Electronmicroscopic Observation on the Carbonated Paste of Cement containing a Large Amount of Granulated Blastfurnace Slag

Abstract

Already, it has been well known that an “absanding phenomenon” occurs at some times after the surface of hardened body which was mainly consisted of the granulated blastfurnace slag, for example, such as super-sulfate cement, was exposed under atmosphere. The practical use of super-sulfate cement is still now obstructing by this phenomenon.
Though it has been suggested that this phenomenon may be mainly caused by the carbonation of calcium compounds in the surface layer of the hardened body made from super-sulfate cement etc., its detailed study has still not been undertaken.
The authors investigated electronmicroscopically the difference of the hardened structure between a neat paste of portland cement and a paste which is mainly consisted of the granulated blastfurnace slag, in the both cases that the hardened body is exposed under atmosphere and not.
In general, all the clinker minerals of portland cement contain a larger quantity of lime than that of lime required for the formation of its normal hydration products. Therefore, these excess lime liberated from each particle of cement in the hydration are present as the crystallized calcium hydroxide or the amorphous ones at every place in the hardend body of portland cement paste. The calcium hydroxide and some portions of the hydration products in the surface of its hardened body react with CO₂ in atmosphere. Then, the tight and continuous coatings consisted of calcium carbonate are formed gradually on the surface of hardened body for some times after its surface comes into contact with CO₂. It is considered that this coatings are very available for the prevention of the penetration of CO₂ inside the hardened body.
On the other hand, the excess lime is not contained in the cement which is mainly consisted of the granulated blastfurnace slag that a large amount of lime is required for the formation of its hydration products. This cement has only a small amount of portland cement or calcium hydroxide as the accelerating agents for the hydration. The tight protective coatings which are observed in the structure of hardened body of portland cement can not be formed by the additional accelerating agents in that of hardened body of super-sulfate cement. Besides, considering the poor rate of hydration, it is natural that the structure of hardened body is so porous that CO₂ can be easily penetrated inside the hardened body. In fact, a large amount of the coarse crystals of calcium carbonate are partially produced on the surface and in the layer under surface of hardened body which is mainly consisted of the granulated blastfurnace slag.
Consequently, the bonds among the hydration products are broken by the formation of these coarse crystals of calcium carbonate and subsequently the “absanding phenomenon” is occurred.