Natural Science in Archaeology

11.3 Cements and Mortars 261
as alabaster. Gypsum is found in nodules in sedimentary rocks, and may be present in
dimensional stone as inclusions. Gypsum is often found as chinking between blocks
or in rubble fill of cavity walls. However, large nodules when plentiful were sometimes
used in construction of desert cobble walls (Ashurst and Dimes 1998, p. 87).
11.3 Cements and Mortars
11.3.1 Lime
Since at least the third millennium BCE, lime has been used as one of the most basic
components of mortar (Dix 1982). Lime can be procured by burning limestone,
marble, shell, coral, or bones (Mazzullo and Teal 1994). Limestone is the most common
source, and both calcitic and dolomitic limestones can be used.
Lime is simple and cheap to prepare. Limestone is heated in a kiln to over 900°C;
the CO 2 is driven off and CaO remains. The resulting product (calcium oxide) is
traditionally referred to as “quicklime” or “unslaked lime”. This slakes with water
and, when mixed with sand, makes mortar or plaster; 100 kg of pure limestone yield
56 kg of lime.
The minimum effective temperature for burning limestone is 880°C, but, in
order to achieve this temperature throughout the limestone, the overall surface temperature
must be about 1000°C. Conversely, excessively high temperatures are also
undesirable. “Excessive temperature produces ‘hard burned’ lime of high shrinkage,
high density, and low chemical reactivity. In contrast, lower burning temperatures
result in ‘soft burned’ lime of low shrinkage, low density, high porosity, and high
chemical reactivity” (Adams et al. 1992).
After burning, lime is further processed by “slaking”, which refers to a hydration
process. The resulting calcium hydroxide is traditionally referred to as “slaked”
lime. There are four basic methods for slaking lime (McKee 1973). These include:
1. “Sprinkling” or “drowning” in which the correct amount of water is sprinkled
over the quicklime.
2. “Immersion” in which the quicklime is placed in a basket and submerged into
water. The basket is drawn up after a time. The slaking is completed by exposure
to moisture in the air.
3. “Exposure” in which quicklime is exposed to the air in a shelter. It absorbs
moisture from the air, but may also absorb any unreacted calcium carbonate,
thus resulting in an inferior product.
4. The “lime paste” method in which the lime is mixed with water in pits. The
slaked lime is aged in the pits or clay jars for weeks, sometimes even months or
years. This method produces the most satisfactory product.
Due to the caustic nature of the lime and the potential for violent chemical reaction,
slaking can be a dangerous task. If not done carefully, the heat of the reaction