The current mining method in Zinkgruvan today is underhand stoping. With this method the ore is mined downwards in disks. When the ore is extracted the stope is backfilled with paste which contains tailings, cement and water. The paste will constitute the ceiling in lower levels and are therefore vital for the safety and the requirements of the strength. The goal of this thesis is to increase knowledge about the paste’s strength after the usual tests within 28 days. Test objects are taken from two different paste’s in Burkland and Nygruvan and compares with old data from the same sights. Besides the test objects that are taken from the mine there are also some studys on the literature with consideration on mine backfill in other mines using a similar mining method. By interviewing experts at Zinkgruvan increased knowledge is acquired about the production and the company. By the results an indication is given regarding the possibility of optimizing the paste containing eight percent cement to a lower amount in order to reduce costs. The results shows that the paste increases in strength after standard testing within 28 days.