Building Materials, Third Edition

PPR f

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The strength of mortar from slaked lime is low: after a month of hardening the compressive
strength becomes 0.5–1 N/mm^2 , rising to 5–7 N/mm^2 after several decades. This is due not only
to greater carbonisation of mortar or concrete, but also to a certain interaction of silicate and
carbonate aggregates with calcium oxide hydrate.
High strength concretes and mortars (30 to 40 N/mm^2 ) can be obtained by artificial
carbonisation. Concretes from ground unslaked lime with addition of up to 0.2 per cent (by
weight) of lime which speeds up carbonization and increases strength are particularly effective.

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It is a gradual conversion of lime mortar and concrete mixes from ground unslaked lime into
a rock-like hard body, resulting from the interaction of lime with water and the formation of
calcium oxide hydrate. First, lime dissolves in water to give a saturated solution, which over-
saturates rapidly because water is absorbed by the remaining unslaked grains. Rapid and
strong over-saturation of a mortar, prepared from unslaked lime, results in formation of
colloidal masses, which appear because calcium oxide hydrate formed by mixing lime with
water consists of particles very close in size to those of the colloids. Colloidal calcium hydrate
coagulates quickly into a hydrogel which glues the grains together. As water is partly sucked
in by the deeper layers of grains and partly evaporates, the hydrogel thickens and thus
increases the strength of the hardening lime. The hydrogel formed in the process of hardening
of slaked lime holds much water and its adhesiveness is poor, which is not so for hardening
unslaked lime. As slaking lime hardens, crystallisation of calcium oxide hydrate increases its
strength. Subsequent carbonisation of calcium oxide hydrate improves the strength of the
hardened mortar.
Thus, mixing of ground unslaked lime with water brings about a hydration hardening,
which is characteristic of other binding materials as well; it consists of the hydration of calcium
oxide and subsequent formation of colloids and crystallisation of the hydration product.
Hardening at normal temperature is also affected by the evaporation of free water in the
process of drying and natural carbonisation.
The conditions which favour hydration hardening are: rapid and uniform extraction of heat
released in the process of hardening, the use of forms to prevent the increase in volume of the
hardening mass and the introduction of admixtures to retard hydration. The coagulation
structure which appears in the process of hydration hardening is retained and serves as a
medium for the crystallisation of new hydrate formation. Should the coagulation structure
disintegrate because of a rise of temperature or increase in volume, the new structure will have
no time to appear because of a high hydration rate of the lime, and the recrystallisation ends
inside non-intergrown particles of lime. Hydration hardening may be improved by uniform
burning and grinding of lime.

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When lime-sand mixtures are treated by high-pressure steam (8-16 atm) corresponding to
temperatures between 175 and 200°C, lime and silica interact in the autoclave and form
calcium hydrosilicate which ensures high strength and durability of manufactured items.
In the autoclave method of hardening lime-sand materials, lime does not play the part of a
binding material, whose hydration and carbonisation gives rise to a stony body of required
strength at usual temperatures. In the given instance, lime is one of the two components that
interact and form calcium hydrosilicate which is the chief cementing substance. The required