Alkali-activated Slag Pastes Research Articles

Effects of mixing water temperatures on properties of one-part alkali-activated slag paste

During hot and cold weather, initial mixing water temperature is a critical parameter in controlling the fresh and hardened properties of cement-based materials. With the advent of Alkali Activated Materials (AAMs), the effectiveness of such a technique becomes questionable as the hydration mechanism differs from that of the ordinary Portland cement (OPC). Therefore, this study focuses on evaluating the effects of initial mixing water temperature on the performance of one-part Alkali Activated Slag (AAS). Various AAS mixtures prepared by different mixing water temperatures (i.e. 0 °C, 10 °C, 20 °C, and 30 °C) were tested. Fresh and hardened properties, including mini-slump, slump loss rate, setting time, the heat of hydration, compressive strength, and shrinkage, were investigated. Results reveal that the initial mixing water temperature plays a dominant role in controlling solubility and dissolution rates for different ingredients (i.e. solid activator and slag). This, in turn, had affected the hydration process and consequently the development of various properties. Slump life and setting time were found to extend as the initial mixing water temperature reduced while the strength was not harmed. Saving energy consumed for warming water during cold weather concreting will also increase concrete sustainability. It is anticipated that these findings will pave the way for a broader implementation of such green sustainable materials in the construction sector.

Behavior of alkali-activated slag pastes blended with waste rubber powder under the effect of freeze/thaw cycles and severe sulfate attack

At present, the need to include waste materials such as waste rubber as a part of building materials is getting more attention rather than any other time. The incorporation of waste rubber as a part of building materials decreases the landfills and reduces the consumption of virgin raw materials. Most of the previous studies incorporated waste rubber as a part of aggregate(s). Thus, this work is the first attempt to study the opportunity of including waste rubber powder (W-R-P) as a part of precursor of alkali-activated slag (AAS) pastes aiming to increase their resistivity against the deterioration resulting from freeze/thaw cycles and severe sulfate attack. For this reason, the slag was partially replaced with W-R-P at ratios from 1% up to 15%, by weight. All the pastes were activated by sodium silicate. After curing, pastes were exposed to 5–20 cycles of freeze/thaw as well as 5–20 cycles of immersion in 5% NaSO4 for 15 h followed by drying at 80 °C for 6 h. The compressive strength before and after exposure was measured. Thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) were conducted to analyze the results. The results showed that despite the incorporation of W-R-P particles have a negative effect on the compressive strength, their particles have a positive effect on mitigating the deterioration resulting from freeze/thaw cycles and severe sulfate attack.

Understanding the early reaction and structural evolution of alkali activated slag optimized using K-A-S-H nanoparticles with varying K2O/SiO2 ratios

So far, little is known about the manner the synthesized K-A-S-H (K2O–Al2O3–SiO2–H2O) nanoparticles affects the hydration of alkali-activated slag (AAS). To understand the effects of adding K-A-S-H nanoparticles with two distinct K2O/SiO2 ratios on reaction and microstructural development of AAS, heat evolution, TGA, MIP and SEM-BSE analyses were measured. Results revealed that incorporation of synthesized K-A-S-H nanoparticles can not only be used as seeding to induce nucleation, thereby significantly promoting the hydration of slag and manipulating the chemical composition of the hydrate phases, but also as fillers to increase the density of alkali-activated slag pastes; then microstructure analysis found that incorporation of synthetic K-A-S-H nanoparticles in AAS could optimize the pore size distribution, leading to a dense matrix structure. Furthermore, the effect of K-A-S-H particles with varying K2O/SiO2 ratios was mainly related to their specific surface area. These important findings provide possible practical implications for controlling the late-stage performance of alkali-activated binders.

Studying the effect of alkali dosage on microstructure development of alkali-activated slag pastes by electrical impedance spectroscopy (EIS)

In this study, electrical impedance spectroscopy (EIS) is applied to investigate the effect of alkali dosage on the microstructural evolution of the alkali-activated slag (AAS) system during the curing process. Pore solution chemistry and pore structure of the paste samples were also determined to get auxiliary interpretation of the EIS results. It is shown by equivalent circuit analysis that the resistance attributed to continuous (R1) and discontinuous pores (R2) increases with curing age due to pore structure development. The value of R1 is more affected by pore solution chemistry while the value of R2 is mainly affected by pore structure. With the increase of alkali dosage, the value of R1 decreases while R2 gradually increases. The exponent n of the constant phase element (CPE), which relates to the capillary pores (n1), decreases with the increase of alkali dosage while the exponent relating to gel pores (n2) is independent of the alkali dosage of AAS paste. The bulk conductivity of AAS paste obtained from the bulk resistance gradually increases with the alkali dosage, partially because of the enhanced conductivity of pore solution. The improvement of microstructure at early age is mainly attributed to the decreased porosity, which is more affected by the pore connectivity with the increase of curing age. These results confirm that EIS is an effective technique to determine the hydration properties and pore structure features of AAS paste.

