Abstract: A method and apparatus for increasing the early and long term strength of underground cemented backfills includes adding heat to the backfill. A cementitious binder is mixed with a sand component and water to form the mine backfill that may be a slurry or a paste. Heat is added to the backfill before the backfill is supplied to a mine cavity and this has been found to increase the early and long term strength of an underground cemented fill resulting from the backfill in the mine cavity. The backfill from the mixing step is mixed to have a pulp density of about 55% to 87%. Heat is added to the backfill so that the backfill has a temperature of about 18 to 60° C. when it is supplied to the mine cavity. Heat is either added by heating the sand or water before the mixing step, or by heating the backfill after the mixing step. The mixing step uses a binder content of about 1.5 to 10% by weight.