2_7 Gypsum and Lime: Plaster of Paris, Quicklime and Slaked Lime

Two calcium compounds do an enormous amount of practical work. Gypsum, hydrated calcium sulphate

\mathrm{CaSO_4 \cdot 2H_2O}

, gives us plaster and plasterboard and helps control cement setting. Lime is a name for calcium oxide (quicklime) and calcium hydroxide (slaked lime), which are made from limestone and used to treat soils, bind bricks, make glass and much more. Their uses follow directly from the solubility and thermal stability trends of group 2 compounds.

From Gypsum to Plaster of Paris

Gypsum holds two molecules of water of crystallization for each

\mathrm{CaSO_4}

. Heated gently to about 120–150 °C, it loses three quarters of that water and becomes the hemihydrate, plaster of Paris:

\mathrm{2CaSO_4 \cdot 2H_2O \xrightarrow{\ \text{gentle heat}\ } (CaSO_4)_2 \cdot H_2O + 3H_2O}

Mixed with about half its mass of water, the powder forms a smooth paste that sets hard within 10–15 minutes as interlocking gypsum crystals grow again. It expands by about 1% as it sets, which pushes it into every corner of a mould and gives sharp detail. That is why it is used for casts of statues and coins, decorative mouldings and, in medicine, for plaster bandages that hold set bones in place. If gypsum is overheated, above about 200 °C, all the water is lost. The resulting anhydrous dead-burnt plaster takes up water so slowly that it is useless for casting.

Gypsum in Industry and Agriculture

• Cement: about 2–5% gypsum is ground into cement clinker. It slows the reaction of calcium aluminates with water, so concrete does not set before it can be placed.
• Building plasters and boards: plaster mixed with retarders such as glue sets more slowly, and hard-finish plasters are made from anhydrous calcium sulphate calcined with alum or borax. Set between paper sheets or with fibres, plaster forms plasterboard for walls and partitions.
• Filler: finely ground gypsum is used as a white filler in paper and paint.
• Soil treatment: gypsum supplies both calcium and sulphur, which plants need. Sulphur is part of the amino acids cysteine and methionine, so it is essential for protein, and sulphur-deficient leaves turn pale because chlorophyll formation suffers. Because calcium sulphate is a neutral salt, gypsum adds calcium without raising soil pH. It also improves sodic soils, where excess sodium ions make clay sticky and waterlogged, because the

  \mathrm{Ca^{2+}}

  ions displace the sodium ions so they can be washed away.

Quicklime and Slaked Lime

Heating limestone to about 900 °C in a lime kiln drives off carbon dioxide and leaves quicklime, CaO. Adding water to quicklime is called slaking. The reaction is strongly exothermic, enough to boil some of the water, and it produces slaked lime, Ca(OH)₂. A suspension of slaked lime in water is milk of lime, used as whitewash.

\begin{aligned} \mathrm{CaCO_3} &\xrightarrow{\ 900\,^\circ\text{C}\ } \mathrm{CaO + CO_2} \\ \mathrm{CaO + H_2O} &\rightarrow \mathrm{Ca(OH)_2} \quad \Delta H < 0 \end{aligned}

Lime in Agriculture

Lime is a base, so its main agricultural job is to neutralize acidic soils. In acidic soil, aluminium and manganese become soluble enough to poison plants, and phosphate gets locked up as insoluble iron and aluminium phosphates. Raising the pH reverses both problems, so more phosphorus becomes available to roots. Calcium itself helps roots and root hairs develop, builds cell walls in leaves and bark, and supports the soil bacteria that produce nitrates. Lime is also boiled with sulphur to make lime-sulphur sprays, which act as fungicides.

Lime in Industry and Building

Lime mortar is slaked lime mixed with three to four volumes of sand and water. It first stiffens as water evaporates, then hardens slowly over weeks as it absorbs carbon dioxide from the air and turns back into calcium carbonate, which binds the sand grains and bricks together. Some slaked lime also reacts slowly with silica in the sand to form calcium silicate.

[Ca+2].[OH-].[OH-].O=C=O>>[Ca+2].[O-]C([O-])=O.O

Heating lime with coke in an electric furnace at above 2000 °C gives calcium carbide,

\mathrm{CaO + 3C \rightarrow CaC_2 + CO}

. Water turns the carbide into acetylene (ethyne), a fuel for welding and a starting material for organic synthesis:

[Ca+2].[C-]#[C-].O.O>>[Ca+2].[OH-].[OH-].C#C

• Glass: lime reacts with sand at high temperature to form calcium silicate,

  \mathrm{CaO + SiO_2 \rightarrow CaSiO_3}

  , which makes soda-lime glass durable and insoluble.
• Metallurgy: lime is a flux that combines with silica impurities to form a removable slag.
• Paper, leather, ceramics and sugar refining: lime removes impurities and adjusts pH.
• Bleaching powder: slaked lime absorbs chlorine to make bleaching powder for fabric and paper pulp.
• Drying agent: quicklime dries ammonia gas and ethanol, and soda lime (NaOH with Ca(OH)₂) removes both water and carbon dioxide from gases.

Tips for remembering

• Plaster loses three quarters: heating gypsum (2 H₂O per CaSO₄) drives off three quarters of the water to give plaster of Paris (½ H₂O per CaSO₄). Adding water back makes it set.
• Lime cycle: limestone → quicklime (heat, lose CO₂) → slaked lime (add water) → limestone again (absorb CO₂), which is exactly how mortar hardens.
• Lime fixes acidity, gypsum fixes sodium: lime raises soil pH, while gypsum adds calcium and sulphur without changing the pH much.