Internal curing by superabsorbent polymers in alkali-activated slag

In this study, internal curing by superabsorbent polymers (SAP) is utilized to mitigate self-desiccation and autogenous shrinkage of alkali-activated slag (AAS) pastes. Absorption and desorption kinetics of SAP incorporated in AAS pastes were studied with X-ray tomography. Internal curing delayed the peak of the rate of heat liberation but increased the total reaction degree of AAS pastes. Internal curing by SAP mitigated effectively the drop of internal relative humidity and the self-desiccation-induced autogenous shrinkage of AAS pastes. The cracking tendency of AAS pastes undergoing shrinkage in restrained conditions was also significantly reduced with SAP. Nonetheless, adding SAP, regardless of their content, cannot eliminate the autogenous shrinkage of AAS pastes, suggesting the existence of other autogenous shrinkage mechanism(s) besides self-desiccation.

Alkali-Activated Slag Paste with Different Mixing Water: A Comparison Study of Early-Age Paste Using Electrical Resistivity.

This paper reports the electrical resistivity measurements on KOH-activated ground-granulated blast-furnace slag, which was mixed with deionized water or natural seawater at three different activator-to-binder ratios (0.4, 0.45, and 0.5). Compressive strength and X-ray diffraction analyses were performed on the samples after the measurement. The type of mixing water did not affect the setting time of samples, whereas the setting time was delayed with an increase in activator-to-binder (a/b) ratio. Regardless of the mixing water type, the increasing ratio of electrical resistivity between a/b 0.45 and 0.5 was larger than that between a/b 0.4 and 0.45. For the same a/b ratio, the pastes mixed with seawater produced higher electrical resistivity and early strength than those with deionized water. The increase in the electrical resistivity in seawater-mixed pastes could be attributed to the formation of Cl-bearing phases such as Cl-hydrocalumite, AlOCl, and aluminum chloride hydrate. It is believed that the reaction products in seawater-mixed samples were helpful in preventing water percolation, and thus, the electrical resistivity increased compared with the deionized water-mixed sample.

Mechanisms of autogenous shrinkage of alkali-activated slag and fly ash pastes

This study aims to provide a better understanding of the autogenous shrinkage of slag and fly ash-based alkali-activated materials (AAMs) cured at ambient temperature. The main reaction products in AAMs pastes are C-A-S-H type gel and the reaction rate decreases when slag is partially replaced by fly ash. Due to the chemical shrinkage and the fine pore structure of AAMs pastes, drastic drop of internal relative humidity is observed and large pore pressure is generated. The pore pressure induces not only elastic deformation but also a large creep of the paste. Besides the pore pressure, other driving forces, like the reduction of steric-hydration force due to the consumption of ions, also cause a certain amount of shrinkage, especially in the acceleration period. Based on the mechanisms revealed, a computational model is proposed to estimate the autogenous shrinkage of AAMs. The calculated autogenous shrinkage matches well with the measured results.

Pore and strength characteristics of alkali-activated slag paste with seawater

This study investigates the pore and strength characteristics of alkali-activated slag paste, using seawater (SW) and tap water (TW) as the mixing water. During the investigation, the water/binder ratio was 0·45 and the activator used was sodium hydroxide (NaOH). Activator concentrations of 2, 4 and 6% of the binder weight were used. The compressive strength was measured and X-ray diffraction, mercury-intrusion porosimetry (MIP) and scanning electron microscopy analyses were conducted to determine the cause of the change in strength and pore structure. The results indicate that the compressive strength for early and later ages improved with the use of SW instead of TW. When SW was used, calcium–silicate–hydrate gel and Friedel's salt were identified as the hydration reactants. Based on the MIP measurements, when the sodium hydroxide concentration was increased, the pore size and number of pores decreased to form a denser matrix. In particular, samples containing SW had an increased number of gel pores (<0·01 μm).

