Properties and performance of lime mortars for conservation

: the role of binder chemistry and curing regime

  • Cristiano Figueiredo

Student thesis: Doctoral ThesisPhD

Abstract

The selection of mortar for conservation of historic and heritage buildings can be challenging. Achieving compatibility with the historic fabric, durability and efficient use of materials within a practical timeframe often requires the use of hydraulic lime-based mortars which set more rapidly than the more traditional air lime mortars. These are considered to be more compatible with historic fabric than cement-based mortars, although, due to the modern production techniques and their natural variability, a deeper knowledge of their chemical and physical properties is needed to minimise damage due to incompatibility and make the decision process easier and safer.Natural hydraulic lime (NHL) binders are currently classified under EN 459-1:2015 in three designations, NHL 2, NHL 3.5 and NHL5, with the suffix representing the minimum compressive strength (in MPa) of a standard mortar mix at 28 days. The performance of NHL binders, manufactured by burning a naturally impure limestone, can be difficult to predict due to the inherent variability of both their physical and chemical characteristics. At the same time, the tolerance values for each classification allow for binders with significantly compressive strength differences to be classified by the same designation. The main aim of this research was to study a range of NHL binders, understand and quantify the variability of their characteristics and to establish how these properties influence the performance of mortars cured under standard and simulated weather conditions. In the first stage of the project, a selection of NHL binders from different origins and distinct designation were rigorously examined through physical, chemical and mineralogical characterisation to elucidate surface area, particle size distribution, oxide composition and crystalline phase composition. The characteristics of the binders were found to vary greatly, particularly amongst binders from the same classification and distinct origins, and in one particular case even from the same origin. A change of properties over time was also identified, binders manufactured in different years could have very different properties, even though, as far as could be ascertained from the packaging, it was the same product.Starting from a selection of 11 NHLs and 1 hydrated lime, the next step involved the manufacture of mortar samples using a sand aggregate appropriate for a conservation mortar with 1:2 ratio (binder:aggregate by volume). Sufficient water was added to produce a spread by flow table of 165 ± 10 mm. These mortars were cured under standard conditions and for a smaller group of binders under simulated weather conditions. For the standard cure conditions, the properties of the binders were compared to the physical properties in terms of strength (from 7 to 1080 days), porosity, capillary water absorption, water vapour permeability and freeze-thaw resistance of mortars made with the binders. The carbonation was also studied by phenolphthalein stain after all the flexural strength tests and after 2 years by XRD. The mortars under climate simulation were studied in terms of mechanical properties (up to 360 days) and carbonation.For comparison purposes, cement-lime (1:1:6 and 1:2:9 cement:lime:aggregate volumetric ratio), lime-metakaolin (MK) (with MK addition of 5, 10 and 20% of the lime mass) and lime putty mortars were manufactured to the same workability as the NHL mortars. These were studied in terms of strength up to 360 days, porosity and water absorption by capillarity action.The strength of the studied mortars does not follow the classification of the binders, with one binder, specified as NHL 2, resulting in a stronger mortar than another binder specified as NHL 5, and one NHL 3.5 mortar surpassing all the other mortars in terms of mechanical strength. The mechanical strength was found to correlate with the hydraulic phases, alite and belite, identified within the binders. The relative long-term performance of the mortars manufactured with the different binders can therefore be predicted based on the mineral properties rather than the standard classification. Pore related properties, such as water vapour permeability and water absorption by capillarity, were found to be related to the water/binder ratio of the NHL mortars.Later in the project, using the standard cured mortars data, a model was developed to predict compressive strength based on the proportion of crystalline phases present in the mortars, the surface area and the water/binder ratio. This model, applied to the studied mortars, was found to predict, with low error, the measured performance of the mortars, meaning that the model can be used as tool to predict mortar strength.The outcomes of this thesis demonstrated that with sufficient knowledge of the underlying chemistry of NHL binders, it is possible to establish the relative performance of mortars, thus making the decision on which binder to use easier and safer for the historic fabric.
Date of Award1 Jul 2018
LanguageEnglish
Awarding Institution
  • University of Bath
SupervisorRobert Lawrence (Supervisor) & Richard Ball (Supervisor)

Keywords

  • mortar
  • natural hydraulic lime
  • conservation
  • historic buildings
  • long-term properties
  • repair mortars
  • mineralogy

Cite this

Properties and performance of lime mortars for conservation: the role of binder chemistry and curing regime
Figueiredo, C. (Author). 1 Jul 2018

Student thesis: Doctoral ThesisPhD