Chemical modification of pinus Kraft pulp with aluminum sulfate for fiber-cement

Abstract

The use of bleached Kraft cellulosic pulp in the manufacture of cementitious composites is used in industry; however, due to the degradation of these fibers and incompatibility of the cementitious matrix with some substances present in vegetable fibers, this work evaluated the effect of aluminum sulfate (chemical modification) on Kraft cellulose. The research aimed to understand treatment effects with aluminum sulfate (AS) at concentrations of 0%, 4%, and 11% on the surface of Kraft pulp of Pinus spp. unbleached refined and derived fiber-cement on the 28th curing day and after 200 cycles of accelerated aging. Fiber surface properties were evaluated through light microscopy analysis, scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), thermogravimetry and its derivative (TGA/DTG), Fourier transform infrared spectroscopy (FTIR) and water retention value (WRV). The composites were evaluated through physical, mechanical and microstructural tests. The results of the analyzes of the treated fibers suggest that the AS is a surface modifying agent, that is, after its hydrolysis, in aluminum hydroxide, it will be deposited homogeneously on the surface of the vegetable fibers. The treatment with AS is capable of reducing water absorption (WA) by approximately 3% and apparent porosity (AP) by approximately 10% of cementitious composites. This effect was more intense after accelerated aging. Furthermore, the presence of AS affected the mechanical properties of the composites before and after accelerated aging, increasing the modulus of rupture (MOR), the limit of proportionality (LOP), and the modulus of elastic (MOE), due to mineralization and re-precipitation of the cement hydration products inside the voids, filling the pores and possibly improving the interface between the fiber and the matrix. Therefore, modification with AS appears to enhance calcium precipitation both superficially and within the plant fibers, playing an important role in minimizing embrittlement and improving the stiffness of the composites during on the 28th curing day and soaking, and drying cycles.