Cellulose ether and latex powder in commercial mortar

The development history of commercial mortar at home and abroad is briefly described, and the functions of two polymer dry powders, cellulose ether and latex powder, in dry-mixed commercial mortar are discussed, including water retention, capillary water absorption, and flexural strength of the mortar. , compressive strength, elastic modulus, and the influence of bond tensile strength of different environmental temperature curing.

Key words: commercial mortar; development history; physical and mechanical properties; cellulose ether; latex powder; effect

Commercial mortar must experience a development process of start, prosperity and saturation just like commercial concrete. The author proposed in “China Building Materials” in 1995 that the development and promotion in China may still be a fantasy, but today, commercial mortar is known by people in the industry like commercial concrete, and the production in China has begun to take shape. Of course, it still belongs to the infancy. Commercial mortar is divided into two categories: pre-mixed (dry) mortar and ready-mixed mortar. Premixed (dry) mortar is also known as dry powder, dry mix, dry powder mortar or dry mix mortar. It is composed of cementitious materials, fine aggregates, admixtures and other solid materials. It is a semi-finished mortar made of accurate ingredients and uniform mixing in the factory, without mixing water. Mixing water is added when stirring at the construction site before use. Different from pre-mixed (dry) mortar, ready-mixed mortar refers to the mortar that is completely mixed in the factory, including the mixing water. This mortar can be used directly when it is transported to the construction site.

China vigorously developed commercial mortar in the late 1990s. Today, it has developed to hundreds of production plants, and the manufacturers are mainly distributed in Beijing, Shanghai, Guangzhou and their surrounding areas. Shanghai is an area that developed commodity mortar earlier. In 2000, Shanghai had promulgated and implemented the Shanghai local standard “Technical Regulations for the Production and Application of Dry-Mixed Mortar” and “Technical Regulations for the Production and Application of Ready-Mixed Mortar”. The Notice on Ready-Mixed (Commercial) Mortar, clearly stipulating that from 2003 onwards, all new construction projects within the Ring Road will use ready-mixed (commercial) mortar, and from January 1, 2004, all new construction projects in Shanghai will use ready-mixed (commercial) mortar. ) mortar, which is the first policy and regulation in my country to promote the use of ready-mixed (commodity) mortar. In January 2003, the “Shanghai Ready-Mixed (Commercial) Mortar Product Certification Management Measures” was promulgated, which implemented certification management and approval management for ready-mixed (commercial) mortar, and clarified the production ready-mixed (commercial) mortar enterprises should achieve Technical conditions and basic laboratory conditions. In September 2004, Shanghai issued the “Notice of Several Regulations on the Use of Ready-Mixed Mortar in Construction Projects in Shanghai”. Beijing has also promulgated and implemented the “Technical Regulations for the Production and Application of Commodity Mortar”. Guangzhou and Shenzhen are also compiling and implementing the “Technical Regulations for the Application of Dry-Mixed Mortar” and the “Technical Regulations for the Application of Ready-Mixed Mortar”.

With the increasing development of dry-mixed mortar production and application, in 2002, China Bulk Cement Promotion and Development Association held a dry-mixed mortar seminar. In April 2004, the China Bulk Cement Promotion and Development Association established the dry-mixed mortar professional committee. In June and September of the same year, national and international dry-mixed mortar technology seminars were held in Shanghai and Beijing respectively. In March 2005, the Materials Branch of China Construction Industry Association also held a national lecture on construction dry-mixed mortar technology and an exchange meeting for the promotion and application of new technologies and new achievements. The Building Materials Branch of the Architectural Society of China plans to hold the National Academic Exchange Conference on Commodity Mortar in November 2005.

