1968 Volume 76 Issue 876 Pages 253-263
A large amount of phosphogypsum by-produced in wet process of phosphoric acid manufacture is being used as an available retarder of portland cement. However, some impurities in phosphogypsums, such as the water-soluble P2O5 and the water-soluble F retard the set of portland cement and hinder the early development of its mortar strength.
In this paper, the authors have revealed the effect of impurities on the hydration of portland cement and alite and have suggested the mechanism of retarding action on the hydration of portland cement.
The obtained results are as follows;
(1) The water-soluble P2O5 and the water-soluble F retard the set of paste and, when they are added over some amount to cement, hinder even the early development of mortar strength. The retarding action on the hydration of portland cement is more strengthened by the co-existence of over two shorts of impurities than by a single existence of any impurities. These results well agreed with that of the preceding investigators.
(2) The Phosphogypsums by-produced in hemihydrate-dihydrate process have a regular form, therefore, most part of the water-soluble impurities which adhered to the surface of gypsum crystals is removed by a simple washing with water after the cake of phosphogypsum was discharged from the filtration step in wet-process. Moreover, the P2O5 in phosphogypsum crystals is also a small amount.
On the other hand, the phosphogypsums by-produced in dihydrate process have an agglomerated form and contain a large amount of impurities which can not be removed by a simple washing with water.
(3) The heat liberation curves of hydration of portland cement containing various phosphogypsums and various artificial phosphogypsums showed that (a) the impurities in phosphogypsums mainly retard the hydration of alite phase, (b) the retarding action of water-soluble F is larger than that of the water-soluble P2O5 and (c) the dormant period of hydration of portland cement is prolonged by the action of impurities, however, the essential part of hydration of alite is scarcely affected.
These tendencies were also confirmed by the measurement of specific surface area of hydration products of portland cement paste containing phosphogypsums and artificial phosphogypsums.
(4) The authors suggested that the retarding action of the water-soluble impurities is caused by the formation of protective film of the difficult soluble compounds resulted from the reaction between the water-soluble impurities and the high-lime surface of cement grains. In high-lime solution of cement-water system, the water-soluble P2O5 forms the difficult soluble basic hydrates of calcium phosphate and sodium silicofluoride as the watersoluble F forms calcium fluoride, silicic acid gel and calcium silicate hydrates. These reaction products are gel-like fine particle and will temporalily coat the surface of cement grains.
