Plaster of Paris

Figure 7 shows a 2-D image for the dry Plaster of Paris powder. A wide variety of different size and shape particles are present in this material. Some of the particles have a very dense structure while others clearly exhibit some sort of microporosity and internal flaws, which could have a large influence on their reactivity with water. In this two-dimensional image, particle sizes are seen to range from a few micrometers up to about 300 µm (a somewhat wider range than that observed for the cement particles).

  

Figure 7: 2-D image (1024 pixels by 1024 pixels) of slice 151 for dry Plaster of Paris powder.

Figure 8 provides a 2-D image of the Plaster of Paris microstructure after 15.5 h of hydration time while Figure 9 provides a 3-D image of a portion of the microstructure after 4 h of hydration. These images are vastly different from that of the dry powder in Figure 7. Numerous needles of calcium sulfate dihydrate reaction product are present in the hydrated systems. Partially reacted calcium sulfate hemihydrate particles and completely reacted particle "shells" can also be clearly observed in the 2-D image. In some cases, reaction products appear to have deposited within a portion of a partially reacted calcium sulfate hemihydrate particle. Because the morphology of calcium sulfate dihydrate crystals is strongly dependent on chemical admixtures added to the mixture and in turn has a large influence on mechanical and other physical properties, microtomography may provide a powerful tool for quantitatively evaluating the influence of these chemical admixtures on microstructure and performance. Because the elastic properties of both forms of calcium sulfate are known, these 3-D images could be used as input into a finite element software package [12] to compute the elastic properties of the material as a function of degree of hydration, w/s, chemical additives, etc., as has been done previously for computer model microstructures designed to mimic the Plaster of Paris system [22].

  

Figure 8: 2-D image (1024 pixels by 1024 pixels) of slice 251 for w/s=1.0 Plaster of Paris hydrated for 15.5 h. Large grey particles are unhydrated calcium sulfate hemihydrate. Grey needles (typically about 50 µm in length) are calcium sulfate dihydrate product. "Pore" impressions of totally and partially reacted starting particles are easily visible.

  

Figure 9: 3-D image (100 voxels by 100 voxels by 100 voxels) of a portion of the w/s=1.0 Plaster of Paris hydrated for 4 h. A variety of needle-like and flat plate crystals of calcium sulfate dihydrate can be seen.