Scanning Electron Microscopy

Typical processed SEM micrographs for the w/c=0.35 and w/c=0.435, 92 d cement pastes are provided in Figs. 5 and 6, respectively. In the figures, unhydrated cement particles are white, calcium hydroxide is light grey, C-S-H and other hydration products are dark grey, and capillary pores are black. A large influence of curing conditions on the hydrated microstructures is observed. For hydration under sealed conditions, there exists a set of large (empty) capillary pores (10 µm² and larger) for both w/c systems. These pores are present due to the chemical shrinkage and self-desiccation occurring during hydration, as the largest "pores" are observed to be the first to empty during either internal or external drying at early ages. ^{21, 22} For saturated and sealed/saturated curing, there are fewer of these "large" pores, although they are not eliminated entirely. For these "saturated" curing conditions, more and coarser pores are observed in the w/c=0.435 system, due to the larger initial spacing (and thus pore size) between cement particles. ^{15, 23}

A quantitative analysis of the coarse porosity was performed by determining the size of each individual two-dimensional pore in the SEM images. Histograms of the obtained pore areas (in pixels) are provided in Figs. 7 and 8, for the w/c=0.35 and w/c=0.435 cement pastes, respectively. The area fractions of the microstructures occupied by pores larger than 7.5 µm² (30 pixels) are provided in Table 2. For both w/c, curing under saturated conditions resulted in the least remaining volume of coarse pores after 92 d, followed by curing under sealed/saturated conditions, and finally by sealed conditions. While curing under saturated or sealed/saturated conditions resulted in basically the same achieved degrees of hydration (Figs. 3 and 4), in the latter case, there is a significantly higher volume fraction occupied by the coarse pores. Many of these pores would have been created due to chemical shrinkage during the initial 7 d of sealed curing, and these results suggest that they are only partially filled in by reaction products after additional curing water is provided and hydration continues. While the size and number of these pores should certainly influence the mechanical properties of these materials, it is more likely that their connectivity (percolation) characteristics will influence the transport properties and durability. The results to be presented in the LTC section to follow will explore these percolation aspects in detail.

As indicated by the results tabulated in Table 3, the degrees of hydration determined for the various cement pastes via the SEM analysis are consistently slightly higher (about 0.05) than those determined by the LOI technique. This could be due to the presence of very small unhydrated cement particles that would not be identifiable in the SEM images. However, the rank order for the different w/c values and different curing conditions is the same for both the SEM and the LOI techniques.