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Views: 0 Author: Site Editor Publish Time: 2023-04-13 Origin: Site

**Abstract:** As a modifier, hydroxypropyl methylcellulose solution is mixed into cement mortar to improve its toughness. The effects of water-binder ratio, concentration and content of cellulose solution on the flexural strength, compressive strength and flexural ratio of cement mortar were studied by orthogonal test, and the optimal mix ratio was obtained. The effects of superplasticizer and cellulose on strength and folding ratio, and the effect of cellulose on setting time were studied by single factor analysis. The final results show that hydroxypropyl methylcellulose can improve the toughness of cement mortar with limited influence on setting time.

**Key words: **hydroxypropyl methylcellulose; cement mortar; flexural strength; compressive strength

**0****.****Preface**

With the continuous application of high-strength concrete and ultra-high-strength concrete in engineering, some scholars have proposed many methods to improve the brittleness of these concretes. Most of these methods are to add some fibers. For example, Wu Zhaoxian found that basalt fibers can improve the compressive strength, splitting strength and tensile strength of BFRC. According to Li Kun's research, single doping of polypropylene fiber and basalt fiber will reduce the compressive strength of recycled concrete, but will increase the splitting tensile strength. Wu Tao found that fibers can toughen concrete. Zhang Xueyuan analyzed the toughening effect of straw fiber on concrete from the perspective of tension-compression ratio, and the results showed that the tension-compression ratio increased with the increase of fiber content, and decreased with the increase of fiber length and fly ash content; fiber modification Although the post-tension-compression ratio decreased, it was still higher than that of the no-fiber group; the effect of straw fiber on increasing the tension-compression ratio was similar to that of other fibers, and the effect of improving the impact strength after modification was significant. The above-mentioned methods are all used to enhance the toughness of dense concrete. However, for porous concrete, its strength is formed by the slurry wrapping the aggregate and then closely combining the bonding points, so it is difficult to use the above-mentioned methods. Enhance its toughness. Li Xiaojun found that increasing the thickness of cement slurry film can improve the strength of porous concrete.

Based on the research results of the above scholars, it is found that to improve the toughness of porous concrete, it is necessary to consider improving the toughness of its cement mortar. Liu Pengfei and other studies believe that hydroxypropyl methylcellulose HPMC can increase the folding ratio of mortar. It can be seen that it is feasible to use HPMC to modify cement mortar and improve its toughness. In this paper, based on orthogonal experiments, the effects of water-binder ratio, HPMC solution concentration and dosage on the toughness of cement mortar were studied, and the optimal mix ratio design was obtained; then the effects of water-binder ratio, water reducer and HPMC on The impact of its toughness, but also the impact of the amount of HPMC on the setting time of cement mortar.

**1. Test overview**

**1.1 Test raw materials**

The cement is Xiujian brand P·042.5 bagged cement produced by Xinxiang Cement Factory. The water reducer is polycarboxylate water reducer, the dosage is 1.5% of the cement dosage. Hydroxypropyl methyl cellulose with 60000mpa. The viscosity specification of S, the viscosity change law experiment of different concentration and temperature is measured by the NDJ-9S digital display rotational viscometer produced by Lichen Technology.

**1.2 Test method**

The test will use the ratio of flexural and compressive strength of cement mortar as a control index of its toughness to carry out experimental research. The size of the flexural strength test piece is 40mm×40mm×160mm, a group of three test pieces; the size of the compressive strength test piece is 40mm×40mm×40mm, a group of six test pieces. The test results were taken as the average value of each group.

Refer to GB/T 17671-1999 "Cement Mortar Strength Test Method (ISO Method)" for the test. The cement mortar is mixed with a planetary NJ-160B cement slurry mixer. After weighing the required materials, pour them into the In the mixing pot of the cement paste mixer, first stir slowly for 30s, then stir rapidly for 90s. After the mortar is loaded into the mold, use a cement mortar vibrator to vibrate for 120s. After the vibration is completed, scrape off the excess cement mortar with a scraper. The curing adopts SHBY-90B cement concrete standard curing box for 3 days under the conditions of humidity above 90% and temperature 20±1°C. The compressive strength test uses a WA-1000B electro-hydraulic universal testing machine to test the peak damage, and the flexural strength uses an electric flexural tester (5000 type) to test.

