Study of the reduction of resistance over time in specimens tested by uniaxial compressive strength for paste filling

Abstract

This paper presents the results of a case analysis which was the product of a conceptual study of the paste fill mix design carried out for a polymetallic mine in southern Peru. Paste fill is a mixture of water, cement and tailings in a high density. It is used in underground mines to fill cavities resulting from rock extraction (galleries, chimneys, pits, among others) to provide the support needed to continue mining and avoid a possible collapse. The strength requirements for the primary paste fill were between 1.0 to 2.0 MPa, and for the secondary filler, between 0.7 to 1.0 MPa after 28 days of setting. The physical, chemical and mineralogical characterisation of the tailings to be used was carried out, as well as uniaxial compressive strength tests (UCS.) of the paste. The mineralogical characterisation allowed us to identify a high presence of siderite (26%) and kaolinite (12%), minerals that do not benefit from obtaining high resistance values. On the other hand, the UCS allowed us to identify that, after 28 days of curing, the resistance of the analysed specimens presented a decrease concerning the values obtained at 7 and 14 days of curing. Given this scenario, tests were carried out at 60 and 90 days of curing and detected that the decrease in resistance fell to values well below those required (a decrease in resistance of more than 40%). Analysing the results identified that the presence of sulfates in the tailings possibly generated gypsum crystals during mixing. When they grow, these crystals break the cement bonds over time, affecting the integrity and resistance of the tested paste.