Dynamic retarder exchange as a trigger for Portland cement hydration

A series of NMR and isothermal calorimetry tests were conducted to depict a retarder exchange mechanism whereby a powerful organophosphonate retarder (nitrilotris(methylene) triphosphonate, or NTMP) is replaced by a much weaker phosphate retarder (sodium hexametaphosphate, or SHMP). The retardation of cement hydration by NTMP is believed to be primarily attributed to the dissolution of calcium from the cement and the subsequent precipitation of a layered calcium phosphonate that binds to the surface of the cement grains and strongly inhibits further hydration. The test results from this study show that the addition of SHMP helps to dissolve the precipitated calcium phosphonate and thus removes the strong retardation effect of NTMP. The proposed retarder exchange mechanism may be employed to develop cement systems with a controlled setting behavior.