Effect of accelerated carbonation on the microstructure and physical properties of hybrid fiber-cement composites

Highlights

• The content of Ca(OH)₂ decreased after accelerated carbonation of the fiber-cement.

• C–S–H and calcium sulfoaluminates contents decreased after accelerated carbonation.

• CaCO₃ formed from the carbonation is precipitated in the pore structure of the matrix.

• The interface between the cellulose fibers and the matrix in the carbonated composites was improved.

• Accelerated carbonation can be effective to mitigate the degradation of cellulose fibers in fiber-cement.

Abstract

Carbonation takes place in the fiber-cement composites through the diffusion of carbon dioxide (CO₂) through the unsaturated pores of the cement matrix, and through its reaction with the hydration products of the Portland cement (mainly calcium hydroxide and CSH phases). The use of this technology in the fiber-cement production consists of an interesting procedure to prematurely decrease the alkalinity of the cement matrix, which is potentially harmful to the cellulose fiber reinforcement. It is also an initiative to CO₂ sequestration and partial replacement of petroleum-based fibers. Therefore, the objective of the present work is to show the impact of accelerated carbonation on the microstructure and physical properties of fiber-cement composites reinforced with cellulose pulp and synthetic fibers. The effectiveness of the accelerated carbonation was confirmed by thermogravimetric (TG) analysis. Accelerated carbonation increased bulk density (BD) and decreased apparent porosity (AP). The SEM micrographs show that the calcium carbonate (CaCO₃) formed from the carbonation reaction is precipitated in the pore structure of the matrix. The interface between the cellulose fibers and the cement matrix in the carbonated composites was improved, decreasing the typical voids around the cellulose fibers that prejudice the fiber-cement performance at long term.

Introduction

Fiber-cement products have been widely used in the world due to their versatility for manufacturing construction materials, such as flat boards for walls, corrugated sheets and water tanks, present in most of houses in developing regions as well as in agricultural and industrial buildings (Ikai et al., 2010).

The restrictions in the use of asbestos led to the investigation of alternative reinforcing and processing fibers, such as vegetable and synthetic fibers. Vegetable fibers are widely available in most developing countries and present several interesting characteristics such as low density, renewable character, biodegradability and availability at low cost and in a wide variety of morphologies and aspect ratios. On the other hand, synthetic fibers are developed from fossil fuels and there is a concern by the fiber-cement industries about the availability of these fibers to the current market demand and about the high costs (as high as approximately 40% of the total cost of raw materials), which can make it impracticable for fiber-cement production at affordable costs. Reducing the content of synthetic fibers is an effective way to reduce production costs in air-cured fiber-cement products (Dias et al., 2010).

One of the drawbacks in the use of vegetable/cellulose fibers is their mineralization in the high alkali Portland cement matrix, with the pH around 13. Mineralization of the cellulose fibers is caused by the free ions from the dissolution of Portland cement phases that penetrated into the fiber cavity (lumen), leading to the re-precipitation of ettringite/monosulphate and calcium hydroxide into the fiber and inducing the stiffening of the cellulose fibers. Secondary (re-precipitated) ettringite formation has been previously suggested as one of the degradation mechanisms of pulp fiber into cement matrix (Mohr et al., 2005, Mohr et al., 2006). This low durability of the vegetable fibers in the cementitious matrix causes reduction of the mechanical performance of the fiber-cement composites due to the fiber mineralization and loss of adhesion between fiber and matrix (Mohr et al., 2005, Tonoli et al., 2010). Hence, the optimal situation would be to protect the cellulose fibers from mineralization with a less aggressive matrix, maintaining the fiber strength and the quality of the fiber bridging that guarantee composite ductility.

Accelerated carbonation of the cement matrix can be employed to improve the durability of the cellulose fiber-cement composites, because it reduces the alkalinity of the cement matrix, lowering the pH, and making it less aggressive to the cellulose fibers (Toledo Filho et al., 2003). Also, the consequences of carbonation on cement are the stability of the chemical hydration products and mechanical properties (Soroushian et al., 2012); densification of the cementitious matrix and reduction of its permeability (capillary) and porosity, which constitutes a positive process with respect to the sealing quality of the cement (Lesti et al., 2013). The carbonation is the reaction of cement hydration products with carbon dioxide (CO₂), which could represent great impact on the sustainable growth of the construction industry in the future (Silva et al., 2009).

In fiber-cement composites, the carbonation of the matrix is enhanced due to its high porosity in consequence of the fibers effect, which facilitates the penetration of CO₂ within the composite. In a previous study, Tonoli et al. (2010) evaluated the effect of accelerated carbonation, applied after 28 days of cure, on the durability of cellulose fiber-cement composites. They concluded that this procedure improved the initial mechanical strength, decreased the calcium hydroxide content and led to a denser matrix, but the embrittlement of the composites after severe aging was not avoided. That study showed the necessity to anticipate the fast carbonation for initial periods of cure, which would not allow time for the alkali to attack the cellulose fibers during the cement hydration. Therefore, the present work contributes to the widespread use of accelerated carbonation as a way to mitigate cellulose degradation into the fiber-cement composites. However, there is still a lack of information about the changes promoted by the accelerated carbonation on the performance and microstructure of hybrid (reinforced with cellulose and synthetic fibers) fiber-cement composites.

The dissemination of the use of accelerated carbonation during the initial cure fits well with the growing necessity of more sustainable technologies and raw-materials, which is associated to the consume of the CO₂ emissions eventually generated by the industrial processes, and allows partial substitution of the more expensive petroleum derivative polymer fibers used to retain the durability of the composites under aggressive climatic conditions. According to Soroushian et al. (2012), CO₂ curing of cellulose fiber-cement has also shown to enhance productivity and engineering properties (including dimensional stability) of the end product.

Carbonation can occur in use naturally by the reaction between alkaline matrix of the fiber-cement and atmospheric CO₂, but the natural reaction is very slow. Then, accelerated carbonation can complete its reaction within hours and has been proposed as an alternative technology to the natural carbonation (Lim et al., 2010). In this context, the aim of this work was to evaluate the effect of accelerated carbonation on the microstructure and physical properties of fiber-cement composites with hybrid reinforcement.