Study On Retarding The Set Of Sulphoaluminate Cement

Sulfoaluminate cement can be used in many fields. However, this kind of cement sets quickly because of the properties of the cement clinker. This makes construction difficult. In project, the retarder is often added to regulate the setting of sulfoaluminate cement. Borax is the common retarder and has a strong effect. However, its effect is unstable and can often bring about some dangers. So it is very important to find out a new retarder, which has a stable effect of retarding on the sulfoaluminate cement.The ratio between borax and sulfoaluminate, which are the important components in the new type of retarder, is 3.6:1. The adding range of the retarder is about 1.1%～1.5%. The new retarder has a stable effect of retarding and makes the loss of fluidity little. The strengths of early age and later age in the cement, which added the new retarder, are higher than the cement without adding the retarder.In the sulfoaluminate cement paste, the situation during the ettringite-crystallizing make it set quickly. According to the mechanism of dissolving-depositing, the crystal of ettringite can separate out from the cement paste. Because of its little super saturation (ΔC) and slow velocity of nucleating, ettringite can be formed into large crystals . On the other hand, the condition of the cement paste can make the ettringite formed in the liquid far away from the surface of the cement particles. These kinds of ettringite are very different in size. This makes ettringite easier to touch together to form a temporary frame. During the hydration, this frame can be strengthened and finally causes the cement paste to set.The retarding mechanism of borax is to form a thick and compact film, which is called calcium borate, and enwraps around the surface of the sulfoaluminate cement particles, and limits the hydration and keeps the paste inside and outside of the film immobile. When the amount of borax is not large enough to enwrap the cement particles, the effect of retarding is not well. Moreover, due to the abnormity of the actual shapes of the cement particles, the solubility on the surface of different curvature radii are different .At the same time, the speeds of dissolving are different. So the concentrations of Ca2+ around the surface of different parts are different. This causes the thickness of the calcium borate film on the surface of different parts to be different. In these places, where the enwrapping film is thin, water can easily filter into the inside of the enwrapping film and causes the clinker to hydrate quickly. The hydrating products of AFm and AFt will make the solid volume larger and then engender stress, and finally cause the enwrapping film to be broken. This makes the effect of retarding unstable.When the aluminum sulfate is introduced into the cement added borax, the concentrations of Al³⁺ and SO₄^2- become higher. With the dissolving of gypsum, the concentration of Ca²⁺ also becomes higher. So the accumulating concentrations of relating ionic in the outside of the enwrapping film will quickly equal the supersaturating, which is required when the ettringite crystallizes. Then the nucleuses of ettringite are formed. When the ettringite grow, they will consume the relating ionic. Actually, the introduction of the aluminum sulfate is to settle an outfit to absorb ionic and reduce the concentrations of different ionic .It will enlarge the difference between the inside paste and the outside paste and accelerate the water to diffuse .This make the clinker which is in the inside of the enwrapping film hydrate quickly . The products formed in the inside of the film will make the solid volume larger and larger, and then engender stress. Finally the enwrapping film will be broken.Through the research, the theory knowledge of the hydrating and hardening of sulfoaluminate cement is enriched. It can provide the base for the further exploring the sulfoaluminate cement.