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Cement paste with natural sand content and the effect of silica fume (SF), class C fly ash (FA) and blast furnace slag (BFS) on the thermal conductivity and compressive strength of mortar was investigated. SF, FA and BFS were added as replacement for cement by decreasing the cement in the ratios of 10, 20 and 30% by weight.The maximum thermal conductivity of 1.186 W/mK was observed with the samples containing plain cement. It decreased with the increase of SF, FA and BFS as replacement for cement. The reductions due to SF were 17, 31 and 40% for 10, 20 and 30% SF (replacement for cement), respectively. FA induced reductions of 14, 26 and 33% for 10, 20 and 30% FA, respectively. Both SF and FA had a decreasing effect on thermal conductivity. BFS effect on the thermal conductivity, was approximately the same at all percent (BFS replacement for Portland cement, PC) and the reduction values were between 12 and 14%. Ten percent SF and 10 and 20% FA and all level BFS increased compressive strength a little at 120 days. However, except 10% SF the other admixters at all level replacement decreased compressive strength at early ages, especially FA decreased compressive strength as function of replacement percent. [Copyright &y& Elsevier]