Calcium carbonate (or limestone) was formed from the skeletons of marine creatures which accumulated at the bottom of a body of water. They were then compressed by the movement of the earth crust and transformed into crystalline limestone of high purity. The impurities present are generally MgCO3, Al2O3, Fe2O3, and silicates. Among the important varieties of limestone are chalk, dolomite, marble, marl, oolite and travertine. Iceland spar is a pure form of calcium carbonate and double refraction (exhibits birefringence).
There are basically three crystal forms (mineralogically known as phases or morphologies): calcite, aragonite and vaterite. Vaterite is not stable and over time transforms into the other two forms. Calcite has a hexagonal structure and aragonite a rhombic structure.
Calcium carbonate is insoluble in water but can dissolve in carbonic acid (water with dissolved carbon dioxide) to give calcium bicarbonate. When the solution of calcium bicarbonate evaporates it regenerate the calcium carbonate. Such process gave rise to the stalactites and stalagmites in caves.
Limestones were used to construct the pyramids of Egypt and shrines in ancient Greek and Roman Empires. After the Industrial Revolution limestone was used as the raw material for cement and to remove impurity (as flux) in the extraction of iron from ironstone. Then came the plastic revolution and it was used as filler in plastic, rubber, paper and decorative paint. Now its use has extended into the agricultural as well as medical domains making calcium carbonate the most used nonsiliceous mineral. It is perhaps the most widely utilised additive mineral today.
The calcium carbonate is grounded to particle size (the minimum size is 1 micron) and sold commercially. It is generally 96% to 99% pure. For smaller and more uniform size distribution Precipitated calcium carbonate (PCC) is used.
PRECIPITATED CALCIUM CARBONATE (PCC)
Precipitated calcium carbonate (PCC) is manufactured by mixing calcium oxide with water to form a slurry, then carbon dioxide gas is passed into the slurry at a control rate to produce very fine precipitated calcium carbonate (a carbonation process). Precipitated calcium carbonates are used in applications that demand specified particle size distribution.
PCCs offer a range of technical effects beyond the capability of grounded calcium carboinate fillers and other more expensive and sophisticated additives. Activated PCCs is surface activated using activator or coupling agent. The product can be produced in dry or wet process. Ultrafine coated precipitated calcium carbonates (CPCC) is PCC coated with calcium stearate.
PCCs are available in a wide range of particle sizes to as low as 0.02 micron. It is a low-cost filler, and provide a widest range, for polymeric systems - rubber, plastic, paper and coating.
CALCINATION
When calcium carbonate, Ca(HCO3)2, is heated it decomposes into the calcium oxide (CaO) and carbon dioxide. Calcium oxide is known in the industry as lime.
| CaCO3 | → 500 − 600˚C |
CaO | + | CO2 | |
| M. Wt. | 100.09 | 56.08 | 44.01 |
In metallurgy, this process is known as calcination. Now the term is used for any heating of solid material to drive off volatile components (except for water).
Calcium oxide is used in making porcelain, glass, mortars and cements; in purifying sugar; in preparing bleaching powder, calcium carbide, and calcium cyanamide. In agriculture it is used for treating acidic soils (liming).
SLAKING
Calcium oxide is a base and it reacts with water to form calcium hydroxide (hydrated lime or slaked lime, Ca(OH)2, known as slaking.
| CaO | + | H2O | → | Ca(OH)2 | |
| M. Wt. | 56.08 | 18.01 | 74.09 |
The reaction is highly exothermic. 63.9 kJ of heat is released when one mole of CaO is hydrated. This is sufficient to increase the temperature of 153 g of water from 0˚C to 1090˚C. So at least 60% excess of water has to be added to compensate for this lost as steam. Too little water will cause "blowing" or "popping" during subsequent hydration (with the unreacted quicklime) in mortar. Too much water will not give a putty (colloidal paste).
Calcium hydroxide reacts with carbon dioxide to form calcium carbonate (carbonation).
| Ca(OH)2 | + | CO2 | → | CaCO3 | + | H2O |
Calcium hydroxide is used as a low cost alkali. It is only slightly soluble in water, about 0.2 g per 100 ml, so it is often used as a suspension in water (milk of lime). It is used to plaster walls of building in the construction industry. When plastered on the wall the lime has first to dry a little before it hardens by carbonation. Carbonation is a slow process and penetration of CO2 into the plaster is only about 1 mm deep.
The reaction of carbon dioxide with calcium hydroxide is also used as a standard test for carbon dioxide. When a gas is bubbled into a calcium hydroxide solution and it turns cloudy (calcium carbonate precipitates) the gas is said to be carbon dioxide.
Calcium hydroxide is also used in liming soil, whitewash building, mortar, and plaster; in sugar refining, in leather tanning to remove hair from hides.
Saturated clear aqueous calcium hydroxide (limewater) is used in medicine to treat acid burns and as an antacid.
Calcium hydroxide can be reverted to calcium oxide by heating above 580˚C.
