04-08-2012, 11:39 AM
MICROSTRUCTURE OF CAST GYPSUM
[attachment=30170]
• The set material consists of a tangled aggregate of monoclinic gypsum crystals, usually acicular in shape, with lengths in the range of 5 to 20 micrometers.
• The aggregate exhibits two types of inherent porosity on a microscopic scale.
1. Microporosity caused presence of residual un reacted water .These voids are roughly spherical and occur between clumps of gypsum crystals.
2. Microporosity resulting from growth of the crystals. These voids are associated with setting expansion and are smaller than first type, they appear as angular spaces between individual crystals in the aggregate.
THE EFFECT OF WATER -POWDER RATIO:
Both type of porosities are affected by W/P ratio of the mix, but in opposite ways.
I. A low W/P ratio that is , a thick mix leaves less residual water in the set mass and decreases the first type of porosity.
II. A low water powder ratio increases the effect of the crystal growth during setting , because available nucleation centers are concentrated in a small total volume of mix ,interaction of growing gypsum crystals occurs earlier and is more effective so the amount of second type of porosity is increased.
III. In any W/P ratio ,the total proportion of inherent porosity in the set mass is the sum of these two types ,
Inherent porosity represents about 40% of total cast volume at water-powder ratio 0.50, and about 20% at W/P ratio of 0.25 (lautenschlager and corbin,1969)
VOLUMETRIC CHANGES DURING SETTING
• Theoretically, calcium sulphate hemihydrate should contract volumetrically.
Explanation:
If equivalent volumes of hemihydrate , water and reaction product ( dihydrate) are compared ,the volume of dihydrate formed will be less than the equivalent volumes of hemihydrate and water.
This represents a linear change in the gypsum object approximately 2.4%
• However setting expansion is observed instead, this phenomenon can be rationalized on the basis of the crystallization mechanism.
[attachment=30170]
• The set material consists of a tangled aggregate of monoclinic gypsum crystals, usually acicular in shape, with lengths in the range of 5 to 20 micrometers.
• The aggregate exhibits two types of inherent porosity on a microscopic scale.
1. Microporosity caused presence of residual un reacted water .These voids are roughly spherical and occur between clumps of gypsum crystals.
2. Microporosity resulting from growth of the crystals. These voids are associated with setting expansion and are smaller than first type, they appear as angular spaces between individual crystals in the aggregate.
THE EFFECT OF WATER -POWDER RATIO:
Both type of porosities are affected by W/P ratio of the mix, but in opposite ways.
I. A low W/P ratio that is , a thick mix leaves less residual water in the set mass and decreases the first type of porosity.
II. A low water powder ratio increases the effect of the crystal growth during setting , because available nucleation centers are concentrated in a small total volume of mix ,interaction of growing gypsum crystals occurs earlier and is more effective so the amount of second type of porosity is increased.
III. In any W/P ratio ,the total proportion of inherent porosity in the set mass is the sum of these two types ,
Inherent porosity represents about 40% of total cast volume at water-powder ratio 0.50, and about 20% at W/P ratio of 0.25 (lautenschlager and corbin,1969)
VOLUMETRIC CHANGES DURING SETTING
• Theoretically, calcium sulphate hemihydrate should contract volumetrically.
Explanation:
If equivalent volumes of hemihydrate , water and reaction product ( dihydrate) are compared ,the volume of dihydrate formed will be less than the equivalent volumes of hemihydrate and water.
This represents a linear change in the gypsum object approximately 2.4%
• However setting expansion is observed instead, this phenomenon can be rationalized on the basis of the crystallization mechanism.