Hydroxypropyl methylcellulose ether on the properties of fly ash mortar

The effect of hydroxypropyl methylcellulose ether on the properties of fly ash mortar was studied, and the relationship between wet density and compressive strength was analyzed. The test results show that adding hydroxypropyl methylcellulose ether to fly ash mortar can significantly improve the water retention performance of the mortar, prolong the bonding time of the mortar, and reduce the wet density and compressive strength of the mortar. There is a good correlation between the wet density and the 28d compressive strength. Under the condition of known wet density, the 28d compressive strength can be calculated by using the fitting formula.

Key words: fly ash; cellulose ether; water retention; compressive strength; correlation

At present, fly ash has been widely used in construction engineering. Adding a certain amount of fly ash in mortar can not only improve the mechanical properties and durability of mortar, but also reduce the cost of mortar. However, fly ash mortar shows insufficient water retention, so how to improve the water retention of mortar has become an urgent problem to be solved. Cellulose ether is a high-efficiency admixture commonly used at home and abroad. It only needs to be added in a small amount to have a great impact on performance indicators such as water retention and compressive strength of mortar.

1. Raw materials and test methods

1.1 Raw materials

The cement is P·O 42.5 grade ordinary Portland cement produced by Hangzhou Meiya Cement Factory; the fly ash is grade Ⅱ ash; the sand is ordinary medium sand with a fineness modulus of 2.3, a bulk density of 1499kg·m-3, and a moisture content of 0.14 %, mud content 0.72%; hydroxypropyl methyl cellulose ether (HPMC) is produced by Shandong Heda Co., Ltd., the brand is 75HD100000; the mixing water is tap water.

1.2 Mortar preparation

When mixing cellulose ether modified mortar, first mix HPMC with cement and fly ash thoroughly, then dry mix with sand for 30 seconds, then add water and mix for not less than 180 seconds.

1.3 Test method

The consistency, wet density, delamination and setting time of freshly mixed mortar shall be measured according to the relevant regulations in JGJ70-90 “Basic Performance Test Methods of Building Mortar”. The water retention of mortar is determined according to the test method for water retention of mortar in Appendix A of JG/T 230-2007 “Ready Mixed Mortar”. The compressive strength test adopts a 70.7mm x 70.7mm x 70.7mm cube bottomed test mold. The formed test block is cured at a temperature of (20±2)°C for 24 hours, and after demoulding, it is continued to be cured in an environment with a temperature of (20±2)°C and a relative humidity above 90% to the predetermined age, according to JGJ70-90 “Building Mortar Basic performance test method “determination of its compressive strength.

2. Test results and analysis

2.1 Wet density

It can be seen from the relationship between the density and the amount of HPMC that the wet density gradually decreases with the increase of the amount of HPMC. When the amount of HPMC is 0.05%, the wet density of the mortar is 96.8% of the benchmark mortar. When the amount of HPMC continues to increase, The decrease speed of wet density is accelerated. When the content of HPMC is 0.20%, the wet density of mortar is only 81.5% of the benchmark mortar. This is mainly due to the air-entraining effect of HPMC. The introduced air bubbles increase the porosity of the mortar and decrease the compactness, resulting in a decrease in the volume density of the mortar.

2.2 Setting time

It can be seen from the relationship between the coagulation time and the amount of HPMC that the coagulation time is gradually increasing. When the dosage is 0.20%, the setting time increases by 29.8% compared with the reference mortar, reaching about 300min. It can be seen that when the dosage is 0.20%, the setting time has a great change. The reason is that L Schmitz et al. believe that cellulose ether molecules are mainly adsorbed on hydration products such as c-S-H and calcium hydroxide, and are rarely adsorbed on the original mineral phase of clinker. In addition, due to the increase in the viscosity of the pore solution, the cellulose ether decreases. The mobility of ions (Ca2+, so42-…) in the pore solution further delays the hydration process.

2.3 Layering and water retention

Both the degree of delamination and water retention can characterize the water retention effect of mortar. From the relationship between the degree of delamination and the amount of HPMC, it can be seen that the degree of delamination shows a decreasing trend as the amount of HPMC increases. When the content of HPMC is 0.05%, the degree of delamination decreases very significantly, indicating that when the content of fiber ether is small, the degree of delamination can be greatly reduced, the effect of water retention can be improved, and the workability and workability of mortar can be improved. Judging from the relationship between the water property and the amount of HPMC, as the amount of HPMC increases, the water retention also gradually becomes better. When the dosage is less than 0.15%, the water retention effect increases very gently, but when the dosage reaches 0.20%, the water retention effect has been greatly improved, from 90.1% when the dosage is 0.15%, to 95%. The amount of HPMC continues to increase, and the construction performance of mortar begins to deteriorate. Therefore, considering the water retention performance and construction performance, the appropriate amount of HPMC is 0.10%~0.20%. Analysis of its water retention mechanism: Cellulose ether is a water-soluble organic polymer, which is divided into ionic and non-ionic. HPMC is a non-ionic cellulose ether with a hydrophilic group, a hydroxyl group (-OH) and an ether bond (-0-1) in its structural formula. When dissolved in water, the oxygen atoms on the hydroxyl group and the ether bond and water Molecules associate to form hydrogen bonds, which makes water lose its fluidity, and free water is no longer free, thus achieving the effect of water retention and thickening.

2.4 Compressive strength

From the relationship between the compressive strength and the amount of HPMC, it can be seen that with the increase of the amount of HPMC, the compressive strength of 7d and 28d showed a decreasing trend, which was mainly due to the introduction of a large number of air bubbles by HPMC, which greatly increased the porosity of the mortar. increase, resulting in a decrease in strength. When the content is 0.05%, the 7d compressive strength drops very significantly, the strength drops by 21.0%, and the 28d compressive strength drops by 26.6%. It can be seen from the curve that the impact of HPMC on the compressive strength is very obvious. When the dosage is very small, it will be greatly reduced. Therefore, in practical applications, its dosage should be controlled and used in combination with a defoamer. Investigating the reason, Guan Xuemao et al. believe that firstly, when cellulose ether is added to the mortar, the flexible polymer in the mortar pores is increased, and these flexible polymers and pores cannot provide rigid support when the test block is compressed. The composite matrix is relatively weakened, thereby reducing the compressive strength of the mortar; secondly, due to the water retention effect of cellulose ether, after the mortar test block is formed, most of the water remains in the mortar, and the actual water-cement ratio is lower than that without Those are much larger, so the compressive strength of the mortar will be significantly reduced.

2.5 Correlation between compressive strength and wet density

It can be seen from the relationship curve between compressive strength and wet density that after linear fitting of all points in the figure, the corresponding points are well distributed on both sides of the fitting line, and there is a good correlation between wet density and compressive strength properties, and the wet density is simple and easy to measure, so the compressive strength of mortar 28d can be calculated through the established linear fitting equation. The linear fitting equation is shown in formula (1), R²=0.9704. Y=0.0195X-27.3 (1), where, y is the 28d compressive strength of the mortar, MPa; X is the wet density, kg m-3.

3. Conclusion

HPMC can improve the water retention effect of fly ash mortar and prolong the operating time of the mortar. At the same time, due to the increase of the porosity of the mortar, its bulk density and compressive strength will drop significantly, so the appropriate dosage should be selected in the application. The 28d compressive strength of mortar has a good correlation with the wet density, and the 28d compressive strength can be calculated by measuring the wet density, which has important reference value for the quality control of mortar during construction.