3.2. Influence of Air Entrainment and Water-Cement Ratio on the Porosity and Strength
The voids within the mortar can be categorized into connected voids and closed voids. Generally, the higher the water–cement ratio is, the higher the porosity is due to free water evaporation [
20,
21,
22]. On the other hand, air-entraining agents can also be used to increase porosity. Both physical and chemical air-entraining agents were added in the mortar mixture to evaluate the porosity and strength of the specimens. As shown in
Figure 4 and
Figure 5, compressive strength decreases with the increasing water–cement ratio, while the porosity does not show a similar trend.
For the specimens with single physical air-entraining agent, the porosity of group S increased from 16.87% (w/c = 0.5) to 23.99% (w/c = 0.9), while compressive strength decreased from 24.84 MPa to 13.89 MPa. The porosity of group K increases from 17.63% (w/c = 0.5) to 20.60% (w/c = 0.9), while the compressive strength decreased from 16.17 MPa to 8.07 MPa. The porosity of the specimens with a single physical air-entraining agent rose monotonically with the increase of the water–cement ratio, but the ascending rate of group S is much higher than that of group K. The compressive strength of S group is much higher than the K group, even with the similar porosity. The admixture SJ-2 is a saponins air-entraining agent with large molecular weight [
32]. The bubbles formed have a relatively thick membrane and good foam stabilization [
33].
Figure 6a shows an SEM (scanning electron microscope) picture of a group S specimen. It can be seen that the pores are relatively small and uniform with relatively high compactness. The bubbles generated will not burst and remained with an increasing water–cement ratio. Hence, the porosity of group S specimens rapidly increased with the increasing water–cement ratio. This is also the reason why group S can achieve relatively high strength. The K12 admixture belongs to alkyl benzene sulfate foaming agent, which generates large amount of rich foam at a high foaming speed. However, its bubbles formation is not stable [
34]. Small bubbles are easy to merge into relatively large ones and overflow. With the increase of the water–cement ratio, the bubbles generated by the K12 air-entraining agent burst in large amounts during vibration and their porosity did not increase significantly. As shown in
Figure 6b, it is seen that part of the small bubbles merge into bigger ones, thus showing both large and small pores. The uneven pore structure and thin cement paste between pores caused a significant decrease in mortar strength.
The same observations also apply to group H with hydrogen peroxide. Group H has the highest strength of all specimens, because its overall porosity is relatively low due to the low amount of bubbles generated [
35], as evidenced in
Figure 7b. Aluminum powder has a relatively high efficiency of generating bubbles. It was observed during the experiment that the volume of the specimens expanded after the aluminum powder was mixed with cement. Large amounts of gas bubbles were evenly generated within the base materials. A vesicular structure with many interconnected pores was generated after hardening which effectively enhanced the porosity of the specimen, as evidenced in
Figure 7. With the increased water–cement ratio, the porosity of group A rose from 16.96% (w/c = 0.5) to 24.58% (w/c = 0.6) and 23.21% (w/c = 0.7), respectively. When the water–cement ratio increased to 0.9, the porosity only rose to 19.74%. This is because when the water–cement ratio increased from 0.5 to 0.7, the fluidity of mortar would increase. Most of the slurry were more mobile than that with w/c = 0.5. Lots of gas would escape from the slurry, leading to high porosity as shown in
Figure 8a. However, when the water–cement ratio was further increased, the slurry would be too thin. The setting speed of the slurry would lag behind the foaming speed of aluminum powder. Thus, it is difficult to stabilize bubbles which in turn decreases the porosity as shown in
Figure 8b. But to those physical air entrainments (SJ-2 and K12), this rule does not exist. The porosity of specimens increased with the water–cement ratio as shown in
Figure 4. This is because the foaming mechanisms between physical and chemical air entrainment are different. The physical air entrainment creates foam although it decreases the surface tension of moisture in mortar; however, the chemical air entrainment creates foam through the chemical reaction between the entrainment and mortar. So the character of the foam created by different air entrainments is totally different. The foam created by physical air entrainment was small and stable, it is hard to gather and form big bubbles to escape from the slurry due to the polar group absorbed to the surface of foam. However, the foam created by chemical air entrainment was big and unstable and can easily gather and escape from the slurry. So the mobility of slurry is much more sensitive to chemical air entrainment than physical air entrainment.
