Sodium Silicate-activated Slag Research Articles

Performance and interaction of sodium silicate activated slag with lignosulfonate superplasticiser added at different mixing stages

This paper investigated the effect of adding lignosulfonate (LS) superplasticiser at the different stages of mixing on the workability and rheological behaviour of sodium silicate activated slag (SSAS) in order to find a practically feasible approach to tackling the incompatibility issue between superplasticiser and alkaline activator. In addition to rheology and minislump tests, adsorption, zeta potential and environmental scanning electron microscopy tests were also undertaken to understand the interactions between the lignosulfonate and the fresh SSAS in order to reveal the mechanisms behind the observation. The results show that adding the LS and the activator separately at the different stages of mixing can significantly improve both the initial minislump and 60-min minislump retention due to the increased adsorption of LS and the improved dispersion of slag particles, with the prior addition of LS better than the delayed addition. However, a nonlinear rheological behaviour of SSAS was observed in the LS-superplasticised SSAS under separate addition and, consequently, modified Bingham model was found to be more suitable for describing this kind of rheological behaviour.

Early structural build-up, setting behavior, reaction kinetics and microstructure of sodium silicate-activated slag mixtures with different retarder chemicals

The utilization potential of citric acid, sodium tetraborate decahydrate and sodium triphosphate pentabasic to control fresh state properties of sodium silicate-activated alkali-activated slag cements (AASC) has been studied. The early structural build-up, reaction kinetics, pore solution chemistry, setting behavior, and hardened properties such as microstructure, pore structure, and mechanical properties of AASC with these chemicals were investigated. Test results showed that early structural build-up, reaction kinetics and setting behavior of AASC can be well controlled by using these chemicals. The citric acid was found the most effective admixture to prolong the setting times of AASC; however, it caused a dramatic decrease in mechanical properties. On the other hand sodium triphosphate pentabasic could provide a slight delay in setting times, while sodium tetraborate decahydrate prolonged the setting times significantly without any detrimental effect on the mechanical properties.

Robustness to water and temperature, and activation energies of metakaolin-based geopolymer and alkali-activated slag binders

The aim of this paper is to evaluate the properties of four alkali-activated materials subjected to precasting and in situ concreting conditions: fluctuations in water content (±5%) and in temperature (10 °C, 20 °C, 30 °C). The initial slump and compressive strengths at 1, 2, 7, 28 and 90 days of a metakaolin-based geopolymer, and of sodium silicate-activated slag, sodium metasilicate-activated slag and sodium carbonate-activated slag mortars were determined, then compared to the properties of an ordinary Portland cement mortar (CEM I). A total of 50 mortars were cast for the five formulations studied and the different conditions. This paper reports that alkali-activated materials are as robust as CEM I in precasting and in situ concreting conditions. However, some adjustments should be made to offset the low reactivity in cold weather, such as hot water or hot curing, or reducing the water/binder ratio. The apparent activation energies were determined for the five pastes to explain these observations.

Chemical and physical effects of high-volume limestone powder on sodium silicate-activated slag cement (AASC)

This study aims at discussing physical and chemical influence of limestone powder (LS) on the AAS system. General properties, including fluidity, setting time, drying shrinkage, and mechanical strength of AAS pastes/mortars, were examined. Pore structure change and phase assemblages of LS-AAS composites were studied. The results showed that the addition of 50 wt% LS increased the fluidity and setting time of AAS pastes, but reduced the total drying shrinkage of AAS mortars. Meanwhile, around 30% reduction in mechanical properties were found at the same replacement ratio. The pore structure was refined in the pastes with LS replacement<25 wt%, but was coarsened in the sample with 40 wt% LS. Limited new phase was formed in the sodium silicate activated slag samples, which was believed to depend on the modulus of the activator. Around 30% hydration degree of LS was achieved in 40 wt% LS-replaced AAS pastes hydrated for 7 ~ 28 d as determined by TG-DSC analysis. The main drawbacks of AAS, fluidity, setting time, and drying shrinkage, are improved by using high-volume LS, so use of high-volume LS in AAS system can be an option for the purpose of environmental protection.

