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.