The impact of the water/lime ratio on the structural characteristics of air lime mortars

Abstract

Lime based mortal's are now widely acknowledged to be generally superior to cement based mortars in the repair of appropriate historic infrastructure. Increasingly the benefits of hydraulic lime mortars are also being realised in new masonry construction. In order to standardise the expected performance of mortars, designers will specify the type of lime, the type of filler (aggregate), the proportions of each and quantity of water or the required workability. Limes can be non-hydraulic (calcium or dolomitic) or hydraulic (natural or artificial). It is well known that the water/binder ratio has a marked effect on the structural performance of cement-based mortars. This is relationship is known as Abrams' rule, which states that when a cement mortar is fully compacted, its strength is inversely proportional to the water/cement ratio. Abrams' rule has also been demonstrated to apply to hydraulic lime mortars. The reason for this is that both cement and hydraulic limes require a minimum quantity of water to produce the chemical set resulting from the hydration of calcium silicates and calcium aluminates. Surplus water eventually dries out, leaving micropores in the matrix which weaken the resulting set mortar. It has generally been assumed that the same relationship applies to non-hydraulic lime (air lime) mortars. This paper reports on results of tests conducted on air lime mortars at early stages of curing. It is known that the form of air lime and the physical and chemical characteristics of the aggregate have a strong impact on the structural performance of air lime mortars. Results to date show that the water/lime ratio has a minimal impact on the structural performance of air lime mortars compared with the impact of lime and aggregate type. 91 day compressive strengths for air lime mortars with a water/lime ratio of 0.56 (a stiff mix) are identical to those with a water/lime ratio of 0.875 (a loose slurry). Whereas Abrams' rule is a key consideration for designers of cement and hydraulic lime mortars, it has been demonstrated that it requires modification in the case of air lime mortars. A relationship between form of lime, type of aggregate, water/lime ratio, and age of mortar is proposed. The resultant equation allows the compressive strength of air lime mortars to be predicted taking into account these factors. The insights gained from this study will allow practitioners to more confidently design and specify air lime mortars.