# Isothermal Drying Process and its Effect on Compressive Strength of Concrete in Multiscale
## 3 Results
### Microstructure Analysis
#### 3.4.3 SEM Analysis  
SEM images of specimen in saturated and dry states are shown in Figure 8. It is evident from Figure 8a that there are clusters of amorphous C-S-H gel, needle ettringite, and plate-like Ca(OH)\({}_{2}\) in the paste zone both of saturated and dry specimen. The difference is that the solid matrix of dry specimen seems to become denser and finer, and there seems more needle-like hydration products. However, from Figure 8b one can see that, in the saturated sample there is a thin adsorption layer around the aggregate, while in the dry sample the aggregate edge is clear and there is no adsorption layer. This indicates that the bond between aggregate and paste was weakened in drying process.  
Figure 7: TGA/DSC curve of hardened cement paste.  
In the drying process of cement-based materials, some changes take place: (1) heating at 105 \({}^{\circ}\)C promotes further hydration of dehydrated cement grains and increases the amount of hydration products, resulting in a denser microstructure; (2) water loss results in the increase of surface free energy [16] and causes solid phase compaction, which was considered to be the result of the packing rearrangement of C-S-H particles [29] or the decrease of interlayer thickness within gel particles; (3) the dehydration of ettringite and C-S-H (Figure 7) leads to changes in the compositions of hydration products and the contraction in their volume; (4) the differences in thermal properties between paste and aggregate allows microcracks to generate and grow in their interface. The expansion of aggregate recovers after the specimen cools to room temperature, while the shrinkage of paste due to water loss is unrecoverable, which weakens the bonding between aggregate and paste and even results in the debonding of them. These physicochemical changes alter the microstructure of concrete and, thus, cause changes in compressive behavior. On one hand, the denser the structure of cement-based materials, the higher its strength, so the densified microstructure of paste is beneficial to concrete strength, as shown in Figure 8a. On the other hand, the weaker the bonding between paste and aggregate, the lower the strength, so the weakened bonding between aggregate and paste is harmful to concrete strength, as shown in Figure 8b. Both of the above opposite effects are associated with drying. Whether the compressive strength increases or decreases after drying depends on which of the two effects dominates.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multirow{2}{*}{**State**} & \multicolumn{4}{c}{**Porosity/\%**} & \multirow{2}{*}{**Average Pore**} \\ \cline{3-3} \cline{5-6}  & & \textless{}100 nm & & \textless{}100\textless{}1000 nm & & \textless{}1000 nm & \multicolumn{1}{c}{**Total Porosity**} \\ \hline \multirow{2}{*}{Cement paste} & Saturated & 11.98 & 0.51 & 0.48 & 12.97 & 10.9 \\  & Dry & 18.70 & 0.45 & 0.89 & 20.04. & 18.8 \\ \multirow{2}{*}{Mortar} & Saturated & 4.33 & 2.60 & 3.00 & 9.93 & 17.7 \\  & Dry & 3.29 & 7.64 & 2.85 & 13.78 & 110 \\ \hline \hline \end{tabular}
\end{table}
Table 6: MIP results (water/cement ratio = 0.41).  
Figure 8: SEM images of dry and saturated specimen.