Chapter One
Construction and Materials
Welcome to Russet and Sage | Surveyor's Library. This content has been created specifically to help bridge the knowledge gap between the lime craftspeople who have spent decades learning and applying their trades, and those who have the unique role of surveying and specifying work to Britain's historic building stock.
The lime cycle is as good a place as most to begin, and it's something that many will recognise with even a passing interest in conservation work. The flow chart to the left is our simple iteration. A vertical flow helps us see more clearly how the process of creating each lime varies. In doing so, we can more carefully understand the impacts of such differences.
Most course content will demonstrate the progression to the left, which is the cycle for lime putty. It begins its life as limestone, which is quarried and then calcined in a lime kiln at approximately 1000°C. This drives off the carbon dioxide content within the stone, and the result is calcium oxide, also known as quicklime. Straight from the kiln, this is in the form of lump lime or kibble. When crushed to a powder, it is available as bagged quicklime.
Quicklime reacts violently with water, producing a strong exothermic reaction that binds the water to the quicklime, thereby hydrating it. For this, there is the term hydrated lime, describing quicklime that has been slaked, not to be confused with lime hydrate, which is described later.
Quicklime can be added to a heat-resistant container of water until the water reaches a boil. The temperature is essential in encouraging a strong chemical reaction. The quicklime will absorb a great deal of the water, and the mixture will need further water until the resultant mixture resembles a thick clotted cream. This is lime putty.
With exposure to air, the lime putty will dry and begin to cure (it must be dry to cure), absorbing carbon dioxide from the atmosphere. Chemically speaking, it ultimately reverts to limestone.
The lime cycle is important because it highlights some key elements of lime mortar, not least the sustainability of lime mortars in the context of carbon costs in the building environment. Lime mortars will sequester up to 80-85% of the carbon dioxide that was initially driven off during the calcination of the limestone. What this cycle doesn't do is describe other limes within the same context. Where does lime hydrate come into this? What is a hot-mixed mortar? What is a hydraulic lime, for that matter? Examining the following cycles helps to classify these lime types and also goes a long way in aiding their specification for conservation and restoration work.
This time we're looking at lime hydrate. Many will have seen bagged lime hydrate at builders' merchants. The best way to think of lime hydrate is that it's a partially slaked putty. In this case, it would be preferable to add water to kibbled lime or lump lime. If enough water is added to introduce a slake, but not enough to hydrate the lime fully (remember hydrated lime is that which has been slaked fully with water), then the result would be a fine white powder, which visually would resemble quicklime powder. It is, in fact, a partially slaked Calcium Hydroxide. Enough of a reaction was achieved to break down the lump lime into powder, with some unslaked percentage left behind.
Lime hydrate is almost exclusively used by the building trade as a plasterciser for cement mortars. A plasterciser is a mortar addition that creates a more 'workable' mortar. That is, it's easier to work with. It is a little subjective and not easy to get across to those who have never worked with mortars. It encompasses a few qualities, a smoothness, a body, sticky but not too sticky! This is discussed further in Modules 2 and 3. Whilst it is possible to use lime hydrate in place of other lime in creating a mortar, what we can take from the flowchart to the left is that at the point of creating lime hydrate, the material is, a large part, Calcium Hydroxide, which, the same as putty, will begin to carbonate when dry.
Lime hydrate is produced as a dry powder, and so it will begin to carbonate immediately. Only very fresh bagged lime hydrate is of any use, which is generally the case because of the trade turnover of the material; however, the nature of the dry production and bagged (not air-tight) storage, means there is a risk that the resultant lime content is too low for a useful mortar that can perform, due to constant carbonation while in storage. It's a lime type that is best left to the builders and their cement mixes.