Innovative Cementitious Composites Produced with Corn Straw Fiber: Effect of the Alkaline Treatments

Recently, numerous studies have been carried out with natural fibers in cementitious composites, due to the viability of using this type of fiber as a substitute for synthetic fibers. In this field of study, the present research aims to evaluate the feasibility of using corn straw fiber for the production of innovative cementitious composites. Mortars with a composition of 1:1:6:1.55 (cement/lime/sand/water) containing 0, 2.5 and 5% corn straw fiber were produced. The corn straw fibers were treated with three different alkaline products: sodium hydroxide (NaOH), potassium hydroxide (KOH) and calcium hydroxide (Ca(OH)2). The compositions were evaluated by means of compressive strength, water absorption, density and porosity and consistency tests. Characterization tests were also carried out on the natural fibers subjected to the different treatments, where it was observed that chemical characterization revealed an increase in crystalline cellulose from 59.03% to 63.50% (NaOH), 62.41% (KOH) and 60.40% (Ca(OH)2), which enhances fiber strength. In the mortars, it was observed that the water absorption results were reduced when the alkaline treatments were used, reducing from 15.95% (composition without fibers) to 6.34% and 6.61% in the compositions with 2.5% and 5.0% of fibers treated with KOH, for example. The effects were also positive in the compositions with fibers treated in NaOH, where the water absorption values were 7.59% and 7.88% for the compositions containing 2.5% and 5.0% of treated fiber, respectively. Alkaline treatments also promote an increase in compressive strength when comparing the results of mortars with natural fibers and fibers treated with NaOH, for example. The result for mortars containing 5.0% untreated fibers was 0.22 MPa, while for the composition containing 5.0% fibers treated with NaOH, it was 3.79 MPa, an increase of more than 15x. This behavior is justified by the effect of the treatment, which, in addition to removing impurities from natural fibers, such as sugar, increases the crystalline cellulose content and the adhesion between fiber and matrix. Based on the results obtained, it is possible to conclude that (i) the treatment with NaOH increases the crystallinity and tensile strength of the fibers, promoting good properties for innovative cementitious composites; (ii) the treatment with KOH degrades the cellulose structure of the fiber, reducing the crystallinity and tensile strength; this promotes greater adhesion of the fiber to the matrix, reducing porosity and water absorption, but promotes a reduction in compressive strength when compared to composites with 2.5% natural fiber; and (iii) the treatment with Ca(OH)2 presents a reduction in water absorption and porosity, due to the impregnation of calcium in the fiber that improves the adhesion between fiber and matrix.