Hydraulicity
is the property of a binder to harden in contact with water. |
Hydraulicity is produced by burning a limestone containing silica, alumina and
iron oxides which above certain temperatures combine, totally or partially, with
the Calcium Oxide. The resulting silicates, aluminates and ferrites give hydraulic
properties to the product. Today as in the past, natural building limes are obtained
by burning and slaking limestone and the more or less hydraulic character of the
finished product is directly related to the percentage of calcium silicates, aluminates
and ferrites formed during burning. The composition of the Earth crust shows the
predominance of silica and its presence is almost inevitable in all limestone
deposits.
The building limes of the past, if the soluble (combined) silica
content is analysed, will almost certainly show some hydraulic property, even
if very feeble. The analysis of historical mortars today rarely takes this factor
into account and, as sometimes the amount of combined silica in a mortar is minute,
a number of findings will not identify the hydraulic component in the mortar.
For example: an amount of 4% of combined silica in a binder represents, in a typical
mortar with a 17.5% binder content, only about 0.7 % of the total mass of the
mortar but still this mortar will be feebly hydraulic. See example below based
on an NHL 2 with a binder/sand ratio of 1:2.5:
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The
existence of pure Calcium Carbonate deposits is not common. High Calcium limes
are mainly exploited for industrial use (i.e. steel industry), where it is essential
to have an almost pure material. Even in metamorphic type calcareous stone such
as marble, silica is found. The little amount of Silica required to combine with
the CaO during burning makes the production of Hydraulic properties almost inevitable
when the raw material is a calcareous stone. | This
mortar will be almost certainly classified as non hydraulic by most analysts.
If a "match" is required, this might be erroneously made by adopting
a non hydraulic high calcium lime instead of a feebly hydraulic lime. |
|
Elasticity
| A factor in building without construction joints.
Important in diminishing shrinkage and cracking. Allows for minor movements. |
Permeability | Good
vapor exchange qualities allow for condensation dispersion. No rot. Great benefits
to the living environment. |
Resistance to
salts | The absence of any potentially damaging addition
(i.e. gypsum or cement) make sulphate attack, alkali-silica reactions impossible.
Existing salts in the building fabric will pass through and eventually can be
washed off. Excellent performance in marine environment. |
Suitable
Compressive Strength | Unlike cement or cementitious
mixes (1:1:6 etc..) the compressive strength will be achieved gradually, allowing
for movement. The availability of a range will permit the making of mortars with
the required strength without having to add or blend. |
Resistance
to weather | Early setting will provide quicker protection
from adverse weather. |
Self Healing | The
available lime provides this quality. A timely light water mist over a minor shrinkage
mark will help to heal it. |
Resistance to
Bacteria and Vegetable growth | The alkalinity
of the binder does not favour their development |
Insulation | The
porosity of the mortar present good insulation values. |
Sand
color | The whiteness of the NHL binders will reproduce
the color of the aggregate used |
Reworking | All
St. Astier mortars can be reworked (8 - 24 hours), reducing wastage and increasing
work speed. This is due to the absence of cement, gypsum or pozzolans. |
Recycling | Materials
built with NHL mortars can be reused. |
CO2
absorption | probably the most Eco friendly contribution
of using limes. Damaging CO2 is re-absorbed during
the carbonation of the free lime. |