Part Two Lime: different varieties explained

It may seem odd to start an article about lime to talk about cement but it warrants a mention because it is such a marvellous material. The use of Ordinary Portland Cement (OPC) has become widespread throughout the construction industry from the Second World War onwards. A type of cement was used by the Romans to make concrete; this was probably hydraulic lime mixed with material to make it set faster. However, these methods died out with the fall of the Roman Empire and were not rediscovered until the eighteenth century. John Smeaton, an engineer, pioneered the use of hydraulic lime. He built the Eddystone lighthouse 14 miles offshore on rocks submerged at high tide. This needed a mortar that set quickly and was incredibly strong. OPC was developed in the nineteenth century by using a new type of horizontal kiln and firing limestone and certain types of clay at higher temperatures. The cement produced using this method was even stronger and faster setting than its predecessors and could be manufactured to consistent quality standards. These qualities make it easy to use and construction a much quicker process. By contrast lime mortar, particularly lime putty based mortar, is slow to set, relatively weak (especially in the early stages of set), time-consuming to mix and apply and has become difficult to obtain.

Limes and cements set by different processes. The incorporation of clay impurities in the limestone used for cement production imparts a chemical process called a hydraulic set. Pure lime putties set by a process known as carbonation. The latter involves the absorption of carbon dioxide and results in a material that has the same chemical make up as limestone. Carbonation requires the presence of moisture to set properly.

Quicklime is the material produced by burning limestone. It is highly alkaline and caustic and was historically used for disposing of dead bodies. Dry hydrated lime, also known as bag lime (or calcium hydroxide for the technically minded), is made by adding water to quicklime (slaking). The result is a dry powder. The slaking process produces a lot of heat and drives off the water content. It is best practice to leave the mortar made with bag lime to mature in order to allow the lime to absorb water and distribute itself as fine particles around the aggregate. It is considered to be inferior to lime putty as it tends to absorb carbon dioxide in the bag and can fully carbonate before use. Lime putty (Hydrated Lime) is a material that has taken the slaking process a stage further and is fully slaked to form a putty-like material. Deprived of carbon dioxide by storage in a sealed container or covered by a layer of water it will remain in this form for many years and is said by many to improve with age. N.B. All putty should be stored for a minimum of 2 weeks before use.

Hydraulic limes have the ability to set without the presence of air, even under water, and combine to a lesser or greater degree both carbonation and a hydraulic set depending on the strength required, which is determined by the class of hydraulic lime used. Hydraulic limes come in three strengths designated by their Natural Hydraulic Lime class, which are NHL 2, NHL 3.5 and NHL 5. The number refers to the material’s compressive strength expressed in N/sq mm and these roughly correspond to the former designations of feebly, moderately and eminently hydraulic. NHL 3.5 is suitable for new construction, whilst NHL 5 is used in more challenging conditions, including sea defence walls, canal walls etc. On an environmental note, the production of cement involves burning limestone at far higher temperatures than lime with the consequent increase in energy consumption. Moreover, the carbonation process involves the absorption of carbon dioxide.