Mortars and Renders
The Relative Merits of Adding Cement
For many years those specialising in historic building repairs have
known the dangers of using hard, cement-based mortars. But the specialist
world has been split between those who advocated the use of small amounts
of Portland cement as an additive to a lime mortar and those who rejected
all cement additives. New evidence sheds light on the controversy, with
some radical conclusions.
The addition of cement to lime mortars is a widespread, almost traditional
practice, but few consider why it is done or the consequences. There
is also confusion over the substances and chemistry involved.
hydraulic lime hardens by a slow process of carbonation, reacting with
atmospheric carbon dioxide over a period of weeks. Hydraulic limes and
cements set rapidly by reacting with water in a matter of hours. A non
hydraulic lime can be made to set much more rapidly by the addition
of an hydraulic or 'pozzolanic' additive. This practice is known as
'gauging'. The additives include finely crushed brick powder, PFA, HTI,
pozzolana, trass or cement (white or OPC). These all contain finely
divided and therefore highly reactive silica and/or alumina, which are
the constituents necessary to obtain a rapid chemical set by reaction
with water. Of these, cement is by far the most widely used in this
country, and the cheapest. Typical proportions, commonly in use, are
1:1:6 (cement: non hydraulic lime: aggregate) and 1:2:9.
are, as one would expect, both advantages and disadvantages in gauging
non hydraulic mortars with cement to make them hydraulic.
imparts a chemical set which occurs before full shrinkage occurs,
thereby reducing the risk of cracking
may be built up more rapidly, without the need to wait a long time
for one to set fully before applying the next
hardens rapidly, thereby providing protection from rain before carbonation
has been completed. This helps to beat the inclement British weather
an artificial substance manufactured under closely controlled conditions,
it is reliable and predictable in use
is available in a choice of colours, useful when it is necessary
to match the colour of an existing mortar or render.
rapid setting time limits the time available to the user in which
to work with the gauged mortar
cements contain appreciable amounts of soluble salts, in particular
potassium sulphate, which may become a source of salt damage to
use of cement tends to lead to the user treating the gauged lime
mortar as if it were a fully hydraulic lime or cement. Too much
reliance on the initial chemical set leads to neglect of the importance
of the longer term carbonation of the non hydraulic component present
danger that segregation occurs, whereby the cement separates from
the lime as the mortar dries and hardens.
is a major hazard of gauging lime mortars with cement. As the mortar
sets, the cement colloid tends to migrate into the pores of the lime
mortar as they form, clogging them and leading to a greatly reduced
porosity. If the proportion of cement is high enough, segregation is
much less likely to occur, but the resulting mortar will be hard. If
the cement proportion is low, the mortar will be less hard, but segregation
is more likely to occur. The resulting mortar will be seriously weakened,
with a poorly formed pore structure leaving it very susceptible to frost
damage and deterioration, even after carbonation of the non hydraulic
lime present has taken place.
Smeaton Project, a research programme commenced by English Heritage
indicates that a 1:1:6 mix, containing a 50 per cent cement binder,
is unlikely to segregate, while a 1:2:9 mix, containing a 33 per cent
cement binder, is almost certainly at risk. Until recently it was considered
good practice to gauge lime mortars with as little as 5 per cent cement,
just enough to impart a chemical set but not enough to make the mortar
appreciably harder. However all of the Smeaton Project test samples
containing less than 25 per cent failed.
the possible hazards of segregation, an un-gauged lime mortar relying
solely on carbonation is likely to be more resilient in the long run
than one gauged with a small amount of cement. This will require care
in its application and careful nurturing to ensure that it carbonates
properly. If a chemical set is required, a safer alternative would be
to use an hydraulic lime. In these the hydraulic components are so closely
associated with the non hydraulic that segregation does not occur. These
tend to be hard and impermeable, but not usually as hard as a 1:1:6
mix. Brick dust is a cheap and highly effective pozzolanic additive,
providing a useful alternative to cement.
is not in itself harmful, but insensitive and indiscriminate use of
it is. It can be used as a useful pozzolanic additive to non hydraulic
mortars, but those specifying and using it should be clear why they
are doing so, and what its effects are likely to be. Given that it is
now widely accepted that mortar should be weaker and more porous than
the material that it is jointing or repairing, it is probably better
in most circumstances to rely on a good non hydraulic lime mortar using
well-matured lime putty and sharp and well-graded aggregate, applied
with care and subsequently well tended to ensure correct carbonation.
- J Ashurst and N Ashurst, Practical Building
Conservation Volume 3: Mortars, Plasters and Renders, English Heritage Technical Handbook, Gower Technical
Press, Aldershot, 1988
- W Czernin, Cement Chemistry and Physics for Civil Engineers, Bauverlag, Wiesbaden, 1980
Cements and Grouts used in the Conservation of Historic Buildings.
ICCROM, Rome, 1982
- B Induni and L Induni, Using Lime, Lydeard St Lawrence,
- JM Tutonico, I McCaig, C Burns, and J Ashurst, The Smeaton
Project AC1: Phase 1 Report, English Heritage, London, 1994
powder - High Temperature Insulation, a finely ground fireclay
containing reactive silica and alumina.
PFA - Pulverized Fuel Ash, a waste product of coal fired
power stations consisting of tiny granules of reactive silica. Effectively
an artificial volcanic ash.
Pozzolana - natural volcanic ash from Italy containing reactive
Trass - natural volcanic ash from Germany containing reactive silica.
This article is reproduced from The Building Conservation Directory, 1995
O'HARE MPhil, PGDip, AIFA
is the Conservation Manager for Wells Cathedral Stonemasons Ltd. After
a career in Italian archaeology he retrained at Bournemouth University
and currently provides a practical conservation and consultancy service
throughout the country.
Lime Mortars and Renders
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