorimetry coupled with thermal analysis was considered sufficient to assess the pozzolanicity and hydraulicity of SCMs.

Application of alkali-activated palm oil fuel ash reinforced with glass fibers in soil stabilization

The feasibility of using palm oil fuel ash (POFA) as a precursor for alkali activation reactions, in combination with glass fibers as a discrete reinforcement, has been investigated. The experimental work was focused on the shear strength (using unconfined compression tests) and the tensile strength (using indirect tensile tests and flexural tests). According to the results, it was found that the peak stress increased and the post-peak behavior was modified from a brittle to a more ductile response depending on the amount of fiber reinforcement in the alkali-activated mixtures. An analysis of the microstructures revealed that the most significant factor contributing to the enhanced behavior of the reinforced mixtures was the interaction between the geo-polymeric matrix and the fiber surface. The present work brings new insights to the soil stabilization industry by providing an effective method for enhancing the properties of soil treated by the alkali activation of POFA (a low-value agro-waste by-product) through the inclusion of glass fibers. This brings advantages over the traditional calcium-based binders (i.e., lime and cement) as their production involves the emission of carbon dioxide, one of the factors significantly contributing to global warming.

Improving the engineering behaviour of residual soil with fly ash and treated natural fibres in alkaline condition

ABSTRACT This study explores the possibility of using sustainable materials in the form of fly ash as a precursor in alkali activation (AA) reactions, as well as coir fibres as discrete reinforcement. For enhancing the longevity of fibres in an alkaline environment and improving the mechanical performance of the soil–fibre composite, an effort was made in this study to physically coat the natural fibre surface with linseed oil and turpentine oil. The effect of surface treatment on coir fibres was initially analysed by field emission scanning electron microscopy and energy dispersive x-ray tests. To evaluate the effects, unconfined compressive strength, indirect tensile strength, and flexural strength tests were performed at 7 and 28 days of curing on AA-treated soil reinforced with treated and untreated coir fibres. The test results demonstrated that treated fibres significantly enhanced the mechanical properties of the AA-treated soil in terms of peak stress and post-peak behaviour.

The effect of mineralogy of calcined shales on the alkali activation and geopolymerization reactions: A case study from Abu-Tartur plateau, Western Desert, Egypt

Abstract The effect of mineralogy of calcined shales on the alkali activation and geopolymerization reactions was studied. Shales under study included black and Dakhla shales, which were obtained from the Abu-Tartur plateau in the Western Desert of Egypt and characterized using optical microscope, SEM, XRF, XRD, and FTIR. The dehydroxylation of shale samples was performed at 550, 650, 750, and 850 °C for one hour. The mineral transformations on calcination were monitored using SEM, XRD and FTIR. The alkali activation process was then conducted on the calcined samples using a mixture of sodium hydroxide and sodium silicate at 1.2 solid/liquid mass ratio. The resultant pasts were molded in steel cubes (50 × 50 × 50mm in diameter) and cured at 70 °C for 48 h. The compressive strength of cured samples was measured to reveal the extent to which the dehydroxylation achieved. The maximum strength values were reported at 4.19 MPa and 2.49 MPa for the calcined black shale and Dakhla shale, respectively. By comparing with literature, the present strength is considered to be too much low. This indicates a deficiency of the dehydroxylation of clay minerals, a little amount of the active amorphous phases, and the dominance of low reactive crystalline phases through the whole range of calcination process.

Incorporating Cs and Sr into blast furnace slag inorganic polymers and their effect on matrix properties

Minimizing harmful effects to the environment in waste-management practices requires continuous innovation. This is especially important in the field of radioactive waste management. Alternatives to the commonly used ordinary Portland cement matrices are being increasingly studied for improved immobilisation purposes. The development of inorganic polymers (IP) from industrial residues has been successfully studied for the immobilisation of caesium (Cs+) and strontium (Sr2+). However, knowledge of the effect of these introduced elements on the IP-matrix is scarce, especially considering that studied effects are dependent on the IP-precursor characteristics and the form in which the Cs+ and Sr2+ are introduced. In this study, IPs containing varying amounts of CsNO3 and Sr(NO3)2 were developed to study the effect of the introduced elements on the IP-characteristics. IP-samples were developed from ground granulated blast furnace slag (GGBFS) and 6 M NaOH activating solution. Cs+ and Sr2+ were added to account for 0.5, 1 and 2 wt% of the total IP-mass. Throughout the entire study, Cs+-addition showed no significant effects on the studied parameters. Calorimetric results showed that Sr2+ severely affects reaction kinetics, consuming hydroxide ions necessary for the alkali activation reaction. Sr2+-addition also caused a severe decrease in compressive strength, increased calcium leaching, and decreased sodium and hydroxide leaching. Micro-chemical analyses showed that Cs+ is almost fully incorporated in the formed IP-matrix, while Sr2+ mainly precipitates as Sr(OH)2 in concentrated regions throughout the IP-structure. The findings presented in this paper give insights on the effect of contaminant elements on the immobilising matrix.