Geoscience Reference
In-Depth Information
L
IME BINDERS
Carbonated (hardened) lime binders have a distinctive
texture comprising finely crystalline calcite with
occasional pockets of coarsely crystalline calcite deriving
from the slow carbonation of trapped slaked lime. The
relative hydraulicity of the lime can be determined by
observing the quantity of small relict grains chiefly
comprising dicalcium silicate ('belite') (
285
). Nonhydrau-
lic limes have very few (if any) of these belite grains, with
the number of grains observed increasing through the
feebly hydraulic, moderately hydraulic, and eminently
hydraulic classes (British Standards Institution, 2000).
The binder of historic lime mortars can comprise
anything up to about 35% of lime inclusions (Leslie &
Hughes, 2002). Usually roughly spherical lumps of 50 μm
to 10 mm in size, these inclusions can consist of unmixed
lime lumps (
286, 298,
and
313
), typically consisting of
underburnt, partially burnt or overburnt quicklime (
287
and
288
). The presence of large (>3 mm sized) unmixed
lime lumps is sometimes interpreted to have been caused
by 'hot mixing' or 'dry slaking', a practice where damp
aggregate is mixed with roughly crushed quicklime
(Hughes
et al
., 2001). Underburnt or partially burnt
inclusions often exhibit the original limestone texture,
enabling identification of the geological source of the
limestone burnt in the kiln.
If the lime has been gauged with Roman cement or
Portland cement this will usually be detected by the
observation of unhydrated relict cement grains composed
of dicalcium silicate crystals and aluminous and ferrous
285
286
285
Relict grain of dicalcium silicate (belite, centre)
within a 1930s hydraulic lime render. The lime binder
appears brown and quartz fine aggregate particles are
shown white; PPT, ×300.
286
Lump of unmixed lime (light brown) within a
lime/Portland cement render. Quartz natural sand fine
aggregate particles appear grey/white and two small
unhydrated Portland cement grains are right of centre
(dark brown); XPT, ×35.
287
287
Partially burnt limestone inclusion within Roman
lime flooring concrete from archaeological
excavations at Piddington Roman Villa, England.
There is a burnt rim (dark brown) around the unburnt
core (lower, light brown); XPT, ×35.