Early reactivity of sodium silicate-activated slag pastes and its impact on rheological properties

The fast loss of fluidity of sodium silicate-activated slag (SS-AAS) systems have been widely studied in the literature. In this study, the rheology of these cements has been correlated with the evolution of the reaction products formed over time. For this purpose, a combination of experimental characterization techniques, to study the pore solution composition and microstructure of SS-AAS pastes and thermodynamic calculations have been applied.The initial precipitation of ill-defined N-A-S-H and C-N-A-S-H producta leads to a fast increase of the storage modulus after 40 min of reaction. These products are formed from the reaction of the Na+ ions and silicate ions of the activator with the Ca2+ and aluminate species dissolved from the slag. However, when a shear is applied, the structure breaks down and fluidity is recovered, that infers that the attractive forces between the particles are low at the early stages of hydration.

Parametric modeling of autogenous shrinkage of sodium silicate-activated slag

In this study, a simple modeling scheme was employed to observe the autogenous shrinkage of sodium silicate-activated slag (SSAS)- one of the most widely used alkali-activated materials. The model parameters, including internal relative humidity, saturation degree of pores, capillary pressure, bulk modulus, and hydration degree were experimentally obtained. The mechanism of SSAS shrinkage was explored using the obtained modeling parameters, which may have some physical quantities, and was discussed comparatively with the shrinkage characteristics of OPC. The results indicated that the most important factors influencing the large shrinkage of SSAS were the high capillary pressure and creep-like behavior, which correlated with pore formation, rather than the stiffness of the matrix and the hydrate type.

On the quantification of degrees of reaction and hydration of sodium silicate-activated slag cements

The present study investigates the correlation between the degree of reaction (DoR) and hydration (DoH) of alkali-activated slag cements, prepared using a sodium silicate activator. The DoR of samples was measured by employing multiple techniques including 29Si magic-angle-spinning nuclear magnetic resonance spectroscopy, scanning electron microscopy image analysis, and selective dissolution, while the DoH was quantified by bound water measurement of samples dried via various drying methods. Due to experimental errors it is recommended that the DoR is described using regression as a function of time, rather than directly using experimentally-obtained values. The DoH values obtained from the measurement of bound water content showed a linear relationship with the DoR values measured by selective dissolution within 20% error margin. From this, it is expected that DoR can be inferred from DoH values obtained by the drying method, which is much easier than selective dissolution.

Shrinkage behaviour, early hydration and hardened properties of sodium silicate activated slag incorporated with gypsum and cement

As a novel low carbon cementitious material, alkali-activated slag (AAS) attracted many researchers’ interests, not only because of its environmental benefits, but also some superior properties than Portland cement (PC). Due to different chemical reactions of AAS, gypsum, a commonly used expansive agent in PC, exhibits a different behaviour in AAS. This paper reports the investigation on the shrinkage behaviour of sodium silicate activated slag (SSAS) incorporated with gypsum and PC. In addition, the early properties, including setting time and early hydration products, were determined by Vicat apparatus, XRD and TG. The hardened properties, namely flexural and compressive strength, were investigated as well. The results showed that, by incorporating gypsum and PC, the drying shrinkage of SSAS could be significantly reduced because of the formation of expansive sulfate-rich and calcium-rich hydration products in terms of ettringite and Portlandite. The initial setting was delayed by blending PC, although adding gypsum accelerated the hydration of SSAS. The addition of gypsum and blend with 20% PC slightly increased flexural and compressive strengths of SSAS at 7 days and 28 days.

Hydration kinetics modeling of sodium silicate-activated slag: A comparative study

A hydration kinetics model for sodium silicate-based alkali activated slag (AAS) is proposed. The hydration kinetics model for AAS is derived either using mass conservation during internal phase transformation, or by considering chemical affinity of hygro- (thermo-) chemical kinetics by adopting parameters obtained from various calculations and experiments. The obtained modelling result indicates that although the amount of water consumed by slag is lower than that by OPC at an identical mass ratio, the slag binder experiences larger chemical shrinkage, which leads to a dramatic reduction in the relative humidity during hydration. In addition, the modelling result suggests that the hydration rate after 1 day is higher when the pores are distributed in a wider range of diameters (i.e., capillary pores and gel pores are both present as in OPC), in comparison with the case when the pore distribution is in a narrower range (i.e., gel pores are dominant as in AAS).

