NATURAL HYDRAULIC LIME


Unilit Natural Hydraulic Lime Mortars, Grouts and Finishes FIg. 1.


Telling is credited with the re-introduction of hydraulic lime to the UK in the early 1990’s and lead the field in quality, range and performance in the use of natural hydraulic lime. Our knowledge and experience is also without equal. Our laboratory facilities together with research programmes commissioned with UK and European Universities have enabled us to understand and replicate the chemical and physical performance characteristics of natural hydraulic lime and to convert this in to a leading market technology for conservation and new build projects. We guarantee our products against failure for 10 years and warrant against defect under a professional indemnity insurance.


FIg. 2.


ABOVE: A comparison of the open structure of hydraulic lime and cement under high magnification, after 6 days. Fig. 1. clearly shows the formation of calcite in the cement which results in strength but low porosity. Fig. 2. indicates the open pore structure of lime.


Using a modern day manufacturing process, which emulates the processes undertaken historically, while ensuring quality control and consistency are maintained, Unilit is produced by the selection of a particular “impure” limestone which when burnt at a specific temperature, produces a lime binder with optimum porosity, an initial set, and slow air carbonation, these being the pre-requisites for a compatible and sympathetic restoration mortar. The Natural Hydraulic lime is then blended with selected aggregates to produce a range of mortars, which replicate the strength, porosity, adhesion and flexibility of historic mortars.


The strength of the mortar is controlled by the grading and sizing of sand and the lime: sand mix ratio and not by its hydraulicity i.e., NHL 2, 3.5 and 5, which indicates the compressive strength of the lime, without sand after 28 days, or feeble, moderate and eminent since these relate more to the timing of the initial chemical set and not as is commonly believed, mortar strength.


The importance of sand grading and binder ratio cannot be over stated in relation to porosity, workability, durability and strength in the production of lime mortars. These characteristics eradicate the risk of failure caused by the incompatibility of the sand, a common cause of shrinkage, de-bonding and cracking.


This is the complete opposite of modern day pure limes since these are the result of selective classification and burning of limestone. This leads to the production of 99.9% pure lime to which pozzolanic material must then be added, sometimes even hydraulic lime to create a set. Where cement is added to a pure lime, even a small proportion will result in a cementitious mortar being produced, and the loss of the desired lime mortar performance. Pure lime is a material intended for chemical and agricultural usage, which was beyond the production capabilities of medieval craftsmen.


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