Effect of cellulose ether on plastic free shrinkage of mortar

Effect of cellulose ether on plastic free shrinkage of mortar

A non-contact laser displacement sensor was used to continuously test the plastic free shrinkage of HPMC modified cement mortar under accelerated conditions, and its water loss rate was observed at the same time. HPMC content and plastic free shrinkage and water loss rate regression models were established respectively. The results show that the plastic free shrinkage of cement mortar decreases linearly with the increase of HPMC content, and the plastic free shrinkage of cement mortar can be reduced by 30%-50% with the addition of 0.1%-0.4% (mass fraction) HPMC. With the increase of HPMC content, the water loss rate of cement mortar also linearly decreases. The water loss rate of cement mortar can be reduced by 9% ~ 29% with the addition of 0.1% ~ 0.4% HPMC. The content of HPMC has obvious linear relationship with free shrinkage and water loss rate of mortar. HPMC reduces the plastic shrinkage of cement mortar due to its excellent water retention.

Key words: methyl hydroxypropyl cellulose ether (HPMC); Mortar; Plastic free shrinkage; Water loss rate; Regression model

Compared with cement concrete, cement mortar cracks more easily. In addition to the factors of raw materials themselves, the change of external temperature and humidity will make cement mortar fast water loss, resulting in accelerated cracking. To solve the problem of cement mortar cracking, it is usually solved by strengthening early curing, using expansion agent and adding fiber.

As a polymer admixture commonly used in commercial cement mortar, cellulose ether is a cellulose derivative obtained by the reaction of plant cellulose and caustic soda. Zhan Zhenfeng et al. showed that when the content of cellulose ether (mass fraction) was 0% ~ 0.4%, the water retention rate of cement mortar had a good linear relationship with the content of cellulose ether, and the higher the content of cellulose ether, the greater the water retention rate. Methyl hydroxypropyl cellulose ether (HPMC) is used in cement mortar to improve its cohesiveness and cohesiveness because of its bonding, suspension stability and water retention properties.

This paper takes the plastic free shrinkage of cement mortar as the test object, studies the effect of HPMC on the plastic free shrinkage of cement mortar, and analyzes the reason why HPMC reduces the plastic free shrinkage of cement mortar.

1. Raw materials and test methods

1.1 Raw Materials

The cement used in the test was conch brand 42.5R ordinary Portland cement produced by Anhui Conch Cement Co., LTD. Its specific surface area was 398.1 m² / kg, 80μm sieve residue was 0.2% (mass fraction); HPMC is provided by Shanghai Shangnan Trading Co., LTD. Its viscosity is 40 000 mPa·s, the sand is medium coarse yellow sand, the fineness modulus is 2.59, and the maximum particle size is 5mm.

1.2 Test methods

1.2.1 Plastic free shrinkage test method

The plastic free shrinkage of cement mortar was tested by the experimental device described in the literature. The ratio of cement to sand of the benchmark mortar is 1:2 (mass ratio), and the ratio of water to cement is 0.5(mass ratio). Weigh the raw materials according to the mix ratio, and at the same time add to the mixing pot dry stirring for 1min, then add water and continue stirring for 2min. Add about 20g of the settler (white granulated sugar), mix well, pour the cement mortar outward from the center of the wood mold in a spiral shape, make it cover the lower wood mold, smooth it with a spatula, and then use disposable plastic film to spread it on the surface of the cement mortar, and then pour the test mortar on the plastic table cloth in the same way to fill the upper wood mold. And immediately with the length of the wet aluminum plate longer than the width of the wood mold, quickly scrape along the long side of the wood mold.

Microtrak II LTC-025-04 laser displacement sensor was used to measure the plastic free shrinkage of cement mortar slab. The steps are as follows: Two test targets (small foam plates) were placed in the middle position of the poured cement mortar plate, and the distance between the two test targets was 300mm. Then, an iron frame fixed with laser displacement sensor was placed above the specimen, and the initial reading between the laser and the measured object was adjusted to be within the 0 scale range. Finally, the 1000W iodine tungsten lamp at about 1.0m above the wood mold and the electric fan at about 0.75m above the wood mold (wind speed is 5m/s) were turned on at the same time. The plastic free shrinkage test continued until the specimen shrank to basically stable. During the whole test, the temperature was (20±3)℃ and the relative humidity was (60±5) %.

