Hydraulic lime: an introduction
Practical Action
concrete. It is generally not recommended for steel-reinforced concrete or for ferrocement
unless, preliminary tests have been carried out and/ or expert advice sought. When preparing
a mortar mix note should be taken of any sandy material already contained in the lime and
only sufficient sand added to produce the desired properties of the mortar mix. Normally 1
part hydraulic lime is added to either 2.5,3 or 4 parts sand by volume depending on the
characteristics required. Four hours is a typical time during which a hydraulic lime mortar is
usable, but mildly hydraulic lime mortars might still be usable considerably later. It is
possible to add a pozzolana if an increase in strength is desired provided that the pozzolana is
mixed in well with the lime before adding sand and water. The plasticity of a hydraulic lime
mortar is intermediate between that of ordinary lime and Portland cement.
Chemical composition, fineness, setting time, compressive strength and soundness are
important properties of cementitious materials. A comparison of values of these properties for
hydraulic lime, drawn from American (ASTM) and Indian standards, is presented below. The
main controlling factor of the degree of hydraulicity or cementing power of a lime is the silica
to lime ratio, i.e. SiO2/CaO or, a more specific expression, the cementation index (CI). The
latter takes into account other minerals which might be present in the lime. Based on the
cementation index hydraulic limes have been classified as follows:
Type
Mildly hydraulic
Moderately hydraulic
Eminently hydraulic
CI
0.3 -0.5
0.5 -0.7
0.7- 1.1
CI is defined as
2.8 x %SiO2+1.1 x %Al2O3+0.7x%Fe2O3
%CaO+1.4x%Mg
The Indian standard recognises only two types of hydraulic lime: Class A, - eminently hydraulic
for structural purposes, and Class B, -semi-hydraulic for masonry use. Hydraulic limes can be
expected to attain compressive strengths of 0.5 to 1.0MPa (or N/mm2) after seven days and 3
to 7MPa after six months for a standard 1:3 lime to sand mix. More exact values will depend
on the degree of hydraulicity and processing characteristics. In comparison Portland cement
will attain compressive strength of 17MPa after 7 days and 28MPa after six months.
Property
Minimum calcium &
magnesium oxides, %
Maximum calcium &
magnesium oxide, %
Maximum magnesium
oxide, %
Minimum silica, alumina
& ferric oxide, %
Maximum cementation
value
Minimum cementation
value
Fineness
Setting time
Indian Standard
Class A (hydrated)
60
5
25
0.6
No residue 2.36mm
sieve, not more than
5% on 850 μm sieve
and not more than
10% of fraction
passing 850 μm sieve
on 300 μm sieve
Within 2 hours to
initial set and within
48 hours to final set
Class B (hydrated)
70
5
15
0.6
0.3
As for Class A
American Standard
65
75
16
Residue of not more
than 0.5% on a
600μ m sieve and
not more than 10%
on a 75 μm sieve
Within 2 hours to
initial set and within
24 hours to final set
3