What is Natural Hydraulic Lime?

Natural hydraulic limes are returning to their rightful position of being perceived as building materials suitable for restoration, conservation and new build. Cement had displaced these materials as it offered a quicker set and more strength when these became the construction industry parameters for building mortars. However, already at the beginning of the 20th century, problems related to the chemical reactions caused by cement and the damaging effects caused by the excessive strength produced in cement mortars were noticed.

Nevertheless the speed at which the cement industry developed and its acquisition of most quarries that produced hydraulic lime to obtain the raw material for cement production was astonishing. Hydraulic limes disappeared.

In building terms this coincided with the appearance of construction joints, damage to old masonry or soft stones due to the hardness of cement mortars, deterioration due to sulphate and alkali-silica reactions, loss of durability and poor compatibility of cement mortars with existing old mortars (which were all lime based).

The lack of hydraulic limes promoted the use of hybrid mixes of cement/lime/sand (1:1:6 and 1:2:9) whenever restorers and builders tried to diminish the excessive strength of pure cement mortars, but the presence of cement, even in these diluted forms, still produced undesirable chemical reactions. To avoid using cement one can use lime putty or hydraulic lime mortars. They can both produce good results, provided that they are applied correctly and that the right quantity is used. The majority of putties on the market today, however, have no resemblance to the putties of the past. Very few are burned traditionally in mix feed kilns. They are too pure and in many cases contain far too much water.

A good putty is months, if not years old, it has been punched through a sieve rather than 'poured' through and, even with pozzolanic additions, requires long curing and protection measures. Its characteristics make lime putty suitable for interior finishes and, at best, exterior work in sheltered areas. Exterior work should be completed well before the Autumn/Winter months otherwise it should be protected throughout this period. Putties demand specialist skills and dedication. Work has to be checked for shrinkage for some time, adding to labour costs. Scaffolds and protection sheeting must remain in place for as long as necessary, adding to the overall cost. The use of putty mortars, pure or with pozzolanic additions, in unsuitable situations or without the required skills has produced an innumerable amount of failures with high financial losses to all concerned.

Pure and natural hydraulic limes (NHLs) are coming back because they offer the right materials and lower costs. They do not contain the damaging chemical components of cement, set quickly, do not require long curing time and are easy to mix, even in conventional mixers. They will produce building mortars of various strengths, matching the BS 5628 classes (II-IV), but allow for movements as they achieve their strength over a longer period of time and have far better elasticity moduli than cementitious mixes. There is no need for construction joints. NHL mortars, although resistant to water penetration have high vapour exchange properties thus avoiding condensation. The fact that they are also available in a range of strengths (NHL 2, NHL 3.5 and NHL 5) eliminates the need for blending. These are perennial materials with centuries of life ahead of them. Once applied with the normal and current working practices and having a well graded sharp sand, they will perform in the harshest climatic conditions.

The cost of pure and natural hydraulic limes is lower than putty/pozzolan mortars. This is due, especially in the case of St. Astier NHL products, to their low bulk density. Limes are bought by weight but mixed by volume. A good putty should weigh between 1350 and 1900 kg per cubic meter. Weights below this figure would mean that the water content is very high and certainly it could make mortars with in-built failure potential. In the case of St. Astier NHL, they weigh between 520-650 kg per cubic meter. The result in a 1:3 mix is that you would use 200-250 kg of St. Astier versus 500kg or more of putty. At today's retail prices this means approximately 120 euro of NHL versus 135 euro for the equivalent amount of putty to make 1 cubic metre of mortar. The use of pozzolans and, in many cases, cement in putty mortars could easily add another 40-50 euro per cubic metre. These costs increase further once consideration is given to the extra care and labour necessary to make a putty mortar succeed (at the right time of year...) and the costs of keeping protective sheeting and, in most cases, scaffolding up for longer periods of time.

There are a number of hydraulic limes available. What the specifier has to demand from manufacturers are exact data on the mechanical and chemical composition of their products, their bulk density and the presence of free lime (a minimum of 3% in accordance with EU norms), and the absolute assurance that if the products are classified as natural hydraulic limes, they contain no addition whatsoever such as gypsum, pozzolans, water retainers, repellents and, most of all, no cement. One should also check their re-working abilities. Some hydraulic lime mortars must be used within a very short time. In St. Astier's case, this time is extendible up to 18 hours.

The St. Astier limes comply in full with the French norms, which are even stricter than the EU norms (for example, fee lime after slacking is minimum 15% and up to 45% and expansion is below 2mm). They are traditionally burned in vertical mixed feed kilns as done for thousands of years. Their pureness, reliability and the availability of a range of strengths to suit the specifier's requirements are the main reasons for their wide adoption in the UK and Ireland.