Celulose nanofibrilada em compósito de cimento Portland: transformações físico-químicas, microestruturais e análise do potencial de mitigação de reação álcali-agregado

Abstract

Several mechanisms generate pathological manifestations in concrete structures, causing a decrease in their service life. Among these processes, the alkali-aggregate reaction (AAR) stands out, in which the amorphous silica of aggregates is dissolved due to the hydroxyls present in the pore solution, generating a hygroscopic gel with expansive characteristics. Using additions has been an alternative to mitigate the development of AAR. In this context, researchers have joined efforts to evaluate how nanofibrillated cellulose (NFC) can influence the durability and properties of cementitious materials. This work aimed to verify the influence of NFC addition in Portland cement matrices focusing on modifications in paste properties and changes in the expansion of mortars subjected to AAR. For this, contents of 0%, 0.05%, 0.10%, and 0.15% of NFC in cement mass were dispersed in pastes and mortars. The NFC suspension was obtained by grinding defibrillation of Eucalyptus sp. The pastes were cured in sealed form for 30 days and then exposed to 1N solution of NaOH for 30 days at (80 ± 2) °C to study the modifications caused by the highly alkaline medium in the nanofibers and chemical, physical and mechanical properties of the pastes. The mortars, in turn, were cured for 30, 60 and 120 days. After that, tests were performed to evaluate how the NFC changes the properties and expansions caused by the presence of reactive aggregate in mortars. Tests of compressive strength, tensile strength, absorption, chemical and physical characterizations, and analysis of linear expansion variation were performed, according to NBR 15577-4. For the pastes, it was observed that NFC causes reductions in consistency and increases in the initial and final setting times. Increases of up to 59% in compressive strength were verified, as the fibers better distribute the stresses inside the material. The fibers’ high tensile strength also contributed to an increase of up to 54% in the pastes’ tensile strength. For the mortars, the NFC was not able to contribute to the mechanical strengths. When exposed to alkaline NaOH solution, the NFC fibers degraded. This degradation reduces the reinforcement provided by the fibers to the matrix and forms voids that did not exist before, which become stress concentration points that affect the mechanical strength. On the other hand, contents of 0.10% of NFC were able to reduce the expansions due to AAR by 10%, both at 60 and 120 days of curing. This is due to two factors: the reinforcement from the NFC in the cement paste tensile strength and the reduction of mortar absorption caused by the nanofibers. So, it is concluded that the CNF can mitigate AAR. It acts as a “stress reserve” that reduces the expansions arising from the reaction, thus reducing the consequences that can be deleterious to structures.