Building Materials, Third Edition

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strength results not from the physical cohesion of the binder hydrate formations with the
grains of the aggregate, but from chemical interaction between the chief components of the raw
materials, lime and quartz sand.
Hardening of autoclave steam-cured lime-silica materials is due to complex physical and
chemical processes in three stages:

1. Formation of crystalline nuclei of hydrosilicates, growth of crystals and increase in their
   number, without any coalescence taking place.


2. Formation of a crystalline concretion.


3. Failure (weakening) of concretion due to recrystallisation contacts among crystals.
   New formations, whose number and composition vary continually harden in the process of
   hydro-thermal curing of items.

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It is obtained by adding hydrated lime to water, stirring to the consistency of a thick cream and,
allowing it to stand and mature for a period of about 16 hours in the case of non-hydraulic lime
before using. The putty so obtained should be protected from drying out.

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The hydrated lime is first thoroughly mixed and ground with the required quantity of sand.
Then water is added and thorough mixing is done. The mix is kept to mature for about less than
16 hours in the case of hydraulic lime. Coarse stuff should be protected from drying out till it
is used.

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A sample of lime is examined for its colour and lumps:

1. white colour indicate fat or pure lime.


2. lumps of lime indicate quick lime or unburnt lime.

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The field tests usually performed on lime at site are as below:

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 X Hydraulic limes are brush grey, brown, or dark coloured. Hydraulic
limestones have a clayey taste and give out earthy smell. White colour of lime is an indication
of pure variety of limestone. Shining particles on the surface of limestones indicate the presence
of free salt.

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X A handful of mortar is thrown on the surface on which it is to be used. The
area covered by the mortar and its quantity is recorded. These data indicate the workability of
the lime mortar. It is a very crude field test performed with the actual mortar.