The modified cement mortar with a water-cement ratio of 0.45, a lime-sand ratio of 1:2.5, and cellulose ether with different viscosities of 0%, 0.2%, 0.4%, 0.6%, 0.8%, and 1.0% was prepared. By measuring the mechanical properties of cement mortar and observing the microscopic morphology, the effect of HEMC on the compressive strength, flexural strength and bond strength of modified cement mortar was studied. The research results show that: with the increase of HEMC content, the compressive strength of modified mortar at different ages decreases continuously, and the decrease range decreases and tends to be gentle; when the same content of cellulose ether is added, The compressive strength of cellulose ether modified mortar with different viscosities is: HEMC20<HEMC10<HEMC5. The flexural strength of cellulose ether modified mortar decreases gradually with the increase of HEMC content. With the increase of the degree of polymerization of cellulose ether, the change of bond strength of modified mortar is: HEMC20>HEMC10>HEMC5.
Key words: cellulose ether; cement mortar; compressive strength; flexural strength; bond strength
1 Introduction
At this stage, the annual demand for mortar in the world exceeds 200 million tons, and the industrial demand is still rising. At present, the traditional cement mortar has defects such as bleeding, delamination, large drying shrinkage, poor impermeability, low tensile bond strength, and incomplete hydration due to water loss, which are difficult to solve, not only causing construction defects, but also leading to hardening Phenomena such as mortar cracking, pulverization, shedding, and hollowing occur.
As one of the most commonly used admixtures for commercial mortar, cellulose ether has the functions of water retention, thickening and retardation, and can be used to improve the physical properties of cement mortar such as workability, water retention, bonding performance, and setting time, such as significantly increasing cement. The tensile bond strength of mortar will be reduced, but the compressive strength, flexural strength and elastic modulus of cement mortar will be reduced. Zhang Yishun and others studied the effect of methyl cellulose ether and hydroxypropyl methyl cellulose ether on the properties of mortar. The results showed that: both cellulose ethers can improve the water retention of mortar, and the flexural strength and The compressive strength decreases in different degrees, while the folding ratio and bonding strength of mortar increase in different degrees, and the shrinkage performance of mortar can be improved. AJenni, R.Zurbriggen, etc. used modern testing and analysis techniques to study the interaction of various materials in the cellulose ether modified thin-layer adhesive mortar system, and observed that cellulose ether and Ca(OH) appeared near the surface of the mortar. 2, indicating the migration of cellulose ethers in cement-based materials.
In this paper, using mortar testing methods such as compressive resistance, flexural resistance, bonding, and SEM microscopic appearance, the influence of cellulose ether cement mortar on mechanical properties such as compressive strength, flexural resistance, and bond strength at different ages is studied, and it is expounded. its mechanism of action.
2. Raw materials and test methods
2.1 Raw materials
2.1.1 Cement
Ordinary laurate cement produced by Wuhan Huaxin Cement Co., Ltd., model P 042.5 (GB175-2007), has a density of 3.25g/cm³ and a specific surface area of 4200cm²/g.
2.1.2 Hydroxypropyl methylcellulose ether
The hydroxyethyl methyl cellulose ether produced by the Hercules Group of the United States has viscosities of 50000MPa/s, 100000MPa/s, and 200000MPa/s in 2% solution at 25 °C, and the following abbreviations are HEMC5, HEMC10, and HEMC20.
2.2 Test method
a. Compressive strength of modified mortar
The compressive strength of the green body specimens was tested with a TYE-300 compressive strength machine from Wuxi Jianyi Instrument Co., Ltd. The loading rate is 0.5 kN/s. The compressive strength test is carried out according to GB/T17671-1999 “Cement Mortar Strength Test Method (ISO Method)”.
By definition, the formula for calculating the compressive strength of the green body is:
Rc=F/S
Where Rc—compressive strength, MPa;
F—the failure load acting on the specimen, kN;
S—pressure area, m².
