In reaction system of CaSO4-CaCO3-(NH4)2CO3-H2O

gypsum dissolution was the rate-controlling step to produce calcium carbonate and ammonium sulfate. Coverage of dissolution surface by newly formed CaCO3 crystal was a significant factor. Based on reaction plane model

adjusting ratio of gypsum dissolution rate to carbonate addition rate could control the reaction-crystallization plane beyond critical distance of solid-liquid diffusion boundary layer of the dissolution surface and avoid CaCO3 coverage on the surface. Pure natural gypsum blocks with constant surface area was used to study CaCO3 coverage on gypsum surface and to predict micro-distance between reaction-crystallization plane and dissolution surface. Ammonium carbonate solution was added into the dissolution system with ratio of gypsum dissolution rate to carbonate addition rate equal to 0.125

0.25

0.5

1

2

4 and 8. The results showed that CaCO3coating on surface was increased as the ratio of gypsum dissolution rate to carbonate addition rate decreased in the range of 0.125

0.25

0.5 and 1

whereas CaCO3particles were formed in the liquid phase without surface coating with the ratio of gypsum dissolution rate to carbonate addition rate in the range of 2

4 and 8. At the ratio of gypsum dissolution rate to carbonate addition rate equal to the critical value 1

the micro-distance was more than 18 µm.