Changes with water
"What's going on?" asked the old lady at the top of the lane.

"We're going to build some new houses in this field," said the man on the bulldozer. All morning he scraped away the turf to expose the light brown soil underneath. In the afternoon the surveyors arrived. They spent the next few days marking out the sites for the houses. As the marking pegs for the last house were being put in position a digger trundled to the top of the lane. Later that day it began scooping up the soil to make trenches. In the bottom of the trenches the foundations to the houses would be laid.

A house weighs many tonnes. If the foundations are weak it may collapse into the ground. Foundations need to be strong to support the house - perhaps for a hundred years or more. The foundations are made from a very strong building material called concrete. It is made from a mixture of gravel, sand, cement and water. Gravel and sand are natural materials. Both are made from the breaking up or weathering of rocks. Cement is a manufactured material. It is made from clay and limestone. These two natural materials are mixed up and poured into a huge metal tube, which is over three metres in diameter and a hundred metres long. The tube lies on its side with one end slightly above the other and is slowly turned by a machine. At the lower end of the tube large flames of burning gas provide heat. As the tube turns, the mixture of clay and limestone slowly tumbles down the tube. With each movement they get hotter as they approach the flames. The heat brings about irreversible changes in the two natural materials and they join together to form lumps of cement. These lumps are called clinkers and when they reach the bottom of the furnace they are taken out and ground to make a powder.

The ingredients in concrete need to be thoroughly mixed. To save time their mixing takes place in two stages. First the ingredients are mixed at a factory and poured into a large drum on the back of the lorry. Next the lorry begins its journey to the building site and, as it travels along, the drum slowly turns. By the time the lorry reaches a building site the slowly-turning drum has mixed the concrete thoroughly.

When the lorry carrying the concrete reached the building site at the top of the lane it was backed into position by the foundations of the first house. The wet concrete flowed quickly down a chute and filled the trenches. As the concrete behaved like a liquid, it took up the shape of the trench and its top surface became flat and level. A builder used a board to beat the surface. This action released any air that was trapped in the concrete to make it rise to the surface and escape. If the air were left in, it would form bubbles and make the concrete weak and useless.

Once the foundations had been laid the concrete was left alone and the lorry moved on to the site of the next house. Inside the concrete irreversible changes began. The cement and water took part in changes, which produced long, needle-like crystals. They grew between the grains of sand and lumps of gravel. As the crystals grew they interlocked with each other and bound the sand and gravel together. When the change was complete the sand and gravel were securely held together and formed a substance as strong as rock.

In a few days bricklayers would arrive to begin building the walls. They would bring bags of cement too. They would mix the sand with cement and water to make a substance called mortar. This would be smeared on the bricks with a trowel. When the bricks were laid, the water and cement in the mortar would make crystals to bind the bricks together.

The chances are you are sitting in a room which is made possible by the irreversible change that takes place between water and cement. The walls of the room may be made of bricks and mortar and beneath your feet is a foundation of concrete.

You sometimes see fresh concrete covered with polythene. Why is that?
The way that crystals form depends on the amount of water in the mixture. In some weather conditions too much water could leave the mixture by evaporation. This would result in poor crystal growth and weak concrete. Polythene does not let water vapour pass through it. When it is placed over the concrete it traps any water vapour escaping from the concrete's surface. This makes the air under the polythene very humid. The humid air slows down evaporation and retains the water in the concrete so a strong network of crystals can form.

Some concrete has metal bars in it. Why?
This kind of concrete is called reinforced concrete. The bars are added to give extra strength. You may see reinforced concrete being used in large buildings in a town, such as a multi-storey car park. Reinforced concrete is widely used in bridge building.

Why are the rocks in some kinds of concrete different colours?
They are made from different kinds of rock. The three main types of rock used in making concrete are granite, limestone and sandstone. Some granite is pink, limestone is grey and sandstone is brown.

Concrete blocks do not have large pieces of rock in them. Why?
Concrete blocks are made from smaller pieces of rock though air may be added to the mixture to make the blocks lighter in weight. Some types of concrete use rock, such as pumice, which has air bubbles in it. Other types of concrete use the rock-like material called slag, which is produced when iron is made in a blast furnace. It does not matter that these kinds of concrete contain air. The blocks are designed so that they can be easily lifted and are used to line the inside of a wall. The air in the concrete acts as an insulator and helps to keep a building warm.

Is plaster of Paris made from cement?
No. It is made from a mineral called gypsum. The gypsum is ground up into a powder. When it is mixed with water it forms needle-like crystals just like the crystals in cement.

Is plaster of Paris used in hospitals?
Yes, it is. It is used to make the 'pots' around broken arms and legs. The damaged limb is covered with a loosely woven cloth and the plaster of Paris is added to it. The plaster of Paris sets quickly so patients do not have to wait long before they can go home. If the 'pots' were made of concrete they could take days to set.

What is the plaster used to cover the walls in rooms?
This is a mixture of plaster of Paris and a substance, such as lime or keratin, which slows down the speed at which the plaster sets. This gives the plasterer time to spread the plaster over a large area and then smooth it out. A smooth surface is needed before the wall can be papered. If plaster of Paris were used on its own it would set too fast and the surface could not be made smooth.