Gypsum building material has excellent fire resistance, air permeability and sound absorption property. The excellent appearance of white color and smooth structure is good for decoration, combined with lightweight and relatively low prices, and gypsum building materials were developing very rapidly in recent years. Currently, the gypsum building material is mainly used to produce plasterboard, ceiling, building blocks, plastering powder and gypsum-based self-levelling materials etc. However, the application of gypsum building material was also limited because it is not good at frost-resistance, water-resistance and hot-resistance. This experiment mainly aimed at improving the water-resistance by adding methyl hydrogen silicone fluid into gypsum and its softening coefficient.
1. Raw Material
Gypsum board supplied by one of the Chinese manufacturers, details show in table 1 and table 2.
Water-Proofing Agent: Methyl hydrogen silicone fluid (Polymethylhydrosiloxane, CAS NO. 63148-57-2), offered by Wuhan Visight International Trade Co., LTD.
2. Test Method
2.1 Testing of Strength and Softening Coefficient
According to GB/T 17669.3-1999 (Building gypsum mechanical properties determination method), the strength test was going to begin immediately after forming 40x40x160mm gypsum block, ageing and drying to constant weight in 40℃ oven. Softening coefficient is a specific value between the strength of the gypsum block in water saturation status and drying status. Typically, the gypsum block will get water saturation status in 2 hours after immersing water.
2.2 Water Absorption Test
Weigh out the constant weight of the gypsum block and write it as B1. After that, put it into 17℃ and 23℃ water, immerse it 30mm, without clinging together with bottom and wall of the water tank. Take out the gypsum block after 2 hours soaking in water and towel off the moisture in surface. Weigh out it as B2 and work out the water absorption rate, accurateness adjusting to 1%.
3. The Results and Analysis
3.1 The Proportion of Gypsum and Water Influences the Water Absorption Rate
Weigh out 300g of gypsum and stirs an amount of water make it into gypsum slurry, drying to constant weight in 45℃ oven. After that, work out the water absorption rate according to the test method mentioned in 2.2. Due to the 45% water demand of normal consistency and quick condensation time, our tests chose 0.45, 0.50, 0.55, 0.60 separately as water/gypsum proportions (Chart 1)
Chart 1: The effect of water absorption by the proportion of water and gypsum
From Chart 1, the water absorption of the gypsum block will go up according to the increase of water/gypsum mix proportion. When the water/gypsum mix proportion is 0.45, the water absorption of the gypsum block in 2 hours is 22.8%. In mix proportion of 0.50, 0.55, 0.60, the water absorption rates are 25.6%, 28.8% and 32.6%, and this is because of the water content in gypsum slurry increasing when water consumption increased. After drying, the water content evaporated and the porosity of gypsum block enlarged. When immersing the gypsum block into the water, the blocks with more significant porosity will be more natural to absorb the water.
3.2 The Effect on Water Absorption of Gypsum Blocks after Adding Methyl Hydrogen Silicone Fluid
We separately tested the addition of silicone fluid in the proportion of 0.2%, 0.4%, 0.6%, 0.8% and 1.0%, to get the results of effect to water absorption of gypsum blocks. At the same condition, we also analyzed the effect of adding silicone fluid into gypsum slurry with different water/gypsum mix proportion. Refer to Chart 2.
Chart 2: The effect to water absorption of adding silicone fluid into gypsum slurry with different water/gypsum mix proportion.
From Chart 2, when the silicone fluid added proportion was increased, the water absorption of the gypsum block will be generally decreased. In water/gypsum of 0.60, the water absorption was continuous decreasing along with the silicone fluid in an added proportion of 0.2%,0.4%,0.6% and 0.8% and reached the minimum in 0.8%. But when the silicone fluid additive proportion went up to 1.0%, the gypsum block’s water absorption was going above 0.8%. In water/gypsum of 0.50, the changing of water absorption is almost the same as in water/gypsum proportion o 0.60. The difference is that the minimum water absorption value is reached when adding 0.6% silicone fluid. In water/gypsum of 0.45, the minimum water-absorption is 7.6%.
The reason: As the water repellent agent methyl hydrogen silicone fluid added into the gypsum, the polysiloxane with -Si-O-Si main molecular chain and organic molecular side chain will combine with -OH molecular chain in gypsum raw material, to form a low tension hydrophobic membrane in the surface of gypsum inner porosity. This hydrophobic membrane will prevent the gypsum block from water or moisture penetrating. When the silicone fluid added proportion is increasing, there are more hydrophobic membranes formed, and water absorption will be decreasing. Because of the tiny air permeability of polysiloxanes, the water absorption will be still going up if it continues to increase the silicone fluid adding after a critical value has been reached.
3.3 The Effect to Strength and Softening Coefficient of Gypsum Block After Adding Methyl Hydrogen Silicone Fluid
Under average consistency water demand (45%), we tested on silicone oil adding a proportion of 0.2%, 0.4%, 0.6%, 0.8% and 1.0% to see the effect of strength and softening coefficient, recorded as Chart 3 and Chart 4.
Chart 3: The effect to strength by adding proportion of silicone fluid
From Chart 3, the strength of the gypsum block was improved after adding silicone fluid. Compressive strength when in silicone fluid proportion of 0.2% and 0.4% was almost the same with the original gypsum block. As the ratio of silicone fluid was continuous increasing, the compressive strength starts to go down. When in silicone fluid2, adding a proportion of 0.8%, the compressive strength reached a minimum of 10.3% lower than in the original gypsum block. The strength of the original gypsum block with water-saturated in 2 hours is only 12.2MPa, but it will go up along with the increasing of silicone fluid. When the proportion of silicone fluid reaches 0.6%, the strength will float in a small scope.
Chart 4: The effect to softening coefficient by adding proportion of silicone fluid
From the Chart 4, adding silicone fluid is efficient to improve the softening coefficient of gypsum block. In adding a proportion of 0.8%, the softening coefficient will reach above 70%. This is the assurance for the application of gypsum board/plasterboard in water-resistant required places.
(1) The water absorption of the gypsum block will increase as the water proportion in gypsum slurry increased. In that case, lowing down the water/gypsum rate will improve the water repellent performance of gypsum block.
(2) As the dosage of silicone fluid increased, the water absorption of the gypsum block has a significant decrease. But the dosage of silicone fluid has a critical value when the dosage is too much. The water absorption will increase because of the tiny breath-ability performance of silicone fluid.
(3) The addition of silicone fluid won’t influence the breaking strength and compressive strength too much, but it will significantly improve the softening coefficient of gypsum block. When the silicone fluid dosage gets to 0.8%, the softening coefficient can reach 70% or more.