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CEMENT
General properties of cements
All cements, mixed with water, set to a hard dense material. The main difference between cements are in setting time, composition, rate of strength gain and ultimate strength, resistance to chemical action, and colour. Normally they are not used alone but are mixed with various types of inert substances (aggregates) such as sand, or sand and larger particles such as gravel or crushed rock. These additions lessen the shrinkage and reduce the coat of resulting material. The chief characteristics of these cement products are given below.
Shrinkage.
Apart from the strength proportion of cement mixes, the shrinkage which takes place with almost all cement products is probably their most important characteristic, because it is this which leads to cracking in buildings if adequate precautions are not taken.
All types of cement shrink appreciably during 'setting. In normal concrete, plaster, or mortar the amount of this shrinkage will depend both on the proportion of cement; in the mix and the quantity of mixing water employed. If enough water is present to enable the chemical action of setting to take place, then the smaller the amount of water the less shrinkage will be. It will be seen later that the type of.aggregate used has an appreciable effect upon both the amount of water and the amount of aggregate that can be mixed with a given quantity of cement to produce certain strength and other properties. Because of this, although the aggregate itself may be subject to moisture movement, it may very largely govern the amount of moisture movement which occurs in the final product of concrete, mortar or plaster. ln addition to the proportion of cement and the water cement ratio of a cement product, the method of curing will also affect the amount of shrinkage. Normally, the slower the drying the less shrinkage there will be. The important point here is to remember that all cement products are liable to a considerable shrinkage during setting and hardening. In addition to the considerable initial shrinkage which occurs in all cement products, subsequent expansion or contraction may arise either from changes in moisture content or from changes in temperature.
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EXERCISE
Answer the following questions
1. What are the main differences between cements?
2. Are cements usually used alone?
3. What lessens the shrinkage?
4. What does shrinkage depend on?
5. What takes place in cement products in addition to initial shrinkage?
6. What does shrinkage lead to?
Cement Strength.
The important thing is the strength of the final cement product rather than the strength of the cement itself. A knowledge of the latter does, however, give some indication of the possible variation in the former, although clearly the strength of the product will also depend upon the type grading of the aggregate used, the proportion of aggregate, and other factors such as water cement ratio and quality of workmanship. The value for tensile strength and for the compressive strength for different cements of different ages illustrate two important points.
First, that the cement improves in strength after the final set occured and, second, that there is a very considerable difference in rate of strength gain of the different types of cement.
