A study of the short-term leaching behaviour of a modified expansive cement in groundwater for mining and mineral extraction

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Process Safety and Environmental Protection


The modified expansive cement, Slow Releasing Energy Material Agent (SREMA), has been introduced as a greener alternative to conventional mining. SREMA is similar to Ordinary Portland Cement (OPC) in its constituents, containing mostly CaO, and Al2O3, SiO2, MgO, Fe2O3, CaSO4 in minute amounts, but including Soundless Chemical Demolition Agents (SCDAs) as the base material. SREMA is an effective method of mining without causing severe environmental damages by undergoing expansion when injected to a borehole, causing fractures in the surrounding materials and exposing the fossil fuel, metals, minerals or ores of interest that may lie within. Nonetheless, its leaching behaviour in underground water conditions has not been explored to date. The aim of this study is to investigate the leaching behaviour of SREMA in typical geological conditions using a serial batch leaching method and propose mitigation techniques to minimise the leaching effect leading to contamination of water. According to the experimental results, calcium leaching is much more prominent than that of other cations present in the solid matrix. When compared with the OPC, the calcium leaching behaviour of both hydrated and hydrating SREMA is comparably higher in all considered leachants, due to the larger mass percentage of CaO and high porosity of SREMA. Overall, hydrated SREMA shows the greatest calcium leaching of 835 ppm in 7 days under saline conditions of 5 wt% NaCl, which is over twice the leaching seen in the presence of water. A similar tendency is seen with hydrating SREMA as well but with a maximum leaching of 539 ppm. An increase in calcium leaching is observed, with over a 2-fold rise in leaching compared to that in water, as the basic pH reduces, with the formation of globular katoite. Moderated calcium leaching is seen under acidic conditions, even though the basic nature of Ca(OH)2 would suggest otherwise. The saturation indices (SIs) and degree of calcium leaching have been calculated to understand the behaviour of solubility in the presence of the considered leachants. In addition, potential modifications are proposed to improve the composition of SREMA to reduce the leaching effects revealed by experimentation, while maintaining its expansive qualities.

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