The right mix for the job
Portland cements are hydraulic cements – in other words, cements that set and harden by reacting chemically with water – and are composed primarily of hydraulic calcium silicates. The reaction process, called hydration, combines cement and water to form a stone-like mass.
Various types of portland cement are manufactured to meet different physical and chemical requirements for specific purposes. In Canada, the Canadian Standards Association (CSA) recognizes six types of portland cement under Standard A3001, as follows.
General Use Hydraulic Cement (GU) is a general-purpose cement suitable wherever the special properties of other types are not required. Its uses in concrete include pavements, floors, reinforced concrete buildings, bridges, tanks, reservoirs, pipe, masonry units and various concrete products.
Moderate Sulphate Resistant Hydraulic Cement (MS) is used where precaution against moderate sulphate attack is important. It is used in normal structures or parts of structures that are exposed to soil or ground waters where sulphate concentrations are higher than normal but not unusually severe.
When sulphates in moist soil or water enter concrete, they cause chemical reactions that result in expansion, scaling and cracking. Wetting and drying in a sulphate environment aggravates the formation of sulphate salts or compounds that have sufficient crystallization pressure to disrupt cement paste.
Concrete exposed to seawater is often made with Moderate Sulphate Resistant cement. Seawater contains significant amounts of sulphates and chlorides. Although sulphates in seawater are capable of attacking concrete, the presence of chlorides reduces the expansive reaction that is characteristic of sulphate attack.
Moderate Heat of Hydration Hydraulic Cement (MH) is specially manufactured to generate less heat, at a slower rate than General Use hydraulic cement. Heat of hydration is the heat generated by the chemical reaction when cement is initially mixed with water. The requirement of moderate heat of hydration can be specified at the option of the purchaser. This type of cement can be used in structures of considerable mass, such as large piers, foundations and thick retaining walls, in which temperature-related cracking may be a problem, especially when concrete is placed in warm weather.
High Early Strength Hydraulic Cement (HE) provides high strength relatively quickly, usually in a week or less. It is chemically and physically similar to General Use hydraulic cement, except that its particles are more finely ground. It is used when forms need to be removed promptly or when the structure must be put into service quickly. In cold weather, its use reduces the length of the curing period.
Low Heat of Hydration Hydraulic Cement (LH) is used where the rate and amount of heat from hydration must be minimized. It develops strength at a slower rate than other cement types. This type of cement is intended for use in massive concrete structures, such as large gravity dams, where the temperature rise resulting from heat of hydration must be minimized. It is generally only available by specific request for large projects.
High Sulphate Resistant Hydraulic Cement (HS) is used in concrete exposed to severe sulphate action, principally where soils or ground waters have high sulphate content. It gains strength more slowly than General Use hydraulic cement. Use of a low water-to-cementing-materials ratio and low permeability are critical to the performance of any concrete exposed to severe sulphate attacks. Like other portland cements, it is not resistant to acids and other highly corrosive substances.
White portland cement differs from the usual gray cement chiefly in colour. It is made to conform to CSA specifications, usually General Use or High Early Strength hydraulic cement. The manufacturing process is controlled so that the finished product will be white. It is made of selected raw materials containing negligible amounts of iron and magnesium oxides, the substances that give cement its gray colour. White portland cement is used primarily for architectural purposes, such as precast curtain walls and facing panels, terrazzo surfaces, stucco, cement paint, tile grout and decorative concrete. Its use is recommended wherever white or coloured concrete, grout, or mortar is desired.