Natural hydraulic lime or non-hydraulic lime putty based mortar must be used when repointing an historic building. Cement should not be added to lime based mortars.Non-hydraulic lime is produced from pure limestone-the type quarried in Derbyshire. Hydraulic Lime Mortar-Hydraulic lime is available in various degrees of strength depending on where the lime is produced. Hydraulic lime sets quickly without exposure to air and will even set under water. Some of the hydraulic limes from overseas can be as impermeable as cement and less suited for use on historic buildings.
The following classification of hydraulic limes is based upon the
speed at which they set under water.
- Feebly hydraulic lime – NHL 2
- Moderately hydraulic lime – NHL 3.5
- Eminently or very hydraulic lime – NHL 5
Feebly and moderately hydraulic types are more suited to conservation work. Manufacturer’s advice should always be sought prior to use. However, a typical mix is a feebly or moderately hydraulic lime in the following ratio:
Lime : Aggregate
1 : 3
The exposure of the joint and the nature of the stone also need to be taken into consideration when choosing a mortar. Lime Putty Mortar The longer the lime putty mortar matures before use, the better it becomes. Lime putty cures on contact with air and so it must be stored in an airtight container. Lime putty mortar can be purchased pre-mixed (coarse stuff) or can be mixed on site to the following ratios:
Lime Putty : Aggregate
1 : 3
If used externally this mix is likely to require a pozzolanic additive (see materials section) to ensure that the mortar hardens. The lime putty option also requires more care when used externally, as it takes longer to harden and is more susceptible to frost damage.