By definition, the formula for calculating the flexural strength of the green body is:
Rf= (3P × L)/(2b × h²) =0.234×P
In the formula, Rf—flexural strength, MPa;
P—the failure load acting on the specimen, kN;
L—the distance between the centers of supporting cylinders, that is, 10cm;
b, h—the width and height of the cross-section of the test body, both of which are 4cm.
b. Tensile bond strength of modified cement mortar
Use ZQS6-2000 Adhesive Brick Adhesive Strength Detector to measure the adhesive strength, and the tensile speed is 2mm/min. The bonding strength test was carried out according to JC/T985-2005 “Cement-based self-leveling mortar for ground”.
By definition, the formula for calculating the bond strength of the green body is:
P=F/S
In the formula, P—tensile bond strength, MPa;
F—maximum failure load, N;
S—bonding area, mm².
3. Results and discussion
3.1 Compressive strength
From the compressive strength of two kinds of cellulose ether modified mortars with different viscosities at different ages, it can be seen that with the increase of HEMC content, the compressive strength of cellulose ether modified mortars at different ages (3d, 7d and 28d) decreased significantly. Significantly decreased and gradually stabilized: when the content of HEMC was less than 0.4%, the compressive strength decreased significantly compared with the blank sample; when the content of HEMC was 0.4%~1.0%, the trend of decreasing compressive strength slowed down. When the cellulose ether content is greater than 0.8%, the compressive strength of the 7d and 28d age is lower than that of the blank sample at the 3d age, while the compressive strength of the modified mortar 3d is almost zero, and the sample is lightly pressed Instantly crushed, the inside is powdery, and the density is very low.
The impact of the same HEMC on the compressive strength of modified mortar at different ages is also different, showing that the compressive strength of 28d decreases with the increase of HEMC content more than that of 7d and 3d. This shows that the retarding effect of HEMC has always existed with the increase of age, and the retarding effect of HEMC has not been affected by the reduction of water in the system or the progress of hydration reaction, resulting in the growth of the compressive strength of the modified mortar being much smaller than that without Mortar samples mixed with HEMC.
From the change curve of compressive strength of cellulose ether modified mortar at different ages, it can be seen that when the same amount of cellulose ether is added, the compressive strength of cellulose ether modified mortar with different viscosities is: HEMC20<HEMC10< HEMC5. With the increase of cellulose ether content, the decrease range of compressive strength of cellulose ether modified mortar changes as follows: HEMC20>HEMC10>HEMC5. This is because HEMC with a high degree of polymerization has a greater effect on the reduction of the compressive strength of mortar than HEMC with a low degree of polymerization, but the compressive strength of the modified mortar mixed with HEMC is much lower than that of the blank mortar without HEMC.
The following three factors lead to the decrease of the compressive strength of the modified mortar: on the one hand, because the water-soluble HEMC macromolecular network structure covers the cement particles, C-S-H gel, calcium oxide, calcium aluminate hydrate and other particles and unhydrated particles On the surface, especially in the early stage of cement hydration, the adsorption between calcium aluminate hydrate and HEMC slows down the hydration reaction of calcium aluminate, resulting in a significant decrease in compressive strength. The retarding effect of the permanent mortar is obvious, which shows that when the content of HEMC20 reaches 0.8%~1%, the 3d strength of the modified mortar sample is zero; on the other hand, the hydrated HEMC solution has a higher viscosity, and the During the mixing process of the mortar, it can be mixed with air to form a large number of air bubbles, resulting in a large number of voids in the hardened mortar, and the compressive strength of the sample decreases continuously with the increase of HEMC content and the increase of its polymerization degree; The mortar system only increases the flexibility of the mortar and cannot play the role of rigid support, so the compressive strength is reduced.
3.2 Flexural strength
From the flexural strength of two different viscosity cellulose ether modified mortars at different ages, it can be seen that similar to the change in the compressive strength of modified mortar, the flexural strength of cellulose ether modified mortar gradually decreases with the increase of HEMC content .
