What Makes a Great Lime for True Lime Mortars?
The lime cycle is a closed loop. Calcium Carbonate [CaCO3, i.e. limestone, oyster shell] is fired to drive off the CO2, making Calcium Oxide [CaO, quicklime]. When water is introduced during the volatile process called slaking, calcium hydroxide is formed [Ca(OH)2]. Lime putty is calcium hydroxide. As water leaves the lime, carbon dioxide from the air begins to take its place, once again reacting with the lime to create Calcium Carbonate again. There is an increase in mass associated with carbonation that counteracts shrinkage from water loss.What if we slake calcium oxide ourselves? Good quality control in manufacturing Sample A produced an optimum lime putty. The same oxide slaked by hand without precise control of water feed rate and temperature created the inferior putties, C and D. Reactivity is created through careful control in firing and slaking. Small particles and high surface area create longer decantation times (the time necessary for a particle to settle out). Viscosity is related to surface area and plasticity. Greater plasticity and water-holding capacity mean better working properties and contribute to low shrinkage from water loss.
True lime mortars set (carbonate or “cure”) by reacting with atmospheric CO2, creating a sort of man-made limestone. Hydraulic mortars (Portland cement, bagged mortar mixes, and hydraulic limes and cements) set by reacting with water. The firing of all mortar constituents drives off CO2, but the reabsorption of this CO2 by lime mortars makes it a more environmentally-responsible building choice. Lime mortars are also less fuel intensive to produce than Portland-based mortars.
Blog post attributed to www.preservationscience.com