Like commercial concrete, commercial mortar has the characteristics of centralized production and unified supply, which can create favorable conditions for adopting new technologies and materials, implementing strict quality control, improving construction methods, and ensuring project quality. The superiority of commercial mortar in terms of quality, efficiency, economy and environmental protection is just as expected a few years ago. With the research and development and promotion and application, it has been increasingly shown and is being gradually recognized. The author has always believed that the superiority of commercial mortar can be summed up in four words: many, fast, good, and economical; Fast means fast material preparation and fast construction; three good is good water retention, good workability, and good durability; four provinces are labor-saving, material-saving, money-saving, and worry-free). In addition, the use of commercial mortar can achieve civilized construction, reduce material stacking sites, and avoid dust flying, thereby reducing environmental pollution and protecting city appearance.

The difference from commercial concrete is that commercial mortar is mostly premixed (dry) mortar, which is composed of solid materials, and the admixture used is generally solid powder. Polymer-based powders are usually referred to as polymer dry powders. Some premixed (dry) mortars are mixed with six or seven kinds of polymer dry powders, and different polymer dry powders play different roles. This article takes one kind of cellulose ether and one kind of latex powder as examples to illustrate the role of polymer dry powder in premixed (dry) mortar. In fact, this effect is suitable for any commercial mortar including ready-mixed mortar.

1. Water retention

The water retention effect of mortar is expressed by water retention rate. The water retention rate refers to the ratio of the water retained by the freshly mixed mortar after the filter paper absorbs water to the water content. The increase of cellulose ether content can significantly improve the water retention rate of fresh mortar. The increase in the amount of latex powder can also significantly improve the water retention rate of the freshly mixed mortar, but the effect is far less than that of cellulose ether. When cellulose ether and latex powder are blended together, the water retention rate of freshly mixed mortar is higher than that of mortar mixed with cellulose ether or latex powder alone. The water retention rate of compound blending is basically the superposition of single blending of one polymer.

2. Capillary water absorption

From the relationship between the water absorption coefficient of mortar and the content of cellulose ether, it can be seen that after adding cellulose ether, the capillary water absorption coefficient of the mortar becomes smaller, and with the increase of the content of cellulose ether, the water absorption coefficient of the modified mortar decreases gradually. Small. From the relationship between the water absorption coefficient of mortar and the amount of latex powder, it can be seen that after adding latex powder, the capillary water absorption coefficient of mortar also becomes smaller. Generally speaking, the water absorption coefficient of mortar decreases gradually with the increase of latex powder content.

3. Flexural strength

The addition of cellulose ether reduces the flexural strength of the mortar. The addition of latex powder increases the flexural strength of the mortar. Latex powder and cellulose ether are compounded, and the flexural strength of the modified mortar does not change much due to the compound effect of the two.

4. Compressive strength

Similar to the effect on the flexural strength of mortar, the addition of cellulose ether reduces the compressive strength of mortar, and the reduction is greater. But when the content of cellulose ether exceeds a certain value, the compressive strength of the modified mortar will not change greatly.

When the latex powder is mixed alone, the compressive strength of the modified mortar also shows a decreasing trend with the increase of the latex powder content. Latex powder and cellulose ether compounded, with the change of latex powder content, the decrease of mortar compressive strength value is small.

5. Modulus of elasticity

Similar to the effect of cellulose ether on the flexural strength of mortar, the addition of cellulose ether reduces the dynamic modulus of mortar, and with the increase of cellulose ether content, the dynamic modulus of mortar decreases gradually. When the content of cellulose ether is large, the dynamic modulus of mortar changes little with the increase of its content.

The variation trend of mortar dynamic modulus with latex powder content is similar to the trend of mortar compressive strength with latex powder content. When the latex powder is added alone, the dynamic modulus of the modified mortar also shows a trend of first decreasing and then slightly increasing, and then gradually decreasing with the increase of the latex powder content. When latex powder and cellulose ether are compounded, the dynamic modulus of mortar tends to decrease slightly with the increase of latex powder content, but the range of change is not large.

6. Bond tensile strength

Different curing conditions (air culture-cured in normal temperature air for 28 days; mixed culture-cured in normal temperature air for 7 days, followed by 21 days in water; frozen culture-mixed culture for 28 days and then 25 freeze-thaw cycles; heat culture-air culture for 14 days After placing it at 70°C for 7d), the relationship between the bonded tensile strength of the mortar and the amount of cellulose ether. It can be seen that the addition of cellulose ether is beneficial to the improvement of the bonded tensile strength of cement mortar; however, the degree of increase in bonded tensile strength is different under different curing conditions. After compounding 3% latex powder, the bonding tensile strength under various curing conditions can be greatly improved.

