This article will briefly chemically describe the basic constituents of concrete. Then it will look at some of the admixtures and how they change the chemistry of concrete.
The three basic ingredients of concrete are aggregate, cement and water. Aggregates are chemically inert and are bound together by the cement. They are there to give bulk to the concrete and are not involved in the chemical processes.
Cement is made by mixing crushed clay and limestone together and roasting it in a kiln. The resulting powder is a mixture of five chemicals. About 50% is tricalcium silicate, 25% is dicalcium silicate, 10% tricalcium aluminate, 10% tetracalcium aluminoferrite and 5% gypsum or hydrated calcium suphate.
When cement is mixed with water, its constituents are hydrated. The calcium silicates form calcium silicate hydrate, calcium hydroxide and heat. These products contribute to the strength of the concrete. Tricalcium silicate reacts quickly producing a lot of heat. Dicalcium silicate reacts more slowly and produces less heat.
The chemistry of these reactions is complex. When the calcium silicates first come into contact with water, a reaction occurs in which calcium ions are formed and the water molecules are broken down to form hydroxide ions. It is this bond breaking that produces heat. Calcium hydroxide is not very soluble so it is soon saturated and forms a solid. At the same time, calcium silicate hydrate is formed which is also a solid. As long as water is in contact with the cement, these reactions continue but they get slower and slower, and can take several years to reach full strength.
Zooming out a level, the aggregate is not involved in the reaction, but forms a surface for the solids to form on. This will be easy at first, as there are large areas of water ad cement mixed together. As the solid is formed, however, there is less and less space between the grains of aggregate and silicate hydrate for the water to move around and reach un-reacted cement. This means that the reaction will slow down as the pores between the aggregate get smaller.
Moving on from this basic chemistry, it is necessary to consider the effects of changing the proportions of water and cement. The concrete is stronger if there is less water added in comparison to the amount of cement. The problem is, however, that if there is not much water, the concrete does not flow very well and is more difficult to pour. A balance need to be found between strength and ease of pouring.
What do admixtures do? Some admixtures improve the workability and plasticity of the concrete so that less water is needed, improving the strength. Some minerals such as fly ash (also known as pulverised fuel ash or PFA) have this property. Synthetic polymer chemicals called superplasticizers are also use for this function.
Other admixtures help construction forms adapt to different environmental factors. There are retarding admixtures to delay the setting time in hot climates (for example sugar). There are also accelerating admixtures to speed up setting in colder climates (for example calcium chloride.
Concrete really is chemistry!
For more general articles on chemistry try these articles:
Chemistry Defined
What is Inorganic Chemistry?
What is Organic Chemistry?
