The oldest cements are as old as construction itself, and as civilization has advanced and its needs have changed, so has cement. One type of modern cement is hydraulic cement. Around the start of the Industrial Revolution, hydraulic cement was developed as a way to meet the changing needs of the construction industry and the people it served. Hydraulic cement refers to any cement that will set and harden after being combined with water. Most construction cements today are hydraulic.
When a dry hydraulic cement is mixed with water, certain chemical reactions take place in the mixture. These reactions form chemical compounds which contain water, and the formation of these compounds causes the mixture to harden. Because of the nature of the compounds formed in these reactions, they are insoluble in water. This means that the hardened cement will retain its strength and hardness even if immersed in water. This makes it ideal for brick buildings in wet climates, harbor structures that are in contact with sea water, and many other applications.
It is important to distinguish cement from concrete. Although these terms are often used interchangeably outside the construction industry, they refer to different products. For example, the most common type of hydraulic cement is called Portland cement. Portland cement is most commonly used in the production of concrete, which contains cement, sand, gravel, and water.
Cement itself is produced by heating limestone with small amounts of other ingredients such as clay. The mixture is brought to a high temperature in a kiln, and the hard substance which results is referred to as “clinker.” Clinker is then ground up into a powder along with a small amount of gypsum. The finished product is ordinary Portland cement. This basic mixture is varied to make a wide variety of Portland cements, suited to many specific applications.
One common application for special hydraulic cement blends is repairing cracks and leaks. Certain types of cement contain an ingredient which causes the cement to expand as it sets, filling in leaks and holes in concrete or other structural materials. There is often a need for this type of repair work because of the fact that ordinary Portland cement shrinks slightly as it dries. Places like basements in areas with wet climates are especially prone to leaks because of this. These special blends can be hard for novices to work with, though, because once mixed, the material only remains workable for 10 to 15 minutes.