A comparison of standard and realistic curing conditions of natural hydraulic lime repointing mortar for damp masonry: Impact on laboratory evaluation

Lucie Fusade,Heather A Viles

doi:10.1016/j.culher.2018.11.011

Abstract

Abstract Environmental conditions can affect the curing and performance of lime mortars. Especially in the case of natural hydraulic lime (NHL) mortars to be used for repointing in exposed conditions, it is essential to assess what if any differences these environmental conditions would make to mortar properties through laboratory evaluations before repointing work begins. This study considers a specific historic environment: traditional masonry exposed to high humidity and rainfall, with a particular focus on Devon. Realistic curing conditions (as likely found on-site) of 15 °C, 85% RH, representing an average of summer climate in Devon were compared with standard recommended laboratory conditions of 20 °C, 65% RH. A range of mixes, representing some conservation pointing mortars, was prepared using NHL 2 (St Astier), quartz sand, and crushed Portland limestone in 1:3 and 1:2 binder to aggregate ratios. The influence of curing conditions on carbonation depth, strength development, internal textural structure, pore structure and water uptake at 28 and 90 days is discussed (called here early and medium ages) and the response of NHL mortars to this humid environment during evaporation and salt crystallisation have been assessed. Results show that significant differences are found in laboratory evaluations of mechanical properties of the same NHL mortar exposed to different curing conditions especially at an early age and for mortar made with quartz sand. Laboratory evaluation should be made on samples cured under realistic conditions if information on the early to medium-term (up to 90 days) characteristics of NHL mortar is required. Overall, realistic humid curing conditions help NHL mortars gain good internal structure more quickly, minimising the risk of early failure of pointing mortar exposed in a harsh humid environment.

Highlights

This study has shown that using realistic curing conditions can have an impact on some mechanical properties, the differences become less pronounced over time
For the same natural hydraulic lime mortar exposed to different curing conditions significant differences are found, in particular on the mechanical properties and internal structure formation
Over the early to medium-term natural hydraulic lime mortars are affected by the surrounding humid environment (RH and temperature)

Summary

Introduction

In England, where the climate is often wet, lime mortar is found in many types of historic masonry built before the 1850s. When dealing with historic buildings highly exposed to rainfall and with interior dampness problems, conservation officers first recommend maintenance of the building features, such as the drainage system, conditions of the roof elements, etc. [2,3] When this has been ensured but interior dampness is still present, conservation work on the masonry is considered, such as rendering, grouting or repointing [4]. Render can be aesthetically disruptive and repointing has two clear advantages: firstly, it is far less disruptive since it ensures a minimal intervention to the historic masonry [6]; secondly, it is less costly [5]

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Results

Conclusion