Coal Gasification Slag as a Green Additive in Supplementary Cementitious Materials: Mechanical Properties and Microstructure

Gasification slag is the solid waste produced in the process of coal gasification. China produces approximately 30 million tons of gasification slag every year, which urgently needs to be recycled in an efficient and sustainable way. This paper discusses the feasibility of using gasification slag as a supplementary cementitious material (SCM). The working properties, mechanical properties, and microstructure of cement paste after the addition of gasification slag were studied and compared with those of pure cement paste. The results indicate that the hydration products of the composite paste contain a significant amount of Ca(OH) and C-S-H gel when the content of gasification slag is less than 30%. However, when the gasification slag content exceeds 30%, the primary hydration product shifts to the C-A-S-H gel. Furthermore, the C-(A)-S-H gel tends to exhibit a lower calcium–silicon ratio and a higher degree of polymerization as the gasification slag content increases. Specifically, the Ca/Si ratio of the 60% C-A-S-H gel is 1.66, with a degree of polymerization of 0.77. When the gasification slag content is maintained at or below 30%, the compressive strength of the gasification slag cement paste decreases by approximately 3.7% to 9.3% compared with that of Portland cement (PC). Nevertheless, the composite cement meets the design requirements of 42.5 composite Portland cement. Thus, gasification slag has emerged as a promising supplementary cementitious material (SCM), with significant potential for widespread application.