Incorporation of CFBC ash in sodium silicate-activated slag system: Modification of microstructures and its effect on shrinkage

Circulating fluidized bed combustion (CFBC) ash was applied in sodium silicate-activated slag system as a secondary activator and/or for the substitution of commercialized expanding admixtures. To understand the effects of this ash on such alkali-activated slag (AAS) system, a series of experiments relating to hydration kinetics, hydration products and microstructure, and engineering properties were performed. It was found that the introduction of the ash resulted in different types of hydration products in AAS, leading to increases in both the size and volume of the pores. Although the setting time was retarded, the autogenous and dry shrinkages of the AAS decreased by the increase in the ash content. The shrinkage and strength of AAS affected by the external moisture conditions during curing could be controlled by substituting the slag with the ash.

Performance of magnesia-modified sodium carbonate-activated slag/fly ash concrete

Various innovative research in the cement industry is looking into improving its environmental sustainability. Sodium carbonate-activated slag/fly ash (NC-SF) binders has recently evolved as a potentially more sustainable binding materials than both Portland cement and conventional alkali activated materials such as sodium silicate and sodium hydroxide activated materials. The reaction mechanism and some microstructural properties of NC-SF cements have been a major area of research recently. However, very few studies have scaled up the investigation of these binders into concrete specimens. This paper, therefore, provides new insight into the strength development and the durability performance of NC-SF concrete and MgO-modified NC-SF concrete. Concrete testing included measurements of compressive strength, split tensile strength, water absorption, depth of carbonation, sulphate exposure, acid exposure and elevated-temperature exposure. Microstructure studies were conducted using Powder X-Ray diffraction (PXRD), Thermogravimetric analysis (TGA) and Attenuated Total Reflectance Fourier Transform Infrared spectroscopy (ATR-FTIR). It is concluded that NC-SF concrete mixes develop acceptable mechanical strength and demonstrate high resistance to sulphate attack. They also showed higher resistance to acid attack than the control mix based on sodium silicate-activated slag concrete. Here, emphasis is placed on the potential of developing NC-SF concrete with excellent performance and less complicated production methods as well as a low carbon footprint. It is also found that the use of reactive MgO enhanced the strength development of NC-SF concrete as well as its resistance to acid and carbonation.

Understanding the roles of activators towards setting and hardening control of alkali-activated slag cement

Sodium silicate-activated slag often gives high early strength but sets too rapidly. Commercial retarders for Portland cement and partial replacement with other cementitious components in alkali-activated slag (AAS) system have been proven ineffective. This study investigated the approach of controlling setting by altering activator ion composition and silica polymer status. The roles of activators during the alkali-activation process were studied via pore solution chemistry analysis and microstructural analysis of hydration products. The addition of Na2CO3 does not, but NaOH does alter the polymerization of silicate ion groups in sodium silicate solution due to the increases in pH and Na2O concentration. The specific effects of Na2CO3 on the setting and hardening of AAS depend on the relative contents of NaOH and Na2O·2SiO2 in alkaline solution. The reaction degree of AAS is dependent on activator compositions, which govern the kinetics of formation and intrinsic characteristics of the reaction products calcium aluminosilicate hydrates. The setting and strength development of sodium silicate-activated slag can be controlled by manipulating the compositions of Na2CO3-NaOH-Na2O·2SiO2.

Solidification of simulated sodium nitrate liquid wastes and ion-exchange resins by sodium silicate-activated slag cements

ABSTRACTIn this study, a mineral matrix based on hydrous sodium metasilicate (NSH5)-activated slag cement (AASC) was found to be suitable for solidification of sodium nitrate solutions with concentrations of up to 700 g/l and nitrate ion-exchange resins with volume of up to 35%. XRD, ТG/DSC, FTIR, and SEM/EDS analyses were used to study the wasteforms. Increasing the nitrate solution concentration decreased the rates of hydration and structure formation process of the system GGBFS-NSH9-NaNO3 solution, the amount of the Ca-, Si-, Al-, and Mg-containing reaction products, and slightly decreased the compressive strength of the wasteforms.

