The environmental simulation method was used to study the effect of cellulose ethers with different degrees of substitution and molar substitution on the water retention of mortar under hot conditions. The analysis of test results using statistical tools shows that hydroxyethyl methyl cellulose ether with low substitution degree and high molar substitution degree shows the best water retention in mortar.
Key words: cellulose ether: water retention; mortar; environmental simulation method; hot conditions
Due to its advantages in quality control, convenience of use and transportation, and environmental protection, dry-mixed mortar is currently more and more widely used in building construction. Dry-mixed mortar is used after adding water and mixing at the construction site. Water has two main functions: one is to ensure the construction performance of the mortar, and the other is to ensure the hydration of the cementitious material so that the mortar can achieve the required physical and mechanical properties after hardening. From the completion of adding water to the mortar to the completion of construction to obtaining sufficient physical and mechanical properties, free water will migrate in two directions besides hydrating the cement: base layer absorption and surface evaporation. In hot conditions or in direct sunlight, moisture evaporates rapidly from the surface. In hot conditions or under direct sunlight, it is essential that the mortar retains moisture quickly from the surface and reduces its free water loss. The key to evaluating the water retention of mortar is to determine the appropriate test method. Li Wei et al. studied the test method of mortar water retention and found that compared with vacuum filtration method and filter paper method, the environmental simulation method can effectively characterize the water retention of mortar at different ambient temperatures.
Cellulose ether is the most commonly used water-retaining agent in dry-mixed mortar products. The most commonly used cellulose ethers in dry-mixed mortar are hydroxyethyl methyl cellulose ether (HEMC) and hydroxypropyl methyl cellulose ether (HPMC). Corresponding substituent groups are hydroxyethyl, methyl and hydroxypropyl, methyl. The degree of substitution (DS) of cellulose ether indicates the degree to which the hydroxyl group on each anhydroglucose unit is substituted, and the degree of molar substitution (MS) indicates that if the substituting group contains a hydroxyl group, the substitution reaction continues to carry out the etherification reaction from the new free hydroxyl group. degree. The chemical structure and degree of substitution of cellulose ether are important factors affecting the moisture transport in mortar and the microstructure of mortar. The increase of the molecular weight of cellulose ether will increase the water retention of mortar, and the different degree of substitution will also affect the water retention of mortar.
The main factors of the dry-mixed mortar construction environment include ambient temperature, relative humidity, wind speed and rainfall. Regarding hot climates, ACI (American Concrete Institute) Committee 305 defines it as any combination of factors such as high atmospheric temperature, low relative humidity, and wind speed, which impairs the quality or performance of fresh or hardened concrete of this type of weather. Summer in my country is often the peak season for construction of various construction projects. Construction in a hot climate with high temperature and low humidity, especially the part of the mortar behind the wall may be exposed to sunlight, which will affect the fresh mixing and hardening of the dry-mixed mortar. Significant effects on performance such as reduced workability, dehydration and loss of strength. How to ensure the quality of dry-mixed mortar in hot climate construction has attracted the attention and research of mortar industry technicians and construction personnel.
In this paper, the environmental simulation method is used to evaluate the water retention of mortar mixed with hydroxyethyl methyl cellulose ether and hydroxypropyl methyl cellulose ether with different degrees of substitution and molar substitution at 45 ℃, and the statistical software is used JMP8.02 analyzes the test data to study the influence of different cellulose ethers on the water retention of mortar under hot conditions.
1. Raw materials and test methods
1.1 Raw materials
Conch P. 042.5 Cement, 50-100 mesh quartz sand, hydroxyethyl methylcellulose ether (HEMC) and hydroxypropyl methylcellulose ether (HPMC) with a viscosity of 40000mPa·s. In order to avoid the influence of other components, the test adopts a simplified mortar formula, including 30% cement, 0.2% cellulose ether, and 69.8% quartz sand, and the amount of water added is 19% of the total mortar formula. Both are mass ratios.
1.2 Environmental simulation method
The test device of the environmental simulation method uses iodine-tungsten lamps, fans, and environmental chambers to simulate outdoor temperature, humidity, and wind speed, etc., to test the difference in quality of the freshly mixed mortar under different conditions, and to test the water retention of the mortar. In this experiment, the test method in the literature has been improved, and the computer is connected to the balance for automatic recording and testing, thereby reducing the experimental error.
