HOW DO ADDITIVES INFLUENCE THE MORPHOLOGY OF THE HYDRATION PRODUCT OF CALCIUM SULFATE HEMIHYDRATE

摘要

Calcium sulfates are used as a retarder for the calcium aluminate hydration in Portland cements, but is also used as a binding material itself. Because the strength of gypsum based binding materials decreases in presence of moisture it could only be used inside of buildings for floor pavement, glue for tiles, stucco or plaster board and reparation mortars. Moreover gypsum based binders are used as a casting material in the ceramic industry because of its porosity, in metal casting or for dental uses. It is also used in plaster bondage or as additive in polymers. For all of these technical uses different technical properties have to be improved and different hydration times are needed. For plaster bondage, floor pavement and ceramic castes a special porosity is needed and in case of dental gypsum or for metal castes a lower porosity has to be used. For plaster bondage time for hardening of the gypsum based binders should be around 5 minutes, for floor pavements it must be much longer. Porosity and hardening time are only some examples for needed technical properties. Besides, the strength, casting accuracy, stiffness or drying time are also taken into account. All of these technical properties could be influenced by additives. Citric acid is a well-known retarder for gypsum based binders and gypsum seeds and potassium sulfate are working as an accelerator. The microscopic methods were used to investigate the influence of additives and its mechanism. During these measurements it could be proved that the additives mostly influence the morphology of the created dihydrate crystals and the morphologies influence technical properties. That is one of the main problems because any additive influences the morphology in a special way and normally there are different additives used in a binder system. For this reason, a technique is needed to check whether a mixture of different additives could work or not. Such methods could be the optical microscopy and the environmental scanning electron microscope (ESEM) technique, because the hydration process and the created gypsum morphology could be investigated beside the solving process of hemihydrate and the seed formation of dihydrate or the reaction time. Especially with the in situ optical microscopy much information about the hydration of hemihydrate with and without additives and mixtures of additives could be created. It could be pointed out, for example, that citric acid creates shorter crystals with less branches due to the poison one of the crystal surfaces caused by citric acid, which was proved by micro-RAMAN-spectroscopy. This phenomenon leads to the porosity of the material and the strengths of the created gypsum normally get lower. Higher temperature or potassium sulfate as an additive is creating longer needles with a smaller diameter. Some polycarboxylate ethers stop the reaction; others change the crystal morphology like citric acid. Optical microscopy could be the good method to check whether a special additive could work for gypsum based binders.