Effect of Hydroxypropyl Methyl Cellulose Ether on the Properties of Machine Sprayed Cement Mortar

Cellulose ether is an essential additive in machine-blasted mortar. The effects of four different viscosities of hydroxypropyl methylcellulose (HPMC) on the water retention, density, air content, mechanical properties and pore size distribution of machine-blasted mortar were studied. Studies have shown that: HPMC can significantly improve the water retention performance of mortar, and the water retention rate can exceed 90% when the amount of HPMC is 0.15%. The most obvious; the air content of mortar increases with the increase of HPMC content: HPMC will obviously reduce the mechanical properties of cement mortar, but the folding ratio of mortar will increase; the pore size of mortar will increase significantly after adding HPMC, The proportion of harmful holes and multiple harmful holes increased significantly.

Key words: mortar; hydroxypropyl methylcellulose ether; water retention; pore size distribution

0. Foreword

In recent years, with the continuous progress of the industry and the improvement of technology, through the introduction and improvement of foreign mortar spraying machines, the technology of mechanical spraying and plastering has been greatly developed in our country. Mechanical spraying mortar is different from ordinary mortar, which requires high water retention performance, suitable fluidity and certain anti-sagging performance. Usually, cellulose ether is added to the mortar, among which hydroxypropyl methylcellulose Plain ether (HPMC) is the most widely used. The main functions of HPMC in mortar are: excellent water retention capacity, thickening and viscosifying and rheological adjustment. However, the shortcomings of HPMC cannot be ignored. HPMC has an air-entraining effect, which will cause more internal defects and seriously reduce the mechanical properties of mortar. This paper studies the influence of HPMC on the water retention rate, density, air content and mechanical properties of mortar from the macroscopic aspect, and studies the influence of HPMC on the pore structure of the mortar from the microscopic aspect.

1. Test

1.1 Raw materials

Cement: commercially available P·O42.5 cement, its 28d flexural and compressive strengths are 6.9 and 48.2 MPa respectively; sand: Chengde fine river sand, 40-100 mesh; cellulose ether: hydroxypropyl alcohol produced by a company in Hebei Methyl cellulose ether, white powder, nominal viscosity 40, 100, 150, 200 Pa·S: Water: clean tap water.

1.2 Test method

According to JGJ/T 105-2011 “Construction Regulations for Mechanical Spraying and Plastering”, the consistency of the mortar is 80~120mm, and the water retention rate is greater than 90%. In this test, the lime-sand ratio is set at 1:5, the consistency is controlled at (93±2)mm, and the cellulose ether is externally blended, and its dosage is calculated according to the cement mass. The basic properties of mortar such as wet density, air content, water retention rate, and consistency are tested with reference to JGJ 70-2009 “Test Methods for Basic Properties of Building Mortar”, and the air content is tested and calculated according to the density method. The preparation, flexural and compressive strength tests of the specimens were carried out with reference to GB/T 17671-1999 “Methods for Testing the Strength of Cement Mortar Sand (ISO Method)”. The pore size was tested by mercury porosimetry. The model of the mercury porosimeter was AUTOPORE 9500, and the measuring range was 5.5 nm to 360 μm. A total of 4 sets of tests were carried out. 0, 0.1%, 0.2%, 0.3% (numbers are A, B, C, D).

2. Results and Analysis

2.1 Effect of HPMC on the water retention rate of cement mortar

Water retention refers to the ability of mortar to hold water. In machine sprayed mortar, adding cellulose ether can effectively maintain moisture, reduce bleeding rate, and meet the requirements of sufficient hydration of cement-based materials.

From the effect of HPMC on the water retention rate of mortar, it can be seen that with the increase of HPMC content, the water retention rate of mortar increases gradually. The curves of cellulose ethers with viscosities of 100, 150 and 200 Pa·s are basically the same. When the content is 0.05% to 0.15%, the water retention rate increases linearly. When the content is 0.15%, the water retention rate is greater than 93%. ．After 20%, the increasing trend of water retention becomes flat, indicating that the amount of HPMC is close to saturation. The influence curve of the amount of HPMC with a viscosity of 40 Pa·s on the water retention rate is approximately a straight line. When the amount is greater than 0.15%, the water retention rate of the mortar is significantly lower than that of the other three kinds of HPMC with the same amount of viscosity. It is generally believed that the water retention mechanism of cellulose ether is: the hydroxyl group on the cellulose ether molecule and the oxygen atom on the ether bond will associate with the water molecule to form a hydrogen bond, so that the free water becomes bound water, thus playing a good water retention effect; It is also believed that the interdiffusion between water molecules and cellulose ether molecular chains allows water molecules to enter the interior of the cellulose ether macromolecular chains and be subject to strong binding forces, thereby improving the water retention of cement slurry. Excellent water retention can keep the mortar homogeneous, not easy to segregate, and obtain good mixing performance, while reducing mechanical wear and increasing the life of the mortar spraying machine.

2.2 The effect of HPMC on the density and air content of cement mortar

From the influence of different viscosities and dosages of HPMC on the density of mortar, it can be seen that when the dosage of HPMC is 0-0.20%, the density of mortar decreases sharply with the increase of HPMC dosage, from 2050 kg/m³ to about 1650kg/m³ , decreased by about 20%; after the HPMC content exceeds 0.20%, the density decrease tends to be flat. Comparing the four kinds of HPMC with different viscosities, it can be seen that the higher the viscosity, the lower the density of the mortar; the density curves of the mortars with the mixed viscosities of 150 and 200 Pa s HPMC basically overlap, indicating that as the viscosity of HPMC continues to increase, the density of mortar no longer decrease.

