Interaction between Ca Ions and Poly(acrylic acid) Chains in Macro-Defect-Free Cements:  A Theoretical Study

Several experimental and theoretical studies have related the peculiar mechanical properties of the macro-defect-free polymer−cement composites with the existence of a cross-linking of the polymer chains by Al or Ca ions coming from the hydration reactions of the cement. In the present study, the interaction of Ca ions with the −COO- groups of one or two poly(acrylic acid) (PAA) chains has been investigated with first-principles theoretical methods in order to achieve evidences of PAA cross-linking. The present results show that a stable CaO4 complex can be formed by a Ca ion and the O atoms belonging to the −COO- groups of two polymer chains. This implies the existence of a quite efficient cross-linking because a Ca ion becomes anchored to four points of two PAA chains. The stretching frequencies of several Ca−O and C═O bonds have also been estimated. The achieved results suggest that indirect evidence of the PAA cross-linking could be obtained by infrared spectroscopy measurements.