Relationship between mortar bond tensile strength and latex powder content under different curing conditions. It can be seen that the addition of latex powder is more conducive to improving the tensile strength of the mortar bond, but the addition amount is larger than that of cellulose ether.

It should be noted that the contribution of the polymer to the properties of the mortar after large temperature changes. After 25 freeze-thaw cycles, compared with normal temperature air curing and air-water mixed curing conditions, the bonding tensile strength values of all proportions of cement mortar were significantly reduced. Especially for ordinary mortar, its bonding tensile strength value has dropped to 0.25MPa; for polymer dry powder modified cement mortar, although the bonding tensile strength after freeze-thaw cycles has also decreased a lot, it is almost still at 0.5MPa above. With the increase of the content of cellulose ether and latex powder, the bond tensile strength loss rate of cement mortar after freeze-thaw cycles showed a decreasing trend. This shows that both cellulose ether and latex powder can improve the freeze-thaw cycle performance of cement mortar, and within a certain range of dosage, the greater the dosage of polymer dry powder, the better the freeze-thaw performance of cement mortar. The bonded tensile strength of the cement mortar modified by cellulose ether and latex powder after freeze-thaw cycles is greater than that of the cement mortar modified by one of the polymer dry powder alone, and the cellulose ether The compound blending with latex powder makes the bond tensile strength loss rate of cement mortar smaller after freeze-thaw cycle.

What is more noteworthy is that under high temperature curing conditions, the bonded tensile strength of modified cement mortar still increases with the increase of cellulose ether or latex powder content, but compared with air curing conditions and mixed curing conditions. It is much lower, even lower than under freeze-thaw cycle conditions. It shows that the high temperature climate is the worst condition for the bonding performance. When mixed with 0-0.7% cellulose ether alone, the tensile strength of the mortar under high temperature curing does not exceed 0.5MPa. When the latex powder is mixed alone, the bonding tensile strength value of the modified cement mortar is just greater than 0.5 MPa when the amount is quite large (such as about 8%). However, when cellulose ether and latex powder are compounded and the amount of the two is small, the bonding tensile strength of cement mortar under high temperature curing conditions is greater than 0.5 MPa. It can be seen that cellulose ether and latex powder can also improve the bonding tensile strength of mortar under high temperature conditions, so that cement mortar has good temperature stability and high temperature adaptability, and the effect is more significant when the two are compounded.

7. Conclusion

China’s construction is in the ascendant, and housing construction is increasing year by year, reaching 2 billion m² this year, mainly public buildings, factories and residential construction, and residential buildings account for the largest proportion. In addition, there are a large number of old houses that need to be repaired. New ideas, new materials, new technologies, and new standards are required for both new construction and repair of houses. According to the “Tenth Five-Year Plan Outline for Building Energy Conservation of the Ministry of Construction” promulgated by the Ministry of Construction on June 20, 2002, building energy conservation work during the “Tenth Five-Year Plan” period must persist in saving building energy and improving the building’s thermal environment and wall reform. Based on the principle of combination, the design standard of 50% energy saving should be fully implemented in newly built heating residential buildings in cities in severe cold and cold regions in the north. All of these require corresponding supporting materials. A large number of them are mortars, including masonry mortars, repair mortars, waterproof mortars, thermal insulation mortars, overlay mortars, ground mortars, brick adhesives, concrete interface agents, caulking mortars, special mortars for external wall insulation systems, etc. In order to ensure engineering quality and meet performance requirements, commercial mortar should be vigorously developed. The polymer dry powder has different functions, and the variety and dosage should be selected according to the application. Attention should be paid to the large changes in the ambient temperature, especially the impact on the bonding performance of the mortar when the weather is high.