Post on 13-Apr-2015
description
Lime Pozzolana cement mortars
by Raghu.T
Faculty of DSCE
Introduction
Nearly half the cement in a building is used for
low strength applications like plasters & mortars
where strength requirements is often as low as
3Mpa.
Contact b/w brick & mortar & the bonding of
brick & mortar are very much improved by the
addition of lime.
Raw Materials for lime pozzolana cements
The basic raw materials for lime pozzolana cement are :
Lime stone & Pozzolana materials
Lime stone
It is one of the basic raw material for all lime
mortars.
Lime stone is essentially calcium carbonate in
nature.
Lime stones are available in 3 forms:1. Natural lime stone2. ‘Kankar’ Lime stone3. Shell lime
Natural lime stone: it is one of the pure form of
calcium carbonate occurring in nature.
‘Kankar’ lime stone:Calcium carbonate is mildly
soluble in water & can be leached out by water &
ultimately deposited with clay as an impurity in the
form of ‘Kankar’ lime stone.
Shell lime : An organic source of lime stone in the
form of shell lime which is usually available in
Coastal areas.
Bureau of Indian standards for Lime use in buildings
The lime used for building activities should
satisfy the guide line drawn by BIS.
IS:712-1973: Specification for building
limes.
IS:1514-1959: Methods of sampling & test
for quick
lime & hydrated lime.
IS:6932-1973: Method of tests for building
limes.
Types of mortars using lime
Two types of mortars can be prepared by using lime they are:
MORTAR
Lime mortarFat lime mortar
Hydraulic lime mortar
Lime pozzolana mortar
Fat lime mortar
when lime stone is of high level of purity the
resulting lime is fat lime.
This type of mortar is obtained by mixing
pure lime with sand.
This type of mortar does not need any
moisture for setting, it sets by absorbing CO2
from the atmosphere.
Hydraulic lime mortar
When the lime stone has clay like impurities, the burning
process leads to hydraulic lime, which is actually mixture of
Calcium oxide & burnt clay.
Mixture of hydraulic lime + sand gives hydraulic lime mortar.
This type of lime sets in the presence of moisture & the
Calcium hydra-oxide reacts with the amorphous silica &
alumina present in burnt clay.
This type of mortar has to be cured by keeping it moist for at
least 28 days.
Hydraulic lime mortars have better strength than fat lime
mortars.
The chemical reactions in the process of lime stone for mortar
The following chemical reactions take place in the processing of lime stone for mortar are:
1.Burning of lime stone Caco3 Cao + Co2 2. Slaking of quick lime Cao + H2o Ca(OH)2 + Heat 3. Setting of fat lime Ca(OH)2 + Co2 Caco3 + H2O
Pozzolana
It’s a material, which is consists of amorphous silica
and alumina.
Its not a cementitious by it self , but forms
cementitious compounds when it combines with
Calcium hydroxide at ambient temperature in the
presence of moisture.
A mixture of finely ground calcium hydroxide and
pozzolana is hence known as a lime-pozzolana
cement.
Commonly used pozzolana’s
The following sources of pozzolana are commonly used :
1. Powdered burnt clay.2. Fly ash3. Rice Husk ash.4. GGBS.
Powdered burnt clay can be made either by
artificially burning clay in a kiln or selecting wastes
by burnt clay materials like bricks, tiles etc…,
Fly ash as pozzolana is usually obtained from thermal
power stations which are pulverized coal as the fuel.
Rice husk ash is a natural source of silica. Normally
rice husk contains about % silica & the rest of it is
combustible materials.
Manufacturing process
An intimate mixture of lime & pozzolana will set in
the presence of water, forming calcium silicate &
calcium aluminate compounds.
In the traditional method lime-surkhi mortar,
slaked lime & brick powder used to be mixed &
ground in the presence of moisture in bullock
driven mortar mills. The ground moist mixture is
then directly used in construction.
Unlike portland cement, which is already a complete
cementitious product, lime pozzolana cement needs an
additional step of mixing lime & pozzolana.
The effeciency of lime pozzolana cements depends on the period
of mixing of lime & pozzolana.
According to the studies made at Karnataka Engineering
Research Station , K.R. Sagar Dam site, it was found that the
mortar strength reaches a maximum when the duration of
grinding is 35 min.
Grinding for longer or shorter durations leads to lower strengths
Two alternative techniques for the processing
of lime pozzolana cement by the Department
of Civil Engineering, IISc. They are:
a. Dry blending of lime & pozzolana
b. Wet blending of lime & pozzolana
Dry Blending of lime and pozzolana
In this method, the slaked lime & pozzolana should be
separately processed as dry powdered.
The pozzolana may be dry ground to fineness of 90 µ.
The two powders may now be blended in a ball mill for a
duration of about 1 hr.
In this method, the size reduction of lime & pozzolana is
carried out first & mixing the two is carried out
subsequently.
This procedure also means that the dry mixture of lime
& pozzolana may have to be stored in bags.
The blended dry mixture of lime & pozzolana should not
be stored for more than 14 days if strength reduction is
to be less than 10%.
Mix stored in gunny bags leads to the reduction of
strength to 60% for 45 days & 35 % for the storage in
impervios plastics.
Wet blending of lime and pozzolana
In this method, the pozzolana has to be produced & ground to
90µ fineness & stored in gunny bags.
The pozzolana can be stored indefinetely.
Slaked lime may be freshly produced by slaking quick lime at site.
The slaked lime & well ground pozzolana may now be mixed in a
sample drum or hand operated concrete mixer in the presence of
water.
After blending the two for about 20min, the lime-pozzolana slurry
may be poured over sand & mixed thoroughly to get the lime-
pozzolana mortar.