INFLUENCE OF RETARDERS ON SHRINKAGE AND COMPRESSIVE STRENGTH IN RAPID SET CONCRETE

Abstract

The purpose of this project is to study the influence and behavior of citric acid on calcium sulfoaluminate (CSA) concrete and compare with industry standard retarders. A review of the available literature on calcium sulfoaluminate (CSA) cements tends to center on the chemical composition and behavior. Current literature fails to effectively address either granulated citric or liquid retarders in CSA cements. Almost all current literature discusses granulated retarders and their interaction with Portland cement. Previously, CSA cements have not been widely used despite the clear advantages of this concrete type possibly due to difficulty in placement. The benefit of using an effective liquid retarder with a CSA concrete would be easier placement while causing no negative long term strength or shrinkage issues. This could greatly increase the use of CSA cements. Laboratory experiments were performed to investigate the effects of the retarders studied. These experiments included compression, shrinkage, and fresh property tests. All compression testing was conducted according to ASTM C-39 using 4”x8” cylinders. Shrinkage testing was conducted via two methods. The first method utilized the ASTM C-157 test. The second method utilized Geokon Vibrating Wire Stain Gauges (VWSG) to monitor the shrinkage continuously without the user bias involved in the standard ASTM test for shrinkage. It has been definitively shown that citric acid acts as an effective retarder for the dosages tested. The citric acid also acts as a superplasticizer, which increases the workability of the concrete. One previously unknown influence is that at large dosages of citric acid, plastic shrinkage occurs as well as permanent strength reduction. An industry retarder called Recover® had absolutely no influence on the retardation of CSA hydration. The retarder Delvo® had moderate retardation influence.