Layered double hydroxides modify the reaction of sodium silicate-activated slag cements

The impact of adding two types of layered double hydroxides (LDHs), commercial hydrotalcite (HT) and its thermally treated form (CLDH), on the reaction kinetics and phase assemblage development of a sodium silicate-activated slag cement was investigated. The reaction kinetics of LDH-modified cements was assessed by isothermal calorimetry, and the results were correlated with in situ attenuated total reflection Fourier transform infrared spectroscopy results collected over the first days of reaction, to identify the structural evolution of the main binding phase forming in these cements: a sodium-containing calcium aluminosilicate hydrate (C-(N)-A-S-H)-type gel. The addition of either HT or CLDH into sodium silicate-activated slag paste accelerates the precipitation of reaction products and increases the formation of HT in these cements, without causing significant changes to the C-(N)-A-S-H binding phase. This is extremely relevant in terms of the durability of alkali-activated slag cements, as a higher content of the HT-like phase has the potential to reduce their chloride permeability and enhance carbonation resistance.

Monitoring the Carbonation-Induced Microcracking in Alkali-Activated Slag (AAS) by Nonlinear Resonant Acoustic Spectroscopy (NRAS)

Abstract This article presents the potential of nonlinear resonant acoustic spectroscopy (NRAS) for noninvasive monitoring of the carbonation progress in alkali-activated slag (AAS) mortars. In the search for sustainable concrete, AAS has emerged as a potential substitute for ordinary portland cement binder. However, carbonation is reported to be an important durability concern for AAS due to the absence of portlandite. In this study, the correspondence between the physical properties and microstructural evolution of sodium silicate-activated slag (SS-AS) and sodium hydroxide-activated slag (SH-AS) mortars were studied over the full course of accelerated carbonation. The measured properties include the following: compressive strength, carbonation depth, porosity, pore size distribution, and phase assemblage. In addition, NRAS was used to track the changes in materials stiffness (linear resonance frequency) and hysteretic nonlinearity (amplitude dependency of resonance frequency). Scanning electron microscopy (SEM) images and porosimetry results showed the formation of microcracks and increased micrometer porosity in a carbonated AAS binder caused by calcium aluminum silicate hydrate decalcification; the cracking was more severe in the SS-AS than in the SH-AS. The NRAS results revealed a close correspondence between the observed microscopic changes in the samples and measured macroscopic test parameters, indicating the potential of acoustic techniques for monitoring the advancement of carbonation fronts in AAS mortars.

Effect of sodium-silicate activated slag at different silicate modulus on the strength and microstructural properties of full and coarse sulphidic tailings paste backfill

In this study, strength and microstructural development of full (FT) and coarse sulphidic tailings (CT) cemented paste backfill (CPB) produced from sodium-silicate activated slag (SSAS) at different silicate modulus (Ms) were investigated in the short- and long-term. SSAS samples (SSASs) with varying Ms for both FT and CT produced 1.5–3.5 fold unconfined compressive strengths in the long-term compared to ordinary Portland cement samples (OPCs). Optimum Ms values were 1.0–1.25 for FT and 1.25–1.50 for CT considering the short- and long-term strength gain and microstructural properties of CPBs based on the polymerization degree and balance of C–S–H gel. Strength losses were observed in OPC-CT and in SSASs at Ms = 0.75 for FT and CT in the long-term. Formation of secondary expansive minerals such as ettringite, decalcification of C–S–H gel and the weakening of microstructure were found to be the main reasons for the strength losses due to the coupled effect of acid and sulphate attack. The use of CT and SSAS together significantly improved the microstructure of CPB. Increase in Ms decreased the porosity, refined the pore structure providing more compact microstructure and alleviated the decalcification of C–S–H gels in consequence of higher rate of polymerization.

