Natural Science in Archaeology

11.3 Cements and Mortars 267
of several layers of calcium carbonate that resulted from the natural alteration of
slaked lime. The Egyptians began using gypsum and lime mortars about 5000 BP.
The Romans improved on lime mortar by adding volcanic ash to make a pozzolana
cement. Following the fall of Rome the technology of cement manufacture was lost
until the Middle Ages.
11.3.6 Artifi cial “Pozzolana”
Artificial pozzolanas are materials that produce a hydraulic reaction similar to the
reaction produced by natural pozzolana. Artificial pozzolana can be organic or inorganic
material. Examples of inorganic materials include calcined clays and shales, furnace
slag, fly ash, and brick powder. Organic artificial pozzolanas include ashes from
rice husks and coffee hulls. The use of rice husk ash is particularly well documented.
After burning, the remaining material is predominantly reactive silica. Mixing these
fine-grained pozzolanic materials with wet lime develops a gel-like structure that
expands as it sets, making it a tight, essentially permanent bonding agent.
Pottery and brick dust are the most popular artificial pozzolanas. Their main
constituent is calcined clay. Clays are products of mechanical and chemical weathering
of igneous rocks. Microscopically, clays are flat plate-like crystalline particles
less than 2 µm in size. Clays can be considered active or inactive depending on how
much water they are capable of absorbing. Thus, the amount of alternate swelling,
shrinkage, warping, and cracking will depend on the type of clay. Clay mineral particles
will stack in layers and adhere to each other much the way two layers of wet
plastic sheets sometimes do. When fired, the clay becomes vitrified or glassy.
If fired at a high enough temperature, calcined clay may make an excellent
pozzolana. High-fired brick and pottery dust makes a much better pozzolana than
low-fired brick and pottery dust (Lea 1973). Research has shown that the size and
distribution of the clay particles in the matrix of the cement can also have an impact
on its strength.
Fired clay in itself possesses “little or no cementitious value, but that in finely
divided form and in the presence of moisture will chemically react with alkali
or alkaline earth hydroxides” (Adams et al. 1992). In other words, in order to be
absorbed into the chemical reaction, the clay product (usually crushed brick or pottery)
must be crushed into a fine powder. “These reactions take place at ordinary
temperatures to form compounds possessing cementitious properties” (Adams et al.
1992). The nature of the pozzolanic reaction can be described as follows:
“The so-called pozzolanic character of crushed brick mortar is attributed to the
adhesion reactions of physicochemical character occurring at the ceramic-matrix
interface; their nature depends both on the type of ceramic and the calcium hydrate
content of the mortar. The observed reactions could probably be attributed to calcium
silicate formations at the interface along the brick fragment, acting as the
silicate source and membrane and the lime, which makes the interfacial surface
alkaline and causes the chemical reaction. The penetration of lime into the ceramic