Civil Engineering Reference
In-Depth Information
slowly on exposure to air (carbonation). These limes also set somewhat stronger, and have
a lower level of permeability to water and water vapour than air limes. These properties
made these types of lime sought after for use in certain applications where a hard set, a fast
set, a set under water or resistance to moisture ingress was required: mainly for signific-
ant structural engineering works and for bridges, dams, docks and canals, water tanks and
drainage systems. The limes were also extensively used for foundations in damp soils, ex-
ternal renders in damp climates, plasterwork in bathrooms, and as a masonry mortar with
certain types of stone (hard stones). Since the 1950s, ordinary Portland cement has been
widely used for most of these applications (see page
47
).
Because they set through hydration in the presence of water, these limes are known as 'hy-
draulic limes'. In early times, the hydraulic properties of the various types of stone were
usually ascribed to visible characteristics, such as the colour of the stone, but from the
eighteenth century onwards there was an explosion of interest and research into the science
of building limes, and it emerged that the hydraulic properties were in fact due to the pres-
ence of impurities, usually in the form of clay, in certain seams of stone. These impurities,
when fired appropriately in a kiln, combine with the lime to form active compounds which
in turn react with water to produce the hydraulic set. This set is
in addition
to the setting by
carbonation of any remaining 'uncombined' or 'free' limes. These limes are always slaked
to a powder because clearly, being hydraulic, they cannot be stored under water like a putty
lime.
The impurities present in the stone that create hydraulic compounds may be soluble silica
(SiO
2
), which is the most active, alumina (Al
2
O
3
), or ferric oxide (Fe
2
O
3
); the presence of
ferric oxide gives a slightly 'buff ' colour to those hydraulic limes when they have set.
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