With the continuous progress of the industry and the improvement of technology, through the introduction and improvement of foreign mortar spraying machines, the mechanical spraying and plastering technology has been greatly developed in my country in recent years. Mechanical spraying mortar is different from ordinary mortar, which requires high water retention performance, suitable fluidity and certain anti-sagging performance. Usually, hydroxypropyl methylcellulose is added to the mortar, of which cellulose Ether (HPMC) is the most widely used. The main functions of hydroxypropyl methylcellulose HPMC in mortar are: thickening and viscosifying, adjusting rheology, and excellent water retention capacity. 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. Shandong Chenbang Fine Chemical Co., Ltd. studied the influence of HPMC on the water retention rate, density, air content and mechanical properties of mortar from the macroscopic aspect, and studied the influence of hydroxypropyl methylcellulose HPMC on the L structure of the mortar from the microscopic aspect. .
1. Test
1.1 Raw materials
Cement: commercially available P.0 42.5 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: produced by Shandong Chenbang Fine Chemical Co., Ltd. Hydroxypropyl methylcellulose 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 80-120 mm, and the water retention rate is greater than 90%. In this experiment, the lime-sand ratio was set at 1:5, the consistency was controlled at (93+2) mm, and the cellulose ether was blended externally, and the blending amount was based on the cement mass. The basic properties of mortar such as wet density, air content, water retention, 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 according to GB/T 17671-1999 “Methods for Testing the Strength of Cement Mortar Sand (ISO Method)”. The diameter of the larvae was measured by mercury porosimetry. The model of the mercury porosimeter was AUTOPORE 9500, and the measuring range was 5.5 nm-360 μm. A total of 4 sets of tests were carried out. The cement-sand ratio was 1:5, the viscosity of HPMC was 100 Pa-s, and the dosage 0, 0.1%, 0.2%, 0.3% (the numbers are A, B, C, D respectively).
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. Effect of HPMC on water retention of mortar.
With the increase of HPMC content, the water retention rate of mortar increases gradually. The curves of hydroxypropyl methylcellulose ether 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%. ; When the amount of grits exceeds 0.20%, the increasing trend of water retention rate 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 hydroxypropyl methylcellulose HPMC on the density and air content of cement mortar
When the amount of HPMC is 0-0.20%, the density of the mortar decreases sharply with the increase of the amount of HPMC, from 2050 kg/m3 to about 1650kg/m3, which is about 20% lower; when the amount of HPMC exceeds 0.20%, the density decreases. in calm. Comparing the 4 kinds of HPMC with different viscosities, 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 no longer decreases.
The change law of the air content of mortar is opposite to the change of density of mortar. When the content of hydroxypropyl methylcellulose HPMC is 0-0.20%, with the increase of HPMC content, the air content of mortar increases almost linearly; the content of HPMC exceeds After 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 a viscosity of 150 and 200 Pa.s is greater than that of HPMC with a 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) and hydrophobic groups (methyl, glucose ring), 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
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 hydroxypropyl methylcellulose HPMC, and the compressive strength can only reach 65% of the blank sample -80%. 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 dosage is too large, it will seriously affect the mechanical properties of mortar, so the relationship between the two should be weighed reasonably.
With the increase of the content of hydroxypropyl methylcellulose HPMC, the folding ratio of the mortar showed an overall increasing trend, which was 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, and the compressive strength of the guide rose mortar decreases sharply, although the flexural strength also decreases to a certain extent; but the cellulose ether can improve the flexibility of the mortar, It is beneficial to the flexural strength, which makes the decrease rate slow down. Considering comprehensively, the combined effect of the two leads to an increase in the folding ratio.
2.4 The effect of HPMC on the L diameter of the mortar
From the pore size distribution curve, pore size distribution data and various statistical parameters of A-D samples, it can be seen that 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. After adding HPMC, the median pore diameter of the cement mortar is significantly larger than that of the blank sample, and the median pore diameter of the sample with 0.3% dosage is increased by 2 orders of magnitude compared with the blank sample.
(2) Divide the pores in concrete into four types, namely harmless pores (≤20 nm), less harmful pores (20-100 nm), harmful pores (100-200 nm) and many harmful pores (≥200 nm ). It can be seen from Table 1 that the number of harmless holes or less harmful holes is significantly reduced after adding HPMC, and the number of harmful holes or more harmful holes is increased. The harmless pores or less harmful pores of the samples not mixed with HPMC are about 49.4%. After adding HPMC, the harmless pores or less harmful pores are significantly reduced. Taking the dosage of 0.1% as an example, the harmless pores or less harmful pores are reduced by about 45%. %, the number of harmful holes larger than 10um 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 hydroxypropyl methylcellulose HPMC content, which may be related to the sample selection in the mercury injection test. related to large dispersion. 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.
Post time: Apr-03-2023