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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 CO bonds have also been estimated. The achieved results suggest that indirect evidence of the PAA cross-linking could be obtained by infrared spectroscopy measurements.

The significant increases in flexural strength achieved in macro-defect-free cements have been attributed to a reduction in the density of defects in the materials as well as to chemical reactions occurring between the organic constituent, i.e., the poly(vinyl alcohol) (PVA) polymer, and inorganic ions coming from the hydration products of the cement. In particular, it has been suggested that PVA chains are partially linked together by Al ions. This phenomenon has been investigated here by studying the structure and the energetics of Al-PVA models with ab initio theoretical methods. The achieved results strongly support the existence of a cross-linking of PVA chains by Al ions, even in the presence of structural deformations of the PVA chains. The vibrational frequency of a stretching mode related to the O atoms bonded with Al has been also evaluated, which may represent a fingerprint for an experimental identification of cross-linking.