The presence of any considerable proportion of mica will destroy the value of any sand for concreting purposes. Its effect is more injurious upon fine sands than upon coarse. It greatly increases the amount of voids, and the smooth surfaces of the mica particles do not permit of a good bond with the surrounding cement. A sand containing as high as 20 percent of mica, will have 67 percent of voids, will require three times the amount of water for mixing that sand without mica would require, and will show a decrease of 20 percent in weight, and a loss of two-thirds in the strength of the mixture.