Mechanical and environmental characteristics of cemented paste backfill containing lithium slag-blended binder

Abstract

Lithium slag (LS), a waste residue produced during the processing of lithium products, has rarely been utilized in the cement industries. The disposal of LS as solid waste causes serious land occupation and environmental pollution. This research work was performed by mixing the LS with NaOH to improve its pozzolanic activity. The blended binder (defined as CPL) was prepared by replacing 30 wt% of Portland pozzolana cement (PPC) with LS-NaOH mixtures. Retardations for early strength development (3 and 7 d) were observed in the CPL specimens, according to the results on the unconfined compressive strength (UCS) test. The specimens richer in NaOH showed higher strength, and the 28 and 56 d strengths of CPL6.0 specimens increased to 32.3 and 39.7 MPa, respectively, which were similar to the strength of PPC specimens (33.4 and 41.8 MPa). It was indicated by thermo-gravimetric analysis (TGA) that the weight-loss rates of CPL6.0 specimens in the intervals 40–200 °C, 400–500 °C, and 600–800 °C were 10.73%, 1.66%, and 1.24%, respectively, which were higher and closer to the PPC specimens than those for CPL0, CPL2.0, and CPL4.0 specimens. Scanning electron microscopy (SEM) micrographs showed that more calcium silicate hydrate (CSH) and less calcium hydroxide (CH) were observed in the specimens richer in NaOH. It was indicated that the degree of pozzolanic reaction increased with the content of NaOH, also in good agreement with the results of UCS tests and TGA. Results on leaching experiments showed that the contents of harmful elements in CPL specimens were far below the standard values for category III of China's quality standard of groundwater (DZ/T 0290–2015). In addition, the feasibility of the engineering application of CPL as a supplementary binder was demonstrated by the industrial experiment. Results on the industrial experiment indicated that the CPL4.0- and CPL6.0-based binders both satisfied the strength requirements for backfill mining in the lead–zinc mine in Gansu Province, China. It may be confirmed that LS, activated effectively by adding NaOH, could be utilized as supplementary cementitious material in the engineering application of backfill mining. The results of the present research can provide an effective and environmental approach to recycling useless LS.