The effects of changing salinity on the sulfate resistance of metakaolin blended seawater mortars

Fresh water is consumed in great amounts in concrete industry. As fresh water is a limited resource on Earth, its excessive use should be avoided. Use of seawater as mix water in concrete is being investigated by researchers. Cement is another main ingredient of concrete, and its production causes high levels of greenhouse gas emissions. Use of mineral admixtures with partial replacement of cement in concrete is one indirect way to reduce emissions. In addition to the essential parameters of concrete such as workability and compressive strength, durability of concrete against harsh exposure conditions is also important for both safety and sustainability. In this study, fresh-state, hardened-state and microstructural properties of cement pastes/mortars containing fresh water and three types of seawater as mix waters were investigated. Natural seawater was gathered from Black Sea, and two additional types of seawaters with higher salinities were obtained by evaporating the gathered seawater. Metakaolin was incorporated in selected mixtures with substitution of 10% cement by weight. Fresh-state properties of mixes were analyzed through determination of setting times, flow, and rheological properties. Compressive strengths and length changes of mortar specimens were monitored during exposure to sodium sulfate solution. Occurred changes in microstructures of the specimens were investigated through X-Ray Diffraction (XRD) and Thermogravimetric Analysis (TGA). In conclusion, workability of mixes reduced with both seawater mixing and metakaolin substitution. Early-age compressive strengths of mortars improved when they were mixed with high salinity seawaters. Metakaolin substitution enhanced the 28-day and 90-day compressive strengths of mortar specimens, but a decrease was observed in fresh water mixed mortar specimens at three days of age. Increasing salinity in seawater, which was used as mix water, deteriorated the durability of mortars against sulfate attacks. Metakaolin substitution enhanced the sulfate resistance of seawater mortars.