Effect of citric acid on setting inhibition and mechanical properties of gypsum building plasters

The role of citric acid, a retarder additive used to prolong the workability and setting time of gypsum binders, was studied at concentrations of 0, 125, 250, 500, 1000, 1500 and 3000 ppm with a constant water/binder ratio. The hydration process (setting) was studied by the knife method, conductivity measurements and attenuated total reflectance infrared spectroscopy, which provided an estimation of the time it took for calcium sulphate dihydrate to be formed. Scanning electron microscopy confirmed that the gypsum microstructure varied when citric acid is used. The mechanical properties of the plaster were also studied by ultrasonic measurements, shore-D hardness, flexure and compressive strength tests. It was found that citric acid partially inhibits binder hydration from hemihydrate to dihydrate, as observed by X-ray diffraction analysis.

► We propose an alternative method using ATR-FTIR to study the plaster setting time.
► We found new evidences about the role of citric acid as retarder by XRD.
► In this study the efficiency of citric acid is maximum at doses of 1500 ppm.
► New evidences on the effect of citric acid are shown by ultrasonic tests and SEM.
► Microstructure interlocking is highly reduced from very low doses of citric acid.