Effect of fly ash and gypsum on drying shrinkage and mechanical properties of one-part alkali-activated slag mortar

One-part alkali-activated materials can be processed into semi-finished products by mixing a solid activator with the powder. They only require the addition of water, which greatly improves their commercial feasibility. So, we used one-part alkali-activated slag mortar to investigate the effects of the dosages of the activator (sodium metasilicate), fly ash (FA), and gypsum on its fluidity, flexural strength, compressive strength, and drying shrinkage. The microscopic mechanisms of the mortar were analyzed using scanning electron microscopy and x-ray diffraction. The below conclusions were drawn. First, the drying shrinkage and compressive strength of the mortar gradually increased with the increase of the activator dosage. Second, the drying shrinkage of the mortar initially decreased and then increased as increasing FA substitution, while the compressive strength decreased gradually in the early stage and then increased and decreased in the later stage. Lastly, by increasing the gypsum dosage, the drying shrinkage of the mortar was significantly reduced. However, the compressive strength gradually decreased. Besides, when adding 20% of FA and 2.4% of gypsum, it was found that ettringite, which underwent expansion, formed in the system. This not only reduced the drying shrinkage by 25.3% but also increased the compressive strength by 1.3%. Furthermore, our investigation has led to the development of a low-shrinkage one-part alkali-activated slag and FA mortar, which not only facilitates the recycling of solid waste materials but also presents commendable environmental and economic advantages.