All lime, including ‘Air Lime’, ’Natural Hydraulic Lime’, ‘Hydraulic lime’ and ‘Cement’ is produced by burning and slaking of limestone.
Air lime is a pure clean limestone when it is quarried, then burnt @ 800oC plus and slaked with water to make quick lime/air lime hydrate.
Natural Hydraulic lime differs and is quarried with a natural proportion of impurity being silica and trace elements. It is the silica and trace elements that when burnt @1200oC [1/3 more energy required over air lime burning], become reactive with water and will cause the hydraulic set of the mortar when mixed with that water into a working mix.
Cement is created by the deliberate and accurate addition of these admixtures to an exact recipe at the burning stage, rather than being found naturally occurring in the limestone – hence ‘natural hydraulic lime’ rather than ‘Hydraulic Lime’ or ‘Cement’. ‘Hydraulic Lime’ is simply anything that has not been made to the exact recognised recipe for the term cement but has a set as a chemical reaction with the addition of water – hence the term ‘hydraulic’. Up until the early 1980s, cement was often called ‘hydraulic cement’.
It is this hydraulic reaction that makes the mortar set faster and harder – in many instances the material becomes too brittle and ‘cement-like’ over time. The cause of this is also the increase in ‘hardness’ from the longer term carbonation of the lime, in addition to the initial hydraulic set. A way of looking at or understanding the ‘hardness or strength rating’ i.e. NHL2, 3.5 & 5 is the speed at which it reaches the final hardness of any hydraulic set. It will all in time attain similar strength, rather than as the designation actually says ‘the strength of set at 48 days’. Simply put 48 days should read 24 months, at which point the ratings would all be 5+ and it all becomes hydraulic lime, including cement.
Carbonation is the intake of atmospheric Co2 and the final completion of the lime cycle (chemical process) which started with Calcium Carbonate (limestone) became Calcium Oxide with the burning process, became Calcium Hydroxide with the slaking process and thence Calcium Carbonate again with the final carbonation – post use.
Air lime only has carbonation in order to achieve its’ final set. It does not have a hydraulic element in its initial set and therefore retains a longterm level of softness and flexibility not found with any hydraulic setting material. It is this very nature that makes ‘Air Lime’ most suitable for building works and especially older buildings. In more recent years, the only method of using ‘air-lime’ was in the form of a wet putty. This is achieved by the ‘over-hydration’ of the quick lime which takes the exothermically reactive dry powder beyond 100% hydration and therefore the material stays wet in the form of a putty. ‘Lime Putty’.
Lime Putty/Air lime can be tricky to use but is widely recognised as the correct version of lime when properly dealing with historic structures and modern sustainable building projects [due to the levels of Co2 absorption and embodied energy]. Lime putty requires maturing for many months [the longer the better] in its wet state. This maturing improves the mortars beyond recognition from younger materials. The skill, knowledge and timescales of storage and site use have rendered Air lime, in form of Putty, realistically unusable in the general building industry and has given the opportunity of the lime-use resurgence of the past 30 years to the hydraulic lime materials.
Vivus is air lime used in a way that has comparable speeds of set to weaker NHL but significantly extends the windows of use over NHL, i.e there is little need to stop work well below freezing and hot direct sun will not overly degrade the set either. We do not limit working environmental conditions – We are able to allow the practitioner to set their own limits of endurance!
