OPC

Ordinary Portland Cement is the most common binder in concrete, mortar, and grout. It is a fine gray powder made by heating limestone and clay to about 1450 degrees Celsius in a kiln, grinding the resulting clinker with gypsum, and milling to a particle size typically between 15 and 45 micrometers. The name comes from its resemblance to Portland stone, a limestone quarried on the English coast; the cement itself contains no Portland stone.

OPC sets through a hydration reaction when mixed with water. Initial set occurs within hours; full strength develops over weeks and months. A typical concrete mix contains 10 to 15 percent OPC by weight, with the rest made up of fine aggregate (sand), coarse aggregate (gravel or crushed stone), water, and sometimes admixtures. The cement develops compressive strength at a predictable rate: about 70 percent of its 28-day strength by 7 days, though this varies with temperature and water content.

Variants and specifications

OPC is graded by strength class in most standards. European EN 197-1 divides it into CEM I 42.5 and CEM I 52.5, referring to minimum compressive strength in newtons per square millimeter at 28 days. American ASTM C150 specifies Type I Portland cement. Early-strength variants with finer grinding or higher silicate content reach strength faster. Sulfate-resisting Portland cement (Type V in ASTM) is used where concrete contacts sulfate-bearing soil or groundwater. White Portland cement is made from raw materials with low iron and manganese oxide content, used for architectural finishes.

The main failure mode is carbonation, where atmospheric carbon dioxide reacts with the cement paste and lowers its pH, allowing steel reinforcement to corrode. Poor curing, high water-cement ratio (above 0.5), exposure to sulfates, and freeze-thaw cycles all accelerate degradation. Alkali-silica reaction can occur if the cement and aggregates contain reactive silica minerals, causing expansion and cracking over years.

OPC has dominated concrete production for over 150 years because it is cheap, reliable, and widely available. It now faces competition from supplementary cementitious materials such as fly ash, slag, and silica fume, which can be blended with OPC or replace portions of it. These blended cements reduce carbon dioxide emissions during production and can improve durability in aggressive environments, but they cure more slowly and require different site practices.