Performance of FA-based geopolymer concretes exposed to acetic and sulfuric acids

The durability of Portland cement concrete, regardless of its composition, in presence of low-pH solutions is limited due to the dissolution and decomposition of the mineral phases of the hardened cement. For example, sewage systems are critical to public health however concrete sewer pipes deteriorate very quickly due to acid attack. This problem, in addition to affecting the service life of the pipes, increase the costs of maintenance and can be cause of environmental impacts. The use of alkali-activated or hybrid concretes can be considered a viable option from the basis of their chemical resistance. This study describes the acidic attack behaviour of alkali-activated concretes, based on a low-quality fly ash (FA), using as sources of calcium granulated blast furnace slag (GBFS) and Portland cement (OPC), which were incorporated in a 20% by weight proportion. A mixture of sodium silicate and sodium hydroxide was used as the activating solution. The specimens were exposed to solutions of sulfuric acid (H2SO4) and acetic acid (CH3-COOH) at concentration 1 M for 360 d. As a reference material, conventional concrete was prepared using general-purpose Portland cement. The effect on the compressive strength and mass loss of each mixture was evaluated. The progress of the degradation by acid action was evaluated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) techniques. The results indicate that alkali-activated concretes show better performance compared with that of OPC concretes with residual strengths at 360 d of exposure between 10 and 20 MPa. In contrast, OPC concrete shows resistance losses of up to 98%. The superior behaviour of alkali-activated concretes is corroborated by the reported lower mass loss (i.e., a maximum of 6%), while that of OPC concrete was up to 19%. In general, gypsum formation is evident in all specimens exposed to H2SO4. In the case of exposure to CH3-COOH, the disappearance of some crystalline products is observed for the alkali-activated concretes. Additionally, it is appreciated the formation of a new crystalline phase, nahcolite. Based in the results, it is possible conclude that this type of alkali-activated binders, (binary and hybrid) based in this type of fly ash, may be used for developing acid-resistant concretes.This study was funded by the Colombian Institute for the Development of Science, Technology, and Innovation COLCIENCIAS (project, “Construction of prototype at the scale of rural housing using innovative materials with low carbon footprint,” Contract 096-2016).Peer reviewe