Effect of Cellulose Ether on Mortar Properties

The effects of two kinds of cellulose ethers on the performance of mortar were studied. The results showed that both kinds of cellulose ethers could significantly improve the water retention of mortar and reduce the consistency of mortar; The compressive strength is reduced in different degrees, but the folding ratio and bonding strength of the mortar are increased in different degrees, thus improving the construction of the mortar.

Key words: cellulose ether; water retaining agent; bonding strength

Cellulose ether (MC) is a derivative of natural material cellulose. 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.

1. Test materials and test methods

1.1 Raw materials

Cement: Ordinary Portland cement produced by Jiaozuo Jianjian Cement Co., Ltd., with a strength grade of 42.5. Sand: Nanyang yellow sand, fineness modulus 2.75, medium sand. Cellulose ether (MC): C9101 produced by Beijing Luojian Company and HPMC produced by Shanghai Huiguang Company.

1.2 Test method

In this study, the lime-sand ratio was 1:2, and the water-cement ratio was 0.45; the cellulose ether was mixed with cement first, and then sand was added and stirred evenly. The dosage of cellulose ether is calculated according to the percentage of cement mass.

The compressive strength test and consistency test are carried out with reference to JGJ 70-90 “Test Methods for Basic Properties of Building Mortar”. The flexural strength test is carried out according to GB/T 17671–1999 “Cement Mortar Strength Test”.

The water retention test was carried out according to the filter paper method used in French aerated concrete production enterprises. The specific process is as follows: (1) put 5 layers of slow filter paper on a plastic circular plate, and weigh its mass; (2) put one in direct contact with the mortar Place the high-speed filter paper on the slow-speed filter paper, and then press a cylinder with an inner diameter of 56 mm and a height of 55 mm on the fast filter paper; (3) Pour the mortar into the cylinder; (4) After the mortar and the filter paper contact for 15 minutes, weigh again The quality of the slow filter paper and the plastic disc; (5) Calculate the water mass absorbed by the slow filter paper per square meter area, which is the water absorption rate; (6) The water absorption rate is the arithmetic mean of the two test results. If the difference between the rate values exceeds 10%, the test should be repeated; (7) The water retention of the mortar is expressed by the water absorption rate.

The bond strength test was carried out with reference to the method recommended by the Japan Society for Materials Science, and the bond strength was characterized by flexural strength. The test adopts a prism sample whose size is 160mm×40mm×40mm. The ordinary mortar sample made in advance was cured to the age of 28 d, and then cut into two halves. The two halves of the sample were made into samples with ordinary mortar or polymer mortar, and then naturally cured indoors to a certain age, and then tested according to the test method for the flexural strength of cement mortar.

2. Test results and analysis

2.1 Consistency

From the effect of cellulose ether on the consistency of mortar, it can be seen that with the increase of the content of cellulose ether, the consistency of mortar basically shows a downward trend, and the decrease of the consistency of mortar mixed with HPMC is faster than that of mortar mixed with C9101. This is because the viscosity of cellulose ether hinders the flow of mortar, and the viscosity of HPMC is higher than that of C9101.

2.2 Water retention

In mortar, cementitious materials such as cement and gypsum need to be hydrated with water in order to set. 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.

From the effect of cellulose ether content on the water retention of mortar, it can be seen that: (1) With the increase of C9101 or HPMC cellulose ether content, the water absorption rate of mortar decreased significantly, that is, the water retention of mortar was significantly improved, especially when mixed with Mortar of HPMC. Its water retention can be improved more; (2) When the amount of HPMC is 0.05% to 0.10%, the mortar fully meets the water retention requirements in the construction process.

Both cellulose ethers are non-ionic polymers. The hydroxyl groups on the cellulose ether molecular chain and the oxygen atoms on the ether bonds can form hydrogen bonds with water molecules, making free water into bound water, thus playing a good role in water retention.

