Rationalizing the Effect of Accelerated Carbonation and Accelerated Ageing on Transition Zone of Cellulose Fibers in Cement Based Composites

The objective of the present work is to show the effect of carbonation at early stages on the transition zone of cellulose fibers in cement based composites The effects of carbonation were also confirmed by X-ray diffraction, thermogravimetric (TG) analysis. Accelerated carbonation increased the content of CaCO₃ and consumed the Ca(OH)₂, C-S-H, AFm, AFt and Mc phases. The SEM micrographs showed that absence of AFm and AFt needles around the cellulose fibers in the carbonated composites, and confirmed by the EDS mapping. The calcium carbonate (CaCO₃) formed from the carbonation reaction is precipitated in the pore structure of the matrix, decreasing diffusion of Si and S during hydration curing, because act as a binder and refine the pore size distribution. Consequently less ettringite is formed around the fibers because of lower diffusion of sulphate-rich phases. 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. Carbonated composites presented denser matrix around the fibers. 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 also improves the mircrostructure of the material and is an initiative to CO₂ sequestration and partial replacement of petroleum-based fibers as is the case of cellulose pulps.