Pore solution, porosity, and microstructure of ternary cement matrices: a holistic and strategic view of concrete durability

Abstract

In this paper, microstructural changes of cementitious matrices, provided by the combined use of silica fume and nanosilica, were investigated and deeply discussed based on the techniques of scanning electron microscopy (SEM), X-ray microtomography (μCT), and X-ray diffraction (XRD). A characterization of the pore solution, reconstituted by the method of ex situ leaching, was also carried out, which allowed the analysis of chemical changes in the interstitial liquid phase of mixtures with silica fume and nanosilica, by means of parameters such as pH, electrical conductivity, and ionic strength. Besides that, characterization tests of the concrete in terms of compressive strength and water absorption were carried out. Throughout the work, it was possible to discuss the systemic alterations that occurred in the internal structure of the concretes modified with these supplementary cementitious materials (SCMs), which have modified chemical and physical parameters of the concrete, in addition to its microstructure, thus improving the mechanical properties and durability behavior of the material. As a result, it was verified that concretes with mineral additions had reduced pH, electrical conductivity, ionic strength, and ions in pore solution, with a strong correlation among these parameters. The most significant reductions in conductivity and ionic strength, respectively, of the order of 50% and 67%, occurred at the water/binder ratio of 0.6 in the SCM-modified concretes. Moreover, a densification of the hydrated matrix in theses modified concretes was observed, with a more refined porosity and a considerably better formed interfacial transition zone (paste–aggregate) being observed, as a result of the supplementary formation of hydrated calcium silicate (C─ S─ H) and the filler effect.