Abstract: The effect of different content of hydroxypropyl methylcellulose ether on the properties of ordinary dry-mixed plastering mortar was studied. The results showed that: with the increase of the content of cellulose ether, the consistency and density decreased, and the setting time decreased. The extension, 7d and 28d compressive strength decreased, but the overall performance of dry-mixed mortar has been improved.
0.Preface
In 2007, the six ministries and commissions of the country issued the “Notice on Prohibiting On-Site Mixing of Mortar in Some Cities within a Time Limit”. At present, 127 cities across the country have carried out the work of “prohibiting existing” mortar, which has brought unprecedented development to the development of dry-mixed mortar. opportunity. With the vigorous development of dry-mixed mortar in the domestic and foreign construction markets, various dry-mixed mortar admixtures have also entered this emerging industry, but some mortar admixture production and sales companies deliberately exaggerate the efficacy of their products, misleading the dry-mixed mortar industry. healthy and orderly development. At present, like concrete admixtures, dry-mixed mortar admixtures are mainly used in combination, and relatively few are used alone. In particular, there are dozens of types of admixtures in some functional dry-mixed mortars, but In ordinary dry-mixed mortar, there is no need to pursue the number of admixtures, but more attention should be paid to its practicability and operability, to avoid excessive use of mortar admixtures, causing unnecessary waste, and even affecting the quality of the project. In ordinary dry-mixed mortar, cellulose ether plays the role of water retention, thickening, and improvement of construction performance. Good water retention performance ensures that the dry-mixed mortar will not cause sanding, powdering and strength reduction due to water shortage and incomplete cement hydration; the thickening effect greatly enhances the structural strength of the wet mortar. This paper conducts a systematic study on the application of cellulose ether in ordinary dry-mixed mortar, which has guiding significance for how to use admixtures reasonably in ordinary dry-mixed mortar.
1. Raw materials and methods used in the test
1.1 Raw materials for the test
The cement was P. 042.5 cement, the fly ash is Class II ash from a power plant in Taiyuan, the fine aggregate is dried river sand with a size of 5 mm or more sieved, the fineness modulus is 2.6, and the cellulose ether is commercially available hydroxypropyl methyl cellulose ether (viscosity 12000 MPa·s).
1.2 Test method
Sample preparation and performance testing were carried out according to JCJ/T 70-2009 basic performance test method of building mortar.
2. Test plan
2.1 Formula for the test
In this test, the amount of each raw material of 1 ton of dry-mixed plastering mortar is used as the basic formula for the test, and the water is the water consumption of 1 ton of dry-mixed mortar.
2.2 Specific plan
Using this formula, the amount of hydroxypropyl methylcellulose ether added to each ton of dry-mixed plastering mortar is: 0.0 kg/t, 0.1 kg/t, 0.2 kg/t, 0.3 kg/t, 0.4 kg/t t, 0.6 kg/t, to study the effects of different dosages of hydroxypropyl methylcellulose ether on the water retention, consistency, apparent density, setting time, and compressive strength of ordinary dry-mixed plastering mortar, in order to guide dry-mixed plastering The correct use of mortar admixtures can truly realize the advantages of simple dry-mixed mortar production process, convenient construction, environmental protection and energy saving.
3. Test results and analysis
3.1 Test results
Effects of different dosages of hydroxypropyl methylcellulose ether on the water retention, consistency, apparent density, setting time, and compressive strength of ordinary dry-mixed plastering mortar.
3.2 Analysis of results
It can be seen from the effect of different dosages of hydroxypropyl methylcellulose ether on the water retention, consistency, apparent density, setting time, and compressive strength of ordinary dry-mixed plastering mortar. With the increase of cellulose ether content, the water retention rate of wet mortar is also gradually increasing, from 86.2% when hydroxypropyl methyl cellulose is not mixed, to 0.6% when hydroxypropyl methyl cellulose is mixed. The water retention rate reaches 96.3%, which proves that the water retention effect of propyl methyl cellulose ether is very good; the consistency gradually decreases under the water retention effect of propyl methyl cellulose ether (the water consumption per ton of mortar remains unchanged during the experiment); The apparent density shows a downward trend, indicating that the water retention effect of propyl methyl cellulose ether increases the volume of wet mortar and reduces the density; the setting time gradually prolongs with the increase of the content of hydroxypropyl methyl cellulose ether, and the content of When it reaches 0.4%, it even exceeds the specified value of 8h required by the standard, indicating that the appropriate use of hydroxypropyl methylcellulose ether has a good regulating effect on the operability time of wet mortar; the compressive strength of 7d and 28d has decreased (The greater the dosage, the more obvious the reduction). This is related to the increase in the volume of the mortar and the decrease in the apparent density. The addition of hydroxypropyl methyl cellulose ether can form a closed cavity inside the hardened mortar during the setting and hardening of the mortar. Micropores improve the durability of mortar.
4. Precautions for the application of cellulose ether in ordinary dry-mixed mortar
1) Selection of cellulose ether products. Generally speaking, the greater the viscosity of cellulose ether, the better its water retention effect, but the higher the viscosity, the lower its solubility, which is detrimental to the strength and construction performance of the mortar; the fineness of cellulose ether is relatively low in dry-mixed mortar. It is said that the finer it is, the easier it is to dissolve. Under the same dosage, the finer the fineness, the better the water retention effect.
2) Selection of cellulose ether dosage. From the test results and analysis of the effect of the content of cellulose ether on the performance of dry-mixed plastering mortar, it can be seen that the higher the content of cellulose ether, the better, it must be considered from the production cost, product quality, construction performance and Four aspects of the construction environment to comprehensively select the appropriate dosage. The dosage of hydroxypropyl methyl cellulose ether in ordinary dry-mixed mortar is preferably 0.1 kg/t-0.3 kg/t, and the water retention effect cannot meet the standard requirements if the amount of hydroxypropyl methyl cellulose ether is added in a small amount. Quality accident; the dosage of hydroxypropyl methyl cellulose ether in the special crack-resistant plastering mortar is about 3 kg/t.
3) Application of cellulose ether in ordinary dry-mixed mortar. In the process of preparing ordinary dry-mixed mortar, an appropriate amount of admixture can be added, preferably with a certain water retention and thickening effect, so that it can form a composite superposition effect with cellulose ether, reduce production costs, and save resources; if used alone For cellulose ether, the bonding strength cannot meet the requirements, and an appropriate amount of redispersible latex powder can be added; due to the low amount of mortar admixture, the measurement error is large when used alone. The quality of dry-mixed mortar products.
5. Conclusions and suggestions
1) In ordinary dry-mixed plastering mortar, with the increase of the content of hydroxypropyl methylcellulose ether, the water retention rate can reach 96.3%, the consistency and density are reduced, and the setting time is prolonged. The compressive strength of 28d decreased, but the overall performance of dry-mixed mortar was improved when the content of hydroxypropyl methyl cellulose ether was moderate.
2) In the process of preparing ordinary dry-mixed mortar, cellulose ether with suitable viscosity and fineness should be selected, and its dosage should be determined strictly through experiments. Due to the low amount of mortar admixture, the measurement error is large when used alone. It is recommended to mix it with the carrier first, and then increase the amount of addition to ensure the quality of dry-mixed mortar products.
3) Dry-mixed mortar is an emerging industry in China. In the process of using mortar admixtures, we must not blindly pursue quantity, but pay more attention to quality and reduce production costs, encourage the use of industrial waste residues, and truly achieve energy saving and consumption reduction.
Post time: Feb-22-2023