To aluminum powder, the results show that when the water–cement ratio is between 0.65 and 0.75, the optimal porosity can be achieved. However, the porosity decreased slightly when the water cement ratio was between 0.75 and 0.95. From a comprehensive analysis of the test results with air-entraining agents, group A with aluminum powder had the highest porosity. As a result, aluminum powder was used along with other air-entraining agents in the subsequent experiments to investigate the combined effect on air entrainment.
From the test results (i.e.,
Table 6 and
Figure 4) of three groups of test specimens (group AH, AS and AK), the porosity of the specimen with mixed air entrainments is larger than those with single air entrainment. The porosity of these three groups did not increase when the water–cement ratio was increased from 0.5 to 0.9. However, the strength of the specimens simply decreased with the increase of the water–cement ratio. In group AH, the porosity stayed fairly unchanged with the highest point at w/c = 0.8. In group AS, the maximum porosity occurs at w/c = 0.7. In group AK, the porosity gradually declined with the increase of the water–cement ratio. This is because with the increase of the water–cement ratio, the mobility of base materials would increase. The expanding volume caused by the chemical reaction of aluminum powder reduced with the increasing water–cement ratio. When the water–cement ratio exceeded the limit value (0.8 for AH group, 0.7 for AS group, 0.5 for AK group), a small bubble caused by aluminum powder merged to form bigger bubbles and overflow, which led to the decrease of the overall porosity. However, with the increasing water–cement ratio, the moisture inside the mortar definitely increased. When the specimen was dried in the drying oven, the water inside the mortar become steam. This would cause the sharp increasing volume, and consequently, cause the micro-crack inside the mortar. This is the main reason why the strength of specimen sharply decreased with the increasing water–cement ratio.
At the same time, test results showed that after mixing with another air-entraining agent, the optimal water–cement ratio for aluminum powder would vary by a certain degree. It is noted from
Table 6 and
Figure 4 that using another air-entraining agent in addition to aluminum powder greatly increases the porosity of the specimen, compared to the single air-entraining agent groups. Test results reveal that mixing aluminum powder can be introduced to interconnected pores based on single mixing to have interconnected pores and independent pores distribute uniformly. It improves the pore structure and pore diameter distribution, and further increases the porosity of the specimen. To verify this observation, scanning electron microscope was used to inspect group AK-5 specimens, as shown in
Figure 9. It can be seen that there are not only parts of the interconnected pores, but also many evenly-located independent small pores on its surface. The independent bubbles were introduced by physical air-entraining agent K12, suggesting that the compound mixing of chemical and physical air-entraining agents can lead to a certain superimposed pores pattern. Therefore, the overall porosity can be increased based on single mixing, making the air-entraining effect superior to that using a single air-entraining agent. Among these three groups (AH, AS and AK), group AK has the highest porosity, but the compressive strength of AK group is lower than the allowable value of the Chinese national standard (GB/T 13545-2014) [
14]; the specimens in group AS and AH both have porosity larger than 20%, but the AS7 has the highest porosity of 25.31% among the two groups; the compressive strength of AS7 is 10.21 MPa, which meets the Chinese national standard (GB/T 13545-2014) [
14].
3.3. Resistivity Influenced by Air Entrainment and Water-Cement Ratio
Dried and hardened cement paste and mortar have an electrical resistivity of approximately 104–107 Ω·m [
36], which is decided by the constituents of cement, humidity, w/c ratio, etc., [
37,
38]. The resistivity of electrically conductive concrete is under 100 Ω·m [
39], depending upon the electronic conduction within the conductive materials, such as steel fibers and graphite. In contrast, the conductivity of the ionically conductive mortar described in this paper solely depends upon the electrolyte dispersion within the mortar.
Table 6 shows the resistivity of specimens at different ages, and the highest resistivity at 28 days of the test specimens is comparable to that of electrically conductive concrete. The resistivity of specimens with different air-entraining agents and water-cement ratios are compared in
Figure 10. The changes in
with water–cement ratios are shown in
Figure 11.
