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Impact of lime mortar properties on masonry strength and behaviour

Lime mortars are increasingly used for building repair and new construction. Mortars influence masonry to such a great extent, that they can either enhance or adversely affect the quality and durability of masonry. This project studies the properties of mortar and masonry and their interactions in order to specify mortars that improve the quality of masonry. In addition, despite the importance of water content, it is often subjectively determined by the mason by assessing the mortar’s workability. This lack of explicit methodology results in a lack of consistency of mortar properties and field performance which adversely affect the large scale uptake of lime mortars into new building and mainstream technology (Figure - Variation of flexural bond strength with respect to water content (165, 185, 195 mm initial flow), hydraulic strength and water retention).

The project studies how mortar water content, curing conditions, compressive/ flexural strength, elastic modulus, air content and water retention impact the flexural, compressive and bond strength of clay brick, sandstone and limestone masonries bound with lime and PC/lime mortars of diverse hydraulic strength and thus different stiffness and deformability.

Flex setup

Flexural strength masonry tests parallel and perpendicular to the joints.

The relationships between bond strength and mortar hydraulicity, water content, workability and water retention can assist the production of mortars designed to reach high bond strengths, and this would improve the quality and performance of lime mortars in construction. Some research results (such as the control of mixing water using initial flow and the impact of varying water content) are being incorporated into masonry practice by OPW. The papers published have resulted in abundant citations and some of the results considered by CEN/TC51/W11/TG1 in the revision of Standard EN 459-2:2010 Building lime.

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Failure of Natural Hydraulic Lime (NHL5) mortar masonry under compression at 56-days.

In order to optimise bond strength, mortars should be mixed to the maximum initial flow that still yields an adequate workability. For NHL2, this optimal flow is close to 165mm; for NHL3.5 slightly below 185mm, and for NHL5 it equals 185mm. These flows are coupled to high water retention and result in strong flexural bond. Regression analysis was applied to the experimental mortar and masonry characteristic compressive strength results, and equations for the estimation of the 28-day characteristic masonry compressive strength proposed. These experimental equations were compared with predictions by previous authors, and Eurocode 6 provided the best predictions not only for cement-lime but also for NHL brick masonry.

Project coordinator: Associate Prof. Sara Pavia, Funded by: Office of Public Works (OPW) & US-Ireland Alliance (G J. Mitchell Scholarship).

SustPhysical properties of magnesian lime mortars

Petrographic examination evidenced that the original mortars of 16th c. Ardamullivan Castle were made with magnesian lime. Repair mortar replicas were to be designed and tested in the laboratory however, background research revealed that the properties of Mg lime mortars were not yet clearly established. There was a lack of agreement on the quality of Mg lime mortars as well as lack of experimental work in the subject; some authors claimed that Mg-lime had no hydraulic properties and produced poor quality mortars while others stated that it produced quality hydraulic mortars.

Medieval paintings on the mortars of Ardamullivan Castle, Co. Clare, c.16th century.

Detail of the dolomite in the original mortars.

The project produced Mg-lime by burning magnesium limestone (dolomite) in a traditional limekiln. Mortars were made with increasing proportions of Mg-lime (several types) and hydrated lime (CL90), and tested according to European, ASTM Standards and RILEM recommendations. Shrinkage, compressive and flexural strengths, absorption, capillary suction, density and porosity were evaluated, and the relationships between workability (measured as initial flow), water demand and strength investigated. This work lead to new discoveries in relation to raw materials and medieval building technologies, in particular the technology of Mg limes. Outcomes of this project are applied by OPW, to Ardamullivan Castle while others assist decision making on the conservation of National Monuments.



The process of lime production evidenced that fabrication parameters are instrumental on the quality of Mg-lime and the subsequent mortar’s performance. Temperatures over 900ºC induced over-burning resulting in clinkering and lack of reactivity. The choice of kiln fuel and burning arrangement proved essential in order to reach homogeneous calcination; and sieving of unslaked and over/underburnt particles as well as trials to determine raw feed proportions were needed in order to avoid poor quality lime. Shrinkage increased with the Mg content however, it did not reach unacceptable values and cracking didn’t occur. Mg lime mortars behave well towards fluids (their capillary suction was lower and their porosity and absorption similar to those of CL90 mortars) and their compressive and flexural strengths equivalent to those of some feebly-hydraulic lime mortars: Mg-lime strength falls within the European Standard strength requirements for natural feebly-hydraulic lime. The research concludes that, provided production is correct, Mg-limes produce reliable masonry mortars which will shrink further but will possess a lower water demand and a slightly higher mechanical strength than hydrated lime mortars.

Project coordinator: Associate Prof. Sara Pavia, Funded by: Office of Public Works (OPW) & Ecole Nationale des Travaux Publics de l’Etat Vaulx en Velin, France.