Study On Action Mechanism Of Water Reducers And Retarders For Building Gypsum

Gypsum-based materials are green building materials popular all over the world. In China, gypsum resource is very abundant, but the technical and economical level of gypsum industry is far more backward. Adding admixtures is one of most efficient ways to improve technological and economical level, and promote scientific progress of gypsum industry. It is also the core technology of high performance gypsum-based materials.In this paper, we have carried out a systemic and deep research on the following contents including relationship between molecule structure of admixtures and their performance, the adsorption characteristics of admixtures, relationship between the structure of adsorption layer and water-reducing effect, influence of admixtures on the hydration process of building gypsum and microstructure of hardened gypsum paste, Negative Effects of Retarders on performance of Building Gypsum and their action mechanism, time dependency of fluidity of fresh gypsum plaster and its inhibition measurement, the aid-grinding and dispersing effect of superplsticizers, the factors affecting the action effect of admixtures, and the composite effect of admixtures.The adsorbed amount and dispersion capacity of water-reducing agents is positively related with the specific surface area of building gypsum, and the appropriate increase in the specific surface area of building gypsum could correspondingly increase the action efficiency of water-reducing agents. As for influences of addition method on water-reducing effect of superplasticizers, the addition before water is the best, the addition after water is inferior and the simultaneous addition is the worst. pH value also have an influence on the dispersing effect of water reducer, and the dispersing capacity is the best in the weakly alkaline condition.The adsorption type of FDN is physical adsorption, with its adsorption heat of 14.71kJ/mol and a relatively high-saturated adsorption amount of 12.67 mg/g, but the adsorption is not stable. In contrast, the adsorption type of HC is chemical adsorption, with its adsorption heat of 20.85 kJ/mol and a relatively low saturated adsorption amount of 4.42 mg/g, but its adsorption is very stable. The adsorption conformation of FDN is in form of lying on gypsum surface and the steric hindrance preventing the aggregation of gypsum particles is very weak, its dispersion depending mainly on electrostatic repulsive force caused by Zeta potential which depends on adsorption amount on the surface of gypsum particle. Due to the shelter effect of new gypsum crystal produced during the rapid hydration of gypsum and desorption of FDN, its dispersion capacity decreases quickly, leading to serious fluidity loss of fresh gypsum plaster. The adsorption conformation of HC is comb-shaped and the steric hindrance caused by the side chain of adsorption layer is relatively high, its dispersion effect depending on the combination effect of steric hindrance and electrostatic repulsive force. Steric hindrance is less affected by new hydration products, its dispersion stability is very good, and fluidity loss is little in macro scale.The adsorption of admixtures changes the solid-liquid interface structure between gypsum particle and electrochemical properties, causing electrostatic repulsive force effect and steric hindrance effect which lead to dispersion of gypsum particles. The adsorption conformation of superplasticizer with straight chain is in form of lying on gypsum surface, its dispersion depending mainly on electrostatic repulsive force caused by Zeta potential which depends on adsorption amount on the surface of gypsum particle. While adsorption conformation of superplasticizers is comb-shaped, its dispersion effect depending on the combination effect of steric hindrance and electrostatic repulsive force. Electrostatic repulsive force isn't stable, while steric hindrance is less affected by new hydration products. The combination effect of steric hindrance and electrostatic repulsive force could increase the dispersing capacity and di...