Internal water curing provides a means to supply hydrating cement paste with needed curing water when conventional external curing is ineffective, usually due to the lower w/b of the binder matrix in the concrete. When this needed water is unavailable, self-desiccation will occur, with the formation of a system of (large) empty pores within the cement paste microstructure (see Figure 2 for example). Concurrently, the increase in capillary tension in the capillary pore water may result in a measurable autogenous shrinkage and possible cracking of the sample. The deficiency in water will also result in a measurable decrease in the ongoing hydration rate and possibly strength development. These various effects can be modeled using existing microstructure models, such as CEMHYD3D [1, 2], HYMOSTRUC [3, 4, 5], and DuCom [6].
Various authors [7, 8, 9] have quantitatively measured the reduction in achieved degree of hydration of samples exposed to sealed as opposed to "saturated" curing conditions, with all other experimental variables being equal. Reductions in measured degrees of hydration of as much as 5 % to 15 % have been observed for low w/b (0.2 to 0.4) cement and blended cement pastes cured for long periods of time (28 d to 150 d). As indicated in Figure 1, microstructural model predictions and experimental measurements are in generally good agreement. In Figure 1, model results are presented for saturated, sealed, and saturated/sealed curing conditions; in the latter case, all pores remain saturated until the capillary porosity in the 3-D microstructural model depercolates, at which point sealed curing conditions are initiated [8].
In addition to a measurable difference in degree of hydration, sealed curing will also dramatically influence the microstructural development of the cement paste. As an example, Figure 2 provides real and model (CEMHYD3D) two-dimensional (from 3-D microstructures) images of a cement paste made with a water-to-cement ratio w/c=0.30 and hydrated under saturated and sealed curing conditions for 90 days at 25 ºC. It is clear that there are more unhydrated cement particles and more and larger capillary pores in the systems hydrated under sealed conditions. In the CEMHYD3D model under sealed curing conditions, empty capillary porosity is created during the hydration simulation to exactly match the volume formed due to the chemical shrinkage present from the various hydration reactions. To simulate the physical reality, pore "sizes" are assessed locally and the largest pores are emptied first regardless of their location within the cement paste microstructure. X-ray absorption measurements have indeed verified that at early ages, small (5 mm thick) cement paste samples dry out "uniformly" as opposed to developing a sharp drying front [10]. The pore sizes and permeability of the fresh (young) cement paste are such that the capillary water can quickly flow to keep the smaller pores filled at the expense of the larger ones. Of course, these phenomena are also of fundamental importance for the technological application of internal water curing. In internal water curing, the water sources are shifted from being the largest pores in the hydrating cement paste to being the water reservoirs in a special internal water curing agent such as saturated lightweight aggregates (LWA) or superabsorbent polymer (SAP) particles.
Figure 1: Experimental and model (CEMHYD3D) degrees of hydration for a w/c=0.35 cement paste cured under "saturated" and sealed conditions at 20 ºC.
In addition to modeling microstructural development under sealed curing conditions, it is also of interest to model the development of the internal relative humidity, as this parameter is an indicator of the magnitude of the stresses developed in the pore fluid and consequently in the solid cement-based material [11, 12]. Neglecting the influence of dissolved salts [12], the Kelvin-Laplace equation quantifies this relation:
 |
(1) |
where σcap = capillary stress (N/m2), γ = surface tension of pore solution (N/m), r = pore radius (m), RH = relative humidity (0 to 1), R = universal gas constant (8.314 J/mol−K), T = absolute temperature (K), and Vm = molar volume of water (m3/mol).
![Figure 2: Scanning electron microscopy (SEM) (top) and CEMHYD3D model (bottom) images for saturated (left) and sealed (right) hydration of a <I>w/c</I>=0.30 cement paste after curing at 25 ºC for 90 d [8]. SEM images are 128 µm by 190 µm; model images are 100 µm by 100 µm. Phases from brightest to darkest are: unhydrated cement, calcium hydroxide, calcium silicate hydrate gel, and porosity (empty and water-filled).](Figure2a.gif)
![Figure 2: Scanning electron microscopy (SEM) (top) and CEMHYD3D model (bottom) images for saturated (left) and sealed (right) hydration of a <I>w/c</I>=0.30 cement paste after curing at 25 ºC for 90 d [8]. SEM images are 128 µm by 190 µm; model images are 100 µm by 100 µm. Phases from brightest to darkest are: unhydrated cement, calcium hydroxide, calcium silicate hydrate gel, and porosity (empty and water-filled).](Figure2b.gif)
Figure 2: Scanning electron microscopy (SEM) (top) and CEMHYD3D model (bottom) images for saturated (left) and sealed (right) hydration of a w/c=0.30 cement paste after curing at 25 ºC for 90 d [8]. SEM images are 128 µm by 190 µm; model images are 100 µm by 100 µm. Phases from brightest to darkest are: unhydrated cement, calcium hydroxide, calcium silicate hydrate gel, and porosity (empty and water-filled).
