Research On The Effect Of Particle Size Distribution On The Hydration And Compressive Strength Of α-calcium Sulfate Hemihydrate

Many efforts have been concentrated on illustrating the relationship between particle size distribution (PSD) and workability of cement and concrete, while studies on gypsum plasters largely lag behind. Grinding and sieving, which lead to the formation of plasters with different PSDs, are essential in the manufacturing process. Thus, research on the relationship between the PSD and properties ofα-calcium sulfate hemihydrate (α-HH) is of great practical significance to improve the workability (especially the compressive strength).PSD ofα-HH was firstly quantitatively analyzed by Rosin-Rammler-Bennett (RRB) distribution function. The hydration and compressive strength development of plaster was correlated with the RRB parameters, focusing on the effect of particle fineness (D_e) and distribution width (n) ofα-HH on water-hemihydrate weight ratio (W/H) for standard consistency, hydration process, compressive strength and microstructure of plaster. Then, the optimum PSD ofα-HH was determined. Furthermore, the plaster was regarded as a grey system and a grey model was developed to quantitatively explore the relationship between PSD ofα-HH and compressive strength of set plaster. Then, the compressive strength of set plaster was predicted by the grey model.The particle fineness and distribution width ofα-HH both have great effect on the W/H for standard consistency, hydration process and compressive strength. Compratively, the particle fineness is more important. Variations of microstructure elucidate the impact of PSD ofα-HH on the hydration and compressive strength development of plaster. Plaster with the optimum PSD has a more reasonable ratio of W/H, giving a complete hydration without a hydration barrier. Thus, homogeneous and well-grown calcium sulfate dihydrate (DH) crystals and a more compact structure are obtained, leading to a higher compressive strength. An optimum PSD is characterized by D_e≈35μm and n≈0.82.The grey model shows that the compressive strength is derived from the combined action of each particle fraction, which could be divided into two groups. One works with positive effect: the particle fractions of (0-20μm), (20-50μm) and (90-140μm). The other works with negative effect: the particle fractions of (50-90μm) and (>140μm). Variations of the compressive strength with the increment of specific particle fractions, which result from actions of particles on the hydration rate and the characteristics of pore structure, are in general agreement with the results given by the grey model. The availability of the grey model is restricted by the W/H giving standard consistency ofα-HH paste. The results indicate that the grey model could provide a potential method to evaluate the relationship.