Views: 2 Author: Site Editor Publish Time: 2023-04-12 Origin: Site
Abstract: 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. When the dosage is 0.15%, the water retention rate can exceed 90%; with the increase of HPMC dosage, the density of mortar decreases gradually, and the density decreases most obviously when the dosage is 0.05%-0.20%; the air content of mortar increases with the increase of HPMC dosage. increase; HPMC will significantly reduce the mechanical properties of cement mortar, but the compressive ratio of the mortar has a general upward trend: after the addition of HPMC, the pore size of the mortar increases significantly, and the harmful and multi-harmful pores
The proportion increased significantly.
Keywords:Mortar；Hydroxypropyl Methyl Cellulose Ether；Water retention rate；Pore size distribution
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.
1.1 Raw materials
Cement: commercially available P·042.5 cement, its 28-day flexural and compressive strengths are 6.9 and 48.2 MPa, respectively; sand: Chengde fine river sand, 40-100 mesh; cellulose ether: KIMA CHEMICAL CO .,Hydroxypropyl methylcellulose ether produced by LTD, white powder, nominal viscosity is 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 mortar consistency is 80-120 mm, 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 based on the cement mass. Refer to JGJ 70 for the basic properties of mortar such as wet density, air content, water retention rate, and consistency. -2009 "Basic Performance Test Methods of Building Mortar" for testing, in which 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 diameter was tested by mercury porosimetry. The model of the mercury porosimeter was AUTOPORE 9500, and the measurement range was 5.5 nm to 360 nm. 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 retain water, reduce bleeding rate, and meet the requirements of full hydration of cement-based materials.
From the influence of different viscosity and dosage 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%-0.15%, the water retention rate increases linearly, and when the content is 0.15%, the water retention rate is greater than 93%. %: After the dosage exceeds 0.20%, the increasing trend of water retention rate becomes gentle, indicating that the dosage 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 the same viscosity and the content of tIPMC on the density of mortar, it can be seen that when the content of HPMC is 0-0.20%, the density of mortar decreases sharply with the increase of HPMC content, from 2050 kg/m3 to 1650 kg/m3 About m3, reduced 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 the change of density of mortar. When the dosage of HPMC is 0.0.20%, with the increase of HPMC dosage, the mortar The air content of the mortar increases almost linearly; after 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 The effect of HPMC on the mechanical properties of cement mortar
From the 28-day flexural strength and 28-day 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. The compressive strength can only reach 65%~80% of the blank sample. When the amount of HPMC exceeds 0.20%, the 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, which causes 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 The effect of HPMC on the pore size of mortar
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 median pore diameter in Table 1, it can be seen that the median pore diameter of the cement mortar after adding HPMC is significantly larger than that of the blank sample, and the median pore diameter of the sample with 0.3% dosage is higher than that of the blank sample. 2 orders of magnitude.
(2) Wu Zhongwei et al. divided the pores in concrete into four categories, namely, harmless pores (≤20 nm), less harmful pores (20–100 nm), harmful pores (100–200 nm) and many harmful pores (t >200nm). From the statistical data of pore size distribution 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%. After adding HPMC, the harmless or less harmful pores are significantly reduced. Taking the dosage of 0.1% as an example, the harmless or less harmful pores are reduced by about 45%, and 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 pore volume of the sample mixed with HPMC tend to increase compared with the blank sample, while the specific surface area decreases.
(1) The water retention rate of mortar increases with the increase of HPMC content, and 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%. ; The water retention rate of the cellulose ether with a viscosity of 40 Pa·s is lower than that of the other three kinds of HPMC when the content is greater than 0.15%.
(2) The density of mortar decreases gradually with the increase of HPMC content, and the content is 0.05%. 0.20% 11-inch density decreased most obviously, decreased by about 20%; the density hardly changed after the content exceeded 0.20%; the air content of mortar increased 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 HPMC content of 0.1% decreased about 45% compared with the empty A sample with no or less harmful pores, and the number of more harmful pores larger than 10 μm increased about 9 times.