Recent studies have been focusing on the carbon sink potential of carbonatable binders as an attempt to reduce CO2 levels. Air lime is a carbonatable binder that fully relies on CO2 absorption to harden and, thus, offers great carbon sink potential. Yet, CO2 absorption is favoured only after the evaporation of the excess water. Therefore, this study investigated the behaviour of air lime-containing mortars regarding water retention and evaporation. Four groups (with different contents of air lime) were monitored for up to 91 days after curing. Results showed that higher contents of air lime yielded greater water retention capacity. Yet, water retention did not prevent the carbonation front from further advancing – especially within lime-cement groups. In this case, greater porosity proved to be an open door for the simultaneous evaporation and ingress of CO2. Thus, hero or villain? It depends on the mixture.

Given the crucial role of carbonation in the hardening of lime-based binders, accurate measurements of carbonation depths are essential for analysing both carbonation kinetics and carbon sequestration capabilities. This study employed both the conventional phenolphthalein spray method and the profile-based method to determine carbonation depths in four types of binders. Unlike the distinct carbonation front observed in cementitious materials, lime-based binders displayed a transition zone between fully carbonated and uncarbonated areas. Meanwhile, the remaining portlandite content in some specimens did not necessarily increase with depth, and typical Liesegang patterns were observed. Compared to phenolphthalein spray method, profile-based methods provide more quantitative evidence for further analyses, but inevitable slice interval can also lead to errors in carbonation depth estimation. Therefore, developing a more precise and convenient method remains essential for a deeper understanding of the carbonation behaviour in lime-based binders.

The service life of wastewater treatment plants is often impaired by the biogenic deterioration caused by sulfuric acid (H2SO4). This study used fly ash (as a Portland cement replacement) and different contents of neutral sodium silicate (0%-10%) as mitigating solutions. One group was immersed in lime-saturated water for 182 days, and the other was submitted to a 0.5% H2SO4 solution for the same interval. The microstructural analysis (SEM/XRD) confirmed the consumption of calcium-based compounds during the pozzolanic reaction, the Na2SiO3 hydration, and the H2SO4 attack. The neutralized depths measured after spraying phenolphthalein reiterated this trend. The H2SO4 front progressed inwards, promoting gradual peeling and the formation of gypsite. The slow diffusion of H2SO4 did not impair the early-age compressive strength, but the depletion of the alkaline reserve hindered the results in the long term. Therefore, Na2SiO3 was not a viable mitigating solution against the H2SO4 attack, contrary to fly ash.

Internal sulfate attack (ISA) occurs in cement composites due to the presence of mineral sulfide in aggregates, such as pyrite (FeS2). The oxidation of the pyrite releases sulfate ions, which react with hydrated phases of Portland cement and produce expansive phases such as ettringite and gypsum. The objective of this article was to study the influence of crystalline admixture and polypropylene microfiber on the internal sulfate attack in Portland cement composites due to pyrite oxidation. Mortars with 10% pyrite content (by sand weight) were evaluated and compared with reference mortars (without pyrite). Pyrite oxidation was stimulated through wetting and drying cycles at 40 °C. Transport properties (water absorption by immersion, sorptivity, ultrasonic pulse velocity, and electrical resistivity), length change, flexural tensile strength, and microstructural properties were monitored for up to 24 weeks. Pyrite promoted degradation in all mortars over time. Mortars with microfibers had the lowest expansion due to the availability of voids for the consolidation of expansive phases. However, the high porosity and permeability of the material facilitated the entry of water and diffusion of oxygen. A large number of expansive phases and cracks were found in these mortars. The presence of soluble alkaline phases promoted the interaction between crystalline admixture and pyrite, accelerating the ISA.

This article evaluated the effect of improved autogenous mortar self-healing in the alkali-aggregate reaction (AAR). Prismatic mortar specimens were cast with different contents of polypropylene microfiber and crystalline admixture. The crack-induction method (for subsequent self-healing) was the AAR accelerated mortar bar test itself. After AAR-testing, the specimens were submitted to wetting and drying cycles to stimulate the self-healing mechanism. These two approaches (AAR and self-healing steps) were alternately repeated four times. The results showed that the reference mortar and the mixture with 1% of polypropylene microfiber had the highest and lowest levels of expansion, respectively. The expansion rate was lower for the combined mixtures, although the initial values of length change were high. The visual inspection confirmed that improved autogenous self-healing could close cracks caused by AAR and promote microstructural densification.