Abstract: Siliceous compositions are prepared from naturally occurring plant materials (particularly rice hulls and rice straw) which have relatively high silica concentration by controlled incineration of the plant material to preserve the original cellular structure of the material and to prevent formation of crystalline forms of silica so that the silica in the product (which may contain from about 49% to about 98% silica remains in the amorphous state; the novel silica containing compositions find utility as a constituent of novel hydraulic cement compositions of high strength and acid resistance, useful in the preparation of mortar and concrete.