Cementitious materials and their classification

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Cementitious materials and their classification

Cementitious materials include the many products that are mixed with either water or some other liquid or both to form a cementing paste that may be formed or molded while plastic but will set into a rigid shape. When sand is added to the paste, mortar is formed. A combination of coarse and fine aggregate (sand) added to the paste forms concrete.

Types of cementitious materials

There are many varieties of cement and numerous ways of classification. One of the simplest classifications is the chemical constituent that is responsible for the setting or hardening of the cement. On this basis, silicate and aluminate cement, wherein the setting agents are calcium silicates and aluminates, constitute the most important group of modern cement. Included in this group are the portland, aluminous, and natural cement.

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 cement and add resilience to mortars and stuccoes. The use of limes is beneficial in that their slow setting promotes healing and the recementing of hairline cracks.

Another class of cement is composed of calcined gypsum and its related products. Gypsum cement is widely used in interior plaster and for the fabrication of boards and blocks, but the solubility of gypsum prevents its use in construction exposed to any but extremely dry climates.

Oxychloride cement constitutes a class of specialty cement with unusual properties. Their cost prohibits their general use in competition with the cheaper cement; but for special uses, such as the production of spark-proof floors, they cannot be equaled.

Masonry cement or mortar cement are widely used because of their convenience. While they are, in general, mixtures of one or more of the above-mentioned cement 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 micro silica, 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 of only 1% of that of a cement particle.