Page 23 - Curriculum Visions Dynamic Book

caustic: a substance that can cause burns if it touches the skin.
dehydration: the removal of
water from a substance by heating it, placing it in a dry atmosphere
or using a drying agent.
 Whitewash paint
This old house is made of natural boulders. It has been painted with whitewash, a mixture of lime and water.
Because lime picks up carbon dioxide from the air, it eventually turns back into chalk. Thus, the surface of whitewashed walls is often dusty.
Whitewash is cheap, but it has to be reapplied more often than modern paints.
Cement: calcium “glue”
 Most apartments are now made
almost entirely with concrete. These are
in Hong Kong. is in the manufacture of cement. Like mortar,
cement is an adhesive (glue) used to bond bricks together or
to bind stones and gravel to make concrete. Most cement is called Portland cement, a general kind of cement mix named after the English limestone (called Portland Stone) that was used in the cement patented by Joseph Aspdin in 1824.
Making cement
Cement is made from a combination of a limestone or chalk and a clay. The raw materials are ground down and then mixed together. Once mixed they are roasted in a kiln that constantly rotates. The roasting temperature is very high, about the same as that used to melt glass (1350°C). The material is then cooled and crushed to make a fine grey powder.
Using cement
Cement powder is mixed with water to make cement, a grey pasty substance. The chemical reaction that takes place happens very quickly; and if nothing were done to slow down the process,
the cement would get hard in a few minutes and so be very difficult to use. The key to slowing down the reaction is to add gypsum (calcium sulphate).
Cement will last for many decades, but it is not as long-lasting as most other building materials. When exposed to the weather, it will eventually return to the calcium carbonate from which it
23 was made. 23
One of the main uses of calcium compounds