The changes in surface area and mesoporosity in aggregates of ∼0.01 μm cross‐section CaO particles when heated in CO2at 686°C were determined from N2 adsorption isotherms. Initially, the surface area decreases rapidly with little change in porosity. When the surface area has decreased below ∼90 m2/g, surface area and porosity variations become consistent with expectations for coarsening by grain‐boundary or bulk diffusion. The initial rapid decrease in surface area must result from CO2‐catalyzed surface diffusion, but the data suggest that surface diffusion is not rate‐limiting. The rate‐limiting step may be reaction of CO2 to form surface CO32‐ ions or decomposition of these ions to O2‐ ions and CO2 gas.

The solid products of decomposing CaCO, powder in vacuum at 510°C (sr-CaO) and of decomposing Ca(OH), powder at 320°C in vacuum (h-CaO) are particles which have approximately the same exterior dimensions as the parent CaCO, or Ca(OH), particles. N, adsorption and desorption isotherms show that srand h-CaO have high internal surface areas which for sr-CaO have cylindrical symmetry, with the most common diameters being -10 nm, and for h-CaO are slit-shaped, with the most common slit width being ~2 . 7 nm. The conclusions reached in earlier investigations, i.e that these decomposition reactions in vacuum initially yield a form of CaO which has the same unit cell dimensions as the parent solid, were in error, probably because water vapor converts much of the CaO to poorly crystalline Ca(OH), before XRD measurements can be completed in air. From the volume of Nz adsorbed by the porous powders, the porosity of h-CaO is calculated to be 36?5% and of sr-CaO 41.525%. These porosities imply that the linear dimensions of the 1 to 20 pm particles of h-CaO and sr-CaO are ~5 % smaller than those of the parent particles. XRD measurements made as a function of time show that particle shrinkage must occur by cooperative, diffusionless movement of crystallites of sr-CaO or h-CaO as they form.

Reaction of water vapor at 25°CC with CaO powder of either high or low surface area yields Ca(OH)2 with broad XRD peaks. Relative rates of reaction of these hydroxides with CO2 at 250°CC depend mainly on the sample surface areas per unit weight. SEM observations show that the exterior shapes of porous 1 to 30 μm particles of CaO formed by decomposition in vacuum of CaCO3(sr‐CaO) or Ca(OH)2 (h‐CaO) are only slightly changed when they are converted to Ca(OH)2, except for surface roughening, which increases with time of exposure to water vapor. The N2 adsorption‐desorption isotherms of h‐CaO and sr‐CaO, and of their hydration products, are used to calculate the average particle expansion, the probable pore shapes, and the changes in pore‐size distributions that accompany the reaction. These data show that the reaction causes expansion perpendicular to ulterior surfaces of the porous powder particles. A possible mechanism is suggested. Both h‐CaO and sr‐CaO dissolve in liquid water to yield Ca(OH)2 by subsequent precipitation.

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volume elements of the kind described.