Hydraulic Lime

There are two types of lime: ‘hydraulic’ lime, which sets under water, and ‘non-hydraulic’ lime, which needs air in which to set. The construction industry commonly uses non-hydraulic lime, which is manufactured by heating pure limestone in a kiln to 1066 degrees centigrade. The resultant ‘quicklime’ is then mixed with water (in a process known as ‘slaking’) to make lime putty, or with less water to produce hydrated lime in the form of a fine, white powder that is delivered to site in 25-kg bags. Since lime is less dense than cement, a 25-kg bag of lime is significantly larger than a bag of cement of the same weight.

Being hydraulic, bags of lime must be protected from damp before use. Preferably, bags should be stored clear of the ground on a wooden pallet in a well-ventilated, rain-proof shed. Bags of lime should be stacked flat, not more than five bags high. If stacking outside cannot be avoided, bags should be raised off the ground and adequately sheeted with polythene, ensuring a good overlap. It is vitally important to keep the bags dry.

Hydraulic Lime

Fig. 18 Size comparison of 25-kg bags of Ordinary Portland cement and hydraulic lime.

Hydraulic Lime

Fig. 19 Hydraulic lime.

Before the widespread use of cement, mortar used in bricklaying was referred to as ‘coarse stuff’, a 3:1 ratio (aggregate to lime) mortar mix using non-hydraulic lime putty, well-graded sharp sand and soft sand. It is still available today from specialist suppliers and is suitable for repairs to historic buildings, blocklaying, bricklaying, stonemasonry, the re-pointing of brickwork, the repair and plastering of internal walls (as a backing or finishing coat), and the repair or rendering of external walls.