Abstract: Cellulose ether is the main additive in ready-mixed mortar. The types and structural characteristics of cellulose ether are introduced, and hydroxypropyl methylcellulose ether (HPMC) is selected as the additive to systematically study the influence on various properties of mortar. . Studies have shown that: HPMC can significantly improve the water retention of mortar, and has the effect of reducing water. At the same time, it can also reduce the density of mortar mixture, prolong the setting time of mortar, and reduce the flexural and compressive strength of mortar.
Key words: ready-mixed mortar; hydroxypropyl methylcellulose ether (HPMC); performance
0.Preface
Mortar is one of the most widely used materials in the construction industry. With the development of material science and the improvement of people’s requirements for building quality, mortar has gradually developed towards commercialization just like the promotion and development of ready-mixed concrete. Compared with mortar prepared by traditional technology, commercially produced mortar has many obvious advantages: (a) high product quality; (b) high production efficiency; (c) less environmental pollution and convenient for civilized construction. At present, Guangzhou, Shanghai, Beijing and other cities in China have promoted ready-mixed mortar, and relevant industry standards and national standards have been issued or will be issued soon.
From the perspective of composition, a big difference between ready-mixed mortar and traditional mortar is the addition of chemical admixtures, among which cellulose ether is the most commonly used chemical admixture. Cellulose ether is usually used as a water-retaining agent. The purpose is to improve the operability of ready-mixed mortar. The amount of cellulose ether is small, but it has a significant impact on the performance of the mortar. It is a major additive that affects the construction performance of the mortar. Therefore, further understanding of the impact of the types and structural characteristics of cellulose ether on the performance of cement mortar will help to select and use cellulose ether correctly and ensure the stable performance of mortar.
1. Types and structural characteristics of cellulose ethers
Cellulose ether is a water-soluble polymer material, which is processed from natural cellulose through alkali dissolution, grafting reaction (etherification), washing, drying, grinding and other processes. Cellulose ethers are divided into ionic and nonionic, and ionic cellulose has carboxymethyl cellulose salt. Nonionic cellulose includes hydroxyethyl cellulose ether, hydroxypropyl methyl cellulose ether, methyl cellulose ether and the like. Because ionic cellulose ether (carboxymethyl cellulose salt) is unstable in the presence of calcium ions, it is rarely used in dry powder products with cement, slaked lime and other cementing materials. The cellulose ethers used in dry powder mortar are mainly hydroxyethyl methyl cellulose ether (HEMC) and hydroxypropyl methyl cellulose ether (HPMC), which account for more than 90% of the market share.
HPMC is formed by etherification reaction of cellulose alkali activation treatment with etherification agent methyl chloride and propylene oxide. In the etherification reaction, the hydroxyl group on the cellulose molecule is substituted by methoxy) and hydroxypropyl to form HPMC. The number of groups substituted by the hydroxyl group on the cellulose molecule can be expressed by the degree of etherification (also called the degree of substitution). The ether of HPMC The degree of chemical conversion is between 12 and 15. Therefore, there are important groups such as hydroxyl (-OH), ether bond (-o-) and anhydroglucose ring in the HPMC structure, and these groups have a certain impact on the performance of the mortar .
2. Effect of cellulose ether on the properties of cement mortar
2.1 Raw materials for the test
Cellulose ether: produced by Luzhou Hercules Tianpu Chemical Co., Ltd., viscosity: 75000;
Cement: Conch brand 32.5 grade composite cement; sand: medium sand; fly ash: grade II.
2.2 Test results
2.2.1 Water-reducing effect of cellulose ether
From the relationship between the consistency of the mortar and the content of cellulose ether under the same mixing ratio, it can be seen that the consistency of the mortar increases gradually with the increase of the content of cellulose ether. When the dosage is 0.3‰, the consistency of the mortar is about 50% higher than that without mixing, which shows that cellulose ether can significantly improve the workability of the mortar. As the amount of cellulose ether increases, the water consumption can gradually decrease. It can be considered that cellulose ether has a certain water-reducing effect.
