Design and properties of seawater coral aggregate alkali-activated concrete

To adequately develop the application of marine resources for island construction and sustainability in the construction industry, this paper explores the feasibility of using alkali-activated materials (AAMs) as alternatives to ordinary Portland cements (OPCs) for the application in seawater coral aggregate concrete (CAC) structures. Artificial seawater, coral aggregates, and slag-based AAMs containing 5 wt.% silica fume and 15 wt.% fly ash, were mixed to develop the alkali-activated seawater coral aggregate concrete (AACAC), with the workability (slump), compressive and splitting tensile strengths being studied. The influence parameters, such as the total cementitious material content, water-to-binder ratio, and sand rate, were considered through Taguchi orthogonal experimental design method. The experimental results indicated that the most important factor herein for the compressive and splitting tensile strengths of the AACAC was the total cementitious material content. The analysis of range and variance demonstrated that an optimal mixture for AACAC was determined to be a total cementitious material content of 500 kg·m−3, a water-to-binder ratio of 0.60, and a sand rate of 55%. Then, this optimal mixture was adopted to analyze the effect of the replacement ratio of sea sand for coral sand (Rs) on the workability, compressive and splitting tensile strengths of AACAC, and the cement-based CAC was selected as the reference. Finally, the microstructures of the paste-aggregate interface for the CAC and AACAC sliced specimens were detected by scanning electron microscopy (SEM). It can be concluded that increasing Rs improved the workability of the AACAC, but exhibited a very slight effect on its mechanical properties. Additionally, the utilization of AAMs can effectively reduce the broken of the coral aggregate inside the concrete due to improved interfacial transition zone (ITZ) between the aggregates and the pastes, thereby exhibiting a higher splitting tensile strength than that of the cement-based CAC.