Properties of gypsum mortar

The influence of cellulose ether content on the water retention of desulfurized gypsum mortar was evaluated by three test methods of water retention of gypsum mortar, and the test results were compared and analyzed. The effect of cellulose ether content on water retention, compressive strength, flexural strength and bond strength of gypsum mortar was studied. The results show that the incorporation of cellulose ether will reduce the compressive strength of gypsum mortar, greatly improve the water retention and bonding strength, but have little effect on the flexural strength.

Key words: water retention; cellulose ether; gypsum mortar

Cellulose ether is a water-soluble polymer material, which is processed from natural cellulose through alkali dissolution, grafting reaction (etherification), washing, drying, grinding and other processes. Cellulose ether can be used as water retention agent, thickener, binder, dispersant, stabilizer, suspending agent, emulsifier and film-forming aid, etc. Because cellulose ether has a good water retention and thickening effect on mortar, it can significantly improve the workability of mortar, so cellulose ether is the most commonly used water-soluble polymer in mortar. Cellulose ether is often used as a water-retaining agent in (desulfurization) gypsum mortar. Years of research have shown that the water-retaining agent has a very important influence on the quality of the plaster and the performance of the anti-plastering layer. Good water retention can ensure that the plaster is fully Hydrates, guarantees the necessary strength, improves the rheological properties of the stucco plaster. Therefore, it is very important to accurately measure the water retention performance of gypsum. For this reason, the author compared two common mortar water retention test methods to ensure the accuracy of the results of cellulose ether on the water retention performance of gypsum, and to evaluate the mechanical properties of cellulose ether on gypsum mortar. The influence of , was tested experimentally.

1. Test

1.1 Raw materials

Desulfurization gypsum: The flue gas desulfurization gypsum of Shanghai Shidongkou No. 2 Power Plant is obtained by drying at 60°C and calcining at 180°C. Cellulose ether: methyl hydroxypropyl cellulose ether provided by Kima Chemical Company, with a viscosity of 20000mPa·S; the sand is medium sand.

1.2 Test method

1.2.1 Test method of water retention rate

(1) Vacuum suction method (“Plastering Gypsum” GB/T28627-2012) Cut a piece of medium-speed qualitative filter paper from the inner diameter of the Buchner funnel, spread it on the bottom of the Buchner funnel, and soak it with water. Put the Buchner funnel on the suction filter bottle, start the vacuum pump, filter for 1 min, remove the Buchner funnel, wipe off the residual water at the bottom with filter paper and weigh (G1), accurate to 0.1g. Put the gypsum slurry with standard diffusion degree and water consumption into the weighed Buchner funnel, and use a T-shaped scraper to rotate vertically in the funnel to level it out, so that the thickness of the slurry is kept within the range of (10±0.5)mm . Wipe off the residual gypsum slurry on the inner wall of the Buchner funnel, weigh (G2), accurate to 0.1g. The time interval from the completion of stirring to the completion of weighing should not be greater than 5min. Put the weighed Buchner funnel on the filter flask and start the vacuum pump. Adjust the negative pressure to (53.33±0.67) kPa or (400±5) mm Hg within 30 seconds. Suction filtration for 20 minutes, then remove the Buchner funnel, wipe off the residual water in the lower mouth with filter paper, weigh (G3), accurate to 0.1g.

(2) Filter paper water absorption method (1) (French standard) Stack the mixed slurry on several layers of filter paper. The types of filter paper used are: (a) 1 layer of fast-filtering filter paper that is directly in contact with the slurry; ( b) 5 layers of filter paper for slow filtration. A plastic round plate acts as a pallet, and it sits directly on the table. Deduct the weight of the plastic disc and filter paper for slow filtration (mass is M0). After the plaster of paris is mixed with water to form a slurry, it is immediately poured into a cylinder (inner diameter 56mm, height 55mm) covered with filter paper. After the slurry is in contact with the filter paper for 15 minutes, re-weigh the slow-filtered filter paper and pallet (mass M1). The water retention of plaster is expressed by the weight of water absorbed per square centimeter of absorption area of chronic filter paper, that is: water absorption of filter paper = (M1-M0)/24.63

