Portland cement

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Portland cement is the most common cement used around the world for making concrete, mortar, stucco, and non-specialty grouts. It is a fine powder produced by heating limestone and clay minerals in a kiln to form clinker, then grinding the clinker with a small amount of gypsum. Most Portland cement is gray, but white Portland cement is also available. It was named by Joseph Aspdin in 1824 because, when it hardens, it resembles Portland stone, a light-colored building stone from the Isle of Portland in Dorset, England.

Portland cement has a long history that builds on earlier cements made in Britain. In the 18th and early 19th centuries, engineers and chemists experimented with different limestones, clays, and additives; the rotary kiln, patented in the 1880s, allowed a stronger, more uniform cement and the start of modern production. Standards and widespread production followed in Europe and North America, helping Portland cement become the dominant binding material for concrete.

How it is made
- Raw materials are primarily limestone (calcium carbonate) mixed with clay or shale to supply aluminum and silicon. The mix is heated in a cement kiln to form clinker, a nodular material.
- In the kiln, calcium silicates form at high temperatures; the main minerals in clinker are alite (C3S), belite (C2S), tricalcium aluminate (C3A), and tetracalcium alumino ferrite (C4AF).
- About 2–8% calcium sulfate (gypsum or anhydrite) is added to control the set time, and the clinker is ground to a fine powder to make finished cement.
- Clinker is usually more than 90% of the cement, with small amounts of minor constituents and sometimes supplementary materials.

Uses and setting
The most common use is in concrete, a composite of cement, water, and aggregates (sand and gravel). When mixed with water, cement hydrates through a complex reaction that forms solid products, including ettringite early on and calcium silicate hydrate (C-S-H) later, which bind the aggregates together. Gypsum added during grinding slows setting to make the concrete workable. Portland cement also makes mortars, plasters, and grouts.

Standards and types
- In the United States, ASTM C150 defines several types of Portland cement: Type I (general purpose), Type II (moderate sulfate resistance and lower heat of hydration), Type III (high early strength), Type IV (low heat of hydration, rarely used now), and Type V (high sulfate resistance). There are also variants with air-entraining agents and other special properties.
- In Europe, the EN 197-1 standard defines classes of Portland cement that differ from ASTM types and allow blends with pozzolanic materials (like fly ash or slag).
- White Portland cement exists for applications requiring a lighter color; it uses low iron content and requires higher kiln temperatures, making it a bit more expensive.

What makes cement white
White cement is produced with very low iron content (Fe2O3) and uses adjustments to the manufacturing process to avoid coloring oxides. It often requires a higher kiln temperature and careful control of raw materials to keep the resulting clinker white.

Safety and health
Cement is highly alkaline and can cause skin burns, eye irritation, and respiratory issues if dust is inhaled. Wet cement is caustic; workers use protective gear and follow exposure limits for dust and fumes. Concrete dust can contain hazards such as crystalline silica and trace metals, so handling and curing practices matter for health.

Environmental aspects and future directions
Cement production requires a lot of energy and releases carbon dioxide. It accounts for a substantial portion of global CO2 emissions. Industry efforts focus on reducing emissions by using alternative materials, improving process efficiency, and replacing some cement with supplementary cementitious materials like slag, fly ash, or natural pozzolans. While cement manufacture remains a major CO2 source, concrete made with Portland cement remains widely used, and research continues to find cleaner, cheaper, and more sustainable options for binding materials.