External Rendering

Information On Lime Putty i.e. The Base Component For Lime Mortars And Renders

The raw material for the manufacture of lime mortars and renders is obtained from naturally occurring sources such as limestone (one form of CaCO3 or calcium carbonate), sea creature shells or coral calcium carbonate/ chalk (another form of CaCO3 or calcium carbonate). For the process of making lime putty (Ca(OH)2 or calcium hydroxide) from the naturally occurring sources is a straight forward procedure.


Usually limestone is placed in a specially built kiln or in a pit or a heap that acts as a furnace. The kiln is layered with fuel e.g. coal and is left to burn the naturally occurring material for approximately 12 hours. At the end of the burning process, white lumps of solid calcium oxide (CaO) are left along with bits of burned and unburned fuel. Overburned limestone appears as black, glassy pieces meaning that carbon soot is also present in the calcium oxide product as a contaminant so overburned limestone is to be removed.

The calcium carbonate material is heated to a maximum temperature of 1,200 degrees Celsius. While the calcium carbonate is heated to this maximum temperature, there are a number of observations to be made, these observations being:

Note: This chemical reaction is usually more complicated as there are other impurities present such as other carbonates and silicates within the limestone which also undergo chemical reactions. However, we should ignore these other reactions and instead concentrate on the chemical breakdown of calcium carbonate.

Calcium oxide product commonly called ‘lump-lime’ or ‘quicklime’ appears as lumpy solids or can be grounded down into fine powder.

Note that calcium oxide (CaO) must be kept dry because it will readily react with water molecules in the air or from the moisture in your skin to form calcium hydroxide (Ca(OH)2). If this reaction is not prevented, the calcium hydroxide solution will react with carbon dioxide, forming calcium bicarbonate (Ca(HCO3)2) which breaks down to calcium carbonate (CaCO3), water (H2O) and carbon dioxide (CO2).

To produce lime putty (the product that is most useful to us), we react the quicklime/ calcium hydroxide (CaO) with water (H2O) to produce aqueous calcium hydroxide/ lime putty (Ca(OH)2) and heat.

There is a technique to making lime putty because usually creating lime putty is a potentially dangerous process. This safer technique is called “slaking” where quicklime is added slowly to water to avoid rapidly release of explosive energy.

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