Performance-based alkali-activated seawater sea-sand concrete: Mixture optimization for mechanical, environmental, and economical objectives

This study aims to explore the viability of employing alkali-activated materials (AAMs) as alternatives for or-dinary Portland cements (OPCs) in seawater sea-sand concretes, thereby promoting the sustainable utilization of marine resources in island-based constructions. Artificial seawater, sea-sand, coral coarse aggregate (CCA), normal limestone aggregate (NLA) and slag-based AAMs incorporating with 15 wt% fly ash and 5 wt% silica fume were mixed to prepare performance-based alkali-activated seawater sea-sand concrete (ASSC). The L9 (33) Taguchi orthogonal method was employed to analyze the influences of the replacement ratio of CCA for NLA, cementitious material content and water-to-binder ratio on the workability, compressive strength, splitting tensile strength, carbon dioxide equivalent (CO2-e) emission, energy consumption and cost of ASSC. Three standardized evaluation indexes and the Gray-Technique of Ordering Preferences by Similarity to Ideal Solution (TOPSIS) model were proposed to estimate the influence degree of each factor, and the suggested optimal mixture for ASSC was determined to be a replacement ratio of CCA for NLA of 50%, a cementitious material content of 440 kg/m3 and a water-to-binder ratio of 0.45. Then, the microstructure characteristics of the interfacial transition zone (ITZ) between the matrix and aggregate of ASSC for the optimal mixture were observed by scanning electron microscopy (SEM). It can be concluded that the main hydration products of ASSC had dense surface pores and internal pores of CCA, reducing the length, width and number of micro-cracks in ITZ. The substrate effect and micro-pump of CCA generate a high dense ITZ between the aggregate and matrix, which can inhibit the crack development of ASSC.