HYDRATION CHARACTERISTICS OF PROMPT CEMENT IN THE PRESENCE CITRIC ACID AS RETARDER

The effects of citric acid (CA), as a retarder, on the properties of prompt cement pastes were investigated for CA contents of 0.50, 0.75, 1.00 and 1.25 mass% of prompt cement. The initial and final setting times, bulk density, compressive strength, total porosity and hydration kinetics including free lime, combined water content and XRD were investigated for selected samples. Results showed that the addition of CA extended the initial and final setting times and reduced the compressive strength and combined water contents, but it increased the total porosity for all hydration periods. CA retarded the liberation of Ca(OH)2 from the cement pastes and their free lime contents were slightly increased for up to 28 days then sharply increased up to 90 days. It was suggested that citrate sorbed onto the clinker surface and formed a protective layer around the clinker grains retarding their dissolution. There was an increase in the compressive strength at later stages after 28 and up to 90 days. The presence of 0.75 mass% citric acid achieved the initial and final setting time of the prompt cement according to the ASTM specification.