Dr. Su, Yufeng has been working with Shanghai Ytong Co. Ltd. for 8 years. Other activities include the School of Materials Science and Engineering of Tongji University since 2005. The current research of the author focuses on new wall construction materials and energy saving measures.        drsyf@189.cn

Li Haifeng is a master candidate from the School of Materials Science and Engineering of Tongji University and has graduated from Chongqing University.

Liang Shiying has graduated from Dalian Jiaotong University as bachelor and from School of Materials Science and Engineering of Tongji University as master. Currently she is working with the Zhengzhou Kangqiao Real Estate Development Co. LTD.

Autoclaved aerated concrete (AAC) products have been widely used in China. At present, there are many studies on the stability, gasing processes, and the thickening of AAC slurry. Wang Jie et al. [2] studied the influence of the fineness of fly ash and quick lime on initial expansion and the gasing process of AAC slurry. Su Yufeng et al. [3] studied the influence of the mass fraction of the NaOH solution and the viscosity of the cellulose ether solution on the bubble stability in AAC slurry, as well as the effect of a foam stabilizer. However, there are few studies on the temperature field of the green cake. 

In order to improve the uneven hardness of the cake during the hardening process, the temperature field in the AAC cake should be studied and analyzed. For this research, referring to the ABAQUS finite element method used to study the temperature field of mass concrete, the temperature field in the cake was tested and simulated. The influence of the ambient temperature and the conditions of the mold on the temperature field of the cake were studied, and an accurate finite element model of the temperature field was established.

Experimental investigation

The percentage mixture ratio of the raw materials cement/lime/sand/gypsum was 15/15/65/5, the water-solid ratio (W/S) was 0.55, and the initial temperature of the raw materials and water was set at 40℃.

Two types of molds were selected for the laboratory investigations (Fig.1). A concrete cube mold was used to simulate the steel frame found under factory conditions. The other mold was a self-made cylinder EPS insulation barrel.

Results and discussions

Influence of ambient temperature on the temperature field of the cake

In order to study the influence of ambient temperatures on the cake during the hardening process, three temperatures were selected: winter temperature (0℃), curing room temperature without heating (40℃), and curing room temperature with heating (60℃). The test results of Model 1 (covered insulation barrel) are shown in Figures 2 and 3.

As shown in Figure 2, on the central axis, the temperature at the measuring points had the following order: Point 4 ＞ Point 5 ＞ Point 3 ＞ Point 2 ＞ Point 1. When the ambient temperature increased from 0℃ to 60℃, the temperature difference between Point 4 and Point 1 decreased from 7℃ to 4℃. As shown in Figure 3, on the side of the cylinder cake, the temperature showed the following order: Point 9 ≈ Point 10 ＞ Point 11 ＞ Point 12 ＞ Point 13. When the ambient temperature increased from 0℃ to 60℃, the temperature difference between Point 9 and Point 13 decreased from 13℃ to 4℃.

The test results show that the ambient temperature does not affect the decreasing trend of the temperature inside the cake, but it does affect the absolute temperature. With an increase in the ambient temperature, the maximum temperature of each measuring point in the cake gradually increased, and the difference between the maximum temperature and the minimum temperature in the same dimension gradually decreased.

Influence of mold conditions on the temperature field of the cake

The test results of Model 2 (iron cube mold) are shown in Figure 4.

When comparing Figure 4(a) with Figure 2(a), it can be seen that the center temperature of the iron cube model (~60℃) is much lower than that of Model 1 (EPS), indicating that the mold material or the thermal insulation performance of the mold has a great influence on the center temperature of the cake.

It can be seen from Figure 4(b) and Figure 4(c) that when the ambient temperature was high during curing, the temperatures at the corner of the cube gradually increased to the ambient temperature and remained at about 60℃. When the ambient temperature was very low, a small temperature increase was observed at the beginning of the curing due to the exothermic reaction of the lime. Subsequently, the temperature gradually approached the ambient temperature (40℃) over time. Considering Figures 3 and 4(b), it is also evident, that the thermal insulation performance of the mold has a great influence on the temperature on the outer sides of the cake.