Based on measured desorption isotherms, Norling Mjörnell has developed a detailed model for predicting degree of hydration and internal relative humidity in ordinary and high-performance concretes [9]. By assuming the following cumulative pore size distribution, as opposed to measuring the desorption isotherms, van Breugel and Koenders [3, 4] have modeled the reduction in internal relative humidity with increasing hydration time in cement-based materials, as exemplified by the results presented in Figure 3.
 |
(2) |
where Vpor = cumulative pore volume in the paste for pores of diameter φpor and smaller (m3), V = the total volume of the paste system (m3), φpor = pore diameter (nm), φ0 = the diameter of the smallest pore in the system = 2 nm, and a = pore structure constant based on experimental data (typically 0.05 to 0.15) [4]. Ye has extended the HYMOSTRUC model to provide an even more realistic pore size distribution curve, as shown in Figure 4 [5].
|
 |
a) | b) |
Figure 3: (a) Schematic representation of cumulative pore size distribution indicating that largest pores empty first during self-desiccation; (b) Modeled relative humidity versus the degree of hydration for three different w/c (wcr) and three different finenesses of cement.
Based on the diameter of the largest water-filled pore in the system, from equation (2) or from a computed pore size distribution such as that in Figure 4b, the relative humidity and capillary stress in the system can be calculated using the Kelvin-Laplace equation (1). Differences in the fineness of the cement cause differences in the formation of the microstructure and lead inherently to changes in the fineness of the pore structure [3, 4, 11]. From the Kelvin-Laplace equation, it can be observed that the relative humidity in the system is directly related to a pore radius. The development of a low relative humidity can thus be opposed by internal water curing, as the largest water-filled pores are shifted from being smaller pores in the hydrating cement paste to being larger pores located in the supplied water reservoirs.
 |
 |
Figure 4: (a) Pore structure (in black) of model cement paste with w/c=0.3; (b) Computed cumulative and differential pore size distribution curves with degree of hydration=0.64 [5].
 |
 |
Figure 5: (a) Local w/c as a function of distance from the aggregate surface for pastes with different overall w/c; (b) local degree of hydration for w/c=0.3 paste at different overall degrees of hydration (DOH) [15].
One microstructural component where a large influence of curing conditions may be observed is in the interfacial transition zone (ITZ) regions surrounding each aggregate (and air void, etc.) in concrete [4, 13, 14, 15]. Because of inefficient packing of the cement particles in the immediate vicinity of the aggregate (the "wall effect"), often there will be larger and more capillary pores in the ITZ, due to its initially higher local w/c ratio, as illustrated in Figure 5a [15]. Thus, particularly for low w/c pastes, the ITZs can behave as water sources for the desiccating bulk cement paste matrix. During sealed curing conditions, these larger pores in the ITZs will likely be the first to empty as indicated by the two-dimensional images in Figure 6 and the plots of empty porosity vs. distance from the aggregate surface in Figures 7 and 8, for simulations conducted using both the CEMHYD3D and the HYMOSTRUC microstructure models. The influences of cement particle size, w/c, and curing conditions are all clearly observed.
 |
 |
 |
 |
Figure 6- Simulated initial (top) and "completely" hydrated (bottom) ITZ microstructures for cements with median diameters of 5 µm (left) and 30 µm (right) and a w/c=0.30 [13]. Images are 100 µm by 100 µm. Hydration conducted under sealed conditions at 25 ºC. In the initial images, phases from brightest to darkest are: C3S, C2S, C3A, C4AF, gypsum, and porosity. In the hydrated images, phases from brightest to darkest are: unhydrated cement, calcium hydroxide and other hydration products, calcium silicate hydrate gel, and porosity. Central bar extending across the microstructure is the flat plate aggregate.
It should be noted that because the smaller cement grains tend to concentrate in the ITZ region (Figure 6) and because the achieved degree of hydration in the ITZ regions can be significantly higher than that in the bulk paste (Figure 5b), the preferential movement of water from ITZ to bulk paste during hydration under sealed conditions is not guaranteed, but will depend on the specific characteristics of the system being considered (w/c, particle size distribution, etc.). However, because in a sealed system the self-desiccation process begins with the setting of the cement, it is likely that the very first pores to empty will be in the ITZ regions. At these early ages, the enhanced hydration in the ITZ relative to the bulk paste will not yet have a significant influence on local pore sizes (DOH = 0.274 curve in Figure 5b). The situation becomes even more complex in the presence of internal water curing. For example, if partially unsaturated LWA are added to the concrete mixture for internal water curing, their initial further absorption of mixing water could significantly densify the local ITZ microstructures [14]. Of course, in this case, it is intended that the pores in the LWA and not those in the ITZ will be the first to empty during hydration.
Figure 7: Empty porosity as a fraction of total paste volume as a function of distance from the aggregate surface for CEMHYD3D hydrated cement pastes with two different particle size distributions (5 µm and 30 µm), two different curing conditions (sealed and saturated/sealed), and w/c=0.3 [13].
Figure 8: Empty porosity as a fraction of total porosity (empty porosity/total porosity) as a function of the distance from the aggregate surface for HYMOSTRUC hydrated cement pastes with three different w/c.
Certain commercially available software products are identified in this paper to specify the computer programs used. In no case does such identification imply endorsement by the National Institute of Standards and Technology, nor does it indicate that the products are necessarily the best available for the purpose.