Jennings Colloidal Model

The behavior of both the SWVE and UWVE specimens can be explained in a manner that is consistent with the microstructural model for 1 nm to 100 nm length scales by Jennings [28] and Jennings and Tennis [29,30]. The Jennings colloidal model (JCM) [28] identifies four types of pores as shown in Table 2. Also shown in Table 2 are the corresponding DSC peak freezing temperatures from Valckenborg et al. [31], Bager and Sellevold [26,27], and this work.

Table 2: Summary of pore size classes and approximate DSC pore freezing temperature Tm.
Pore Class	Pore Size (nm)	Description	Tm (ºC)
(a)	(a,b)	(b)	(a)	(c)	(d)
C-S-H Gel	-	chemically bound	-
Mono-layer	0.25 - 0.45	globule pores	-70
Dense Gel	1.2	inter-globule pores within LD structure	-60	-45	-45
Open Gel	1 - 10	pores between LD and HD units	-20	-25	-30
Capillary	>10	larger than gel pores	-10	-10	-20

(a) Valckenborg et al. [31]
(b) Jennings [28]
(c) Bager and Sellevold [26,27]
(d) This work

A schematic of the JCM is shown in Fig. 10. According to the JCM, the basic unit of C-S-H is a colloidal particle having a radius on the order of 1.5 nm. During the middle period of hydration (4 h to 24 h), these smallest units form globules. These globules, in turn, condense to form low density (LD) C-S-H clusters having pores that are emptied of water only below 20 % RH [28]. The globules probably form first, and condense to form the LD structure during the period between 10 h and 24 h [28].

During the late period of hydration (after 24 h), collections of LD C-S-H clusters aggregate to form a microstructure having inter-LD cluster pores. These pores are characterized as being full of water at 90 % RH and empty by 40 % RH [28]. Therefore, the JCM is consistent with the DSC results for SWVE specimens assuming that the -45 ºC peak corresponds to water within the LD C-S-H pores, and the -30 ºC peak corresponds to water that is only accessible via the inter-LD pores. These freezing temperatures are denoted in the JCM schematic in Fig. 10.

Figure 10: Schematic of JCM. LD C-S-H is composed of globules, and the inter-globule pores freeze at -45 ºC. The Inter LD C-S-H microstructure is composed of LD C-S-H clusters, and the inter LD C-S-H pores freeze at -30 ºC.

The JCM forms a foundation for understanding the DSC results for the 100 % RH exposure. From the JCM and the DSC results, a self-consistent conceptual model (CM) is developed to explain the observations. The CM is first developed based on the SWVE specimens, and then used to explain the DSC results for the UWVE specimens.