Abstract
The very early stages of alkaline activation of slag control its rheology and setting, but also affect its hydration, which occurs later. Simultaneously, these parameters are dictated by the nature and dose of the alkaline activator. Therefore, we investigated and compared the changes in slag particles (SEM, BET, laser diffraction), as well as in the pore solution composition (ICP–OES), pH, and conductivity, of alkali-activated slag (AAS) pastes containing the three most common sodium activators (waterglass, hydroxide, and carbonate) and water during the first 24 h of its activation. To ensure the best possible comparability of the pastes, a fairly nontraditional mixture design was adopted, based on the same concentration of Na+ (4 mol/dm3) and the same volume fraction of slag in the paste (0.50). The results were correlated with the pastes’ hydration kinetics (isothermal calorimetry), structural build-up (oscillatory rheology), and setting times (Vicat). Great differences were observed in most of these properties, in the formation of hydration products, and in the composition of the pore solution for each activator. The results emphasize the role of the anionic groups in the activators and of the pH, which help predict the sample’s behavior based on its calorimetric curve, and offer data for further comparisons and for the modelling of AAS hydration for specific activators.
Highlights
The search for ways to reduce the CO2 footprint in the cement and building industry is a very current topic
One possibility is the use of alternative binders, including alkaliactivated materials (AAMs), which can reduce CO2 emissions by up to 80% compared to ordinary Portland cement or clinker-based materials [1]; this strongly depends on the mix design [2]
The common issue for both systems is the use of plasticizing additives to control their rheology, which is associated with their chemical stability and miscibility in the activating solution, as well as their competitive adsorption with activator anions [24]
Summary
The search for ways to reduce the CO2 footprint in the cement and building industry is a very current topic. One possibility is the use of alternative binders, including alkaliactivated materials (AAMs), which can reduce CO2 emissions by up to 80% compared to ordinary Portland cement or clinker-based materials [1]; this strongly depends on the mix design [2]. For low-calcium systems such as alkali-activated fly ash (AAFA), elevated temperatures are needed to ensure their hydration, while for alkali-activated slag (AAS)—as the main representative of high-calcium systems—excessive drying and autogenous shrinkage [16], and sometimes rapid setting [17,18], are a problem Both AAS and AAFA are often blended to overcome their individual disadvantages [19,20,21,22,23]. The common issue for both systems is the use of plasticizing additives to control their rheology, which is associated with their chemical stability and miscibility in the activating solution, as well as their competitive adsorption with activator anions [24]
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