Cement-based materials improved by surface-treated admixtures

Cement-based materiah containing solid admixtures such as silica fume and short carbon fibers were improved by surface treatment prior to using the admixtures. Consistency, static and dynamic mechanical properties, specific heat, and drying shrinkage were improved. Mortar with high consistency, even without a water-reducing agent, was obtained by using silica fume that had been surface treated with silane. The treatment also increased the strength and modulus, both under tension and compression. In particular, the tensile strength was increased by 31% and the compressive strength was increased by 27%. Moreover, Jlexural storage modulus (stiffness), loss tangent (damping capacity), and density were increased. The tensile strength of cement paste was increased by 36%, and the modulus and ductility were increased by 39% using silane-treated carbon bers 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 approximately equally to the strengthening effect. Silane treatment of fibers and silica fume also decreased the air void content. The effects on strengthening and air void content reduction were less when theber treatment involved potassium dichromate instead of silane, and even less when the treatment involved ozone. The addition of short carbonbers to cement paste containing silica fume and methylcellulose caused the loss tangent under flexure (1 Hz) to decrease by up to 25% and the storage modulus (2 Hz) to increase by up to 67%, such that both effects increased in the following order: as-received fibers, ozone-treated fibers, dichromatetreated'fibers, and silane-treated fibers. The addition of methylcellulose to cement paste containing silica fume caused the loss tangent to increase by up to 50% and the storage modulus to decrease by up to 14%. Silane treatment of silica fume had little effect on the loss tangent, but increased the storage modulus by up to 38%. The specific heat of cement paste was increased by 12% and the thermal conductivity was decreased by 40% by using silane-treated silica fume and silane-treated carbonbers. The specific heat was increased by the carbon fiber addition, due to fiber-matrix interface slippage. The increase was also in the previously described order, due to the increasing contribution of the movement of the fiber-matrix covalent coupling. The specific heat was increased by the silica fume addition, due to slippage at the interface between silica fume and cement. The increase was enhanced by silane treatment of the silica fume. Silane treatment of carbonbers decreased the thermal conductivity. Silane treatment of carbon bers and silica fume increased the effectiveness of these admixtures for reducing the drying shrinkage of cement paste.