When water is withdrawn from concrete mass stored in the unsaturated air, drying shrinkage is occurred. This is a consequence of moisture movement, a part of which is reversible due to altering of wet and dry condition.Some portion is irreversible and clear distinguishing is required. For this we have to learn about mechanism of shrinkage.
The simple relation of change in volume with removal of water is not valid here i.e. volume change is not equal to removal volume of water. The early loss of free water doesn’t result shrinkage, if any, is very small.
When drying continues, water adsorbed in hydrated cement paste is continued to remove. At this stage, the unrestrained paste loss its volume equal to (aprox.) loss of water layer having thickness of one molecular space from surface of gel particles as a whole.
Water molecule has thickness is considered as 1% of particles size of gel which indicates a linear dimension change of cement paste upon complete drying. The actual value observed was up to 400 X 10-6 but theoretically these may be 10000 X 10-6.
But specific surface of cement paste cured by high pressure steam is low; the microcrystalline structure suffer lesser shrinkage of (5~10) times than normally cured cement paste. Sometimes 17 times less shrinkage are observed. The coarse natural stone shows less drying shrinkage relative to fine grained stone, which is the example of influence of gel particles on drying.
We can relate shrinkage or fraction of it with removal of water from intercrystalline structure. It is found to change of space between lattice of hydrate of calcium silicate from (1.4~0.9) nanometer on drying. Hydrated tetra calcium aluminate & calcium sulfo-aluminate also show identical behavior but at different extent. Thus we are not sure that whether intra-or-intercrystalline moisture movement produces shrinkage. But as both Portland cement and high-alumina cement, sometimes pure ground granulated calcium mono aluminate also used to make paste and all exhibit similar behavior especially in shrinkage
When drying continues, water adsorbed in hydrated cement paste is continued to remove. At this stage, the unrestrained paste loss its volume equal to (aprox.) loss of water layer having thickness of one molecular space from surface of gel particles as a whole.
Water molecule has thickness is considered as 1% of particles size of gel which indicates a linear dimension change of cement paste upon complete drying. The actual value observed was up to 400 X 10-6 but theoretically these may be 10000 X 10-6.
But specific surface of cement paste cured by high pressure steam is low; the microcrystalline structure suffer lesser shrinkage of (5~10) times than normally cured cement paste. Sometimes 17 times less shrinkage are observed. The coarse natural stone shows less drying shrinkage relative to fine grained stone, which is the example of influence of gel particles on drying.
We can relate shrinkage or fraction of it with removal of water from intercrystalline structure. It is found to change of space between lattice of hydrate of calcium silicate from (1.4~0.9) nanometer on drying. Hydrated tetra calcium aluminate & calcium sulfo-aluminate also show identical behavior but at different extent. Thus we are not sure that whether intra-or-intercrystalline moisture movement produces shrinkage. But as both Portland cement and high-alumina cement, sometimes pure ground granulated calcium mono aluminate also used to make paste and all exhibit similar behavior especially in shrinkage
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