Figure 10 and
Figure 11 clearly show the resistivity of specimens mixing with two air-entraining agents to be lower than that of specimens with a single air-entraining agent. And
in the same group of specimens decreased with the increasing of porosity. With respect to the moisture absorption data shown in
Table 5, it can be concluded that the specimen with higher porosity can absorb more electrolyte solution, and consequently, will have lower resistivity in most circumstances. For instance, from
Table 5 and
Table 6, the porosity of A5 and A6 are 16.96% and 24.58%; the electrolyte absorption of A5 and A6 are 9.66 g and 14.67 g; and the resistivity of A5 and A6 are 30.29 Ω·m and 10.82 Ω·m, respectively. When electrolyte solution could easily permeate into the mortar, the porosity of the mortar would increase and the resistivity of the mortar would decrease.
However,
from different groups does not always decrease with the increasing of porosity. Three specimens, S-8, AS-8 and AH-7 are chosen herein for discussions. The porosity of these three specimens were 22.31%, 22.20% and 22.28%, respectively. However, the resistivity of these specimens were 20.54 Ω·m, 6.56 Ω·m and 10.21 Ω·m, respectively. The relation between pore structure and permeability of cement mortar was studied, and the results showed that the permeability did not just rely on porosity [
40]. It also depends upon what causes the change of porosity, different water–cement ratios and/or hydration time [
41]. In other words, it also relates to the pore aperture size, pore size distribution and pore pitch coefficient, etc.
Figure 12 shows the apparent images of these three specimens. It can be seen in
Figure 12 that the shape and distribution of the voids on the surface of the three specimens are totally different even if the porosity is approximately the same. Relative to another two specimens, S-8 is much more compact and less porous on the surface; it makes it more difficult for an electrolyte to penetrate and consequently yields the highest
of the three specimens. AS-8 and AH-7 are specimens fabricated with two air-entraining agents; the porosity and resistivity of these two specimens are fairly close. However, as shown in
Figure 12b,c, the aperture of the voids on the AH-7 surface appears to be bigger than that on the AS8 surface. Even though the number of voids on the AS-8 surface is seen to be more than that on the AH-7 surface, the cracks caused by connected voids are observed on the AS-8 surface. The electrolyte solution could penetrate into the mortar easily through these cracks, and this process caused the lower
of AS-8 than AH-7.
is the resistivity of specimens at 28 days and
is the gradient of resistivity at 28 days, which is defined in Equation (4). These two values indicate the availability of the electrolyte solution inside the specimens, and they are also an indicator of the stability of conductivity. There is a trend shown in
Table 6 and
Figure 10 that
increases with decreasing of porosity. For example, most porosity of K group specimens are less than 20%, while the
of K group specimens exceeds 1000%.
of K-6 reached 1955.61%. AK group had the highest porosity of all specimens, the highest porosity of the AK group exceeded 30%, and the
of AK group was also the lowest with
only at 156.8%. It is noteworthy that this trend is not very obvious when the porosity of the specimens are approximately the same. This is because
of ionically conductive mortar is influenced by not only porosity but also many other factors such as the degree of hydration of mortar, C/S and H/S ratios of hydration products, evaporation of water, and distribution of electrolyte within the mortar, etc., [
40].
To determine the optimized water–cement ratio and combination of air-entraining agents, the AS and AK groups were chosen to compare for their relatively low
and
.
Figure 13 shows the changes of these two groups’ resistivity with ages. Both AS-7 and AK-5 had excellent
and
, and those values also met the demand of traditional conductive concrete [
39]. AK-5 had the lower resistivity, with
= 8.25 Ω·m. The microstructure images of these specimens are compared in
Figure 14.
Figure 14a,c shows the images of the two specimens magnified 50 times, and the images show that pore structure and numbers are similar. However, when showing these images magnified 2000 times, different results emerge.
Figure 14b shows that there are many micro-cracks inside AK-5 and hydration products C-S-H (calcium silicate hydrate) which contribute to most of the strength of mortar, much less than those in AS-7 as shown in
Figure 14d. In addition, there are some Ca(OH)
2, which are hexagonal crystals dispersed inside the mortar. This component is easily broken when the specimen is stressed under loading. As a result, it can be predicted that the strength of AK-5 specimens would be low due to these drawbacks. The number and structure of C-S-H inside the AS-7 shows that the hydration process is fully developed and there are no micro-cracks observed. Therefore, the AS-7 specimen which is fabricated with aluminum powder and SJ-2 air-entraining agent (with w/c = 0.7) was determined to be the optimized mixing ratio for heating applications.