The above test results show that, by improving the thermal insulation performance of the mold, the heat loss of the green cake can be reduced, the surface temperature of the cake can be increased, the surface (direct exposure to environment) temperature can be increased, and the uniformity of the temperature field distribution in the green cake can be improved.

Simulation of the temperature field in the green cake

When ABAQUS was used to simulate and analyze the temperature field in the green cake, the initial temperatures of the raw materials and the mold were set at 40℃, and the ambient temperature during the curing was set at 60℃ (the same as the ambient temperature in the curing chamber). The selected material parameters are shown in Table 1.

Table 1: Material parameters for the finite element model

Temperature field simulation of Model 1

In Model 1, the convective heat transfer coefficient between the EPS barrel and the air is 90 W/(m²·K). The simulation results for the temperature field of the cake in Model 1 in the curing process are shown in Figure 5 (temperature field cloud diagram of half of the cake). The comparison between simulation results and measured data is shown in Figure 8.

As can be seen from Figure 5, the temperature of the green cake increased gradually with the curing time, the temperature at the bottom was the highest, and the temperature was lower towards the upper surface. There was still heat dissipation in the gap between the lid of the mold, resulting in lower temperatures on the upper cake.

As can be seen from Figure 6, the maximum difference between the simulated temperature and the measured temperature of the cake of Model 1 was 0.81℃, indicating that the simulated results are in good agreement with the measured data.

Temperature field simulation of Model 2

In Model 2, the convective heat transfer coefficient between the steel barrel and the surrounding air was 4000 W/(m²·K). The simulation results for the temperature field in the cake in Model 2 in the curing process are shown in Figure 7. The comparison between simulation results and measured data is shown in Figure 8.

As can be seen from Figure 7, in the first 80 minutes of curing, the temperature at the corner of the upper surface of the cube was the highest and gradually increased from the initial temperature to the ambient temperature, indicating that the higher ambient temperature was rapidly transmitted to the steel mold and the surface of the cake. After 80 minutes, the temperature inside the cube cake gradually increased, the temperature in the lower part was the highest, and temperatures decreased towards the upper surface. The temperatures inside the cube cake increased as a result of the heat released by the hydration of lime and cement. When the temperature of the cake was higher than the ambient temperature, the heat dissipated at the uncovered surface into the environment, resulting in a lower temperature after 80 minutes. The surface temperature of the cake gradually matched the ambient temperature.

As shown in Figure 8, the maximum difference between the simulated results and the measured data at each point was no more than 2.0 ℃, indicating that the established finite element model for the temperature development is accurate.

Conclusions

The development and distribution of temperature in the curing process for autoclaved aerated concrete slurries have not been well studied. In this paper, the factors that may affect the development of the internal temperature of the green cake, such as mold materials and ambient temperature, were studied experimentally under laboratory conditions and through a finite element simulation using ABAQUS software. The conclusions are as outlined in the following.

1.     The ambient temperature in the curing chamber does not affect the distribution of the body temperature, but it affects the absolute value of the temperature in the inner green cake. With an increase in ambient temperature, the maximum temperature in the cake gradually increases, and the difference between the maximum and the minimum temperature in the same dimension gradually decreases. That is, the higher ambient temperature is conducive to reducing the temperature differences inside the green cake.

2.     The mold material and thermal insulation performance have a great influence on the temperature field in the green cake. In the direction of the height of the cake, the amplitude and distribution of temperature are affected by the mold conditions on the upper surface of the cake (with or without a lid). In the direction of the radius (Model 1), temperatures are affected by the mold conditions on the side of the cake.

3.     Finite element software was used to simulate the temperature field inside the green cake, with the cake situated either in an EPS thermal mold with a lid or in a steel cube mold. The maximum difference between the simulated results and the measured values was less than 2℃, indicating good accuracy and confirming that the finite element model established was effective.

References

[1] Wang Jie, Yan Chaoyong. Effect of Fineness of Fly ash and Quicklime on Properties of Lightweight Autoclaved Aerated Concrete Slurry and its product[J]. Brick and Tile, 2015 (4): 8-11.(in Chinese)

[2] SU Yufeng, SUN Xuan, ZHANG Hui. Effect of Slurry Viscosity on bubble Stability with Aluminum Powder [J]. Journal of Building Materials, 2017, 20 (4): 506-510. (in Chinese)