Geoscience Reference
In-Depth Information
As it is produced by heating at relatively low
temperatures, it is likely that gypsum was the earliest
deliberately manufactured cement, although exactly when
it was first used is not known. It was frequently used in
Egypt during Pharaonic times, extending back at least
4000 years, and it is found in the construction of the Giza
pyramids. When used as mortar, gypsum is relatively weak
and has limited adhesive power. As testament to this, the
Egyptians often used gypsum mortars more as a means to
slide large stone blocks into place, rather then to set them
together (Arnold, 1991). A drawback of gypsum is that it
is water soluble and therefore not suited to external use,
particularly in wet climates. Gypsum plaster was not
introduced into Britain until the 13th century when it was
brought over from France as plaster of Paris (Davey, 1961).
Gypsum plasters were then used sporadically as an
alternative to, or as an addition to, lime plaster. In the 20th
century, gypsum became the material of choice for internal
plaster applications in the developed world. Gypsum
plasters are perceived to have a number of advantages
over lime, including easily controlled setting time
according to function, less time delay between application
of coats, they do not shrink on drying, and have excellent
fire resistance.
A major development in binder technology was the
introduction of lime mortars with greater strength and
durability than gypsum. The earliest known examples
comprise lime wall and floor plasters from Neolithic sites
in the Near East dating from 7000-6000
BC
(Hughes &
Válek, 2003). Construction with lime spread through the
Middle East, North Africa, Europe, and the Far East, with
lime technology apparently developed independently in
Central America by the Mayans at around 1000
BC
(Hansen, 2005). The Greeks and Romans refined the use
of pozzolanic additives to lime, intentionally producing
hydraulic lime mortar for underwater works (Vitruvius,
trans 1960). In the UK, lime:sand mortars, plasters, and
renders using nonhydraulic or feebly hydraulic lime as
the binder proved to be durable over many centuries and
were used routinely until the late 19th century.
Natural cements are made from deposits of calcium
carbonate rich in clay minerals, such as septarian
nodules. They are quick setting, very hydraulic, and were
popular for use in civil engineering and stucco work in
19th century Britain (Kelsall, 1989). Portland cement,
patented in Britain in 1824 is produced by 'over burning'
a mix of calcium carbonate (limestone or chalk) and an
aluminosilicate (clay or shale) forming a very hydraulic
cement clinker.
With the development of natural and then hydraulic
cements, lime mortars became uneconomic from the
builder's viewpoint, as they set and harden slowly, and
were perceived to be unsuited for use in wet situations.
Cement gauged or pure cement mortars with their rapid
hardening and high strength properties then superseded
lime mortars and renders. Portland cements are now the
dominant binder for masonry mortars and external
renders in new buildings of the developed world.
Despite the phenomenal growth in its use, the generic
Portland cement has a number of shortcomings,
restricting its usage in certain ways. A wide range of
other ingredients has been combined with Portland
cement to improve its performance when used for special
applications. Specialist mortars have been developed for
a plethora of special circumstances, including concrete
repair, grouting, tiling, mortar jointing, rendering, self-
levelling floor finishes, and water-leak stoppers. These
tend to be complicated premixed formulations containing
multiple ingredients that are manufactured commercially
using confidential mix designs. In the 20th century,
cement-rich mortars were used in the repair and
restoration of historic buildings, where they had most
often not been part of the original fabric. The resulting
lack of compatibility between dense, impermeable
cement-based repairs and traditional construction built
in, and protected by, lime-based materials has resulted
in an unacceptable level of damage to historic buildings.
In the last few decades there has been a renewal of
interest in the use of lime, owing to recognition of its
superior properties of vapour permeability (breathability),
flexibility, and appearance (Induni, 2005). It is now
recognized that repair of historic masonry structures
should be carried out with a traditional craft-based
approach, ideally using materials similar to those used
originally to ensure compatibility (British Standards
Institution, 1998).
Since the 1990s there has been a modest revival of use
of lime mortars for new masonry buildings in the UK.
Using hydraulic lime enables rapid construction and does
away with the movement joints needed to cope with the
shrinkage of strong Portland cement mortars (Beare,
2004). With climate change becoming a global concern,
lime binders also have the potential to regain ground as
they are more environmentally friendly (Pritchett, 2005).
