3.1. Capillary Water Absorption Test Analysis
The relationship between the cumulative absorption per unit area and the square root of the capillary absorption time was obtained from the results of the capillary water absorption test. The results are illustrated in
Figure 2.
As shown in
Figure 3, the linear correlation between the water absorption mass per unit area and the time square root of each test group was strong, with a correlation coefficient above 0.999. This result agrees with the theoretical derivation. The capillary water absorption coefficient of group K was slightly smaller than that of group J. The difference between the two groups was discernible, which indicated that the waterproof effect of the KIM waterproof agent during the initial stage of capillary water absorption is not obvious. The capillary water absorption coefficient of group KS was significantly lower than that of groups J and K. This indicated that nano-SiO
2 causes crystal nucleation and micro-aggregation, which considerably improved the compactness of the test samples. The modification effect of nano-SiO
2 in the cement mortar with waterproof agent was significant, and the capillary water absorption rate was slower.
According to the linear expression obtained from the fitted curve, the capillary water absorption coefficients of the J, K, and KS groups were 1.0245, 1.0216, and 0.7678 (kg/m2)/h1/2, respectively.
3.2. NMR Saturation Test Analysis
Before using NMR technology to visually obtain the internal moisture distribution of the material, calculation and simulation were carried out according to Equation (12) to predict the saturation at a long distance from the water absorption surface.
From the root formula, the value of
can be expressed by Equation (19):
Then, the relationship between the coordinate
of a point in the test sample and its relative water content is:
In the first half, the saturation of the function gradually decreases with distance, whereas in the second half, it decreases rapidly. The curve of the function is parabolic. The data in [
49] also perform well with this function. However, there is a limitation in formula simulation. In the prediction of Equation (20),
has a maximum value. When the distance exceeds the maximum value, its saturation defaults to 0.
The NMR technique uses the MSE sequence to perform spatial encoding by reading the gradient. The acquired data images are shown in
Figure 4.
After the cement mortar samples were saturated with water, the saturation was set to 100%, while that at 0 h was set to 0%. Then, the saturation curve as a function of the spatial position was calculated from the signal value ratio.
As shown in
Figure 5, the internal moisture distribution of the material obtained by the NMR technique is significantly different from that obtained by the formula calculation simulation. Moreover, the curve does not show a downward trend as the water absorption distance increases. Although the magnitude of the nuclear magnetic signal on the ordinate cannot be directly substituted into the calculation, it can be used to reflect the quality of pure water in the sample. After fitting, the relationship between the semaphore and pure water quality was roughly a function. As the test conditions changed, the scale factor changed as well. Here, the unknown coefficient was defined as
γ and was retained, not specifically calculated. From signal value of 0 h and saturated state, the water content
in the saturated state of each test group was calculated. The capillary water absorption coefficient
of each group and the water content
in the saturated state of each group were substituted into Equation (13) to obtain
. Subsequently, the
of the exponential function and power function were obtained using Equations (15) and (17), respectively (see
Table 5).
Substituting the saturation value calculated from the signal value obtained in the NMR test into Equations (8) and (9), the water diffusion coefficient
at each depth section at different time intervals was obtained, as shown in
Figure 6 and
Figure 7.
Figure 6 and
Figure 7 indicate that the amount of water absorption in the concrete and the water diffusion coefficient
increase. The water diffusion coefficient curves of group J are above the curves of groups K and KS, which indicate that the water diffusion capacity of group J was relatively strong. This implies that the impermeability of cement-based materials is improved by the addition of a waterproofing agent or a combination of a waterproofing agent and nano-SiO
2. Comparing the curves of groups K and KS, the water diffusion coefficient of group KS was significantly lower than that of group K at each section position, which suggested that after nano-SiO
2 is added to the mixture, the cement mortar structure is more uniform and denser than that with only the waterproofing agent. Moreover, there were fewer connected pores, which weakened the water diffusion ability.
, the water diffusion coefficient, is a function that describes the strength of water transport and is positively correlated to porosity. Consequently, the number of pores in the section where the peak surface appears can be assumed to be high. Comparing the curves of group J at 1 and 4 h, as shown in
Figure 7, a new peak appeared at 20–30 mm; at 49 h, another new peak appeared near 40 mm. Essentially, as the test time increases, the surface moisture continuously spreads into the material. When it is transmitted to a relatively loose section, it appears as a more obvious peak on the curve. Therefore, by observing the position and time of each peak in the graph, the distribution of the sample density can be evaluated and the approximate depth of water infiltration can be determined. The group K curve also followed a similar law. However, the undulation of the curve was small, which implied that the waterproofing agent effectively improved the compactness of the cement mortar. The group KS curve showed almost no obvious peak during the entire test period, which indicated that nano-SiO
2 further improved the pore distribution of the cement mortar sample and reduced the pore size.
The average water diffusion coefficient
of each group exhibited three curves—high, medium and low—related to the degree of modification of the material, as shown in
Figure 8. Although the initial waterproofing effect of the KIM waterproofing agent was not obvious, the test sample had lower saturation during the initial stage of the water absorption test, and the curve differed from the capillary water absorption fitting curve. However, as the test proceeded, the effect of the waterproofing agent was more pronounced and, consequently, the growth rate of the curve was lower than that of group J. The presence of nano-SiO
2 further increased the waterproofing effect in group KS, and the curve increased even more slowly than that of group K. Compared to
Figure 2, the slope of the curve began to change after about 49 h of the capillary water absorption test, which indicated that the cumulative water absorption gradually entered a stable state and the water content of the test sample began to saturate. In
Figure 8, the three curves have a noticeable transition at about 50 h, indicating that the internal saturation of the test samples changed significantly. This indicates that the water content was at a higher level, which confirms the results of the capillary water absorption test.
Figure 9 shows that the relationship between the water diffusion coefficient of the three test groups. The curves of groups J and K are very close, and the growth rate is significantly larger than that of group KS. This corroborates the curve fitting result of the capillary water absorption test (
Figure 2 and
Figure 3). Overall, the results of the two tests are in good agreement with each other.