Hydraulic lime is made from kiln burnt limestone which has a clay content typically of 6 – 25%. Hydraulic lime sets partly by reaction with water (Hydraulic set) and partly by reaction with air (Carbonation). It comes in a powdered form and has a shelf life of 6 months. It cannot be stored as putty. Hydraulic lime has a greater compressive strength than lime putty, the strength increasing as the clay content increases. Hydraulic lime is produced in three strengths NHL 2, NHL 3.5 and NHL 5 (the higher the number the stronger the material).
Hydraulic lime mortars have some advantages over lime putty mortars. When building in wet conditions hydraulic lime mortars will still set due to their reaction with water as well as carbonation. This makes them more suited to certain costal works, bridges and buildings in exposed areas etc.
Hydraulic lime can fulfil the structural requirements that Portland cements once answered. Once craftsmen/builders have been introduced to the working qualities and appearance of lime mortars there is usually no turning back. Furthermore, the movement towards using locally produced materials for local buildings is growing in practice and will continue to do so as we become ever more environmentally aware.