Preparation and microstructure of coal gasification slag concrete for road construction

In order to utilize the huge stock of coal gasification slag (CGS) as an alternative to river sand, the feasibility of coal gasification slag concrete for road construction is studied, and the influence of CGS on the performance of cement-based material is investigated. Using the CGS (particle size below 4.75 mm) as fine aggregate, the CGS mortar is made by replacing river sand with the same mass CGS. Replacement ratios are 0%, 20%, 40%, 60%, and 80%, respectively. Through the mortar fluidity test, compressive strength test, X-ray diffraction analysis, mercury intrusion analysis, scanning electron microscopy, and energy spectrum analysis, the influence of CGS replacement ratio on the rheological property, mechanical property, hydration product, pore structure and micro morphology of the mortar are systematically studied. The optimal substitution rate of CGS is determined through comprehensive comparison and selection, and the mix design of CGS concrete for road construction is carried out using this substitution rate. The mechanical performance reliability of CGS concrete is evaluated based on 28 d flexural tensile strength. The results show that as the substitution rate of CGS increases, the flowability of CGS mortar gradually decreases and the compressive strength at 28 days gradually increases. Taking into account the fluidity and mechanical properties of the mortar, the optimal substitution rate for CGS is 80%, and the corresponding compressive strength of CGS mortar is 34 MPa. The microscopic analysis shows that the CGS can promote the cement hydration reaction of CGS mortar, increase the relative content of C—S—H gel, and reduce the content of C₂S and C₃S. The addition of CGS reduces the critical pore size, average pore size, and median pore size of mortar, increases the proportion of small pore size, improves its pore size distribution type, and thus increases the proportion of small pore size, resulting in a decrease in pore connectivity and higher compactness. After adding CGS, the CGS mortar does not produce new hydration products, which reduces the calcium silica ratio of C—S—H gel, enhances the crystallinity of gel, and improves the durability of mortar. With variation coefficient 0.1, the CGS concrete with a substitution rate of 80% can ensure the use of cement concrete pavement for rural road without standard axle load for 10 years.