We are proud to have been producing hot mixes for 20 years!

Date published: 12/02/2018

What is a Hot Mix?

A hot-mixed lime mortars is a traditional lime mortar, prepared in the traditional way. This in turn has led to greater understanding of the ‘active ingredient’, which made traditional lime mortars robust, durable, and economical and a pleasure to use. ‘Why use lime mortar?’ The issue is one of functional behaviour. Traditional lime mortars should be used for the care and repair of traditional masonry because they are objectively the right material for the job.


The active ingredient in traditional lime mortars that imparts poultice functionality, vapour permeability, intimacy of bond and deformability is calcite: carbonated free lime. A technical understanding of how and why traditional lime mortars function in the way they do enables new lime mortars to be specified that will perform in the same way. With regard to the mix composition of traditional lime mortars (typically fat lime at 1:1.5 mix proportion), the specifier considers how such a lime-rich and binder rich mortar. The answer is found in hot mixing, the only known viable way of making mortar with the historic ingredients at the historic proportions.


Given the lime richness observed in the historic example, the ML90 binder appears to be the best starting point. Such rich mix proportions using this binder type can only be achieved using the hot mixed method (the traditional method of mortar preparation for many historic mortars). As quicklime doubles in lime yield during slaking, the typical starting mix would be 1:3 quicklime: aggregate to replicate the 1:1.5 historic example. In order to reproduce the required degree of hydraulicity of the original mortar and meet the environmental/service requirements of a particular context, the quicklime mortar can be gauged with a pozzolan or by partial binder replacement with NHL. Pozzolans create a hydraulic set by consuming a proportion of the free lime in the mortar mix: the ‘lime demand’ of the respective pozzolan is described by its reactivity under the Chapelle test. Particularly reactive pozzolans, such as metakaolin, broadly consume their own weight in free lime. Research indicates that by keeping the Pozzolan content to around 10 per cent of the weight of the lime binder, a microstructure very close to that of an aerial lime mortar can be produced – with all the practical benefits of a hydraulic set. The pozzolan inclusion should be limited to ‘just enough’ according to its lime demand and the context in which the mortar is to be used. Quicklime-NHL hybrid lime preparation is a very practical way of overcoming the markedly low free lime content in an otherwise NHL-only mortar. Recent work has demonstrated excellent replication of the original mortar’s physical and chemical properties and the resulting functionality.48 Work by the Building Limes Forum Ireland,49 Historic Environment Scotland and Historic England demonstrates the opportunities presented by the traditional method of mortar preparation using a contemporary palette of materials. Masonry heritage testifies to the enduring reliability of the material and the method.

The functional behaviour exhibited by traditional lime mortars is best replicated by hot-mixed lime mortars today. This is entirely unsurprising. The basic chemistry behind lime mortar is no different to that found in home baking: if the same recipe is followed, using the same ingredients and prepared in the same way, for all intents and purposes the same result can be expected. Hot-mixed lime mortars use the historic ‘recipe’, the right ‘ingredients’ and the traditional method of mortar preparation, thus ensuring the mortars are made in the same way.


There is renewed interest in hot-mixed non-hydraulic lime mortars with many proponents being of the view that lime mortars prepared in this way have advantages over the normal practice of mixing lime putty and sand. Hot mixed mortars are those where quicklime, aggregate and water are mixed together at the same time, and either used immediately while still hot, or stored and allowed to cool for later use. The quicklime is therefore slaked and mixed in the same operation and much heat produced, hence the term ‘hot-mix’.


If you have any questions please do not hesiate to contact our technical team on 01874 611 350 or email tymawr@lime.org.uk