There are many varieties of cements and numerous ways of classification. One of the simplest classifications is by the chemical constituent that is responsible for the setting or hardening of the cement. On this basis, the silicate and aluminate cements, wherein the setting agents are calcium silicates and aluminates, constitute the most important group of modern cements. Included in this group are the portland, aluminous, and natural cements.
Limes, wherein the hardening is due to the conversion of hydroxides to carbonates, were formerly widely used as the sole cementitious material, but their slow setting and hardening are not compatible with modern requirements. Hence, their principal function today is to plasticize the otherwise harsh cements and add resilience to mortars and stuccoes. Use of limes is beneficial in that their slow setting promotes healing, the recementing of hairline cracks.
Another class of cements is composed of calcined gypsum and its related products.
The gypsum cements are widely used in interior plaster and for fabrication of boards and blocks; but the solubility of gypsum prevents its use in construction exposed to any but extremely dry climates.
Oxychloride cements constitute a class of specialty cements of unusual properties.
Their cost prohibits their general use in competition with the cheaper cements;
but for special uses, such as the production of sparkproof floors, they cannot be equaled.
Masonry cements or mortar cements are widely used because of their convenience.
While they are, in general, mixtures of one of more of the above-mentioned cements with some admixtures, they deserve special consideration because of their economies.
Other cementitious materials, such as polymers, fly ash, and silica fume, may be used as a cement replacement in concrete. Polymers are plastics with long-chain molecules. Concretes made with them have many qualities much superior to those of ordinary concrete.
Silica fume, also known as microsilica, is a waste product of electric-arc furnaces.
The silica reacts with limes in concrete to form a cementitious material. A fume particle has a diameter only 1% of that of a cement particle.