Properties and hydration mechanism of eco-friendly cementitious material prepared using coal gasification slag and circulating fluidized bed fly ash

Coal gasification slag (CGS) and circulating fluidized bed fly ash (CFBFA) are industrial solid wastes discharged from coal utilization process. They are difficult to be reused as secondary resources and stockpiled on the land in large numbers, which threatens local ecological environment. In this study, they were employed to prepare an eco-friendly cementitious material under the activation of the ordinary Portland cement (OPC). The binary cementitious systems of OPC-CGS (CS) and OPC-CFBFA (CF) and the ternary cementitious system of OPC-CGS-CFBFA (CSF) were designed to reveal the role of the wastes in the properties of cementitious materials. The workability, mechanical property, and hydration mechanism of these systems were studied by using various methods. Results showed that in the binary cementitious system, the addition of CFBFA had an adverse effect on the flowability and shortened the setting time, whereas the CGS had a positive effect on the flowability and delayed the setting time. The compressive strengths of the binary cementitious systems were much lower than that of the P·O 42.5 cement because the potential hydration reactivity of the aluminosilicates in the two wastes cannot be well activated by the OPC in the binary systems. However, in the ternary cementitious system, their respective shortcomings were greatly improved. Not only the workability can meet the technical requirements of the P·O 42.5 cement, but also good mechanical properties were obtained. The 28 days and 90 days compressive strengths of the best recipe (40%OPC-40%CGS-20%CFBFA, denoted as CSF3) could reach 43.3 and 54.9 MPa, respectively. The formation of large amounts of the hydration products including the ettringite (AFt) and amorphous gels played an important contribution to the development of matrix strength. The combination of the CGS and the CFBFA in the ternary system exhibits a notable synergistic impact on the hydration process under the OPC activation. Furthermore, the composite cementitious materials had low autogenous shrinkage coefficients compared to the OPC. These findings not only provide a theoretical guidance for the large-scale consumption of the CGS and CFBFA, but also help to reduce CO2 emission in the construction field.