Natural Science in Archaeology

11.3 Cements and Mortars 265
11.3.4 Hydraulic Reactions
There are two methods by which limes and mortars “set” or harden. (1) Nonhydraulic
limes and mortars set by evaporation of water vapor. For example, a slurry
of nonhydraulic lime, quartzite sand and water hardens as the water evaporates. (2)
Conversely, some mortars, some limes (those which contain more than 10% silicates)
and all cements set by nonevaporative chemical reactions that do not require
atmospheric evaporation. These are hydraulic reactions in which water is one of the
constituents of the chemical reaction.
Cements and hydraulic mortars are classified as ceramics. The hardness of
ceramics is a result of partially covalent bonding. Ceramics can be usefully divided
into three groups: naturally occurring ceramics, ceramics produced by chemical
reaction, and ceramics produced by heat treatment (Cotterill 1985). Cements and
hydraulic mortars are ceramics produced primarily by chemical (i.e., hydraulic)
reaction, and thus fall into the second group.
Thorough mixing of a cement or hydraulic mortar is important to the final product.
Simply turning over the mixture is inadequate. In ancient times, the mixture was
traditionally chopped, beaten, and rammed with wooden paddles. The result was
improved workability and performance of the mixture. “The value of the impact is
to increase the overall lime-aggregate contact and to remove the surplus water by
compaction of the mass” (Ashurst and Dimes 1998).
Cements develop in two stages. During the initial setting reaction, the cement
achieves only a small percentage of its ultimate strength. During the subsequent
curing stage, the cement mixture experiences slower hydration and eventual conversion
to a hydrated calcium silicate glass. In modern Portland cements this process
can take years (Lechtman 1986). In fact, the slower the cement cures, the stronger
the final product. This is one reason cement can achieve a higher strength under
water. The absence of evaporation encourages maximum water absorption into the
chemical reaction. However, there are many factors that can affect strength, and
there are currently no data available regarding the curing time of Roman cements.
Lime-based compounds formed by chemical reaction are said to be hydraulic
or pozzolanic, and the materials that drive these reactions are called pozzolana.
The term has been used since ancient times and derives from the name of the town
Pozzuoli in southern Italy. The ancient Greeks and Romans were familiar with this
technology, although they did not understand the chemical reaction. The ancients
knew that not all soils and rocks produced a hydraulic reaction with slaked lime.
Thus, they had to depend on trial and error when attempting to make cement. The
Romans discovered that by adding certain volcanic materials to lime, a very hard
cement was produced, which would harden even under water.
In order to understand hydraulic lime reactions, it is important to understand
the nature of silica. Silica plays the central role in chemical reactions that produce
cements. Reactive silicas have weak ionic bonds that can be broken easily. This
enables the silica to react chemically in the cement mixture. Conversely, nonreactive
silica, such as quartz, has very strong ionic bonds.