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.