Effect of ceramic waste powder on alkali-activated slag pastes cured in hot weather after exposure to elevated temperature

Extensive amounts of ceramic and slag wastes are produced every year by ceramic and iron industries. Recycling of these wastes are one of the operational solutions to eliminate their disposal. It is appropriate to employ these wastes as a binder material in the field of construction to save the virgin natural raw materials and limit environmental pollution problems. This work aims to incorporate ceramic waste powder (C–W–P) into alkali-activated slag (AAS) pastes activated with sodium silicate. The specimens were cured at a high ambient temperature of 45 °C to simulate curing in hot weather. Slag was partially replaced with C–W–P in the range of 5–50%, by weight. The effect of C–W–P on the workability, water absorption and compressive strength were measured. After 91 curing days, some specimens were subjected to elevated temperatures in the range of 200–1000 °C with a step of 200 °C for 2 h and the residual compressive strength was monitored. The results were analyzed by X-ray diffraction (XRD), thermogravimetric analysis and its derivative (TGA/DTG) as well as scanning electron microscopy (SEM). The results revealed that C–W–P has a negative effect on the workability, whilst it has a positive effect on the water absorption as well as the compressive strength before and after exposure to elevated temperatures.

An investigation on alkali-activated slag pastes containing quartz powder subjected to elevated temperatures

In this work, the opportunity of employing crystalline silica in the form of quartz powder (QP) to increase the fire resistance of alkali-activated slag (AAS) has been explored. The ground granulated blast-furnace slag (designated as slag) was partially replaced with QP at levels changed from 5% to 30% with a step of 5%, by weight. After curing for 28 days, the specimens were subjected to elevated temperatures varying from 400 oC to 1000 oC with a step of 200 oC for 2 h. The compressive strength before and after heating was measured. The new formed phases were evaluated by using X-ray diffraction (XRD), whilst the decomposition phases were evaluated by using thermogravimetric (TGA). The changes in the microstructures were monitored by using scanning electron microscopy (SEM). The results showed a positive effect of incorporation of QP, up to 30%, in AAS system before and after exposure to elevated temperatures.

Mitigating shrinkage of alkali-activated slag by polypropylene glycol with different molecular weights

In this work, the influence of polypropylene glycol (PPG), as the shrinkage-reducing admixture (SRA), on the fresh and hardened properties of alkali-activated slag (AAS) paste is studied. The flowability, setting time, compressive strength, pore structure, and drying shrinkage performance of AAS with PPG of three different molecular weights (i.e., PPG400, PPG1000, and PPG2000) at two addition dosages (i.e., 0.5% and 1.0% by slag mass) are evaluated. The results show that PPG addition slightly affects the flowability and setting behaviors of AAS, but retards its strength development likely due to the inhibiting effect of polymer agglomerates in PPG on slag dissolution and reaction. The PPG400 shows a higher shrinkage mitigation efficiency in AAS systems than its counterparts with a higher molecular weight. Nevertheless, the beneficial shrinkage mitigation role of PPG addition is merely significant in the AAS dried under a relatively low relative humidity level. The mechanism by which PPG addition mitigates shrinkage of AAS is attributed to the combinational effects of surface tension reduction of pore solution and coarsening of pore structure.

Interpreting the early-age reaction process of alkali-activated slag by using combined embedded ultrasonic measurement, thermal analysis, XRD, FTIR and SEM

In the present study, the early-age reaction process of alkali-activated slag (AAS) pastes was in-situ monitored by using embedded piezoelectric transducer based ultrasonic monitoring system. The effects of different types of activator (sodium hydroxide and sodium silicate) on reaction process, products and their microstructure evolution were investigated. The experimental results reveal that the early-age reaction process of AAS can be effectively in-situ observed by use of the ultrasonic method. It is found that the early-age reaction process of AAS pastes made with sodium silicate can be divided into five stages: (Ⅰ) dormant period, (Ⅱ) acceleration period, (Ⅲ) deceleration period, (Ⅳ) second acceleration period and (Ⅴ) second deceleration period. However, only three featured stages could be identified in those activated with sodium hydroxide. The soluble SiO44− ions in activator solution are critical to the formation process of reaction products and the microstructure evolution, especially for the former four stages. In specific, when activation is with sodium silicate, the increase of its modulus (Ms, molar ration of SiO2 to Na2O) leads to the improvement of reaction extent in stage Ⅰ, prolongs the reaction time of stage Ⅱ; meanwhile, the deceleration period of stage Ⅲ is shortened due to the advancement of stage Ⅳ.

