Performance evaluation and microstructure characterization of seawater and coral/sea sand alkali-activated mortars

To adequately develop the application of marine resources for island construction and effectively promote the sustainability of industrial resources, this paper investigates the feasibility of using seawater and coral or sea sand as replacements for freshwater and river sand in alkali-activated mortars (AAMs) containing ground blast furnace slag (GBFS), fly ash (FA), and silica fume (SF). Five different mortar mixtures were designed: seawater and coral sand alkali-activated mortar (SC-AAM), seawater and sea sand alkali-activated mortar (SS-AAM), freshwater and river sand alkali-activated mortar (FR-AAM), seawater and coral sand cement mortar (SC-OPC), and freshwater and river sand cement mortar (FR-OPC). The differences in the workability (slump), setting time, compressive strength, flexural strength, and drying shrinkage of the prepared cement mortars and AAMs were compared. Then, the microstructure and crystalline phases of the mortar samples were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectrometry. The experimental results indicated that the utilization of seawater and coral or sea sand promoted the formation of C-S-H gel phases. Additionally, the existence of coral sand decreased the slump and initial setting time of the mortars due to the high water absorption and porosity of the coral aggregate. The SC-AAM exhibited higher compressive and flexural strengths at an early age and had a lower drying shrinkage than other AAMs, which can be attributed to the natural internal curing effect of the coral sand and the dense interfacial transition zone between the aggregate and the binders. The weak strength of coral sand had no effect on the strength development of the mortars in AAM production. However, the SS-AAM had a relatively low compressive strength and a large drying shrinkage in comparison to other mortar samples due to the existence of more free water. (C) 2020 Elsevier Ltd. All rights reserved.