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Calcium and its Compounds, CaCO3, Ca(OH)2, CaO, Cement

 

Calcium is an alkaline earth metal. It does not occur freely in nature due to its high reactivity. It is found in the following minerals as CaCO3 in limestone, marble, aragonite, calcite, chalk, coral and sea shells.

It is found as CaSO4 in gypsum and anhydrite. It is also found as a double carbonate, such as CaCO3.MgCO3 in dolomite; as calcium floride in fluorspar; as several silicate(IV) in the soil; and as phosphate(V) in bones and teeth.

Calcium Oxide

Calcium oxide, CaO, is also known as quicklime and can be prepared from sea shells.

Sea shells are composed of calcium trioxocarbonate(IV), CaCO3.The action of strong heat upon CaCO3 produces calcium oxide, together with carbon(IV) oxide.

CaCO3(s) → CaO(s) + CO2(g)

Properties of Calcium oxide

Here are properties shown by calcium oxide.

1. It is a white solid.

2. It has a high melting point (about 2600oC).

3. It is hygroscopic and is used to dry ammonia gas.

4. When water is added drop wise onto CaO, it cracks with a hissing sound and breaks up into a powdery form, with the liberation of enormous heat.

The product formed is Ca(OH)2 and is known as slaked lime and the reaction process is called slaking.

CaO(s) + H2O(l) → Ca(OH)2(s)

5. It is a strong base. It reacts with acids to form salts and water only. It displaces ammonia from ammonium salts.

CaO(s) + 2HCl(aq) → CaCl2(aq) + H2O(l)

CaO(s) + 2NH4Cl(aq) → CaCl2(aq) +H2O(l) + 2NH3(g)

Uses of Calcium oxide, CaO

CaO can be used in the following ways:

(1). In the manufacture of slaked lime, Ca(OH)2

(2). In the building industry for the preparation of mortar and for the manufacture of cement.

(3). In the manufacture of calcium carbide.

(4). In smelting processes.

(5). In the manufacture of refractory furnace linings.

(6). In the manufacture of glass.

(7). For drying ammonia in the laboratory.

The Chemical Composition of Cement

Cement is composed of the following materials:

(1). Lime or CaO - from limestone, chalk, shells, calcareous rock.

(2). Silica, SiO2 - from sand, old bottles, clay or argillaceous rock.

(3). Alumina, Al2O3 - from bauxite, recycled aluminium, clay. 

(4). Iron, Fe2O3 - from clay, iron ore, scrap iron and fly ash.

(5). Gypsum, CaSO4 . 2H2O. Gypsum is found together with limestone.

In the manufacture of cement, the above materials are crushed, milled and proportioned.

The materials, without the gypsum are proportioned to produce a mixture with the desire chemical composition and then ground and blended by one or two processes - dry or wet process.

The materials are then fed through a kiln at 2,600oF to produce grayish-black pellets known as Clinker.

The cement clinker formed has the following typical composition:

Tricalcium aluminate - Ca3Al2O6

Sodium oxide, Na2O

Tetracalcium alumino ferrite, Ca4Al2Fe2O10

Potassium oxide - K2O

Belite or dicalcium silicate, Ca2SiO5

Alite or tricacium silicate, Ca3SiO4

These compounds contribute to the properties of cement in different ways. By mixing them appropriately, manufacturers can produce different types of cement to suit several construction environments.

An example is the common Portland cement. The alumina and iron act as fluxing agents, which lower the melting point of silica from 3000 to 2600oF.

The Clinker is cooled, pulverized and gypsum is added (gypsum is added last) to regulate setting time. It is then thoroughly ground to produce cement.

Cement gets its strength from chemical reactions between the cement and water. These reactions, that is, hydration and hydrolysis continue for many years.

Setting Mortars

Mortar is the mixture of lime (CaO or Ca(OH)2 and sand and water to set bricks, stones, ceramics, tiles to walls, e.t.c., in building.

Mortars have plastic and hardening properties. The first step in the “setting” of mortars is the loss of water by evaporation and by absorption into the bricks.

The ultimate hardening is due to the action of the atmospheric carbon(IV) oxide, producing insoluble calcium carbonate.

Ca(OH)2 + CO2 → CaCO3 + H2O

Note: lime (CaO or Ca(OH)2) is a natural plasticiser and 100% cementitious. That is, it sets things firmly.

Calcium Hydroxide (slaked lime), Ca(OH)2

As has been stated, Ca(OH)2 is made by adding water to quicklime (CaO).