(3) Filter paper water absorption method (2) (“Standards for the basic performance test methods of building mortar” JGJ/T70) Weigh the mass m1 of the impermeable sheet and the dry test mold and the mass m2 of 15 pieces of medium-speed qualitative filter paper. Fill the mortar mixture into the trial mold at one time, and insert and pound it several times with a spatula. When the filling mortar is slightly higher than the edge of the trial mold, use the spatula to scrape off the excess mortar on the surface of the trial mold at an angle of 450 degrees, and then Then use a spatula to scrape the mortar flat against the surface of the test mold at a relatively flat angle. Erase the mortar on the edge of the test mold, and weigh the total mass m3 of the test mold, the lower impermeable sheet and the mortar. Cover the surface of the mortar with a filter screen, put 15 pieces of filter paper on the surface of the filter screen, cover the surface of the filter paper with an impermeable sheet, and press the impermeable sheet with a weight of 2kg. After standing still for 2 minutes, remove the heavy objects and impermeable sheets, take out the filter paper (excluding the filter screen), and quickly weigh the filter paper mass m4. Calculate the moisture content of the mortar from the ratio of the mortar and the amount of water added.

1.2.2 Test methods for compressive strength, flexural strength and bond strength

Gypsum mortar compressive strength, flexural strength, bond strength test and related test conditions are carried out according to the operation steps in “Plastering Gypsum” GB/T 28627-2012.

2. Test results and analysis

2.1 Effect of cellulose ether on water retention of mortar – comparison of different test methods

In order to compare the differences of different water retention test methods, three different methods were tested for the same formula of gypsum.

From the test comparison results of three different methods, it can be seen that when the amount of water-retaining agent increases from 0 to 0.1%, the test result using the filter paper water absorption method (1) drops from 150.0mg/cm² to 8.1mg/cm² , decreased by 94.6%; the water retention rate of the mortar measured by the filter paper water absorption method (2) increased from 95.9% to 99.9%, and the water retention rate only increased by 4%; the test result of the vacuum suction method increased by 69% .8% increased to 96.0%, the water retention rate increased by 37.5%.

It can be seen from this that the water retention rate measured by the filter paper water absorption method (2) cannot open up the difference in the performance and dosage of the water retaining agent, which is not conducive to the accurate test and judgment of the water retention rate of gypsum commercial mortar, and the vacuum filtration method is because of There is forced suction, so the difference in data can be forcibly opened to reflect the difference in water retention. At the same time, the test results using the filter paper water absorption method (1) fluctuate greatly with the amount of water-retaining agent, which can better widen the difference between the amount of water-retaining agent and the variety. However, since the water absorption rate of the filter paper measured by this method is the amount of water absorbed by the filter paper per unit area, when the water consumption of the standard diffusivity of the mortar varies with the type, dosage and viscosity of the water-retaining agent mixed, the test results cannot accurately reflect the true water retention of the mortar. Rate.

To sum up, the vacuum suction method can effectively distinguish the excellent water retention performance of mortar, and it is not affected by the water consumption of mortar. Although the test results of the filter paper water absorption method (1) are affected by the water consumption of the mortar, due to the simple experimental operation steps, the water retention performance of the mortar can be compared under the same formula.

The ratio of fixed gypsum composite cementitious material to medium sand is 1:2.5. Adjust the amount of water by changing the amount of cellulose ether. The influence of the content of cellulose ether on the water retention rate of gypsum mortar was studied. From the test results, it can be seen that with the increase of the content of cellulose ether, the water retention of mortar is significantly improved; when the content of cellulose ether reaches 0% of the total amount of mortar ．At about 10%, the water absorption curve of filter paper tends to be gentle.

The cellulose ether structure contains hydroxyl groups and ether bonds. The atoms on these groups associate with water molecules to form hydrogen bonds, so that free water molecules become bound water, thus playing a good role in water retention. In mortar, in order to coagulate, gypsum needs water Get hydrated. A reasonable amount of cellulose ether can keep the moisture in the mortar for a long enough time, so that the setting and hardening process can continue. When its dosage is too large, not only the improvement effect is not obvious, but also the cost will increase, so a reasonable dosage is very important. Considering the performance and viscosity difference of different water retaining agents, the content of cellulose ether is determined to be 0.10% of the total amount of mortar.