From the change curve of flexural strength of cellulose ether modified mortar at different ages, it can be seen that when the content of cellulose ether is the same, the flexural strength of HEMC20 modified mortar sample is slightly lower than that of HEMC10 modified mortar sample, while When the content of HEMC is 0.4%~0.8%, the 28d flexural strength change curves of the two almost coincide.
From the change curve of the flexural strength of cellulose ether modified mortar at different ages, it can also be seen that the change in the flexural strength of modified mortar is: HEMC5<HEMC10<HEMC20, that is, the flexural strength of HEMC20 modified mortar is slightly lower, and the reduction Smaller and larger. The flexural strength of modified mortar is lower than that of unmodified mortar.
3.3 Bond strength
It can be seen from the variation curves of the bond strength of the three cellulose ether modified mortars at different ages that the bond strength of the modified mortar increases with the increase of HEMC content and gradually tends to be stable. With the extension of age, the bond strength of modified mortar also showed an increasing trend.
It can be seen from the 28-day bond strength change curves of the three cellulose ether modified mortars that the bond strength of the modified mortar increases with the increase of HEMC content, and gradually tends to be stable. At the same time, with the increase of the degree of polymerization of cellulose ether, the change of bond strength of modified mortar is: HEMC20>HEMC10>HEMC5.
This is due to the introduction of a large number of pores into the modified mortar with high HEMC content, resulting in the increase of the porosity of the hardened body, the decrease of the density of the structure, and the slow growth of the bond strength; in the tensile test, the fracture occurred in the modified mortar Inside, there is no fracture at the contact surface between the modified mortar and the substrate, which indicates that the bond strength between the modified mortar and the substrate is greater than that of the hardened modified mortar. However, when the amount of HEMC is low (0%~0.4%), the water-soluble HEMC molecules can cover and wrap on the hydrated cement particles, and form a polymer film between the cement particles, which increases the flexibility and flexibility of the modified mortar. Plasticity, and due to the excellent water retention of HEMC, the modified mortar has sufficient water for hydration reaction, which ensures the development of cement strength, and the bond strength of modified cement mortar increases linearly.
3.4 SEM
From the SEM comparison images before and after the cellulose ether modified mortar, it can be seen that the gaps between the crystal grains in the unmodified mortar are relatively large, and a small amount of crystals are formed. In the modified mortar, the crystals grow fully, the incorporation of cellulose ether improves the water retention performance of the mortar, the cement is fully hydrated, and the hydration products are obvious.
This is because cellulose ether has been treated with a special etherification process, which has excellent dispersion and water retention. The water is gradually released over a long period of time, only a small amount of water escapes from the capillary pores due to drying and evaporation, and most of the water hydrates with the cement to ensure the strength of the modified cement mortar.
4 Conclusion
a. As the content of HEMC increases, the compressive strength of modified mortar at different ages decreases continuously, and the range of reduction decreases and tends to be flat; when the content of cellulose ether is greater than 0.8%, the 7d and 28d The compressive strength of the 3d-aged blank sample is lower than that of the blank sample, while the 3d-aged compressive strength of the modified mortar is almost zero. The sample breaks when lightly pressed, and the inside is powdery with low density.
b. When the same amount of cellulose ether is added, the compressive strength of cellulose ether modified mortar with different viscosities changes as follows: HEMC20<HEMC10<HEMC5. With the increase of cellulose ether content, the compressive strength of cellulose ether modified mortar changes as follows: HEMC20>HEMC10>HEMC5.
c. The flexural strength of cellulose ether modified mortar decreases gradually with the increase of HEMC content. The change of flexural strength of modified mortar is: HEMC5<HEMC10<HEMC20.
d. The bonding strength of modified mortar increases with the increase of HEMC content, and gradually tends to be stable. At the same time, with the increase of the degree of polymerization of cellulose ether, the change of bond strength of modified mortar is: HEMC20>HEMC10>HEMC5.
e. After the cellulose ether is mixed into the cement mortar, the crystal grows fully, the pores between the crystal grains are reduced, and the cement is fully hydrated, which ensures the compressive, flexural and bonding strength of the cement mortar.