Mineralogical and microstructural changes promoted by accelerated carbonation and ageing cycles of hybrid fiber-cement composites

摘要

Carbonation takes place in the fiber-cement composites by the diffusion of carbon dioxide (CO_2) through the unsaturated pores in the cement matrix, and through its reaction with the hydration products of the Portland cement (mainly calcium hydroxide - Ca(OH)_2 and calcium silicate hydrate - C-S-H phases). The use of this technology during the fiber-cement production consists of an interesting procedure to prematurely decrease the alkalinity of the cement matrix, which is potentially harmful to those reinforcing fibers that are vulnerable to the alkali attack. It is also an initiative to CO_2 sequestration and partial replacement of petroleum-based fibers as is the case of cellulose pulps. Therefore, the objective of the present work is to show the effect of accelerated carbonation and ageing on the mineralogical composition and microstructure of fiber-cement composites reinforced with both cellulose pulp and synthetic fibers. The effectiveness of the accelerated carbonation was confirmed by X-ray diffraction (XRD) and thermogravimetric (TG) analysis. Accelerated carbonation increased the content of calcium carbonate (CaCO_3) and consumed the Ca(OH)_2, C-S-H, monosulfate (AFm), ettringite (Aft) and monocarboaluminate (Mc) phases. The SEM micrographs showed that absence of AFm and AFt needles around the cellulose fibers in the carbonated composites, confirmed by the absence of the peaks that represents these phases in the XRD spectra. The CaCO_3 formed from the carbonation reaction is precipitated in the pore structure of the matrix also acting as a binder and refining the pore size distribution. The interface between the cellulose fibers and the cement matrix in the carbonated composites was improved, decreasing the typical transition zone around the cellulose fibers that prejudice the fiber-cement performance at long term.