Structural Build-Up and Breakdown of Alkali-Activated Slag Pastes with Different Order of Lignosulfonate and Activator Addition

One of the crucial issues of alkali-activated materials is the application of conventional plasticizers. Lignosulfonate plasticizer (LS) works well for sodium hydroxide-activated slag but loses its efficiency in sodium waterglass-activated slag because of competitive adsorption with silicates. Therefore, the effects of the timing of LS and activator addition on the structural build-up and breakdown of AAS pastes with low-silicate moduli activator (0 to 0.50) were investigated, particularly by means of non-traditional strain sweep tests, which are thoroughly discussed. The rheological tests were correlated with the Vicat needle test and isothermal calorimetry. The results showed the high efficiency of LS in NaOH-activated slag paste over time, regardless of the order of addition of the components of the paste. It was accompanied by a considerable retardation effect of LS on the hydration of AAS. The beneficial effect of LS on the rheological parameters and hydration kinetics decreased rapidly with an increase of the silicate modulus of the activator. The delayed addition of the concentrated silicate-containing activators completely counteracted the slightly beneficial effect of LS and even accelerated the initial structural build-up.