How is cement made?

The basic characteristic of cement is that after it has been mixed with water, it will set hard as rock, and will bind together any rock or mineral fragments mixed into it. Mortar is made from a mixture of sand and cement, and bonds together bricks in a wall. Most cement is mixed with both sand and aggregate to make concrete.

It seems that the Romans knew about cement, but the technique was lost until Smeaton built the Eddystone Lighthouse in 1756, using a mixture of fired ground limestone and clay. Cement was patented by John Aspdin in 1824 as Portland cement because he though it looked rather like limestone from Portland, which was used a lot in buildings. This type of cement is used a lot today, and is still known as OPC (ordinary Portland cement).

What are the ingredients?

Calcite (CaCO₃) from limestone

Silica (SiO₂)

Alumina (Al₂O₃)

+ minor amounts of iron.

All of these ingredients can be assembled by mixing limestone and shale. After grinding them together, they are fired in a kiln to about 1400°C. Water is given off first, showing that the shale is decomposing, and then CO₂, when the limestone starts to decompose. The other materials react to produce cement clinker.

The four most important anhydrous components of Portland Cement are, in decreasing order of abundance: tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite.

The cement clinker is obviously anhydrous and decarbonated. It is ground up into a powder to form ordinary OPC. When this is mixed with water, new hydrated minerals form. These grow as long crystals, locking the cement and any inert particles into a hard mass.

These new hydrated minerals form slowly, and the cement needs to be kept moist while this is occurring. Portland cement starts to harden a few hours after mixing, because the hydrated tricalcium aluminate grows rapidly. Cement doesn’t start to become strong until some days later when the hydrated tricalcium silicate forms. It reaches 70% of full strength after about a month, and doesn’t reach full strength until several years later because the dicalcium silicate hydrates so slowly. This reaction is non-reversible.

Geology and raw materials for making cement

Occasionally, sediments already have roughly the correct proportions of minerals present to make cement. Cementstones are impure limestones that contain quartz and clay minerals and some iron. Sometimes they are interbedded thin limestones and shales that will together give the right composition for making cement. Usually, a cement works will be built where limestone and clay crop out next to each other. The raw material is then ground up in the right proportions for making the cement. It is common in southern England for a cement works quarry to have chalk at one end, and clay at the other. The mixing becomes more tricky where the composition of the limestone or shale is variable.

Case Study

There is an interesting example of land restoration at a limestone quarry in Dunbar, where land is restored behind the excavation as the working face advances. There is a very expensive bridge conveyor that moves the waste from one end of the quarry to the back, but the quarry life is about 50 years, which is a long time, and it allows the quarry waste to fill up the old workings continuously, so that only a narrow strip of the quarry is exposed at any one time.

Uses of cement

Mortar for bonding bricks is made up of 1 part by volume of cement powder with 3 to 6 parts of sand, and the minimum of water to make the mix workable. Cement and sand can also be used to produce a thin skin or render, to protect the outside of the buildings. The most important use of cement is in making concrete, where it is used to stick together a mixture of sand and rock fragments, i.e. aggregate.