2.2 The effect of cellulose ether content on the mechanical properties of gypsum

2.2.1 Influence on compressive strength and flexural strength

The ratio of fixed gypsum composite cementitious material to medium sand is 1:2.5. Change the amount of cellulose ether and adjust the amount of water. From the experimental results, it can be seen that with the increase of the content of cellulose ether, the compressive strength has a significant downward trend, and the flexural strength has no obvious change.

With the increase of cellulose ether content, the 7d compressive strength of mortar decreased. Literature [6] believes that this is mainly because: (1) when cellulose ether is added to the mortar, the flexible polymers in the mortar pores are increased, and these flexible polymers cannot provide rigid support when the composite matrix is compressed. effect, so that the compressive strength of the mortar decreases (the author of this paper believes that the volume of cellulose ether polymer is very small, and the impact made by the pressure can be ignored); (2) with the increase of the content of cellulose ether , its water retention effect is getting better and better, so that after the mortar test block is formed, the porosity in the mortar test block increases, which reduces the compactness of the hardened body and weakens the ability of the hardened body to resist external forces, thus reducing the compressive strength of the mortar (3) When the dry-mixed mortar is mixed with water, the cellulose ether particles are first adsorbed on the surface of the cement particles to form a latex film, which reduces the hydration of the gypsum, thereby reducing the strength of the mortar. With the increase of cellulose ether content, the folding ratio of the material decreased. However, when the amount is too large, the performance of the mortar will be reduced, which is manifested in the fact that the mortar is too viscous, easy to stick to the knife, and difficult to spread during construction. At the same time, considering that the water retention rate must also meet the conditions, the amount of cellulose ether is determined to be 0.05% to 0.10% of the total amount of mortar.

2.2.2 Effect on tensile bond strength

Cellulose ether is called a water-retaining agent, and its function is to increase the water retention rate. The purpose is to maintain the moisture contained in the gypsum slurry, especially after the gypsum slurry is applied to the wall, the moisture will not be absorbed by the wall material, so as to ensure the moisture retention of the gypsum slurry at the interface. Hydration reaction, so as to ensure the bond strength of the interface. Keep the ratio of gypsum composite cementitious material to medium sand at 1:2.5. Change the amount of cellulose ether and adjust the amount of water.

It can be seen from the test results that with the increase of the content of cellulose ether, although the compressive strength decreases, its tensile bond strength gradually increases. The addition of cellulose ether can form a thin polymer film between the cellulose ether and the hydration particles. The cellulose ether polymer film will dissolve in water, but under dry conditions, due to its compactness, it has the ability to prevent The role of moisture evaporation. The film has a sealing effect, which improves the dryness of the mortar. Due to the good water retention of cellulose ether, sufficient water is stored inside the mortar, thus ensuring the full development of hydration hardening and strength, and improving the bonding strength of the mortar. In addition, the addition of cellulose ether improves the cohesion of the mortar, and makes the mortar have good plasticity and flexibility, which also makes the mortar well able to adapt to the shrinkage deformation of the substrate, thereby improving the bond strength of the mortar. With the increase of the content of cellulose ether, the adhesion of gypsum mortar to the base material increases. When the tensile bonding strength of the plastering gypsum of the bottom layer is >0.4MPa, the tensile bonding strength is qualified and meets the standard “Plastering Gypsum” GB/T2827.2012. However, considering that the cellulose ether content is 0.10% B inch, the strength does not meet the requirements, so the cellulose content is determined to be 0.15% of the total amount of mortar.

3. Conclusion

(1) The water retention rate measured by the filter paper water absorption method (2) cannot open up the difference in the performance and dosage of the water retaining agent, which is not conducive to the accurate test and judgment of the water retention rate of gypsum commercial mortar. The vacuum suction method can effectively distinguish the excellent water retention performance of the mortar, and is not affected by the water consumption of the mortar. Although the test results of the filter paper water absorption method (1) are affected by the water consumption of the mortar, due to the simple experimental operation steps, the water retention performance of the mortar can be compared under the same formula.

(2) The increase in the content of cellulose ether improves the water retention of gypsum mortar.

(3) The incorporation of cellulose ether reduces the compressive strength of the mortar and improves the bonding strength with the substrate. Cellulose ether has little effect on the flexural strength of mortar, so the folding ratio of mortar is reduced.