Optimization of mix proportion of alkali-activated slag mortars prepared with seawater and coral sand

Abstract To effectively utilize marine resources and promote sustainable development in the construction industry, an innovative building material, namely, alkali-activated slag mortars prepared with seawater and coral sand (SC-AAMs), was proposed in this paper, and its workability, setting time, compressive strength, flexural strength, and drying shrinkage were studied. The effects of the modulus of sodium silicate (Ms), Na2O-to-binder (N/B) ratio by weight, replacement ratio of the coral sand for sea sand (Rs), and water-to-binder (W/B) ratio were considered using the Taguchi orthogonal experimental design method. The experimental results indicated that an increased alkali activator modulus and alkaline contents improved the compressive and flexural strengths, however, resulting in a decreased setting time and an increased drying shrinkage. According to the results from the aforementioned orthogonal experiments, the optimum mixtures for SC-AAMs were determined to be an Ms = 1.2, a N/B = 4%, a W/B = 0.45, and a Rs = 100%. Then, the microstructure and crystalline phases in the SC-AAMs prepared with this optimum mix proportion were analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively, and the cement mortar mixed by seawater and coral sand was selected as the reference. It can be concluded that the utilization of the alkali-activated materials changed the reaction hydrate products of the mortars and improved the interfacial microstructures between the coral sand and slurry. In addition, the existence of the coral sand reduced the drying shrinkage of the AAMs due to the self-curing effect inside the coral aggregate.