Effect of borax on the hydration process of FGD building gypsum and its mechanism analysis

As FGD building gypsum has a character of fast solidity,the effect of borax on the hydration process and crystal morphology of dehydrate of FGD building gypsum and relationship between pH value and retarding effect were analyzed.Furthermore,by using the X-ray photoelectron spectroscopy technique,its retarding mechanism was also studied.The results indicate that the addition of borax retards the early hydration of FGD building gypsum,which leads to a delay in the hydration heat evolution.It can also prolong the setting time but has a little influence on strength development.Borax has a better retarding effect in alkaline hydration conditions and the setting time reaches the peak point when pH=10.The addition of borax will lead to the transformation of dihydrate crystal from needlelike to short prismatic.It could cause the large crystal sizes and few links between crystals,in that the strength will be lower.Borax interacts with the Ca element on the crystal nuclei surface of dehydrates and forms a chemical adsorption layer,inhibiting its growth,and consequently slowing down the hydration process.