Abstract:

In order to further promote the application of phosphorus building gypsum，with ash calcium as the alkaline regulator，a comparative study was conducted on the effects of ash calcium content on the working performance，mechanical properties，phase composition，and micro⁃morphology of phosphorus building gypsum with or without polycarboxylate superplasticizer（PCE）.The results showed that，without PCE，as the ash calcium content increased，the pH of the phosphorus building gypsum was gradually increased，the water⁃soluble P₂O₅ content was gradually decreased，the fluidity was decreased by more than 59.5%，the standard consistency water consumption was increased，and the strength was firstly decreased and then increased，but the strength was still lower than the original phosphorus building gypsum.The flexural and compressive strengths were reduced by 63.3% and 54.4%，respectively，compared to those without ash calcium.When PCE was added，the incorporation of ash calcium could increase the adsorption capacity of phosphorus building gypsum for PCE by more than 10 times，thus improving the adverse effects of ash calcium incorporation on the fluidity，standard consistency water consumption，and strength of phosphorus building gypsum.The strength was firstly increased，then decreased，and then increased again as the ash calcium content increased.The flexural and compressive strengths were increased by 11.1% and 48.9%，respectively，compared to those without ash calcium.The results of XRD and SEM analysis showed that ash calcium was mainly reacted with H₃PO₄ and H₂PO₄^- in phosphorus building gypsum to form insoluble and amorphous small particles，reducing the mutual overlap degree between dihydrate gypsum crystals，resulting in a decrease in strength.In the production and processing of phosphorus building gypsum，the ash calcium content should be greater than 1.3%，and should be used with PCE.

Key words: phosphorus building gypsum, ash calcium, polycarboxylate superplasticizer, strength, microstructure