This paper reports the results of a study conducted to evaluate the influence of high-volume class C fly ash (FA), blast furnace slag (BFS) and both FA + BFS on the thermal conductivity (TC), compressive strength, water absorption and density of concrete. TC decreased with the increase of FA, BFS and FA + BFS as replacement for Portland cement. The reductions in TC due to FA, BFS and FA + BFS were, respectively, up to 39%, 18% and 31%. The addition of FA, BFS and FA + BFS in the concrete had a decreasing effect on TC. Their addition also decreased compressive strength as a function of replacement percent. However, this reduction in compressive strength decreased with increasing curing period. (c) 2005 Elsevier Ltd. All rights reserved.