Mortars based on lime

Pure lime hardens in contact with CO2 dissolved in moisture. This hardening process takes place from the outside inwards, with the carbonation process depending on temperature and moisture content.

Lime as binder

The abundance of limestone worldwide is the main reason for it having been used often as a binder historically. Easy processing as well as its fitting strength and flexibility in different applications are also of importance. Modern production and increased demands in hardening time and strength have led to the experimentation with and development of additives and new binders (cement). But even still - to comply with older buildings, their behavior and building technique - it is important to use and produce materials that behave as the original. New knowledge and interest in the last 20 years have led to a more diversified use of the available binders, using the correct mixture of binders based upon the original construction and materials used.

Mortars based on pure lime (hydrated/aerated lime) are the original and basic mortars. The binder itself can be used as a dry powder (hydrated lime) or as a wet slaked lime (lime putty). Mortars based on hydrated lime are characterised by their high flexibility, high damp diffusion openness, low strength and self-healing properties. In comparison, modern and cementitious products are brittle, high strength and less prone to damp diffusion. Too often, modern products and solutions have been used resulting in damaged bricks, joints and renders. In renovation it is therefore of high importance that renovation mortars behave as the original mortars used. In this way, the renovation itself will not induce new damage.

Historically, lime mortars were based on local resources, using traditional recipes and mixtures, sometimes with additives to enhance their properties e.g. strength, workability, colour, etc. These additives have shown to be based on organic matter e.g. blood, hair, straw, or inorganic materials e.g. pozzolans, clays, etc to obtain hydraulicity properties. Matching the original mortars is therefore often a difficult task, even after analyzing the original composition. This is because the binder and sand/aggregates themselves are today produced with a more stable quality and from different sources than the original.

Mortars based on hydrated lime are often used in conservation and renovation projects where there is a clear indication of low-hydraulicty lime. Specialized mixes are delivered to these projects both fitted for local resources of sand and ready-made premixes ready for use. The benefit of the latter is a stable and prescribed quality.

Mortars based on wet slaked lime

Wet slaked lime / lime putty is known to have perfect properties when preparing mixtures for limewash, milk of lime, renders and plasters. Its long maturing time results in small, high-plasticity particles that yield products with high workability. It is said that the longer the maturing time, the better the workability and quality of the work. Low smitting and a long durability of surfaces are also said to be an effect. The main reason for using wet slaked lime, however, is the absence of unslaked/hard-burnt particles, therefore excluding the possibility of lime popping. Most probably wet slaked lime was the original binder used. And still today it is the basis for the production of everything from bedding mortar to limewash and pigmented lime paint. Traditionally mixes of 1:3 lime putty / sand were used for mortars, and a 1:5 lime putty / limewater ratio for limewash. Local traditions should be followed when using lime putty.

Mortars based on Natural hydraulic Lime (NHL)

Historically, hydraulic lime mortars are found widespread in Europe, both based on natural hydraulic binders, but also as a result of additives. Research has found the use of hydraulic limes in Roman construction and in Northern Europe dating back to the 12th century. As described in the lime technology chapter, natural hydraulic lime (NHL) is produced from clay-containing limestones. This resource is the basis for binders that have the benefits of both hydrated lime binders and cementitious binders. In a mixture of hydrated lime and additives, these binders are called "hydraulic limes" (HL, ref ISO-EN 459-1). The build-up of strength is quicker and it can be used in humid construction environments (e.g. dikes, wells and channels). These properties also make the use of NHL mortars suitable for projects where time and especially climate is an issue. Compared to a hydrated lime mortar that needs up to 4 months of carbonation before being ready to handle frost, an NHL mortar needs only half that time.

Mortars based on Lime and Cement

Portland cement was introduced as a binder in 1824 by Vicat. Todays cements cannot be compared with the original cements, as the development has aimed for more strong binders with a shorter hardening time. Cement was introduced slowly and used firstly in military installations and high-cost constructions. Industrialization in the last half of the 19th century made it possible to produce locally and therefore available for local – also low-cost use. In the start of 20th century the normal was” Lime with a dash of cement” to enhance strength- later developed to what we know today as standard LC-based mortars. Lime is today used to increase workability and reduce strength in mortar mixes both for bedding mortars, joints/pointing,and especially renders. A specific construction date/ timeframe for when to allow LC-based mortars is not possible as the introduction and use were controlled by the local economy and availability of cement. But it is said that after WW1, most bedding mortars were Lime-cement-based. During a few decades lime-mortars were totally absent in construction in parts of Europe- even renovation of old 16th century properties were done with modern, brittle and strong mortars. New knowledge and interest in the last 20 years have led to a more diversified use of the available binders, using the correct mixture of binders based upon the original construction and materials used.

Special Mortars

From time to time there is a need for mortars with specific properties. Especially when dealing with corrosion of steel and high-strength applications, modern techniques and binders have to be used. Binders based on specialised cements, organic binders and modern additives are then available. But as for all renovation projects, documentation for its use, where and why, should be easily accessible for future generations.