Natural Science in Archaeology

268 11 Building, Monumental, and Statuary Materials
and the consequent reaction transform the microstructure of the ceramic by transforming
pore radii into smaller pores, decreasing the total porosity and augmenting
the apparent density. The reduction of the pore radii confirms the cementitious
character of the mortar matrix, imparting high strength to the mortar” (Moropoulou
1995a).
The reaction between the brick dust particles and the lime has also been described
as a “boundary reaction.” Microscopically, the rims of these events are manifested
at the ceramic-matrix interface “in the form of veins along the matrix, as if they fill
the vacancies and discontinuities of its structure” (Moropoulou 1995b, p. 762).
11.3.7 Modern Portland Cement
It is important to point out the differences between the ancient cements and modern
Portland cements. These differences begin with the materials and the manufacturing
techniques.
As previously mentioned, the reactivity of the pozzolana can be altered by
exposing the material to high temperatures before mixing it with the lime. This is
the key to the manufacture of modern Portland cement. Clay containing iron oxide
and calcium carbonate is fired to approximately 1400°C in a rotary kiln. The silica
begins to soften and fuse, resulting in the formation of “clinker”. This material is
mixed with gypsum and ground to form Portland cement powder. Portland cement
is extremely hard and has a high compressive strength. The chemical reaction that
produces modern Portland cement results in interlocking crystals. The structures
produced by pozzolanic cements are more difficult to analyze, since the product is
very fine-grained and not easily observed under a microscope.
Like its predecessors, the main catalyst for the chemical reaction in Portland
cement is colloidal silica. However, it is imperative to control the pH of the mixture
carefully in this modern material. In highly alkaline Portland cements, the silica
can become gelled by the lime (Ilers 1979). Gelling occurs when the silica particles
of the sol become linked together in branched chains. These chains fill the whole
volume of the sol so that there is no increase in the concentration of silica in any
macroscopic region in the medium. Instead, the overall medium becomes viscous
and then is solidified by a coherent network of particles that, by capillary action,
retains the liquid (Ilers 1979). In a Portland cement, the result of this process will be
swelling and cracking of the material.
11.4 Masonry
Masonry units can be laid dry or with mortar. For example, the Inca laid enormous
polygonal masonry blocks without any mortar. In some types of construction, mortar
does not play a role in the building’s structural strength. For example, in ancient