Resumen de Utilització de fibres vegetals per a l'elaboració de morters de ciment d'altes prestacions (shcc)
Vegetable fibers have been used throughout history to improve the strength of brittle materials. During World Wars I and II these fibers were considered as an option to replace the asbestos in the manufacture of fibrocement due to the scarcity of this material. Afterwards, at the end of last century, as a consequence of the health problems caused by its handling, many developed countries banned the use of asbestos. Then, many manufacturers of fiber cement materials used again vegetable fibers as reinforcement. Nevertheless due to lack of durability of the resultant materials, the manufacturers began to replace the vegetable fibers in favor of the new synthetics ones, with higher durability, but much more expensive. Nowadays, the growing need to develop more environmentally friendly construction materials has raised again the interest in vegetable fibers as reinforcement of more sustainable fiber cement materials.
This doctoral dissertation analyzes the interaction between the cement matrix and the vegetable fibers in order to propose a method of improving the durability of strain-hardening fiber-reinforced cement based composites (SHCC).
The results indicate that the fibers that not contain plant extractives interfere less the normal process of cement hydration and that the calcium hydroxide is the main responsible of the of the vegetable fibers¿ degradation due to a migration phenomenon from the matrix to the fiber. In porous matrix, in which there is an exchange of moisture between inside and outside of the material, the solubility of the hydroxide along with the ability of the fiber to absorb and retain water, generates a continuous effect of the particles¿ precipitation around the fiber. Moreover, the alkalinity of the medium causes a reduction of the degree of polymerization of the cellulose and therefore a reduction of the reinforcement capacity of the vegetable fibers. To reduce this degradation process this research proposes to limit the capacity of the fibers to absorb and retain water with a treatment of hornification consisting in subjecting the fibers to a various wet-dry cycles. This treatment causes an irreversible shrinkage which dimensionally stabilizes and constrains the ability of the fibers to absorb and retain water.
Results indicated that the previous treatment of hornification of the fibers improves the durability of the resulting cementitious composites. Nevertheless these materials still have an excessive reduction of this resistance capacity and, therefore, such treatment must be complementary to other ones.