Chloride and heavy metal binding capacities of hydrotalcite-like phases formed in greener one-part sodium carbonate-activated slag cements

The treatment of industrial solid wastes requires a proper binder that can efficiently solidify and stabilize the harmful containments. Previous studies have shown that the incorporation of calcined dolomite (CD) in the one-part sodium carbonate-activated blast furnace slag binder promotes the formation of hydrotalcite-like phases, which exhibit a potentially high ion-binding capacity. This study investigates the chloride and heavy metals binding capacities of the hydrotalcite-like phases formed in the binder. The results show that this binder has higher resistance to chloride penetration than the Portland cement and other alkali-activated slag pastes. The solidification efficiencies of Pb2+, Cu2+ and Cr3+ ions for the binder with 10% CD dosage are in the range of 66–88%. The hydrotalcite formation mechanism proposed in this work suggests that the carbonate ion-exchange in the interlayer of hydrotalcite reduces the available binding sites, so the binding capacity is mainly attributed to the physical surface adsorption, rather than interlayer ion-exchange. The CO2 emission and energy consumption of this binder are around 80% lower than the PC paste. This study provides a guidance for the production of greener binder for the use in the treatment of industrial solid wastes.

Investigation of the Effects of Magnesium-Sulfate as Slag Activator.

This study is about the mechanical and microstructural properties of alkali-activated slag (AAS) paste using magnesium sulfate (MS) as an activator. MS is 2%, 4%, 6%, 8% and 10% contents of binder weight and water-binder ratio is 0.35. Compressive strength, X-ray diffraction, mercury-intrusion porosimetry, and thermal analysis were performed for analysis. The MS contents at which the maximum compressive strength appeared varied according to the measurement age. Hydration products affecting compressive strength and pore structure were ettringite and gypsum. As a result, the changes of ettringite and gypsum depending on the contents of MS have a great influence on the pore structure, which causes the change of compressive strength. The high MS contents increases the amount of gypsum in the hydration products, and the excess gypsum causes high expansion, which increases the diameter and amount of pores, thereby reducing the compressive strength.

The influence of superabsorbent polymer on the properties of alkali-activated slag pastes

This paper discusses the interaction between superabsorbent polymer (SAP) and alkali-activated slag pastes and the influence of SAP on the properties of the pastes. SAP showed higher absorption in pore solutions extracted from alkali-activated slag pastes compared to that extracted from portland cement paste. Hydrogel desorption in alkali-activated slag matrix activated with Na2CO3 was noticeably delayed compared to alkali-activated slag matrix activated with NaOH matrix or portland cement matrix; this is attributed to a significant delay in the solid skeleton development in this matrix. Addition of SAP was shown to be effective in reducing autogenous shrinkage in alkali-activated slag pastes. A general reduction in compressive strength and electrical resistivity was observed in alkali-activated slag pastes when SAP was added; macrovoid formation and increased overall water/binder are possible reasons for this reduction, respectively.

Effects of CO2 Curing on Alkali-Activated Slag Paste Cured in Different Curing Conditions.

The effect of CO2 curing on alkali-activated slag paste activated by a mixture of sodium hydroxide and sodium silicate solutions is reported in this paper. The paste samples after demolding were cured in three different curing environments as follows: (1) environmental chamber maintained at 85% relative humidity (RH) and 25 °C; (2) 3-bar CO2 pressure vessel; and (3) CO2 chamber maintained at 20% CO2 concentration, 70% RH and 25 °C. The hardened samples were then subjected to compressive strength measurement, X-ray diffraction analysis, and thermogravimetry. All curing conditions used in this study were beneficial for the strength development of the alkali-activated slag paste samples. Among the curing environments, the 20% CO2 chamber was the most effective on compressive strength development; this is attributed to the simultaneous supply of moisture and CO2 within the chamber. The results of X-ray diffraction and thermogravimetry show that the alkali-activated slag cured in the 20% CO2 chamber received a higher amount of calcium silicate hydrate (C-S-H), while calcite formed at an early age was consumed with time. C-S-H was formed by associating the calcite generated by CO2 curing with the silica gel dissolved from alkali-activated slag.