CaO(s) + H2O(l) → Ca(OH)2(s)

When excess water is added, a suspension, known as the milk of lime or whitewash is obtained. On filtration, the filtrate (saturated solution) obtained is known as limewater.

Note: slaked lime, milk of lime and lime water are all composed of calcium hydroxide.

Properties of Calcium hydroxide

Calcium hydroxide shows the following properties:

1. Calcium hydroxide dissolves in water to give an alkaline solution.

2. It absorbs CO2.

Uses of Calcium hydroxide

Calcium hydroxide can be used as follows:

(1). Used in treating acidic soils.

(2). Used in softening water.

(3). In the manufacture of bleaching powder.

(4). In the manufacture of mortar.

Calcium Trioxocarbonate(IV), CaCO3

Calcium trioxocarbonate(IV) occurs abundantly in nature as limestone, chalk and marble. It is also found in coastal caves as stalactites (deposits of CaCO3 which grow downwards from the top of the cave to its floor) and stalagmites (deposits of CaCO3 which grow upwards from the floor of the cave to its top).

It is also found in the bones of animals, and in the external skeleton of marine organisms; and in natural ores, such as calcite, dolomite and Iceland spar.

CaCO3 can be produced in the laboratory as a precipitate when Na2CO3 solution is added to a solution of CaCl2.

Na2CO3(aq) + CaCl2(aq) → CaCO3(s) + 2NaCl(aq)

Properties of Calcium Trioxocarbonate(IV)

Below are properties shown by calcium trioxocarbonate(IV)

1. It is a white solid which is insoluble in pure water.

2. It dissolves in water that contains dissolved CO2 , to form calcium hydrogen trioxocarbonate(IV)

CaCO3(s) + H2O(l) + CO2(g)  reversible reaction arrow Ca(HCO3)2(aq)    

Recall that the presence of calcium hydrogen trioxocarbonate(IV), Ca(HCO3)2, is the cause of temporary hardness in water.

3. CaCO3 decomposes into CaO and CO2 when strongly heated.

4. CaCO3 is attacked by dilute acids to liberate CO2

CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)

Uses of Trioxocarbonate(IV)

Trioxocarbonate(IV) is useful in the following ways:

1. In the manufacture of sodium carbonate, quicklime, cement, glass and steel.

2. For neutralizing acids in acidic soils.

3. In the extraction of iron.

4. As building materials.

5. In the manufacture of pigments, putty and paper.

Test for Ca2+ Ion

(1). Flame test

Calcium compounds, when heated in a non-luminous flame, produce a brick-red colour. The compound under investigation is prepared for the flame test by moistening it with some concentrated hydrochloric acid.

A reaction occurs, leading to the formation of the chloride of the metal. It is necessary to convert the sample compound (i.e. the compound under investigation) to a chloride because chlorides are more volatile, and would therefore give a satisfactory result on heating.

To confirm the presence of calcium ions, the brick red colour produced will appear green when viewed through a blue glass.

(2). With sodium hydroxide

A solution of calcium salt gives a white precipitate with NaOH solution. The precipitate is insoluble in excess sodium hydroxide solution. The white precipitate is calcium hydroxide.

2NaOH(aq) + Ca2+(aq) → Ca(OH)2(s) + 2Na+(aq)

Note: NaOH is a stronger alkaline than Ca(OH)2, hence it displaces it from solutions of its salts.

(3). With ammonium ethanedioate (ammonium oxalate)

A calcium salt solution produces a white precipitate when reacted with ammonium oxalate solution. The precipitate is calcium ethanedioate, which is soluble in dilute HCl, but insoluble in ethanoic acid (this indicates the presence of Ca2+ ion).

(NH4)2C2O4(aq) + Ca2+(aq) → CaC2O4(s) + 2NH4+(aq)

Note: this test is also used to identify barium and strontium.

(4) With ammonium trioxocarbonate(IV)

A white precipitate is also produced when a solution of calcium salt is reacted with that of ammonium trioxocarbonate(IV). This indicates the presence of the Ca2+ ion.

(NH4)2CO3(aq) + Ca2+(aq) → CaCO3(s) + 2NH4+(aq)

Note: this test also indicates the presence of barium and strontium ions. Calcium salts may be distinguished from barium and strontium salts by the following test:

1. With potassium chromate(V) solution - no precipitate is produced from solutions of calcium salts, while barium salt solutions form precipitates.

2. With saturated CaSO4 solution - no precipitate is produced from solutions of calcium salts, while barium and strontium salt solutions produce precipitate.  

  

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