# Experiment and Analytical Model for Pore Structure of Early-Age Composite Cement Pastes by LF-NMR
## 3 Results and Discussion
### Pores of Composite Cement Pastes  
The PSD curves of composite cement pastes are shown in Figure 6. The PSD curves of all the composite cement pastes contain two peaks. Compared to the PSD peaks of neat cement paste, the ones of composite cement pastes are much higher and thinner. In addition, the left peak and right peak of all the composite cement paste PSDs are right next to each other, while the ones of neat cement paste are separated. At the same time, the effect of admixture is significant. The PSDs of each composite cement are markedly different before 14 days, however, the PSDs of the composite cement tend to be similar at 28 days. Compared with the peak of neat cement paste, the addition of fly ash shifts the peaks to the right before 14 days, but shifts the peaks to the left at 28 days. Furthermore,  
Figure 4: W/c ratio effect on the most probable pore size. Figure 5: Aging effect on the most probable pore size. the effect of slag shifts the peaks to the right at 3 days and then shifts the peaks to the left. However, the peaks of paste blended with silica fume are on the left of those of the neat cement paste from the beginning. The relationship between the MPPD and age is plotted in Figure 7. The MPPD of cement paste blended with fly ash is larger before the age of 14 days and smaller at 28 days than that of neat cement paste. Due to the inert nature of fly ash, composite cement pastes produce fewer and more widely spaced hydration products compared to neat paste, which results in an increase in the gel pore size at the early stages. Thereafter, the hydration products of secondary hydration caused by the pozzolanic reaction of fly ash gradually fill the interspace at the later stage and refine the pore structure. The experimental results are consistent with the results of [PERSON] et al. [34] and [PERSON] [31]. The MPPD of cement paste blended with slag is bigger at 3 days and then smaller than those of neat cement paste. The functions of slag blended in cement are similar to those of fly ash, though the refinement of slag occurs earlier than that of fly ash. This may be due to the higher activities of slag than that of fly ash. These test results are in agreement with the work of [PERSON] [35]. On the contrary, the pore diameter of silica fume paste is smaller than that of neat paste cement from 3 to 28 days. This may be because of the high activity and microaggregate filling effect of silica fume, as reported in the literature [36; 37]. The decrease rate of the MPPD of pastes blended with fly ash or slag is rapid before 7 days and after 14 days, and the rate is slow between 7 days to 14 days in Figure 6. In contrast, the decrease rate of the MPPD of neat paste and silica fume paste is gentle over the whole time. Finally, the four curves of cement paste with admixtures almost intersect at a point with an approximate pore size of 8 nm at 28 days. Figure 6: The pore size distributions of composite cement paste. Figure 7: The most probable pore diameter of composite cement paste. Figure 8 presents the pore size distribution (PSD) curves for composite cement pastes with varying admixture contents. As seen from the figure, for cement pastes containing 15% and 30% fly ash, the MPPDs exceed those of neat cement paste before 28 days, but fall below them at 28 days. With an increase in the fly ash content, the proportion of gel pores rises, yet the MPPD shows little change. This finding contrasts with the results of [PERSON] et al. [31], who observed that MPPD grew progressively as fly ash content increased, indicating a direct correlation between the proportion of fly ash and the development of the MPPD. It is worth noting that in [PERSON] et al. 's study, the fly ash proportions differed slightly from those in this study, being 30%, 50%, and 70%, respectively. Figure 8: The pore size distributions of composite cement paste with different content. For cement pastes blended with 15% and 30% slag, the pore structure improves before 7 days.