Influence of Wastewater Content on Mechanical Properties, Microstructure, and Durability of Concrete

In this study, high concentration wastewater from ready-mixed concrete plants was used to replace potable water as mixing water of concrete, with replacement rates of 0%, 25%, 50%, 75%, and 100%, by weight. The solid content of the wastewater was 12%. Five groups of C20 concrete mix proportions were designed. Different concrete properties, including workability, compressive strength and durability under freeze–thaw cycles, carbonization, and drying conditions, were studied, and the effect of the increase in the proportion of wastewater as a replacement for potable water was investigated. The microstructural attributes of the developed C20 concrete were studied through X-ray diffraction (XRD), thermal analysis (TG-DSC), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS), and mercury intrusion porosimetry (MIP). Finally, the economic benefits of replacing potable water with wastewater were analyzed. The results indicate that using wastewater for concrete mixing reduces workability and a superplasticizer is needed to ensure adequate concrete workability. At the ages of 7, 28, and 56 days, with the increase in the proportion of wastewater as a replacement for potable water (0, 25%, 50%, 75%, 100%), the compressive strength of concrete shows a trend of first decreasing, then increasing, and then decreasing. When the proportion of wastewater replacing potable water is 75%, the concrete compressive strength is the highest. The microstructure showed that the main products of wastewater-mixed concrete are calcite (CaCO3), portlandite (Ca(OH)2), ettringite (Aft), and calcium silicate hydrate (C-S-H). Adding wastewater to concrete does not lead to the formation of new products in the concrete. Wastewater can fill the concrete pores well, thus optimizing the pore structure. When the proportion of wastewater replacing potable water is 75%, C20 concrete has the densest microstructure, lower porosity, and better pore structure. Durability properties further indicate that 25%, 50%, and 75% of wastewater replacing potable water can improve the concrete’s frost resistance. However, there is a negative impact on the carbonation resistance of wastewater. Wastewater replacing 75% potable water by weight can improve the drying shrinkage of concrete. The recycling of wastewater is not only green and environmentally friendly but also has good economic and environmental benefits.