Mechanism and assessment of the pozzolanic activity of melting-quenching lithium slag modified with MgO

Abstract

The current energy consumption and waste emission are mainly produced from the process of cement manufacturing. Lithium slag (LS), a solid waste from lithium smelting, has been utilized as concrete additives with a low rate of approximately 10 %, and the remaining nearly 90 % of LS are stored as industrial solid waste, which not only occupies large amounts of land, but also causes serious resource waste and environmental pollution. For the purpose of developing a kind of cleaner binders using LS to reduce cement consumption and environmental pollution, a novel activation method of “chemical modification-melting granulation-mechanical pulverization” was conducted in this present study. Alkali cations, such as Mg2+, Ca2+, etc., are beneficial to improve the pozzolanic activity of LS according to previous results. Thus, a chemical modification was first performed by preparing the melted-quenched mixtures of LS and MgO in different ratios, and coded as MGLS. The characterization results of MGLS demonstrate that MgO modification can enhance the pozzolanic activity of LS by accelerating the dissolution of the amorphous networks. Pastes of PPC and P-MGLS (70 % PPC + 30 % MGLS) were prepared for UCS tests (a maximum allowable rate of 30 % for LS as concrete additives, according to engineering experience), showing clear retardations of strength development for P-MGLS pastes due to the low extent of pozzolanic reaction in the early period of 3 ∼ 7 d. The 7 ∼ 90 d strengths significantly increase with the content of MgO for P-MGLS pastes, and the P-MGLS15 paste cured for 90 d gains the UCS value of 52.49 MPa, even higher than that of PPC paste at 90 d, due mainly to the enhanced pozzolanic reaction with large amounts of CSH/CAH gels formed by MgO modification, and a compact microstructure, as well as higher values of Ca/Si and Al/Si ratios are measured in the CSH/CAH gels of P-MGLS15 paste, indicating that more reactive constituents can be released to enhance the formation of pozzolanic products for the pastes richer in MgO. Results of the present study may provide a potential way for the applications such as construction building, backfill mining, etc., that can be developed using MGLS, which also promotes the sustainability for cement and environmental industries.