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Abstract This paper presents the results of a study about the mechanical behaviour, before and after aging, of two cement-based composites reinforced with long sisal fibres cured at early age in a CO 2 environment. After curing, samples of both composite systems were submitted to wetting and drying cycles to accelerate the aging of the materials allowing the evaluating of the efficiency of the CO 2 treatment. Besides promoting CO 2 capture, the used cure also allowed the increase in the durability of the sisal fibre-cement based composites by the Calcium Hydroxide (CH) consumption and pH decrease promoted by the carbonation reactions. The lime-pozzolanic material composite was also treated with CO 2 at early stages of formation, with the carbonation and pozzolanic reactions acting simultaneously to deplete the Ca(OH) 2 thus improving its durability. Four-point bending test, thermogravimetry analysis of the matrix and SEM analysis of the fibres were done before and after aging to study the durability mechanisms of the composites. The obtained results showed that the CH free cement-based composite containing metakaolin and fly ash presented the best mechanical behaviour before and after accelerated aging. The early age carbonation of the lime-pozolan composites improved its mechanical response, before and after accelerated aging, when compared with the non-carbonated samples. The thermogravimetry test showed that the increase of the carbonation time of the reference Portland cement-based composite consumed the C S H, compromising the ductility and strain capacity of this composite family after aging. The SEM pictures showed petrified fibre effect by the migration of hydrated products after 10 and 20 cycles of ageing for this composite system. [ABSTRACT FROM AUTHOR]

Several fiber treatments are used to mitigate the high water absorption of vegetable fibers. Wetting and drying cycles are usually performed in the industry of paper and cellulose to reduce the volume variation of these fibers. This procedure stiffens the polymeric structure of the fiber-cells (this process is known as hornification) resulting in a higher dimensional stability. The aim of this study is to determine the effect of the hornification on the chemical and mechanical behavior of natural fibers and how these properties influence the fiber matrix bond. For this purpose, 5 and 10 cycles of wet and drying were applied to curauá, jute and sisal fibers. Fiber pull-out tests were performed in the embedment length of 25 mm. Direct tensile tests were performed in natural and hornified fibers. Furthermore, X-ray diffraction, thermogravimetry analysis, infra red spectroscopy and nuclear magnetic resonance were used to investigate the influence of the hornification on the chemical properties of the studied fibers. Modifications on the fiber morphology were observed with a scanning electron microscope. The results indicate changes on the tensile strength and strain capacity of the studied fibers, showing that morphology, physical aspects and chemical composition play an important role on the efficiency rate of hornification. Significant improvement in the fiber-matrix interface was observed through the pullout tests. It was concluded that 5 cycles promotes a better performance to curauá and sisal fibers. Only the sisal fibers show improvement on its bond mechanisms after 10 cycles. [ABSTRACT FROM AUTHOR]

• Humidity variations barely altered the mechanical properties of jute fiber. • Sisal fiber presented better adhesion under the proposed humidity levels. • Humidity effect on fibers can be assessed by their cellulose crystallinity index. • Sisal fiber presented better adhesion under the proposed humidity levels. The aim of this study is to determine how the mechanical behavior of natural fibers is affected by the environmental and internal relative humidity, especially on the fiber-matrix interface. For that, curauá, jute and sisal fibers were selected. The experimental program commenced determining the fibers' mechanical properties under tensile stresses, followed by X-ray diffraction measurement, thermogravimetric analysis, infra-red spectroscopy and dynamic water vapor sorption analysis, as well as a microscopic examination of fibers' rupture under tensile stresses. In addition, a 2D finite element analysis was carried out to assist the interpretation of the mechanical behavior of the pull-out tests. The results indicate that the absorption capacity of each fiber can be correlated to the amount of hemicellulose and cellulose cristalinity, and the fibers' swelling process was influenced by each microstructure and hemicellulose content. Results from the mechanical tests showed a strength enhancement when the fibers were submitted to low levels of relative humidity, while a drastic decrease in strength was observed along with higher levels. The development of finite element models helped understanding the geometric particularities of each fibers. From the results it is possible to state a unique correlation between each fiber and its respective moisture content. It is believed that this data can help the use of such fibers as reinforcement of cement-based systems and their durability under a wider range of relative humidity levels. [ABSTRACT FROM AUTHOR]

• Temperatures until 100 °C promote an improvement on tensile strength. • Temperature does not promote substantial modification on fiber cross-section. • Temperatures over 300 °C promote a breakage on microfibrils bonding. • Sisal fiber present a good adhesion to cement until 100 °C. This experimental investigation aims to study the influence of temperature on natural fibers mechanical performance and on its interface to cement based systems. Natural sisal fiber was used on this study. Temperatures of 20, 100, 150, 200, 250 and 300 °C were used as conditions for this research. The influence of temperature on this fiber was evaluated by scanning electron microscopy (SEM), x-ray diffraction patterns (XRD), thermogravimetric analysis (TGA). The mechanical properties were evaluated by tensile test. Pullout test were performed in order to evaluate bonding on cement-based matrix after exposition to higher temperatures. The results indicate a minor modification on sisal morphology after temperature exposition until 200 °C. At 250 °C a few microcracks were observed on fiber surface, indicating a debonding of sisal fibercells and microfibrils. Sisal mechanical behavior was improved until 150 °C. This may happen due the water evaporation and shrinkage, promoting a closed and densified fiber structure. Cement main component degradation was observed before degradation of the natural fiber (around 250 °C). Bonding to cement matrix decreased after 100 °C and was maintained stable until 150 °C, and resulted in pullout forces reduction of 75% in comparison to fiber tested at 20 °C. [ABSTRACT FROM AUTHOR]