Durability of concrete incorporating blended binders and alkali-activated materials to sulfuric acid environments

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Hussien Mahmoud, Mohamed
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Acidic attack on concrete imparts unique set of damage mechanisms and manifestations compared to other durability issues of concrete. Sulfuric acid attack limits the service life of concrete elements and, thus, results in increased expenditures for the repair or in some cases replacement of the whole structure. To date, there is lack of standardized tests for specifically evaluating the resistance of concrete to sulfuric acid attack, which has caused great variability, for example in terms of solution concentration, pH level/control, etc., among previous studies in this area. Accordingly, there are conflicting data about the role of key constituents of concrete (e.g. supplementary cementitious materials [SCMs]), and uncertainty about building codes’ stipulations for concrete exposed to sulfuric acid. Hence, the first objective of this thesis was to assess the behaviour of the same concretes, prepared with single and blended binders, to incremental levels (mild, severe and very severe) of sulfuric acid solutions over 36 weeks. The test variables included the type of cement (general use [GU] or portland limestone cement [PLC]) and SCMs (fly ash, silica fume and nano-silica). The severe (1%, pH of 1) and very severe aggression (2.5%, pH of 0.5) phases caused mass loss of all specimens, with the latter phase providing clear distinction among the performance of concrete mixtures. The results showed that the penetrability of concrete was not a controlling factor, under severe and very severe damage by sulfuric acid attack, whereas the chemical vulnerability of the binder was the dominant factor. Mixtures prepared from PLC performed better than that of counterparts made from GU. While the quaternary mixtures comprising GU or PLC, fly ash, silica fume and nanosilica showed the highest mass losses after 36 weeks, binary mixtures incorporating GU or PLC with fly ash had the lowest mass losses. Several studies reported that the improved chemical resistance of alkali-activated materials (AAMs) over concrete based on portland cements. However, AAMs have technical limitations, which might deter its widespread use in cast-in place applications. These limitations include need for heat curing, slow setting, and slow strength development, which might be mitigated by further improving the reactivity of AAMs during early-age with nanoparticles; however, this area remains largely unexplored. Hence, the second objective of this thesis was to develop innovative types of AAMs-based concrete [alkali activated fly ash (AAFA), alkali activated slag (AAS) and their blends incorporating nanosilica] and evaluate their resistance to two different sulfuric acid exposures over 18 weeks for potential use in repair of concrete elements vulnerable to acidic attack. While AAFA specimens, produced without heat curing, experienced rapid ingress of the acidic solution and a significant reduction in the bond strength with substrate concrete, fly ash based AAMs comprising slag and or nanosilica (AAFA-S and AAFA-S-NS) had improved performance due to discounting the ingress of acidic solution and continued geopolymerization reactivity. Comparatively, specimens from the slag group exhibited high levels of swelling, internal cracking and mass loss due to chemical deterioration. The overall results suggest that AAFA-S and AAFA-S-NS mixture, without heat curing, may be a viable option for repair applications of concrete elements in acidic entrainments, but field trials are still needed to further verify their performance.
Concrete, Alkali-Activated Materials, SCMs, Sulfuric Acid