Experimental Study of Slag Changes during the Very Early Stages of Its Alkaline Activation

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.

Influence of activator type and slag volume fraction on properties of alkali-activated slag pastes

“…One of the main disadvantages of these materials, limiting their wider use in practice, is considered to be extensive shrinkage, both drying and autogenous. Rapid setting may also occur when some activators are used [5,8,9].…”

“…The type of activator, particularly its anionic part, greatly affects the hydration process of AAS and consequently the final properties. Activation with sodium water glass (SWG) results in high compressive strength, high workability (low yield stress) [10], but unfortunately, the greatest autogenous shrinkage occurs [9,11]. When sodium hydroxide (SH) is used, usually high initial compressive strengths are obtained due to the fast hydration rate in the early stages, but they do not show their significant increase over time compared to other activators, because of a slow decrease in capillary porosity [3,10,12].…”

“…The structure of the C-A-S-H gel is similar to the C-S-H gel formed by hydration of OPC, but in the C-A-S-H-gel, some Si 4+ ions are substituted by Al 3+ ions. The charge change is compensated by Na + ions from the alkali activator [3,9,14]. Although the C-A-S-H gel is the main hydration product of AAS regardless of the type of alkali activator, its structure, morphology, and timing are significantly different [9,15].…”

“…The charge change is compensated by Na + ions from the alkali activator [3,9,14]. Although the C-A-S-H gel is the main hydration product of AAS regardless of the type of alkali activator, its structure, morphology, and timing are significantly different [9,15]. Also, various reaction by-products are formed, strongly depending on the alkaline activator composition [3].…”

Properties and Microstructure of Na2CO3-Activated Binders Modified with Ca(OH)2 and Mg(OH)2

“…However, with the excessive CO 3 2− ions released from Na 2 CO 3 , the moderate alkalinity conditions developed in the pastes usually result in much longer setting times, up to around 4 days in some formulations, than the pastes activated with the strong bases of activators [ 19 ]. The Ca 2+ dissolved from slag initially react with the CO 3 2− to precipitate as CaCO 3 , and the formation of the C-A-S-H gel phase occurs after the CO 3 2− ions are exhausted, accounting for the prolonged hardening process [ 18 , 19 , 22 ]. For the purpose of serving the one-part Na 2 CO 3 -activated slag binders as practical cementing materials in general applications, recent studies are extensively focused on the use of auxiliary activators to accelerate the kinetics of alkali activation [ 23 , 24 , 25 , 26 , 27 , 28 ].…”