Studies of the surface and bulk reactivity of calcium hydroxide
The physical characteristics and reactivity of a number of commercially available samples of calcium oxide and hydroxide were investigated. 'Fresh' and 'aged' (Le. exposed to the atmosphere) samples were characterised by Fourier Transform Infra-Red Spectroscopy, Thermo-Gravimetric Analysis, Nitrogen Adsorption, Water Adsorption, Scanning Electron Microscopy and Microprobe Analysis. From this slight differences in the 'fresh' samples were found to be due to various degrees of atmospheric ageing. The main process involved in the ageing process was confirmed to be carbonatation (or carbonation). This ageing process was then followed by analysing a sample of calcium hydroxide over a long time period (350 days) and by looking at variations in the process by exposure of the sample to various gases and vapours, i.e. water, dried carbon dioxide, deuterium oxide, ethanol and methanol. These experiments showed that the reaction probably proceeded by a direct reaction between the carbon dioxide gas and the solid and confirmed that water was important when considering the rate of the carbonatation reaction. This was thought to be due to its influence in disrupting the calcium hydroxide structure, thus allowing new surface to become available for reaction and allowing the formation of a bulk calcium carbonate structure. Water adsorption isotherms carried out on the calcium hydroxide at elevated temperatures (250°C and above) showed that outgassing at such temperatures caused a partial decomposition to calcium oxide but that the original calcium hydroxide structure remained intact. These isotherms also had negative hysteresis which was thought to be due to the take up of water in an expanded skeletal calcium hydroxide structure which was expelled from the structure when it fully reconstituted at higher relative pressures to reform calcium hydroxide.