What is cement?
A powder, which by hydraulic reaction (i.e. with water) forms a solid, cohesive mass. There are several types of cement;

What is Rapid Hardening Portland Cement?
It is chemically very similar, but ground finer. It still sets as slowly as OPC, but its strength gain is more rapid after it sets. For most, if not all, DIY purposes, OPC is perfectly OK.

What is Sulphate Resisting Portland Cement (SRPC) ?
SRPC is for underground work, particularly with "aggressive" i.e. sulphate containing groundwater.

What is High Alumina Cement (HAC)?
It is a special cement which develops its strength very rapidly and has high chemical resistance. It can also be used for refractory concretes, e.g. in steelworks, using the white version. It suffers from a phenomenon known as "inversion", in which if it gets hot and wet it disintegrates. There were a number of 1960s high-rises which had autoclaved HAC panels and started shedding them in the 1980s. I do not believe that anyone was killed, but a hundredweight panel falling 20 stories was obviously very dangerous. It is also useful in cold regions as it can be laid in sub-zero temperatures. Lafarge, a French company are the principal manufacturer of HAC.

There are also "fast set" cements based on mixtures of OPC and HAC, but these do not achieve the same final strength as either OPC or HAC alone. There are also a few cements with "weird" chemistries, but these are not used for DIY.

How does cement set?
Ordinary Portland Cement sets by hydraulic (i.e. water) reaction. It is a complex mixture of components, probably the most important of which are dicalcium and tricalcium silicates (C2A and C3A to cement chemists). The water/cement ratio is of paramount importance to the final set strength of the concrete, and the cement/aggregate ratio and aggregate size distribution are also important. I am fairly sure the optimum w/c ratio is .28, but up to .35 is commonplace. It is important to avoid water loss, especially in the first 7 days, and the old approach was wet sacking laid on the surface. Nowadays polythene sheet is used, and there are thin foam products which will prevent light frosts from causing damage. The actual mechanism is more or less as follows; When the cement grains become wet, they absorb the water and various chemical reactions begin. Acicular (needle shaped) crystals begin to grow from the grains over the next few hours and each grain begins to look rather like a microscopic sea-urchin. The crystals from adjacent grains begin to interlock and intergrow and the concrete at this stage takes on an initial set. This typically takes between 2 and 6 hours, although the reaction continues, albeit at a slower rate for some time after this. Even after 28 days the strength is increasing slightly. If a cement mix stiffens, it should not be knocked back/up (water added and a good stir) because this knocks off the acicular crystals, and the final strength will be reduced. This will not make too much difference for a patching mortar, though. The hydration reactions of cement are exothermic (Give out heat) as mentioned elsewhere, and for really large pours there are a number of ways to prevent overheating of the concrete. To tie another thread in, cement set is accelerated in a number of ways, The only common DIY method being calcium chloride. In concrete, calcium formate is often used as the chloride (mistakenly IMHO) was blamed for the corrosion of reinforcing bars.

The mechanism of this is as follows: New concrete contains abundant free lime. (which you would know if you followed the link above) This inhibits the corrosion of steel. However, atmospheric carbon dioxide reacts with the free lime, in a process called carbonation, to form calcium carbonate. Calcium carbonate does not inhibit the corrosion of steel, so that when the carbonation penetrates to the depth at which the reinforcing steel is, the steel corrodes. Worse still, the rust resulting from this occupies a bigger volume than the steel it is formed from, and as the concrete is weak in tension, it spalls away to reveal the reinforcement, thus accelerating the corrosion. This sort of damage is very expensive to repair. Carbonation proceeds at a rate more or less inversely proportional to the square of its depth. Most modern reinforced concrete is designed to last at least fifty years. The majority of reinforcement corrosion seems to be promoted by the practise of salting roads in the winter.
 
 

Can cement irritate my hands?
Every bag of cement Blue Circle sells has a warning to this effect. Some people are rather more sensitive to the free lime than others. It can be especially irritating if you get it in your eyes. I am not aware of any long- term health problems, but it is possible that cracked skin will leave you vulnerable to dermatitis.

 

What is a mortar?
A material composed of sand and cement (and possibly other additives) used to build brickwork or blockwork with, can also be applied as a screed or a render, screed is a mortar applied to a floor, render is a mortar applied to a wall. A screed is also a term used for batten nailed to the floor for the purposes of getting a flat application of mortar.

Particularly for rendering (though not for building brick/blockwork) priming the surface with a PVA bonding agent, diluted 4:1, and using this mixture instead of pure water as gauging liquid, will make the render tend to stick to the wall both when wet, and after setting.

What other types of mortar are there?
Cement Mortar; Made with cement, sand and (not necessarily) lime.

Lime Mortar; Made with lime and sand. Regarded as aesthetically the nicest as the mortar is lighter than the brick.

Black Mortar; Made with cement, ash, and (some) sand.

Surely neat cement will be really strong?
No, neat cement undergoes shrinkage as it cures, and forms masses of internal cracks. mixes stronger than 3:1 should be avoided.
 

Why a plasticiser?
The amount of water required to reach a specific workability will be reduced. In theory this will make the mortar stronger, so you could increase the sand content, but this puts you in uncharted territory, so I would not bother.
 

What are the differences between 'builders' and 'sharp' sand?
Sharp sand is coarser, and the particles are more angular, than Soft sand. Silver sand is of a nearly white colour, and builders' sand is not very specific, but normally is synonymous with sharp sand. Soft sand feels like beach sand and is fine like table salt. Sharp sand feels sharp and gritty.
 

What is concrete?
A mixture of a cement (q.v.) gravel, and sand, gauged with water, which may contain admixtures, and other pozzolanic material(s) normally poured into formwork to create a specific shape. Metal reinforcement may be included in the structure to improve its tensile/flexural performance. Concrete can also be divided into two categories by its intended use, but the composition will be much the same. Mass concrete e.g. for foundations does not necessarily contain reinforcement, whereas structural concrete (e.g. for bridge spans) does. In this case the reinforcing steel is likely to be tensioned. If this is the case, it will either be pre- or post-tensioned. Pre-tensioned reinforcement is tensioned before the concrete is poured, and when the concrete sets, the tension is maintained by the bond between concrete and reinforcement. Post-tensioned reinforcement is tensioned in a duct running through the concrete after the concrete has achieved sufficient strength. Both systems have their advantages and disadvantages.

How long should it take to set?
Dependent on temperature, mix specification and other things 2-4 hours for initial set, 24 for final set depending on the cement type, and the presence of retarders or accelerators, approximately 90% of final strength will be achieved within 28 days.

How dense is wet concrete?
Concrete and mortar both have a density of about 2 to 2.1 tonnes per cubic metre. This figure applies both to wet and cured material. There are special lightweight concretes as low as 1.4 t/cu m.

What is plaster stuff?
Plaster is calcium sulphate hemihydrate. Gypsum is the dihydrate, and is roasted to extract the water of crystallization and ground. Certain plasters contain additives to improve texture, adhesion etc. Old plaster (even a couple of months) is best thrown away, as it either sets very fast, or not at all. Confusingly, in some parts of the world "plaster" is a term used to describe a sand/cement render.