Components, systems and materials › Construction materials › Ordinary Portland cement
Ordinary Portland cement
What it is
Ordinary Portland cement (OPC) is the basic hydraulic cement made by grinding clinker with a little gypsum.
Where it is used
All concrete and mortar.
How it works
When water is added, the clinker minerals hydrate. Tricalcium silicate reacts quickly and gives most of the early strength; dicalcium silicate reacts slowly and adds strength over months. The main product, calcium silicate hydrate, is a gel that binds aggregate particles together, while calcium hydroxide released alongside keeps the pore water strongly alkaline, which protects reinforcing steel. Gypsum controls the early reaction of tricalcium aluminate so the paste does not stiffen too quickly.
Types
- Grades 33, 43 and 53 by 28-day strength in MPa
Materials
Clinker and gypsum.
The grade number is the minimum 28-day compressive strength in MPa of standard mortar cubes, so OPC 53 reaches at least 53 MPa in that test. Higher grades gain strength faster, which helps early formwork removal and precast work, but also release more heat and can crack more in thick sections if not cured and cooled. The grade of cement is not the grade of concrete: concrete strength depends mainly on the water-cement ratio and is designed by mix proportioning. IS 269 specifies OPC in India; ASTM C150 and EN 197-1 are the American and European counterparts.
Installation and QA steps
- Check the manufacturing week and grade on each bag or bulk delivery
- Store bags off the floor on pallets, away from walls, stacked to a limited height and covered against moisture
- Use on a first-in, first-out basis
- Sample and test setting time, fineness and mortar cube strength at the frequency in the quality plan
- Reject cement with hard lumps that do not break between the fingers
- Partly hydrated, lumpy cement from damp storage, giving low strength
- Cracking in mass pours from high heat of hydration
- Wrong grade used where a blended cement was specified for durability
- False set from poor gypsum control, mistaken for real setting
Codes cited
- IS 269 (India): Ordinary Portland cement: requirements.
- ASTM C150 (USA): Portland cement specification.
- EN 197-1 (Europe): Cement: composition and conformity of common cements.
Cited by number only. We do not reproduce code text, tables or figures.
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