# Isothermal Drying Process and its Effect on Compressive Strength of Concrete in Multiscale
## 4 Discussion  
The composition and properties of concrete at each scale play a role in its strength and affects it at the upper scale due to the multiscale nature of concrete. In this section, the drying effects on compressive strength will be discussed form four scales--C-S-H, cement paste, mortar, and concrete. In C-S-H scale, C-S-H gel is made up of the aggregation of C-S-H granules. The interaction between water and C-S-H gel determines the mechanical properties of cementitious materials [15]. Water removal from the interlayer space and intraglobular pores (IGP) causes the C-S-H globule collapse and results in the changes of C-S-H sheets in shape and orientation, relative to their nearest neighbors [28]. As shown in Figure 13 in Reference [30] by [PERSON] et al., the removal of interlayer water and gel water caused by heating leads to the C-S-H grain shrinkage, and further, to shrinkage of concrete at three scales. Furthermore, the impact of water on the performance of C-S-H gel was interpreted from the perspective of molecular dynamics. Water molecules can change the connections in C-S-H gel by screening the Ca-O connections and replacing the ionic-covalent bond with unstable H-bond connections, which can be shown from Figures 8 and 11 in Reference [31]. The hydrolytic weakening of water molecules reduces the stiffness and cohesive force of C-S-H gel, as a result of the reduction in compressive strength. The molecular simulation of [PERSON] et al. [31] had indicated that compressive strength of dry C-S-H was 33.3% higher than that of saturated C-S-H. As the dominant cement hydrate, the reduction of compressive strength in C-S-H scale provides an explanation for the strength changes caused by drying in higher scales of concrete. Cement paste scale is composed of hydration products (C-S-H, CH, AFt,...), dehydrated clinkers, and capillary pores. According to the Kelvin-Laplace equation, water confined in capillary pores is under compression. When paste is exposed to drying conditions, capillary water is gradually lost and smaller and smaller pores are emptied. The water loss from large capillary pores has less contribution on shrinkage than that from small capillary pores, so in the early stage of the drying process more water loss just results in relatively small shrinkage. The drying shrinkage of paste is only about 1/4 of the total shrinkage when about 70% of the water is lost, as shown in Figure 6a. On the other hand, the dry and saturated paste behaves differently under compression loading. For the dry paste, which has a higher stiffness than saturated paste, more energy is required when microcracks are driven to extend through, and microcracks in other weak regions may propagate or new microcracks may initiate to reduce local stress intensity. This will result in a higher compressive strength a columnar failure mode, and the development of more microcracks (Figure 5b). For the saturated paste, however, the growth of microcracks is easier because water reduces the surface free energy of solid phase [5]. Thus, the saturated paste fails in a lower compressive strength and presents a failure mode with a main crack on surface (Figure 5a). The mortar scale is composed of cement paste, sand, and interface transition zone (ITZ) between paste and sand, of which cement paste is the origin of strength and ITZ is the weakest phase [15]. Due to the introduction of sand, mortar has higher stiffness and lower water content than cement paste with the same water/cement ratio, and thus presents the smaller shrinkage with the loss of water. When subjected to compression loading, microcracks in mortar grow mainly along the ITZ as shown in Figure 9. They propagate and coalesce into macro-cracks. On one hand, drying makes paste stiffer and stronger, on the other hand, it causes the bonding between sand and paste to get weaker, as well as increasing the porosity within 100-1000 nm.