Natural Science in Archaeology

266 11 Building, Monumental, and Statuary Materials
Reactive silicas tend to become “colloidal” when exposed to highly alkaline substances
such as lime. Colloidal silica refers to stable dispersions or sols of discrete
particles of amorphous silica. Ideally, reactive silica has a high specific surface area
that enables it to be absorbed into the chemical reaction. Put another way, the aluminosilicates
that comprise pozzolanic sands are chemically reactive, in part because
the grains of sand preserve the highly porous internal structure and large surface
area that results upon solidification of the molten, gassy magma generated during
volcanic eruption (Lechtman 1986).
11.3.5 Natural Pozzolana
There are a variety of materials that will produce hydraulic reactions of varying
intensity. Pozzolana can be divided into two groups: natural and artificial. Natural
pozzolana includes diatomaceous earth, pumice, volcanic tuffs, and some shales.
Natural pozzolana can be further subdivided based on how it formed. In one
group of natural pozzolanas, the main component is glass produced by fusion. This
group includes volcanic ashes and tuffs, pumice, scoria, and obsidian. The second
group of natural pozzolanas includes those derived by the weathering of rocks. The
ingredients common to all natural pozzolana are reactive silica and alumina.
The proportion of silica has a significant impact on the strength of the resulting
cement. For example, pumice and tuff have a high reactivity since they are high in
silica. Basalts and other mafic volcanic rocks will not react as well, because they are
too low in silica. Rocks that resulted from explosive volcanism are generally higher
in silica than those that resulted from lava flows.
In pozzolanic materials derived from rock weathering, the “silica constituent
contains opal, either from precipitation of silica from solution or from the remains
of organisms. Examples of these are diatomaceous earth, chert, opaline silica, lava
containing substantial amounts of glassy component, and clay which has been
naturally calcined by heat from a flowing lava” (Hill et al. 1992). Again, not all
varieties of these materials will make good pozzolana; it depends on their specific
constituents. Most natural pozzolanas owe their activity to one or a combination of
five substances: (1) volcanic glass, (2) opal, (3) clay minerals, (4) zeolites, and (5)
hydrated oxides of aluminum (Price 1975).
Certain types of limestone can also be pozzolanic by themselves. These limestones
are used to manufacture “pozzolanic” or “natural” cements. Lea (1956)
gives the following definition: “Natural cements are materials formed by calcinating
(heating) a naturally occurring mixture of calcareous and argillaceous (clay)
substances at a temperature below that at which sintering (in a fusible powder, just
below the melting point, particles do not melt but are fused together) takes place.”
Natural cements obtain all of their physio-chemical properties from the limestone
without the aid of other pozzolanic additives (Insley 1955:197).
The production of lime cement appears to go back about 9000 years at Galilee
in Israel where a mortar floor was discovered (Bentur 2002). The floor consisted