From the influence of different viscosities and dosages of HPMC on the air content of mortar, it can be seen that the change of air content of mortar is opposite to that of the density of mortar. The air volume almost rises in a straight line; when the HPMC content exceeds 0.20%, the air content hardly changes, indicating that the air-entraining effect of the mortar is close to saturation. The air-entraining effect of HPMC with viscosity of 150 and 200 Pa·s is greater than that of HPMC with viscosity of 40 and 100 Pa·s.

The air-entraining effect of cellulose ether is mainly determined by its molecular structure. Cellulose ether has both hydrophilic groups (hydroxyl, ether groups) and hydrophobic groups (methyl groups, glucose rings), and is a surfactant. , has surface activity, thus having an air-entraining effect. On the one hand, the introduced gas can act as a ball bearing in the mortar, improve the working performance of the mortar, increase the volume, and increase the output, which is beneficial to the manufacturer. But on the other hand, the air-entraining effect increases the air content of the mortar and the porosity after hardening, resulting in the increase of harmful pores and greatly reducing the mechanical properties. Although HPMC has a certain air-entraining effect, it cannot replace the air-entraining agent. In addition, when HPMC and air-entraining agent are used at the same time, the air-entraining agent may fail.

2.3 Effect of HPMC on mechanical properties of cement mortar

From the 28d flexural strength and 28d compressive strength, it can be seen that when the amount of HPMC is only 0.05%, the flexural strength of the mortar decreases significantly, which is about 25% lower than that of the blank sample without HPMC, and the compressive strength can only be Reach 65% of the blank sample. 80%. When the content of HPMC exceeds 0.20%, the degree of decrease in the flexural strength and compressive strength of the mortar is not obvious. The viscosity of HPMC has little effect on the mechanical properties of mortar. HPMC introduces a lot of tiny air bubbles, and the air-entraining effect on the mortar increases the internal porosity and harmful pores of the mortar, resulting in a significant decrease in compressive strength and flexural strength. Another reason for the decrease in mortar strength is the water retention effect of cellulose ether, which keeps water in the hardened mortar, and the large water-binder ratio leads to a decrease in the strength of the test block. For mechanical construction mortar, although cellulose ether can significantly increase the water retention rate of mortar and improve its workability, if the amount is too large, it will seriously affect the mechanical properties of mortar, so the relationship between the two should be weighed reasonably.

From the 28-day folding ratio, it can be seen that with the increase of the HPMC content, the overall folding ratio of the mortar shows an increasing trend, which is basically a linear relationship. This is because the added cellulose ether introduces a large number of air bubbles, causing more defects inside the mortar, resulting in a sharp decrease in the compressive strength of the mortar, and although the flexural strength also decreases to a certain extent; but the cellulose ether can improve the flexibility of the mortar and resist The folding strength is favorable, which makes the decrease rate slow down. Considering comprehensively, the combined effect of the two results in an increase in the folding ratio.

2.4 Effect of HPMC on the pore size of mortar

The pore size distribution curves of the four groups of samples A, B, C and D were measured by mercury intrusion porosimetry.

According to the pore size distribution curve, pore size distribution data and various statistical parameters of A-D samples, HPMC has a great influence on the pore structure of cement mortar:

(1) After adding HPMC, the pore size of cement mortar increases significantly. On the pore size distribution curve, the area of the image moves to the right, and the pore value corresponding to the peak value becomes larger. Also from the statistical data of pore size distribution and the median pore size in the test results of various statistical parameters, it can be seen that the median pore size of the cement mortar after adding HPMC is significantly larger than that of the blank sample, and in the sample with 0.3% dosage The value aperture is 2 orders of magnitude higher than that of the blank sample.

(2) Wu Zhongwei et al. divided the pores in concrete into four types, which are harmless pores (≤20 nm), few harmful pores (20–100 nm), harmful pores (100–200 nm) and many harmful pores (≥200 nm). 200 nm). From the pore size distribution statistical data and the test results of various statistical parameters, it can be seen that the number of harmless pores or less harmful pores is significantly reduced, and the number of harmful pores or more harmful pores is increased after adding HPMC. The harmless or less harmful pores of the sample without HPMC are about 49.4%, and the harmless or less harmful pores are significantly reduced after adding HPMC. Taking the dosage of 0.1% as an example, the harmless or less harmful pores are reduced by about 45%. , the number of harmful pores larger than 10 μm increased by about 9 times.

3) The median pore diameter, average pore diameter, specific pore volume and specific surface area do not follow a very strict change rule with the increase of HPMC content, which may be related to the large dispersion of sample selection in the mercury injection test. But on the whole, the median pore diameter, average pore diameter and specific pore volume of the sample mixed with HPMC tend to increase compared with the blank sample, while the specific surface area decreases.

3. Conclusion

(1) The water retention rate of mortar increases with the increase of HPMC content. The curves of cellulose ether with viscosities of 100, 150 and 200 Pa·S are basically the same, and the water retention rate is greater than 93% when the content is 0.15%. When the content of 40 Pa·s cellulose ether is greater than 0.15%, the water retention rate is lower than that of the other three kinds of viscosity HPMC.

(2) The density of mortar decreases gradually with the increase of HPMC content, and the content is 0.05%. The density decrease is the most obvious at 0.20%, about 20%; when the content exceeds 0.20%, the density hardly changes; the air content of mortar increases with the increase of HPMC content.

(3) The increase of HPMC content will obviously reduce the mechanical properties of cement mortar, but the corresponding folding ratio of mortar will increase, and the flexibility of mortar will become better.

(4) After adding HPMC, the pore size of the mortar increases significantly, and the proportion of harmful pores and multiple harmful pores increases significantly. The sample with 0.1% HPMC content reduced about 45% compared with the blank sample with no or less harmful pores, and the number of more harmful pores larger than 10 μm increased about 9 times.