The test was carried out in a standard laboratory [temperature (23±2)°C, relative humidity (50±3)%] using a non-absorbent base layer (plastic dish with an inner diameter of 88mm) at an irradiation temperature of 45°C. The test method is as follows:
(1) With the fan turned off, turn on the iodine-tungsten lamp, and place the plastic dish in a fixed position vertically below the iodine-tungsten lamp to preheat for 1 h;
(2) Weigh the plastic dish, then place the stirred mortar in the plastic dish, smooth it according to the required thickness, and then weigh it;
(3) Put the plastic dish back to its original position, and the software controls the balance to automatically weigh once every 5 minutes, and the test ends after 1 hour.
2. Results and discussion
Calculation results of water retention rate R0 of mortar mixed with different cellulose ethers after irradiation at 45°C for 30 min.
The above test data were analyzed using the product JMP8.02 of the statistical software group SAS Company, in order to obtain reliable analysis results. The analysis process is as follows.
2.1 Regression analysis and fitting
Model fitting was performed by standard least squares. The comparison between the measured value and the predicted value shows the evaluation of the model fitting, and it is fully displayed graphically. The two dashed curves represent the “95% confidence interval”, and the dashed horizontal line represents the average value of all data. The dashed curve and The intersection of dashed horizontal lines indicates that the model pseudo-stage is typical.
Specific values for fitting summary and ANOVA. In the fitting summary, the R² reached 97%, and the P value in the variance analysis was far less than 0.05. The combination of the two conditions further shows that the model fitting is significant.
2.2 Analysis of Influencing Factors
Within the scope of this experiment, under the condition of 30 minutes of irradiation, the fitting influence factors are as follows: in terms of single factors, the p values obtained by the type of cellulose ether and the molar substitution degree are all less than 0.05, which shows that the second The latter has a significant impact on the water retention of the mortar. As far as the interaction is concerned, from the experimental results of the fitting analysis results of the impact of the type of cellulose ether, the degree of substitution (Ds) and the degree of molar substitution (MS) on the water retention of mortar, the type of cellulose ether and the degree of substitution, The interaction between the degree of substitution and the molar degree of substitution has a significant effect on the water retention of mortar, because the p-values of both are less than 0.05. The interaction of factors indicates that the interaction of two factors is more intuitively described. The cross indicates that the two have a strong correlation, and the parallelism indicates that the two have a weak correlation. In the factor interaction diagram, take the area α where the vertical type and the lateral substitution degree interact as an example, the two line segments intersect, indicating that the correlation between the type and the degree of substitution is strong, and in the area b where the vertical type and the molar lateral substitution degree interact , the two line segments tend to be parallel, indicating that the correlation between type and molar substitution is weak.
2.3 Water retention prediction
Based on the fitting model, according to the comprehensive influence of different cellulose ethers on the water retention of mortar, the water retention of mortar is predicted by JMP software, and the parameter combination for the best water retention of mortar is found. The water retention prediction shows the combination of the best mortar water retention and its development trend, that is, HEMC is better than HPMC in type comparison, medium and low substitution is better than high substitution, and medium and high substitution is better than low substitution in molar substitution, but There is no significant difference between the two in this combination. In summary, hydroxyethyl methyl cellulose ethers with low substitution degree and high molar substitution degree showed the best mortar water retention at 45 ℃. Under this combination, the predicted value of water retention given by the system is 0.611736±0.014244.
3. Conclusion
(1) As a significant single factor, the type of cellulose ether has a significant impact on the water retention of mortar, and hydroxyethyl methyl cellulose ether (HEMC) is better than hydroxypropyl methyl cellulose ether (HPMC). It shows that the difference in the type of substitution will lead to the difference in water retention. At the same time, the type of cellulose ether also interacts with the degree of substitution.
(2) As a significant single factor influencing factor, the molar substitution degree of cellulose ether decreases, and the water retention of mortar tends to decrease. This shows that as the side chain of the cellulose ether substituent group continues to undergo etherification reaction with the free hydroxyl group, it will also lead to differences in the water retention of mortar.
(3) The degree of substitution of cellulose ethers interacted with the type and molar degree of substitution. Between the degree of substitution and the type, in the case of low degree of substitution, the water retention of HEMC is better than that of HPMC; in the case of high degree of substitution, the difference between HEMC and HPMC is not large. For the interaction between degree of substitution and molar substitution, in the case of low degree of substitution, the water retention of low molar degree of substitution is better than that of high molar degree of substitution; The difference is not huge.
(4) The mortar mixed with hydroxyethyl methyl cellulose ether with low substitution degree and high molar substitution degree showed the best water retention under hot conditions. However, how to explain the effect of cellulose ether type, degree of substitution and molar degree of substitution on the water retention of mortar, the mechanistic issue in this aspect still needs further study.