In the late 1920s, portland cement manufacturers began formulating special combinations of portland cement and plasticizers for use in the masonry industry. Premixed and packaged in bags, these cements came to be called masonry cements. All masonry cements today must comply with ASTM C 91. It covers requirements for fineness, autoclave expansion, setting time, compressive strength, air content, and water retention. C 91 defines three types of masonry cements: N, S, and M. Type N is a general use cement, Type S is a moderate-strength cement, and Type M is a high-strength cement. BUT WHAT DO THEY CONTAIN? Masonry cement manufacturers don't reveal the ingredients of their trade products, but they usually contain three things: portland cement for high early strength; plasticizers for workability; air-entraining additives for workability and durability. HOW DO THEY PERFORM? The bonding ability of masonry cement mortars and all air-entrained mortars is controversial. Those opposed to air-entraining agents contend that the air bubbles in the mortar interfere with bonding. Those favoring air-entraining agents praise the increased workability (which improves bond), water retentivity, and durability of mortars made with them.