Adding hydrated lime (CH) into blended cement incorporating high volume of Supplementary Cementitious Materials (SCMs) is a viable method to provide the necessary calcium hydroxide for the pozzolanic reaction, thereby improving the mechanical performance at later stages. However, the effects of relatively small dosages of CH on the rheological properties and resulting microstructure of limestone-calcined clay cement (LC3) remain unclear. This paper aims to investigate the influence of a small CH addition on the fresh and hardened properties of LC3 systems, in which Portland cement is largely replaced (80 wt%) by limestone and calcined clay. The results indicate that the additional CH notably reduces the water film thickness, leading to increased dynamic yield stress, plastic viscosity and re-flocculation. A delay in the elasticity development and static yield stress evolution within the first 1.5 h was observed with the addition of 2.5 wt% CH, attributed to the initial dissolution of CH, which is mitigated by using 10 wt% CH. Furthermore, additional CH accelerated early-age hydration and facilitated long-term pozzolanic reactions, resulting in the increased amount of C-(A)-S-H gel and AFm phases, and reduced porosities after 7 and 28 days. These chemical effects could well compensate the high air void content caused by the high viscosity, and therefore contributes to mortars with higher compressive strengths than plain LC3 at later ages.

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.

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.