Microstructural behaviour and shrinkage properties of high-strength fiber-reinforced seawater sea-sand concrete

The use of seawater (SW) and dredged sea-sand (DSS) in concrete manufacturing could bring considerable environmental and economic benefits. However, due to lack of information on properties of the concrete made up of SW and sea-sand (SS), their application in industrial practice is rather limited. Shrinkage is an important property that affects concrete long-term strength and durability. To better understand and improve shrinkage performance, the behavior of high-strength fiber-reinforced seawater sea-sand (SWSS) concrete was investigated in this study. Polypropylene (PP) and polyvinyl alcohol (PVA) fibers with varying volume content (0.1% similar to 0.5%) were used for concrete reinforcement. The autogenous and drying shrinkages of fiber reinforced SWSS concrete were examined and compared with those of the plain SWSS and freshwater natural sand (FWNS) concretes. The microstructural characteristics of the concrete mixes were also studied using various approaches including thermogravimetric analysis, X-ray diffraction analysis, nitrogen adsorption test and scanning electron microscopy images. Experimental results indicated that using SW and DSS resulted in an increase in the autogenous and drying shrinkage of the concrete due to existence of chloride, sodium, and potassium ions. These ions led to cement hydration acceleration, higher activation of ground granulated blast furnace slag and formation of C-A-SH, consequently, a higher shrinkage value. Addition of either PP or PVA fibers could decrease the concrete shrinkage and a higher dosage of fibers caused a higher reduction in the shrinkage values. While both PP and PVA showed almost similar reduction in autogenous shrinkage, PP fibers resulted in a lower level of drying shrinkage. Compared with FWNS, lower cementitious hydration products (C-H and C-S-H) were observed in SWSS concrete. Effects of fibers on the hydration products were insignificant. Although SWSS concrete porosity was lower than that of conventional concrete at early-age, higher porosity was observed for SWSS after 63-days of curing. Finally, this study shows that the incorporation of fibers in SWSS concrete increased its long-term total porosity.