The water retention of cellulose ether mainly depends on its viscosity, particle size, dissolution rate and addition amount. In general, the greater the amount added, the higher the viscosity, and the finer the fineness, the higher the water retention. Both C9101 and HPMC cellulose ether have methoxy and hydroxypropoxy groups in the molecular chain, but the content of methoxy in HPMC cellulose ether is higher than that of C9101, and the viscosity of HPMC is higher than that of C9101, so the water retention of mortar mixed with HPMC is higher than that of mortar mixed with HPMC C9101 large mortar. However, if the viscosity and relative molecular weight of cellulose ether are too high, its solubility will decrease accordingly, which will have a negative impact on the strength and workability of the mortar. Structural strength to achieve excellent bonding effect.

2.3 Flexural strength and compressive strength

From the effect of cellulose ether on the flexural and compressive strength of mortar, it can be seen that with the increase of the content of cellulose ether, the flexural and compressive strength of mortar at 7 and 28 days showed a downward trend. This is mainly because: (1) When cellulose ether is added to the mortar, the flexible polymers in the pores of the mortar are increased, and these flexible polymers cannot provide rigid support when the composite matrix is compressed. As a result, the flexural and compressive strength of the mortar is reduced; (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 , the flexural and compressive strength will be reduced; (3) when the dry-mixed mortar is mixed with water, the cellulose ether latex particles are first adsorbed on the surface of the cement particles to form a latex film, which reduces the hydration of the cement, thereby also reducing the the strength of the mortar.

2.4 Fold ratio

The flexibility of the mortar endows the mortar with good deformability, which enables it to adapt to the stress generated by the shrinkage and deformation of the substrate, thus greatly improving the bond strength and durability of the mortar.

From the effect of cellulose ether content on mortar folding ratio (ff/fo), it can be seen that with the increase of cellulose ether C9101 and HPMC content, the mortar folding ratio basically showed an increasing trend, indicating that the flexibility of mortar was improved.

When the cellulose ether dissolves into the mortar, because the methoxyl and hydroxypropoxyl on the molecular chain will react with the Ca2+ and Al3+ in the slurry, a viscous gel is formed and filled in the cement mortar gap, thus It plays a role of flexible filling and flexible reinforcement, improving the compactness of the mortar, and it shows that the flexibility of the modified mortar is improved macroscopically.

2.5 Bond strength

From the effect of cellulose ether content on the mortar bond strength, it can be seen that the mortar bond strength increases with the increase of cellulose ether content.

The addition of cellulose ether can form a thin layer of waterproof polymer film between cellulose ether and hydrated cement particles. This film has a sealing effect and improves the “surface dry” phenomenon of mortar. Due to the good water retention of cellulose ether, sufficient water is stored inside the mortar, thereby ensuring the hydration hardening of the cement and the full development of its strength, and improving the bond strength of the cement paste. 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.

2.6 Shrinkage

It can be seen from the effect of cellulose ether content on the shrinkage of mortar: (1) The shrinkage value of cellulose ether mortar is much lower than that of blank mortar. (2) With the increase of C9101 content, the shrinkage value of mortar decreased gradually, but when the content reached 0.30%, the shrinkage value of mortar increased. This is because the greater the amount of cellulose ether, the greater its viscosity, which causes an increase in water demand. (3) With the increase of HPMC content, the shrinkage value of mortar decreased gradually, but when its content reached 0.20%, the shrinkage value of mortar increased and then decreased. This is because the viscosity of HPMC is greater than that of C9101. The higher the viscosity of cellulose ether. The better the water retention, the more air content, when the air content reaches a certain level, the shrinkage value of the mortar will increase. Therefore, in terms of shrinkage value, the optimal dosage of C9101 is 0.05%~0.20%. The optimum dosage of HPMC is 0.05%~0.10%.

3. Conclusion

1. Cellulose ether can improve the water retention of mortar and reduce the consistency of mortar. Adjusting the amount of cellulose ether can meet the needs of mortar used in different projects.

2. The addition of cellulose ether reduces the flexural strength and compressive strength of the mortar, but increases the folding ratio and bonding strength to a certain extent, thereby improving the durability of the mortar.

3. The addition of cellulose ether can improve the shrinkage performance of mortar, and with the increase of its content, the shrinkage value of mortar becomes smaller and smaller. But when the amount of cellulose ether reaches a certain level, the shrinkage value of the mortar increases to a certain extent due to the increase of the air-entraining amount.