3.1. Temperature Change Process of the Reinforced Concrete Wall
The temperature change process of the reinforced concrete wall could be divided into three stages, namely, the rapid temperature rise stage, rapid temperature drop stage and change stage with ambient temperature. The concrete temperature (in the depth of 5 cm) measured by three strain gauges was basically the same. In this paper, the temperature measured by No. 2 strain gauge was taken as an example to introduce the temperature variation. The detailed temperature variation is shown in
Figure 6.
The first stage was the stage of intense heating. During this period of time, the hydration reaction of active components such as cement, mineral powder and MEA was intense, and the temperature of reinforced concrete wall increased sharply. This stage lasted about one day, with the highest temperature reaching 56 °C. During this period, the concrete is in the state of thermal expansion, and the stress in the concrete is the compressive stress generated by the restraint of the reinforcement and the foundation, which will not cause the concrete to crack. Comparing the data of the reinforced concrete wall in this paper with the temperature of the dam in other publications, it is found that its temperature change is very different from that of the dam [
24]. For dams, the heat generated by hydration is large due to the large amount of cement. Moreover, the large width of the dam makes it difficult to dissipate the hydration heat, which makes the temperature rise of the dam continue for 1800 h. After 1800 h, the temperature gradually decreases. So, the activity of MEA in the dam needs to be low to be able to compensate for the later shrinkage. MEA140 in this paper, in contrast, can produce large expansion at an early stage.
The second stage was the rapid cooling stage. During this period of time, the rate of hydration and heat release of cement slowed down significantly. Owing to the large temperature difference between the inside and the outside, the heat from the inner core slowly diffused into the air, thus gradually cooling down. This stage lasted about 7 days, and the internal temperature of the concrete wall was significantly higher than the outside air temperature, resulting in continuous heat dissipation from the wall. In this process, the CR-Ref concrete was in a state of shrinkage due to the temperature drop, and the stress in the concrete was tensile stress owing to the constraint of the steel bar and foundation. In the process, the concrete wall would crack if the tensile stress exceeded the limit of the tensile strength.
The third stage was accompanied by the change of atmospheric temperature. During this period, the temperature of the reinforced concrete wall was basically the same as the atmospheric temperature, which changed with the atmospheric temperature.
3.2. Volumetric Deformation of Reinforced Concrete Wall
Figure 7 and
Table 7 show the volumetric deformation curve of concrete walls at different positions after deducting the influence of temperature. The deformation was the comprehensive result of the concrete self-contracting and expansion of the MEA. According to the measured deformation data, the whole deformation process could be divided into three stages. The first stage was a stage of drastic volume change. At this stage, the shrinkage of the concrete dominated, lasting about 10 days. The second stage, the moderate expansion stage, lasted about 90 days. The third stage was the anaphase stage of expansion.
In the first stage, intense contraction appeared in CR, producing microstrains of −138 με, −106 με, and −50 με, respectively, at three locations. This was owing to the early high temperature accelerated the hydration rate of cement, leading to greater self-shrinkage of the concrete. After the first stage, the contraction rate of CR slowed down, and the cause of CR contraction changed from self-shrinkage to dry shrinkage, finally reaching a maximum of −235 με in 150 days. On the other hand, the concrete shrinkage at three different positions was not exactly the same, and the shrinkage at position 3 was significantly less than that of the other two. This was mainly owing to the fact that position 3 was closer to the foundation, which constrained the shrinkage and deformation of the concrete, resulting in a relatively smaller shrinkage.
For concrete mixed with the MEA, it showed a completely different deformation. The CMM expanded significantly during the first phase. For example, at position 1 and position 2, the deformation was 39 με and 36 με, respectively. At position 3, owing to the foundation constrained the expansion of MEA, its expansion was reduced relative to the other two positions, and the 10 days expansion was 19 με. On the other hand, the expansion produced by CMR and CMS at this stage cannot completely compensate for the shrinkage of concrete, and the shrinkage was −7 με~−25 με.
In the second stage, the expansion of the MEA was greater than the shrinkage of concrete, resulting in micro-expansion of the reinforced concrete walls. During this period, for CMM, the expansion gradually increased, reaching a maximum value of 75 με at 90 days. At the same time, the expansion of position 3 was also smaller than that of the other two positions owing to the constraint of the foundation.
