Abstract The tensile strength of cement paste was increased by 56% and the modulus and ductility were increased by 39% by using silane-treated carbon fibers and silane-treated silica fume, relative to the values for cement paste with as-received carbon fibers and as-received silica fume. Silane treatment of fibers and silica fume contributed about equally to the strengthening. Silane treatment of fibers and silica fume also decreased the air void content and the drying shrinkage. The strengthening, air void content reduction and drying shrinkage reduction were less when the fiber treatment involved either potassium dichromate or ozone. Silane’s effectiveness is due to its hydrophilic nature.