Comportamento do cimento álcali ativado quando exposto ao ataque combinado de sulfatos

Abstract

Alternative cements to Portland cement have been widely studied in the field of civil construction in order to mitigate the environmental effects produced by this conventional binder. In this context, activated alkali cement (AAC) has stood out due to the consumption of industrial by-products for its production and the good mechanical and durability results. At the same time, the behavior of AAC under sulfate attack is little discussed and needs clarification in order to improve the durability of buildings exposed to this condition. The sulfates studied in this work were sodium sulfate and magnesium sulfate. These two sulfates have their effects independently addressed in the literature, through separate solutions, since both sodium and magnesium sulfate present harmful results, but with different mechanisms from Portland cement. However, in AAC, although the exclusive attack by magnesium sulfate is harmful, the attack by sodium sulfate is beneficial, improving the mechanical strength of the cementitious material. In this way, it becomes feasible to study the attack on cements of both sulfates, combined in a single solution. Therefore, the present work aimed to evaluate the performance and durability of activated alkali cement, against the combined attack of sodium and magnesium sulfates, through the accelerated test methodology, NIST test, for 91 days. Pastes of AAC and Portland cement resistant to sulfates (CPIV-32 RS) were molded, in the proportion 1:0.45, by mass. AAC was produced from blast furnace slag and activated with sodium hydroxide (NaOH) at a content of 5%. The evaluations made for the two mixtures include microstructural (XRD, SEM and TG/DTG) and macrostructural analyzes through visual inspections, dimensional and mass variation, compressive strength, water absorption, in addition to durability by the NIST test. The results regarding the combined etching of sulfates were positive, since it improved the mechanical strength of the pastes and attenuated the expansive effects caused by the independent etching of each sulfate. The AAC showed higher mechanical strength at all ages, lower weight gain, assuming a limited amount of calcium in the AAC, preventing the large formation of components of the cement reaction with water and sulfates. In addition, there was greater expansion in AAC specimens than in Portland cement.