Effects of gypsum and phosphoric acid on the properties of sodium silicate-based alkali-activated slag pastes

In this paper, the effects of gypsum and phosphoric acid on the properties of sodium silicate-based alkali-activated slag paste were examined. Alkali-activated slag is recognized as a good performing binder. However, it suffers from fast setting and drying shrinkage. Phosphoric acid has been used as a retarder and gypsum has been used as a drying-shrinkage inhibitor in previous studies. However, it is not known what will happen if one adds both of them to alkali-activated slag. In this study, the effects of gypsum and/or phosphoric acid on the compressive strength development, setting time and drying shrinkage of alkali-activated slag were examined. Experimental results indicated that adding gypsum shortened the setting time, and at the same age, the compressive strength reached a higher value when the amount of used gypsum is higher. Drying-shrinkage decreased when the amount of used gypsum increased; however, when 0.82 M phosphoric acid was added in the activator with gypsum the results were somewhat different. With added phosphoric acid, adding more gypsum shortened the setting time as well. The compressive strength of specimens with added gypsum was lower than that of control specimens under 28 days. Drying-shrinkage increased with phosphoric acid as the amount of used gypsum increased.