More about cement

Last post I showed some large scale application of cement in building a harbour wall. Cement is made from limestone and shale with other components containing aluminium and iron minerals. These are carefully mixed and heated to high temperatures with coal to form a "clinker". The clinker is then crushed and its composition is adjusted by adding gypsum to control setting times. Up to 5% of other additives may be added.
The main components of cement are 3CaO.SiO2 and 2CaO.SiO2 (tricalcium silicate and dicalcium silicate). These compounds make up the majority of the cement with the iron and aluminium clinker compounds and the gypsum making up the rest.

Cement can be made to set faster or slower by adding accelerators (e.g. calcium chloride or sodium nitrate)or retarders (e.g. citric acid, sodium gluconate).

Other additives can be added depending on the desired use. E.G. adding latex makes the cement harder and improve the workability.

When this mixture that we call cement is mixed with water it reacts with the water and sets. The reaction is called a hydration reaction, and cement can be used under water - it is hydraulic. In fact it needs the water for it to work well. Plaster is a mixture of cement and fine sand that sets hard over the brick of a wall making it smooth. If your builder is impatient and tries to speed up the drying of your cement it will be weaker, or it may crack as the water needed for hydration is lost.

When I had some building done at home my wife and the builder thought I had gone insane when they found me perched on the wall watering it with then garden hose! I wanted it hydrated.

Some times you may see concrete (cement and large pieces of stone and fine sand mixed) being covered with plastic or wet rags - these cover remain on for 48 hours - this allows the hydration to further strengthen the cement or concrete before it dries. Actually cement gradually gets stronger the longer it stays wet - the strength will double from 8 hours to three days and after another 27 days the strength will double again.
From this you can see that the impatience of builders to complete the job can lead to weakened structure.

Here is a chemistry based tip for you if you build. Keep an eye on the builder and staff - they should only mix as much cement as can be used in 1 hour (the cement bag normally reminds them anyway - printed user-directions). Do not let them mix the cement sand and water just before lunch. The cement gets started reacting and the hydration reaction is well under way while they rest in the shade for that hour or so - your structure ends up being weaker if the builder violates this procedure. Two years later your plaster is falling off the wall and the builder is long gone. The same goes for tile adhesive and grouting  as these materials are also cement based.

Cement is stronly alkaline - in the manufacture the limestone breaks down to carbon dioxide gas and calcium oxide rendering it alkaline. Some alkalinity is released during hydration as well. Keep cement off your skin or it will slowly dissolve your skin.

Fact: Did you know that you can make lime Ca(OH)2 by heating sea shells or egg shells to 850 deg C?

Cement reacts with water and give off heat at the same time - in the building of large concrete structure water cooled pipes may be included to prevent cracking of the structure.

So there you have a bit of everyday chemistry - we rely on the chemical reactions of the past for our structure of the present. There is more water in your walls that you may think.