Natural Science in Archaeology

264 11 Building, Monumental, and Statuary Materials
Gypsum can also be added to mortars and cements. A gypsum mortar ideally has
25–30% gypsum. Gypsum itself often contains clay impurities that contribute to
the chemical properties of the resulting mortar (Almagro 1995). Gypsum mortars
develop an interlocking crystal structure in the presence of water (Adams et al.
1992). Gypsum used in this manner controls shrinkage and slows setting time in
mortars and cements (Lechtman 1986). The slower the setting time, the stronger the
mortar or cement will be. Gypsum-lime mortars have been shown to be superior in
strength to lime mortars, especially in tension.
11.3.3 Aggregates
Aggregate is a mass of rock or mineral particles used for mixing with a cementing
material to form mortar or plaster. Aggregates can include bits of crushed pottery
or terracotta (grout), crushed stone, or even debris from old buildings (Lechtman
1986). However, the most common aggregate used in cements and mortars is quartz
sand. Vitruvius divided sand into three categories: sea sand, river sand, and pit sand.
He recommended the use of the latter, because the other two categories can contain
harmful impurities. This is particularly true of sea sand containing salt, which can
recrystallize in the mortar or concrete causing accelerated deterioration.
Ideally, a binder is mixed with the aggregate in a proportion that will fill all
the voids present in the volume of the aggregate. The ratio of voids will increase
according to the aggregate’s coarseness and uniformity of size. Relatively coarse
particles of varying sizes are preferable for filling in voids and producing a strong
mortar (Ashurst and Dimes 1998).
Aggregates used as filler have an important influence on the final physical properties
of mortars and cements. In addition to potential reactivity, aggregates also
influence porosity and strength. The qualities that the aggregates impart to mortars
and cements are based on mineralogical composition and the size and shape of the
aggregate particles, as well as preparation of the material before it is added to the
mortar or cement mixture.
Aggregates can be either reactive (i.e., pozzolanic) or nonreactive (see Sect.
11.3.4). The level of reactivity depends on the amount of amorphous silica and alumina
in the aggregate as well as the size of the particles. Smaller particles increase
the surface area with which the lime can react chemically. Thus, a cement with
small particles of a reactive aggregate is stronger in compressive strength than a
mixture of the same composition with larger particles. The use of well-sorted small
volcanic aggregates insures maximum reactivity.
Vitruvius recommended washing aggregates before using them in cements. This
turns out to be very important. Unwashed volcanic sands contain large amounts of
nonreactive earth materials. The Romans added the volcanic sand based on proportional
measurements. The use of unwashed sand could thus result in the inclusion
of a smaller amount of reactive material, resulting in an inferior grade of cement
(Lechtman 1986).