Micromechanical properties of alkali-activated slag cement binders

An experimental investigation into the micromechanical properties of alkali-activated slag cement (AASC) binders was carried out using targeted and grid nanoindentation. The results of grid indentation techniques were deconvolved using Gaussian mixture modeling with Bayesian model selection to determine the appropriate number of component phases for the model. The information given by the resulting mixture models and from targeted indentation experiments was disseminated in the context of existing information about the composition and development of the microstructure in AASC binders. The microstructure of sodium silicate-activated slag cement contains only two components (ground mass gel and unreacted slag cement) upon microscopic examination, but indentation data suggest that it is much more complex and varied. The microstructure of sodium hydroxide-activated slag cement contains ground mass gel, unreacted slag cement, and an inner product ring surround the unreacted slag. The inner product is denser, harder, and stiffer than the surrounding product phases. The micromechanical properties in sodium hydroxide-activated slag cement are not affected by activator molarity; the macroscale strength is similarly unaffected. Conversely, the micromechanical properties of sodium silicate-activated slag show a slight improvement with increased silica modulus, while the macroscale strength shows a significant improvement. The macroscale improvement is likely due to the increased size of unreacted slag cement grains, which are shown to be very hard and stiff.

Advances in near-neutral salts activation of blast furnace slags

The utilisation of near-neutral salts as activators to produce alkali-activated slag cements offers several technical advantages, including reduced alkalinity of the binders, minimising the risk associated with handling of highly alkaline materials, and better workability of the fresh paste compared to that of sodium silicate-activated slag cements. Despite these evident advantages, the delayed setting and slow early-age mechanical strength development of these cements have limited their adoption and commercialisation. Recent studies have demonstrated that these limitations can be overcome by selecting slags with chemistry which is more prone to react with near-neutral salts, or by adding mineral additives. A brief overview of the most recent advances in alkali-activation of slags using either sodium carbonate or sodium sulfate as activators is reported, highlighting the role of material design parameters in the kinetics of reaction and phase evolution of these cements, as well as the perspectives for research and development of these materials.

Advances in near-neutral salts activation of blast furnace slags

The utilisation of near-neutral salts as activators to produce alkali-activated slag cements offers several technical advantages, including reduced alkalinity of the binders, minimising the risk associated with handling of highly alkaline materials, and better workability of the fresh paste compared to that of sodium silicate-activated slag cements. Despite these evident advantages, the delayed setting and slow early-age mechanical strength development of these cements have limited their adoption and commercialisation. Recent studies have demonstrated that these limitations can be overcome by selecting slags with chemistry which is more prone to react with near-neutral salts, or by adding mineral additives. A brief overview of the most recent advances in alkali-activation of slags using either sodium carbonate or sodium sulfate as activators is reported, highlighting the role of material design parameters in the kinetics of reaction and phase evolution of these cements, as well as the perspectives for research and development of these materials.

Temperature and activator effect on early-age reaction kinetics of alkali-activated slag binders

The early-age reaction kinetics of alkali-activated ground granulated blast-furnace slag (GGBFS) binders as determined by in-situ isothermal calorimetry are discussed in this paper. Particular attention is paid to the effects of activator type (sodium hydroxide and sodium silicate) and concentration, as well as curing temperature (23°C and 50°C). The mechanical strength development, microstructure, and product phase composition are also discussed to provide context for the phenomena observed in the kinetics results. It is shown for both activators that elevated temperature curing greatly accelerates hydration, resulting in more rapid product formation and strength development. High-molarity sodium hydroxide activators are shown to accelerate early hydration at ambient temperature, but tend to present a barrier to advanced hydration thereby limiting the later-age strength. Elevated temperature curing is shown to remove this barrier to advanced hydration by improving solubility and diffusivity. Hydration of sodium silicate-activated slag is comparatively slow, resulting in the delayed formation of very dense products with higher mechanical strength. Increasing sodium oxide tends to accelerate hydration, resulting in improved early- and later-age strength, while increasing the silica tends to retard the reaction, resulting in slower, more complete hydration as well as improved mechanical strength.