Potencial use of colloidal silica in cement based composites: evaluation of the mechanical properties

Abstract

The properties of cement based composites depend not only on the properties of their individual components but also on their interfacial characteristics and transition zone between fiber and matrix. There has been a renewed interest in the use of cellulosic pulp as micro-fiber reinforcement in cement based composites. The addition of nanoparticles, such as colloidal silica, to fiber-cement could allow a better control of its microstructure and the enhancement of the matrix/fiber interface. The objective of this work is to evaluate the effects of colloidal silica on the microstructure and mechanical performance of cementitious matrices and fiber-cement composites. These cementitious materials were prepared with 0%, 1.5%, 3%, 5% and 10% w/w colloidal silica suspension content. Cementitious matrices without fibers were produced by vibration. Fiber-cement composites with unbleached Eucalyptus kraft pulp as a micro-fiber reinforcement were produced by the slurry dewatering technique followed by pressing. All composite materials were cured by water immersion. A splitting (Brazilian) test was carried out to determine the tensile strength of cementitious matrices. Mechanical behavior of the fiber-cement composites was evaluated via modulus of rupture and fracture toughness based on load-displacement curves (L-d curves) under continuous loading and 3-point bending arrangement. The energy of fracture was measured through a stable crack propagation test with SENB (single-edge notched bending) configuration also under a 3-point bending arrangement. The matrix with highest content of colloidal silica suspension (10% w/w) presented high values of water absorption and consequently presented the lowest splitting tensile strength. The average values of modulus of rupture and fracture toughness of fiber-cement tend to decrease with increasing colloidal silica content. However, the pullout mechanism increased significantly in the fiber-cement composites with additions between 3% and 10% w/w of colloidal silica suspension as compared to that without any addition, noted by degree of improvement in the energy of fracture and by scanning electron microscopy micrographs (SEM). These findings show the potential use of colloidal silica to improve the transition zone between the cellulosic fiber and the cementitious matrix. The results of this study show an important way to engineer and control the fracture process of the composites.