Chemical Interaction of Di-Phosphonate Terminated Monofunctional Polyoxyethylene Superplasticizer with Hydrating Tricalcium Silicate
C. Comparet, A. Nonat, S. Pourchet, J. P. Guicqukro,
E. Gartner, and M. Mosquet
Appears on pages(s):
hydration; portlandite; superplasticizer
Certain di-phosphonate terminated monofunctional polyoxyethylene are widely used as concrete superplasticizers. In order to understand its action mode, its effect on hydration of pure tricalcium silicate suspensions has been investigated by conductimetry, isothermal calorimetry and ionic and total organic carbon (TOC) analyse of the liquid phase. The polyoxyethylene di-phosphonate modifies the nucleation and growth process of C-S-H by reducing the number of initial nuclei, decreasing the growth rate in the accelerated period and increasing the rate during the difision limited period comparatively to control samples. The evolution of ionic concentrations in the solution during hydration reveals an apparent increase of the critical supersaturation required to nucleate both C-S-H and portlandite. This is due to the formation of a calcium-diphosphonate polyox complex. The calcium complexation constant and pKa of the phosphonate polyox have been determined from pH and conductivity studies of calcium-hydroxide and sodium-hydroxide polymer solutions and well account for portlandite solubility in presence of phosphonate. The polyox phosphonate does not seem to adsorb on C3S surface but is rather adsorbed on C-S-H. This is probably the origin of the decrease of the growth rate leading to the modification of the texture of C-S-H and subsequently the modification of the rate during the diffusion limited period.