Cement-based materials improved by surface-treated admixtures

Cement-based materials containing solid admixtures such as silica fume and short carbon fibers were improved by surface treatment prior to using the a dmixtures. 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 strengt h and modulus, both under tension and compression. In particular, the tensi le strength was increased by 31% and the compressive strength was increased by 27%. Moreover, flexural storage modulus (stiffness), loss tangent (damp ing capacity), and density were increased The tensile strength of cement pa ste was increased by 56%, and the modulus and ductility were increased by 3 9% using silane-treated carbon fibers and silane-treated silica fume, relat ive to the values for cement paste with as-received carbon fibers and as-re ceived silica fume. Silane treatment of fibers and silica fume contributed approximately equally to the strengthening effect. Silane treatment of fibe rs and silica fume; also decreased the air void content. The effects on str engthening and air void content reduction were less when the fiber treatmen t involved potassium dichromate instead of silane, and even less when the t reatment involved ozone. The addition of short carbon fibers to cement paste containing silica fume and methylcellulose caused the loss tangent under flexure (less than or equ al to 1 Hz) to decrease by up to 25% and the storage modulus (less than or equal to 2 Hz) to increase by up to 67%, such that both effects increased i n the following order: as-received fibers, ozone-treated fibers, dichromate -treated fibers, and silane-treated fibers. The addition of methylcellulose to cement waste containing silica fume caused the loss tangent to increase by up to 50% and the storage modulus to decease by up to 14%. Silane treat ment of silica fume had little effect on the loss tangent, but increased th e storage modulus by up to 38%. The specific heat of cement paste was incre ased by 12% and the thermal conductivity was decreased by 40% by using sila ne-treated silica fume and silane-treated carbon fibers. The specific heat was increased by the carbon fiber addition, due to fiber-matrix interface s lippage. The increase was also in the previously described order, due to th e increasing contribution of the movement of the fiber-matric covalent coup ling. The specific heat was increased by the silica fume addition, due to s lippage at the interface between silica fume and cement. The increase teas enhanced by silane treatment of the silica fume. Silane treatment of carbon fibers decreased the thermal conductivity. Silane treatment of carbon fibers and silica fume increased the effectivene ss of these admixtures for reducing the drying shrinkage of cement paste.