Concrete is a manufactured material that is mainly a mixture of aggregate (gravel, sand or a mix of these), cement and water. The aggregate forms the bulk of the concrete. The cement reacts with water to harden and bind together the aggregate it is mixed with. The hardened material is a cheaper replacement for stone in building.
The concrete is delivered as a liquid to the building site and is poured into a mould where it sets over a few hours. The reaction of cement with water is exothermic (gives out heat) and the heat produced must be controlled so that it doesn’t affect the way the concrete hardens. Hardening takes several weeks and usually the faster the concrete sets and hardens the weaker it is.
The basic mix of the concrete also will affect its final hardness and strength. The amount of water used is important: too little and not all the cement reacts, making the concrete weaker than it could be; too much and some of the water evaporates, making the concrete porous and weaker. Different aggregates contain different amounts of chemicals. High levels of sodium or potassium salts create a highly alkaline mix which makes the concrete swell while hardening, leaving cracks which can weaken it.
Environmental conditions will also affect how the concrete sets and hardens. For example, frost can cause setting concrete to crack while high air temperatures can make wet concrete set too quickly and also make it weaker.
Many different chemicals, called ‘admixtures’, can be added to the cement mix to help reduce some of these problems. For example, lithium nitrate can reduce the swelling caused by an alkaline mix and ice can help to keep the mix liquid long enough to deliver the concrete to the top of a super-high skyscraper in a hot climate.
Despite all the problems with concrete, it can be a more reliable material to use than stone. Even though quarried stone is thoroughly tested to British Standards before supply to a building site, the quality of stone throughout a quarry varies and it can be difficult to predict how it will react under some kinds of pressure.
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