What is the role of paraffin emulsion and polyvinyl alcohol (PVA) on the water resistance of gypsum?

With the increase of the amount of paraffin added, the water absorption rate of gypsum decreases significantly, because the paraffin wax forms extremely fine spherical particles after emulsification, which are suspended in water and become a continuous phase emulsion of oil-in-water. When the paraffin wax emulsion and the gypsum slurry are uniformly mixed After that, the hydrophobic substances in the emulsion are immediately dispersed in the continuous phase of the gypsum slurry. When the hemihydrate gypsum slurry is coagulated and hardened, it will absorb the water in the surrounding hydrophobic substances, causing the hydrophobic substances after water loss to agglomerate into one. The layer of waterproof film is adsorbed in the microporous wall and fine network of the gypsum hardened body structure. The paraffin wax emulsion plays the role of coating the gypsum particles in the gypsum slurry, thereby reducing the water absorption rate. In addition, the addition of paraffin emulsion can also reduce the porosity of gypsum, increase the compactness of gypsum, and reduce the water absorption of gypsum.

The addition of PVA also reduces the water absorption of gypsum as a whole. With the increase of the amount of PVA, the water absorption does not change much when the amount of PVA is 0.3% to 5%. With the increase of the amount, the water absorption decreases. The amplitude starts to increase. This is because PVA is an organic polymer that can dissolve in water and form a gel. When the gypsum slurry is fully stirred, the polyvinyl alcohol shrinks, and the gel is uniformly dispersed in the gypsum slurry and formed in the gypsum slurry. The network structure reduces the water absorption of gypsum. The water absorption rate of gypsum showed a decreasing trend with the increase of the amount of water reducing agent added, and the water absorption rate reached the lowest when the amount of water reducing agent was 0.7%.

When the amount of borax added is less than 0.01%, the water absorption rate of gypsum gradually increases, and the water absorption rate of gypsum increases gradually, and then gradually increases. Because borax is an inorganic retarder, it can play a retarding role in gypsum. In addition, since polyvinyl alcohol is very sensitive to borax, adding a small amount of polyvinyl alcohol can greatly reduce the influence of borax on the fluidity of gypsum slurry. Adding borax to the gypsum in an appropriate range will increase the flexural strength of the gypsum test block, while the water absorption of the gypsum tends to increase slightly.

The change of admixture content has obvious effect on the absolute dry strength of gypsum, and the increase of the content of paraffin and PVA reduces the absolute dry strength of the gypsum test block. The gypsum particles are surrounded by paraffin wax emulsion and PVA polymer before immersion and condensation, which isolates the effect of gypsum and water and hinders the normal development of the crystal structure of dihydrate gypsum. A large number of defects are generated between them, resulting in a decrease in the absolute dry strength of gypsum crystals.

The addition of paraffin and PVA increased the softening coefficient of gypsum, and the softening coefficient gradually increased with the increase of the content of the two.

The most important indicators of gypsum water resistance are softening coefficient and water absorption rate. The orthogonal test data is compared with the blank gypsum sample data. When adding paraffin 3.0%, PVA 0.3%, water reducing agent 1.0%, borax 0.02%, The softening coefficient of gypsum is greatly improved, and the strength retention rate is large. Using this mixing ratio scheme, the softening coefficient of gypsum is increased from 0.362 to 0.835, and the water absorption rate at 72h is reduced by about 8%, the dry strength retention rate is close to 70%, and the wet strength is also greatly improved (50%). When adding paraffin wax 5.0%, PVA 0.1%, water reducing agent 1.0%, borax 0.01%, the softening coefficient is increased by 0.05, although the softening coefficient increase is not obvious, but the water absorption rate at 72h is reduced by about 9%, and the bending resistance and resistance The overall compressive strength has been improved, of which the wet flexural strength is increased by 25%, the wet compressive strength is increased by 20%, the dry flexural strength is increased by 32%, and the dry compressive strength is increased by 16%.