Indirect methods as quality control of cemented hydraulic fill : Renström mine, Boliden mineral AB

Abstract: The Renström mine, Boliden Mineral AB, uses, among other methods, the Transverse long hole stoping method with backfill. The mine uses a cement-stabilized hydraulic fill (CHF), which is a mixture of enrichment sand, cement and water. The most important factor in relation to strength is the water cement ratio. Today, only tests have been performed on the surface and in laboratory tests. The purpose of this work is thus to study the possibility of using other types of methods that can give a better understanding of the strength of the entire filling volume. Studies on backfill containing Portland cement and sulphur have shown that the UCS strength decreases with time, which can cause problems for the mine. A leaching method (MRM leaching test) used to evaluate potential sulphide soils has been used in this study on the backfill to investigate the sulphurs impact on the cement mixture. Electrical conductivity, pH, and redox potential were investigated and gave similar results, where the higher cement levels (8%, 10%, and 12%) were indistinguishable while the lower cement content (4%) differed significantly from the other levels. This probably indicates that the higher cement levels have a greater resistance to the influence of the sulphur. The most promising results were given when the method was used on enrichment sand alone. The linear lowering of the pH value of the enrichment sand, from pH 4.9 to about pH 3, point to that the enrichment sand contains large amounts of sulphur, as previously chemical analysis has shown. This suggests that the method can best be used in an early stage before the sand is mixed with cement. The strength results on the samples showed a strength after 7 days that was in line with the 90-day strength. However, all 90 day samples had decreased in strength during the time sequence, which may be an effect of the sulphur content, but should be further investigated to be determined. The smaller size of the samples was also experienced as a challenge during the temperature measurements due to small temperature variations and environmental effects. A maturity method for predicting the strength of concrete called "The maturity method" has been investigated in this study. To determine the strength maturity relationship, both the temperature related to time and the equivalent age must be evaluated. The study showed that the method cannot be used on the mine's backfill without first making adjustments related to the lower cement content used in the backfill compared with concrete constructions, since it is not possible to determine the strength growth in the early stage as the method requires.