Comparing the two stages, the following conclusions can be drawn. For CMR, because the activity is too high, its hydration rate is too fast, and many expansions occur at the plastic state of the concrete, which cannot effectively compensate for the shrinkage of concrete. For CMS, due to its low activity, the early hydration rate is so slow that it cannot compensate for the shrinkage, but it compensates well at the later stage due to the continuous hydration expansion of the MEA. For CMM, the shrinkage of concrete is well compensated for the shrinkage at all the stages due to its moderate activity.
In the third stage, due to the large consumption of the MEA in the early stage, there are few remaining MEAs that can continue to produce expansion. At this stage, contraction and expansion were basically equal, and concrete expansion gradually decreased. The deformation of CMR tended to be flat and even shrink. The later deformation of CMM and CMS was still expansion deformation, which indicated that the hydration expansion of MEA-M and MEA-S could compensate for the self-shrinkage caused by hydration and the shrinkage caused by drying.
Figure 8 shows the distribution of cracks on the surface of the reinforced concrete walls at 28 days. The shrinkage of concrete was limited by steel bars and generated tensile stress. And cracks occurred when the tensile stress exceeded the tensile strength of concrete. At 28 days, many slender cracks (
Figure 8a) were distributed on the surface of CR, which indicated that there was a large tensile stress due to shrinkage in the concrete. On the other hand, since the expansion of MEA compensated for the contraction, and the compressive stress was generated under the constraint conditions, this would not cause cracks in the wall. At this time, the wall surface was smooth, and no cracks were found (
Figure 8b).
According to classical computational mechanics, when a wall is constrained by a foundation, cracks appear at the bottom of the wall, and cracks are perpendicular to the foundation. However, in many large-volume concrete structures, cracks are often randomly distributed and do not always appear near the foundation [
25,
26]. In our opinion, this is due to the fact that the walls are affected not only by their own temperature, but also by sunlight, which is also related to the construction method. Moreover, the shrinkage of the wall is not evenly distributed, and cracks often appear first where the quality of concrete pouring is poor. Therefore, we speculate that the concrete shrinkage limited by the foundation is an important cause of wall cracking, but this is not the only reason, and it is likely that the reinforcement will also affect the shrinkage.
3.3. Degree of Hydration of MEA
The hydration of MEA depends on its own activity, maintenance temperature and humidity. As shown in
Figure 9, owing to the large thickness of the wall, up to 0.8 m, the water inside the wall was hard to volatilize, which lead to the humidity inside the wall to remain at a high level. The relative humidity after 150 days still reached 92%, indicating that humidity was not the main factor affecting MEA hydration in this experiment. Therefore, this paper mainly discussed the effects of activity and temperature on MEA hydration.
Figure 10 shows DSC/TG curves of the MEA cement paste buried in reinforced concrete walls at different curing age. According to the figure, the dehydration peak of magnesium hydroxide was 310~400 °C. Equation (2) was used to calculate the content of Mg(OH)
2 in the cement paste, and the calculated results were shown in
Figure 11. The hydration degree and Mg(OH)
2 content of the MEA at different curing ages are showed in
Table 8. As shown in
Table 8, the hydration rate of the MEA at the early stage (0–27 days) was very fast, and the hydration degree of the three samples all exceeded 89%, specifically 96.4%, 93.0% and 89.0%, respectively. This was mainly owing to the fact that a large amount of heat was released during cement hydration at the early stage, and the high temperature accelerated the hydration rate of MEA, which was consistent with Li Hua’s research results. He found that curing temperature had a great influence on the MEA’s hydration and the hydration rate of the MEA at 80 °C was nearly 4.5 times of 20 °C at 28 days [
27].
On the other hand, the activity of MEA also affected the hydration rate of MEA. At 28 days, compared with the content of Mg(OH)
2 in cement paste with different activities of the MEA under the same content, Mg(OH)
2 produced by MEA-S (PMS) with low activity was the least. However, at the later stage (160 days), PMS produced more Mg(OH)
2 than the other two samples. As the expansion of the MEA was generated by MgO hydration to generate Mg(OH)
2, it indicated that the expansion of the MEA with low activity was smaller in the early stage, but it produced greater expansion in the later stage, which was consistent with the laboratory test results of many other researchers [
28,
29].