Physicochemical mechanism of structure formation and strengthening in the backfill massif when filling underground cavities

Abstract

The strength and microstructural properties of the backfill massif have been studied and assessed when filling underground cavities that pose a threat of mine rocks collapsing in the process of mining mineral deposits. It is suggested that due to a tendency to mechanical destruction by crushing Ca–O ionic bonds rather than Si–О covalent ones, the backfill mixture composition is saturated with a large amount of Ca2+ ions. This leads to the formation of a highly-basic type of hydrated calcium silicates and a decrease in the massif strength properties. To study the mineral composition of the components of the mixture and solidified massif and to investigate the microstructure and chemical composition of new formations in the backfill massif, infrared spectroscopy and scanning electron microscopy were used. Laboratory studies of the strength properties of backfill massif were also conducted. The minerals of the mixture components, melilite and pseudowollastonite, have been revealed that perform the main function of the new formations occurrence. It was found that the strength of the backfill massif is by 16% less than the required standard value of 7.0 MPa at the age of 90 days. It was determined that highly-basic jellylike hydrated silicates of tobermorite type of the group CSH (II) with variable composition and a ratio of CaO/SiO2=2–3 are formed in the studied structure of the backfill massif after 90 days of hardening. There are no strong low-basic hydrated calcium silicate bonds that could have a reinforcing effect. Providing the conditions for occurrence of low-basic hydrated calcium silicates in the structure is one of the ways to create a hard backfill massif.