1.2.2 Test method of water evaporation rate

Considering the influence of the composition of cement-based materials on the water evaporation rate, the literature uses small specimens to simulate the water evaporation rate of large specimens, and the relationship between the ratio Y of the water evaporation rate of large-plate cement mortar and small-plate cement mortar and the time t(h) is as follows: y= 0.0002 t+0.736

2. Results and discussion

2.1 Influence of HPMC content on plastic free shrinkage of cement mortar

From the effect of HPMC content on the plastic free shrinkage of cement mortar, it can be seen that the plastic free shrinkage of ordinary cement mortar mainly occurs within 4h of accelerated cracking, and its plastic free shrinkage increases linearly with the extension of time. After 4h, the plastic free shrinkage reaches 3.48mm, and the curve becomes stable. The plastic free shrinkage curves of HPMC cement mortar are all located below the plastic free shrinkage curves of ordinary cement mortar, indicating that the plastic free shrinkage curves of HPMC cement mortar are all smaller than that of ordinary cement mortar. With the increase of HPMC content, the plastic free shrinkage of cement mortar gradually decreases. Compared with ordinary cement mortar, the plastic free shrinkage of HPMC cement mortar mixed with 0.1% ~ 0.2% (mass fraction) decreases by about 30%, about 2.45mm, and the plastic free shrinkage of 0.3% HPMC cement mortar decreases by about 40%. Is about 2.10mm, and the plastic free shrinkage of 0.4% HPMC cement mortar decreases by about 50%, which is about 1.82mm. Therefore, in the same accelerated cracking time, the plastic free shrinkage of HPMC cement mortar is lower than that of ordinary cement mortar, indicating that the incorporation of HPMC can reduce the plastic free shrinkage of cement mortar.

From the effect of HPMC content on the plastic free shrinkage of cement mortar, it can be seen that with the increase of HPMC content, the plastic free shrinkage of cement mortar gradually decreases. The relationship between plastic free shrinkage (s) of cement mortar and HPMC content (w) can be fitted by the following formula: S= 2.77-2.66 w

HPMC content and cement mortar plastic free shrinkage linear regression variance analysis results, where: F is the statistic; Sig. Represents the actual significance level.

The results show that the correlation coefficient of this equation is 0.93.

2.2 Influence of HPMC content on water loss rate of cement mortar

Under the condition of acceleration, it can be seen from the change of water loss rate of cement mortar with the content of HPMC, the water loss rate of cement mortar surface gradually decreases with the increase of HPMC content, and basically presents a linear decline. Compared with the water loss rate of ordinary cement mortar, when the HPMC content is 0.1%, 0.2%, 0.3%, 0.4%, respectively, The water loss rate of large slab cement mortar decreased by 9.0%, 12.7%, 22.3% and 29.4%, respectively. The incorporation of HPMC reduces the water loss rate of cement mortar and makes more water participate in the hydration of cement mortar, thus forming enough tensile strength to resist the cracking risk brought by the external environment.

The relationship between cement mortar water loss rate (d) and HPMC content (w) can be fitted by the following formula: d= 0.17-0.1w

The linear regression variance analysis results of HPMC content and cement mortar water loss rate show that the correlation coefficient of this equation is 0.91, and the correlation is obvious.

3. Conclusion

The plastic free shrinkage of cement mortar decreases gradually with the increase of the content of HPMC. The plastic free shrinkage of cement mortar with 0.1% ~ 0.4% HPMC decreases by 30% ~ 50%. The water loss rate of cement mortar decreases with the increase of HPMC content. The water loss rate of cement mortar with 0.1% ~ 0.4% HPMC decreases by 9.0% ~ 29.4%. The plastic free shrinkage and water loss rate of cement mortar are linear with the content of HPMC.