Waterproofing Plasticizing Admixture when used as an admixture, forms an internal barrier against water penetration. STP No.1 also increases the plasticity of mortar, reduces water absorption and thereby guards against freeze-thaw damage. It is available in powdered form without chlorides or in a liquid formula which contains additives for additional densification and acceleration of the mortar or concrete. STP No. 1 will not increase the air content of mortar or concrete.