Natural Hydraulic Lime Mortar (NHL)

Natural hydraulic limes (NHL's) differ from non-hydraulic limes in that they have a chemical set as well as the process of carbonation.

Natural Hydraulic Lime

Natural hydraulic limes (NHL's) differ from non-hydraulic limes in that they have a chemical set as well as the process of carbonation. The limestones from which NHL's are formed, naturally contain a varied range of minerals of which silica and alumina are the main ones for creating NHL. When these limestones are heated in the kiln, the resulting lime has different properties. The burning process produces calcium oxide and calcium silicates and calcium aluminates.The resulting material is ground, hydrated and carefully packaged.

From the pure limestone, the silica and alumina combine with the lime to form active compounds. These compounds combine in water to create a chemical set. The percentage of silica and alumina contained in the limestone will determine the main characteristics of the lime and of course the resulting mortar or plaster. Carbon dioxide present in the air also contributes to this hardening process.

Natural Hydraulic Lime is mixed on site with suitable aggregates to give compatible, flexible, breathable and beautiful plasters and mortars. As well as this, its hydraulic set gives the added benefit of robustness and long-term durability.

NHL's come in three different strengths:

NHL 2 - Feebly Hydraulic Lime - Some internal/external work, pointing, bedding, rendering on soft brick or stone backgrounds in sheltered locations, grouts.

NHL 3.5 - Moderately Hydraulic Lime - The most common NHL, it is used for external rendering, pointing, bedding, in normal exposure conditions.

NHL 5 - Eminently Hydraulic Lime - Areas of extreme exposure on hard dense stonework, sea defences, canal work, limecrete, coping, pointing (very strong mortar).

Natural Cements - Run stucco work, casting, and underwater works.

Hydraulic Limes (HL's) are different to Natural Hydraulic Limes (NHL's). Hydraulic limes consist of lime and additives such as cement that change the properties of the binder. It is important to take care when using HL's as the additives do not have to be declared.

Natural Hydraulic Limes are very useful where a quick set is required as it sets much quicker than lime putty. NHL mortars also have a higher compressive strength than lime putty mortars, which can be beneficial in some areas of work. In some instances it is preferable to use lime putty mortars rather than NHL mortars, as it is important the mortar is weaker than the masonry.

Restoration works on Elder Hall were completed with Roundtower lime mortars.


The second part of a mortar is sand. Sand and larger sized aggregates make up the larger proportion of most mortars. Colour, texture and overall strength are all strongly affected by the choice of aggregate.

The aggregates most commonly used with hydraulic lime are sand and grit, although for the purpose of matching historic mortars various impurities may have to be added. A good sand should be a washed sharp sand with angular grains to ensure good bonding qualities. Soft building sands should be avoided as their rounded grain shape can result in excessive shrinkage.

Sands used should be well graded with a range of grain sizes, which for most plaster, render and mortar work will range from 5mm down to 75 micron. Larger sized aggregates may be used in some mortar or pointing work. As a rule of thumb for pointing, the maximum size of aggregate should be no bigger than one third of the joint width. Sands, which contain a clay or silt content of more than 4% should be avoided, as these will inhibit the contact between lime binder and aggregate.

Sands which have a high fines content should also be avoided as the larger surface area of these will require more water in the mixing. This higher water content will induce shrinkage and can affect flexural and compressive strengths. Monogranular sands should be avoided as they will possess poor workability qualities and will inhibit good vapour exchange i.e. the ability to breathe.


The third and final part of making a mortar is water. Use clean water. The addition of water should be considered carefully, as it will directly affect the ultimate strength and durability of a mortar. The more water introduced into the mortar mix, the weaker will be the final result. However too little water will prevent the chemical processes taking place and weaken the material. Generally, water should be added sparingly, until a useable consistency is achieved. Adjust quantities to give a workability suitable for the application. It is important to use the minimum amount of water necessary so as to reduce shrinkage.

Explore our lime binders here.