**1.3 Experimental design**

The research experiment adopts the orthogonal test design. According to the trial mix, when the water-binder ratio is less than 0.3, the fluidity of the cement slurry is poor and it is not suitable for concrete. Therefore, the water-binder ratio is selected at three levels: 0.30, 0.35, and 0.40. In order to reflect the adhesiveness of hydroxypropyl methylcellulose, it is mixed by solvent method; the concentration of hydroxypropyl methylcellulose solution is 0.5%, 1.0%, 2.0%, hydroxypropylmethylcellulose The mass of cellulose solution is 25, 50, 100g. Since the solution is added, the quality of water in the solution should be taken into account when designing the mixing ratio to keep the water-binder ratio as the design water-binder ratio.

**2. Test results and analysis**

**2.1 Orthogonal test**

From the results of range analysis, it can be seen that the compressive strength and flexural strength are most affected by the water-binder ratio. The second is the concentration and dosage of hydroxypropyl methylcellulose solution respectively. Both the compressive strength and the flexural strength decrease with the increase of the water-binder ratio, and the flexural strength decreases with the increase of the concentration of hydroxypropyl methylcellulose solution, and the compressive strength first increases and then decreases, while with the With the increase of solution content, the flexural strength first decreases and then increases, and the compressive strength increases.

In the influence of basalt fiber volume content on the flexural and compressive strength of concrete, Zhou Hao et al. found that the relationship curves of the toughness index and folding ratio are consistent with the basalt fiber volume content, so the paper uses the folding ratio to characterize cement mortar. toughness. The folding ratio decreases with the increase of the water-binder ratio, and there is almost no change in the folding ratio when the water-binder ratio is greater than 0.35; the folding ratio decreases with the increase of the concentration of the cellulose solution; The amount first decreases and then increases. This shows that the water-binder ratio has a limited effect on the folding ratio, and the addition of cellulose has a certain effect on the strength of cement mortar, but the effect is far lower than that of the water-binder ratio when the amount is small.

Taking the compressive strength as the test index, the best mix ratio is A1B2C3, and taking the flexural strength and folding compression ratio as the test indicators, the best mix ratio is A1B1C3. The folding ratio is almost the same when the concentration of hydroxypropyl methylcellulose solution is 1.0% and the dosage is 100g. Comprehensive consideration of the best mix ratio is A1B1C3.

**2.2 Single factor analysis**

According to the above test results, the optimal mix ratio is designed as water-cement ratio 0.3, cellulose solution concentration 0.5%, and dosage 100g/kg cement. In order to further study the relationship between the folding ratio and the three factors of water-binder ratio, water reducing agent and cellulose, a single factor analysis was carried out.

After 3d natural curing, the flexural strength and compressive strength were tested respectively.

From the single factor test results, it can be seen that the addition of water reducing agent has a significant effect on improving the flexural strength and compressive strength, but its folding ratio will decrease, indicating that the addition of water reducing agent will increase the brittleness of cement mortar. After adding cellulose solution, its strength will still be improved, but the folding ratio is also higher than that before no addition, indicating that the addition of cellulose solution can improve the toughness of cement mortar.

**2.3 Effect of coagulation time**

Hydroxypropyl methylcellulose is a chemical dispersant with certain water retention. It has a certain influence on the setting time of cement mortar, and the setting time is tested by using the Weibo consistency meter.

From the test results of the influence of cellulose on the setting time of cement mortar, it can be seen that the addition of hydroxypropyl methylcellulose will affect the setting time of cement mortar, the initial setting time is reduced by 30 minutes, and the final setting time is increased by 5 minutes, indicating that cellulose does have Good water retention, and the effect on setting time does not change with the change of cellulose concentration when the concentration is lower than 0.5%. The addition of hydroxypropyl methylcellulose does not affect the setting time of cement mortar too much, and has little effect on engineering applications.

**3. Conclusion**

(1) Considering the improvement of cement mortar toughness, the best mix ratio of water-binder ratio, cellulose solution concentration and dosage is A1B1C3, that is, water-binder ratio is 0.3, cellulose solution concentration is 0.5%, and dosage is 100g /1kg cement.

(2) Hydroxypropyl methylcellulose can not only improve the strength of cement mortar but also improve its toughness.

(3) When the concentration of hydroxypropyl methylcellulose is lower than 0.5%, the initial setting time of cement mortar can be reduced by 30 minutes, and the final setting time can be increased by 5 minutes, which has little impact on engineering applications.

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