Published: Jan 2002
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Full scale calcination of high calcium limestone using traditional methods was performed in the batch process Experimental Lime Kiln (ELK). The ELK is equipped to monitor temperature, air flow and gas compositions and also has significant insulation to ensure minimum lateral energy loss during burning. Internal kiln wall temperatures of up to 550°C, and kiln core temperatures of 850°C, within the range of calcium carbonate disassociation have been achieved for several hours with predictable temperature/time gradients. A mixed feed solid fuel : stone ratio of 1:9 was employed resulting in limestone conversion to quicklime of 75% ± 9. The heat balance efficiency is approximately 45%. However, this may not be a useful indicator of overall efficiency of binder production, especially in a open-top batch process traditional kiln. The hydration behaviour of the low-temperature traditional quicklime is more varied than high-temperature commercially produced quicklime, made from the same stone. The low temperature material hydrates more slowly, reaches a lower temperature and maintains a peak temperature plateau for longer than the high-temperature quicklime. The traditional quicklime also produces as much as 50% non-hydrated residue during the tests. This suggests an origin for “lime inclusion” texture in historic mortars. Mortars produced using traditional hot mixing processes are petrographically similar to historic mortars. The recognition of distinctive quicklime microstructures may also provide a diagnostic tool for quality control in small-scale traditional lime production.
masonry conservation, lime:sand mortar, lime kiln, calcination, quicklime, heat of hydration