Comparative study of lime-based mortars for conservation and restoration interventions
B.A. Silva, Elsa Guerreiro, António P. C. Duarte
Abstract
To overcome the limitations of pure air lime mortars, various strategies have been used since ancient times, including the addition of hydraulic binders, pozzolanic materials, and additives. Despite extensive research, uncertainty remains regarding the most appropriate lime-based mortar for each conservation or restoration intervention. To address this, the study compares nine lime-based formulations: a pure air lime mortar (reference), two with natural pozzolan, three with superplasticizer, two blended air lime-hydraulic lime mortars, and a pure natural hydraulic lime mortar. Results indicate that the natural pozzolan used was not beneficial for air lime mortars due to its low reactivity. Conversely, air lime mortars with superplasticizer presented reduced porosity and water absorption, and increased strength (but also stiffness), particularly at higher dosages, showing potential as alternatives to blended air lime-hydraulic lime mortars. The latter displayed the greatest colour differences compared to the reference mortar but similar mechanical and physical properties, especially for 25 % hydraulic lime (HL 5). Finally, the pure natural hydraulic lime (NHL 3.5) mortar exhibited the highest strength at early ages but presented a distinct unimodal pore structure. • Nine lime-based mortars were compared for restoration applications. • Azorean pozzolan was not beneficial for lime mortars due to its low reactivity. • NHL 3.5 mortar presented the highest early strength but a distinct unimodal pore structure. • Blended lime mortars with HL 5 showed a good performance but noticeable colour differences. • Lime mortars with superplasticizer show potential as alternatives to blended mortars.