In making mortar, sand is mixed with lime with a twofold purpose. Lime without sand sets or hardens so slowly as to be almost useless as mortar; but the addition of sand makes the mixture porous, and the carbonic acid in the atmosphere obtaining access to the lime sets up chemical action and causes the mixture of lime and sand to set or harden. The action of setting causes pure lime to contract largely in bulk; the admixture of sand with the lime prevents such contraction. Sand is added to Portland cement for economical reasons. This cement, used without sand, is exceedingly strong; for all ordinary purposes, this strength is unnecessary, and when the addition of sand does not unduly reduce the strength of the mixture, cement and sand may be economically used together.