1984 Volume 92 Issue 1070 Pages 554-561
The hydration of 3CaO⋅Al2O3 in the presence of CaSO4⋅2H2O and Ca(OH)2 is remarkably retarded. The study aimed to clarify the mechanism of the excessive retardation. When 3CaO⋅Al2O3 was dispersed in suspension of CaSO4⋅2H2O and Ca(OH)2, the interaction between solid and liquid phases as a function of hydration time was investigated. The results are summarized as follows:
(1) The concentration of Ca2+ and SO42- in liquid phase give usually 1.80g CaO/l, 1.02g SO3/l, respectively, independent on mixing amounts of CaSO4⋅2H2O and Ca(OH)2. On the other hand, Al(OH)4- eluted from 3CaO⋅Al2O3 is extremely little.
(2) The retention period at the highest concentration differed depend on mixing amounts of CaSO4⋅2H2O and Ca(OH)2.
(3) After the end of this period, the coprecipitate which suggested solid solution consist of 3CaO⋅Al2O3⋅CaSO4⋅12H2O-4CaO⋅Al2O3⋅xH2O system as shown 7.6-9.6Å phase was rapidly produced on 3CaO⋅Al2O3 surface.
Therefore, the added CaSO4⋅2H2O and Ca(OH)2 describes two roles as follows:
(A) CaSO4⋅2H2O and Ca(OH)2 are required to keep at the highest concentration of Ca2+ and SO42- in liquid phase.
(B) The remained CaSO4⋅2H2O and Ca(OH)2 except for above (A) is present.
Since CaSO4⋅2H2O and Ca(OH)2 corresponding to above (B) is reacted with 3CaO⋅Al2O3, AFt and AFm phases are immediately formed, the resulting 3CaO⋅Al2O3 surface is partly coated by these phases. However, this cause was not essentially on the excessive retardation. Because 3CaO⋅Al2O3 is covered by the highest concentration of CaSO4⋅2H2O and Ca(OH)2 corresponding to above (A), consequently, during this period, the elution of Ca2+ and Al(OH)4- from its surface has not been able to recognize. Therefore, the hydrates is not produced, and the hydration stops apparently.