Effect of microwave curing as compared with conventional regimes on the performance of alkali activated slag pastes

Increasing the rate of strength development can greatly facilitates and accelerates numerous essential procedures in the construction field. Rapid strength development in alkali-activated materials composites could be reached by various techniques. In recent era, alternative techniques for accelerated heating, and curing have been investigated for cementious composites. The microwave heating is one of those methods, which is the main concern in the proposed research. As for the alkali-activated materials concrete, it has not been yet extensively studied, the aim of the conducted research program was to investigate the microwave curing effect on the compressive strength, and microstructure of alkali-activated slag pastes as compared with other curing regimes; air, water, and conventional heating. The experimental results revealed that, microwave curing is beneficial for achieving high early strengths over all of the other curing regimes studied, furthermore, the heat curing time of application was significantly shortened by the microwave radiation. Results demonstrated that microwave energy increased the dissolution of slag in the alkaline media. Widespread formations of the hydration gels were identified in the microscopic analyses. This led to a denser, well compacted, and stronger matrix resulting in higher compressive strength gain compared to those pastes cured under air, water, and conventional heating regimes. The microwave significantly accelerated the activation, so it can reach around 90% of its 28 days strength at the first 7 days.

Preparation, characterization of highly dispersed reduced graphene oxide/epoxy resin and its application in alkali-activated slag composites

A highly dispersed reduced graphene oxide/epoxy resin composites (HD-RGO/EP) were synthesized and applied in alkali-activated slag based composites firstly. The synthesis, characterization and application in alkali-activated slag cementitious materials of this synthesized graphene are reported in this paper. The dispersion states of the reduced graphene oxide in epoxy resin and alkali activated slag paste are proved to be sound by observation by TEM and Metallographic Microscope respectively. Both FTIR and Raman spectrograms verifies that there is no new reaction products generated under HD-RGO dispersing in the epoxy resin. It is confirmed that HD-RGO/EP increases the fracture toughness of alkali-activated slag mortar 75 times the most compared with the one blend with EP, the bonding behavior of the alkali-activated slag composite blended with HD-RGO/EP is enhanced significantly as well. HD-RGO/EP alkali-activated slag mortar is believed to have broad application prospects in repair and rehabilitation of reinforced concrete structures.

Understanding the aqueous phases of alkali-activated slag paste under water curing

The chemical compositions of the aqueous phases in alkali-activated slag (AAS) paste and Portland cement (PC) paste were determined up to 28 d with the aim of obtaining a better understanding of the stability of the hydration products in the two binder systems. The saturation levels with respect to the hydration products of PC and AAS were obtained by thermodynamic modelling. The main findings of this study were: the effective saturation index for portlandite in the AAS system was always below zero and the sulfate-bearing phases were not stable in the AAS system compared with those in the PC; strätlingite and hydrotalcite phases were stable in the AAS paste due to the high magnesium and silicon concentrations in the pore solution; both the ionic strength and alkalinity of the AAS pore solution were higher than those of the PC pore solution, which were responsible for more severe efflorescence in the AAS paste and the higher conductivity of the AAS pore solution. According to thermodynamic estimations, tobermorite-based C–S–H was dominant in the AAS system after 7 d, while jennite-based and tobermorite-based C–S–H gels were present in the PC system up to 28 d. The results suggest that in PC and AAS pastes, different solid phases are formed during the hydration, which change with time, and the reactions and equilibria in both binders are completely different.

Ambient-cured alkali-activated slag paste incorporating micro-silica as repair material: Effects of alkali activator solution on physical and mechanical properties

In this paper, the optimum alkali activator solution properties in alkali-activated pastes based on ground granulated blast furnace slag (GGBFS) and micro-silica (MS) were investigated. For this purpose, sodium hydroxide (NH) solutions with molarities of 8 M, 12 M, and 16 M were prepared and mixed with sodium silicate (NS) solutions at NS:NH ratios of 1.0, 2.5, and 4.0 to produce the final activator solution. Two sets of pastes were manufactured: (1) pastes based on GGBFS, and (2) pastes in which 10% by weight of GGBFS was replaced with MS. The fresh density increased with increasing NH molarity and NS:NH ratio; whereas, the final setting time reduced with an increase in the aforementioned properties. With regard to the hardened paste, the tests of compression strength, porosity, and electrical resistivity were performed. According to the results, pastes activated with a 12 M NH solution and NS:NH ratio of 2.5 showed the highest compressive strength and electrical resistivity and the lowest porosity. Furthermore, the use of MS increased the compressive strength and electrical resistivity of the optimum paste by 27% and 64%, respectively, over those of the same paste with 100% GGBFS. In the final part, linear regression models were developed to predict the properties of GGBFS and GGBFS/MS pastes. Based on the findings of this study, it is possible to produce a zero-cement paste based on GGBFS and MS, with final setting time of 25–35 min and 1-day compressive strength of 60–70 MPa by using the optimal alkali activator solution, which can be used for repair purposes due its proper setting and strength properties.