What is Portland cement?
Portland cement is a hydraulic binder made of finely ground Portland cement clinker with a small addition of gypsum. It is the base cement from which practically every other cement type is composed. In the European cement standard EN 197-1, Portland cement is CEM I and contains at least 95% clinker.
The name dates from 1824, when the English bricklayer Joseph Aspdin patented a binder that, once hardened, resembled the well-known building stone from the Isle of Portland in Dorset.
How it is made
- Limestone and clay (or marl) are ground and blended.
- The mix passes through a rotary kiln and is heated to around 1,450 °C. This produces hard nodules: clinker.
- The clinker is cooled rapidly and ground very fine.
- A few per cent of gypsum is added. That gypsum is not a filler but a brake: without it the cement would flash set almost immediately.
What sets it apart
- Rapid strength gain. CEM I develops strength quickly, which suits precast work, winter working and early striking of formwork.
- High heat of hydration. That same speed generates heat. In thick sections it can cause thermal cracking; there, cements containing ground granulated blast furnace slag or fly ash are preferred.
- Light colour. Portland cement is grey; blast furnace cement is noticeably lighter. White Portland cement also exists, for architectural concrete and pale jointing mortars.
- Less resistant to sulfates and chlorides. Sulfate-resisting grades exist for aggressive ground conditions, as do alternative cement types.
- High carbon footprint. Calcining limestone in the kiln makes cement production one of the larger industrial sources of CO₂. Cements in which part of the clinker is replaced by slag or fly ash perform considerably better on this count.
Strength classes
The designation is followed by a strength class: 32.5 / 42.5 / 52.5, with N (normal) or R (rapid) early strength. CEM I 52.5 R is therefore a finely ground Portland cement that hardens quickly — good for precast, less good for a thick slab on a hot day.
Practical points
- Cement burns. Wet cement is strongly alkaline and causes burns to skin and eyes. Gloves, goggles and long sleeves are not optional.
- Store it dry. Cement absorbs moisture from the air and lumps. An opened bag has a limited shelf life.
- More cement is not stronger. Concrete strength depends mainly on the water/cement ratio. Excess mixing water weakens the mix more than extra cement strengthens it.
- Cure it. Fresh concrete and mortar must stay damp while hardening. Drying out in the first few days permanently costs strength and causes shrinkage cracking.
Related terms
- Cement
- Blast furnace cement
- Clinker
- Concrete
- Water/cement ratio
- Hydration
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