2.2.2 Water retention
The water retention of mortar refers to the ability of mortar to retain water, and it is also a performance index to measure the stability of the internal components of fresh cement mortar during transportation and parking. The water retention can be measured by two indicators: degree of stratification and water retention rate, but due to the addition of water retaining agent, the water retention of ready-mixed mortar has been significantly improved, and the degree of stratification is not sensitive enough to reflect the difference. The water retention test is to calculate the water retention rate by measuring the mass change of the filter paper before and after the filter paper contacts with the specified area of mortar within a certain period of time. Due to the good water absorption of the filter paper, even if the water retention of the mortar is high, the filter paper can still absorb the moisture in the mortar, so. The water retention rate can accurately reflect the water retention of the mortar, the higher the water retention rate, the better the water retention.
There are many technical ways to improve the water retention of mortar, but adding cellulose ether is the most effective way. The structure of cellulose ether contains hydroxyl and ether bonds. The oxygen atoms on these groups associate with water molecules to form hydrogen bonds. Make free water molecules into bound water, so as to play a good role in water retention. From the relationship between the water retention rate of mortar and the content of cellulose ether, it can be seen that within the range of the test content, the water retention rate of mortar and the content of cellulose ether show a good corresponding relationship. The higher the content of cellulose ether, the greater the water retention rate. .
2.2.3 Density of mortar mixture
It can be seen from the change law of the density of the mortar mixture with the content of cellulose ether that the density of the mortar mixture gradually decreases with the increase of the content of cellulose ether, and the wet density of the mortar when the content is 0.3‰o Decreased by about 17% (compared with no blend). There are two reasons for the decrease in mortar density: one is the air-entraining effect of cellulose ether. The cellulose ether contains alkyl groups, which can reduce the surface energy of the aqueous solution, and have an air-entraining effect on cement mortar, making the air content of the mortar increase, and the toughness of the bubble film is also higher than that of pure water bubbles, and it is not easy to discharge; on the other hand, the cellulose ether expands after absorbing water and occupies a certain volume, which is equivalent to increasing the internal pores of the mortar, so it causes the mortar to mix Density drops.
The air-entraining effect of cellulose ether improves the workability of the mortar on the one hand, and on the other hand, due to the increase of the air content, the structure of the hardened body is loosened, resulting in the negative effect of decreasing the mechanical properties such as strength.
2.2.4 Coagulation time
From the relationship between the setting time of mortar and the amount of ether, it can be clearly seen that cellulose ether has a retarding effect on mortar. The greater the dosage, the more obvious the retarding effect.
The retarding effect of cellulose ether is closely related to its structural characteristics. Cellulose ether retains the basic structure of cellulose, that is to say, the anhydroglucose ring structure still exists in the molecular structure of cellulose ether, and the anhydroglucose ring is the cause of The main group of cement retarding, which can form sugar-calcium molecular compounds (or complexes) with calcium ions in the cement hydration aqueous solution, which reduces the calcium ion concentration in the cement hydration induction period and prevents Ca(OH): And calcium salt crystal formation, precipitation, and delay the process of cement hydration.
2.2.5 Strength
From the influence of cellulose ether on the flexural and compressive strength of mortar, it can be seen that with the increase of the content of cellulose ether, the 7-day and 28-day flexural and compressive strengths of mortar all show a downward trend.
The reason for the decrease in mortar strength can be attributed to the increase of air content, which increases the porosity of the hardened mortar and makes the internal structure of the hardened body loose. Through the regression analysis of the wet density and compressive strength of mortar, it can be seen that there is a good correlation between the two, the wet density is low, the strength is low, and vice versa, the strength is high. Huang Liangen used the relationship equation between porosity and mechanical strength derived by Ryskewith to deduce the relationship between the compressive strength of mortar mixed with cellulose ether and the content of cellulose ether.
3. Conclusion
(1) Cellulose ether is a derivative of cellulose, containing hydroxyl,
Ether bonds, anhydroglucose rings and other groups, these groups affect the physical and mechanical properties of mortar.
(2) HPMC can significantly improve the water retention of mortar, prolong the setting time of mortar, reduce the density of mortar mixture and the strength of hardened body.
(3) When preparing ready-mixed mortar, cellulose ether should be used reasonably. Solve the contradictory relationship between mortar workability and mechanical properties.
Post time: Feb-20-2023