Restoration and conservation mortars

Today all of us are responsible for ensuring the survival of our historical and vernacular built heritage through careful and considered conservation, restoration and renovation. All too often buildings are repaired with inappropriate materials and poor skills. Many are subject to interventions that in time lead to their total loss. Others are simply changed out of all recognition.

In many cases, the mortars used for restoration or conservation work are not suitable. Suitability should not be confused with “like for like”. Most of the time it is simply not possible today to replicate the mortars of the past: the binders are different, the sands might not be the same and in many instances the buildings have changed.

The properties that one should look for when qualifying a mortar as suitable are:

Compatibility should be both chemical and mechanical. In chemical terms what is important is that mortars introduced into existing structures do not react with existing mortars and the surrounding masonry. Introducing soluble salts such as sulphates and aluminates or alkalis (potassium and sodium) into existing masonry can cause sulphate Attack or an alkali-silica reaction. In mechanical terms, mortars should not be too dense to impede breathability or have poor elasticity resulting in cracks that will allow water/moisture penetration.

Low capillarity & shrinkage
High capillarity allows moisture penetration and if a mortar has poor breathability (such as cementicious mortars) condensation and eventually damp will occur. The main causes of high capillarity are poor sands, void structure and a high quantity of Free Lime in the binder. Free Lime crystallises in the voids, reducing their size and therefore increasing their capillary action.

Poor building details and drainage increases the amount of water in contact with the masonry which can then be absorbed by capillary action.

High Shrinkage is also a source of cracking and is to be avoided.

Shrinkage is caused by:
– Inadequate suction control of the masonry units.
– High Free Lime content. This is because Free Lime is very fine and demands more water when mixing the mortar. The evaporation of this water leaves micro-cracks in the mortar (Shrinkage).
– Wrong dosage in the mortar mix. Common when too much binder is used. Over-binding produces a heave effect which causes cracks.
– Poor mixing of the mortar can cause over-binding in certain areas and low binding in other resulting in cracks.
– Poor protection from drying wind, direct sun, driving rain and frost when the mortar is still fresh will also cause cracking and shrinkage. So will poor curing: mortars, especially lime mortars, need time to cure.

Adequate Ca(OH)2 or free lime content
FREE LIME / HIGH CALCIUM LIME / AVAILABLE LIME / AIR LIME/ HYDRATED LIME / PORTLANDITE/ FAT LIME/SLAKED LIME are basically all the same and consist of Calcium Hydroxide – Ca(OH)2

Free lime makes mortars more workable and this is appreciated by the mason. However Free Lime affects a number of important properties in mortars such as:
– Setting and hardening time > an early set and a relatively quick hardening are important to be able to build with some speed. Initial setting of a hydraulic mortar is within 1 or 2 hours, the final set is within 24 hours. mortars made with binders with high Free lime content (CL and some NHLs in the BS 459 Standard) need contact with Air to re-absorb CO2 and to harden (by reconstituting itself to CaCO3 or limestone). This process, called carbonation, may take a very long time. Damp environment, rain, location of the mortar (exposure to air), contribute to slowing and in some cases even stopping carbonation therefore greatly increasing the setting and hardening time.
– Free lime, because of its fineness, demands more water in the mortar. It is one of the causes of shrinkage, as already discussed.
– It also increases capillarity by crystallising in the voids, reducing breathability
– The reduction of the voids’ size also affects frost resistance, as we will shortly see.

High free lime mortars such as 1:1:6 and 1:2:9 mixes or hydrated (powder or putty) lime mortars are not always as suitable as they are thought to be.

Careful judgement has to be made in relation to their use. This also applies to lime mortars made with hydraulic lime with very high free lime content.

Resistance to frost
The reduction in void size due to free lime crystallisation or a poor void structure caused by poor sands plus the slow carbonation of free lime, makes the mortar more susceptible to frost damage.

Frost always starts from the surface and goes inward. Frozen water particles on the surface voids push the water further into the voids behind and so on. A hardened mortar with a good void structure that accommodates the water movement will resist frost.

Resistance to salts
The same mortar will also resist the outward migration of salts contained in the masonry.

A macro void structure in the mortar will accommodate the pressure exercised by salts when moving or crystallising.
When structures contain salts it is almost impossible to control them if they become unstable. Their behaviour is linked to the relative humidity at which each salt is stable. A lower RH will cause crystallisation, a higher RH will cause the salt to become liquid and migrate.

If the void space is sufficient, these phenomena will not cause a damaging heave. If crystallisation occurs, they will stay in the wall and if there is no space in the voids the pressure can be great enough to crack the mortar. However, if they become liquid they will eventually come out and can be cleaned off the wall.

Vapour permeability (breathability) and good sands
A breathable mortar is essential to controlling condensation, thereby preventing damp and rot and creating a much better living environment.
It also helps reduce energy consumption for heating: if a room is damp, a lot of the heating will go on drying out the damp before heating the room….
The two main factors in achieving good breathability are: a relatively low Free Lime content in the Binder to avoid too much crystallisation in the mortar voids and the use of well graded sands.

Good setting, elasticity and workability
Setting and hardening determine the work rate. They are also related to the mortar dosage, water addition and the weather conditions during execution of the work. Protection and curing methods are directly connected to setting, hardening and curing mortars.
See Protecting lime mortars.

Elasticity determines the amount of movement the mortar will take before cracking. It is also relevant in calculating the positioning of joints. It helps to absorb thermal movements.

The workability of a mortar is often left up to the mason doing the job. Invariably this results in excessive water addition to obtain the “plasticity” that masons like. This is a mistake as adding too much (or too little) water can have serious effects on setting time, shrinkage, capillarity and so on.

Suitable compressive and bonding strength
It is no good to look just at quick setting and hardening. A very hard mortar, especially when made with cement, is not the solution to all requirements.

Since the advent of cement quick setting and strong hardening have become synonymous with “a good mortar” but in restoration and conservation, setting and hardening should only be one of the criteria studied when choosing the most suitable mortar for the job.

Of course we need setting and hardening, especially in adverse climatic conditions and seasonal work but this should never be to the exclusion of all other considerations such as compatibility, breathability, elasticity and so on.

Environmental considerations
It is common knowledge that lime mortars re-absorb part of the CO2 emitted. Saint-Astier® NHL mortars re-absorb between 38% and 49% of the CO2 emitted in the production of the binder.

NHL mortars are also non-toxic and once hardened they do not increase the water pH. Cementicious mortars can contain Chromium VI, directly related to skin allergies. Depending on the type of cement used they can also contain components like Pulverised Fly Ash (PFA), Granulated Ground Blast furnace Slag (GGBS) and a number of dangerous elements such as Heavy Metals and toxic components.

NHL mortars can also be easily removed from masonry units and re-cycled. The breathability of NHL mortars eliminates condensation and contributes to a better living environment.

Good workmanship & site practice
Good materials are nothing without respect and adhesion to the correct site practices. Good workmanship and supervision are essential.

If mortars are not dosed or mixed properly, if they are not cured and protected correctly, if they are applied badly, if the application surfaces are not properly prepared, if suction is not controlled, if salt movements are not taken into consideration and so on, the result will be a failure.

Architects, engineers – specifiers in general – should work with the contractor to ensure that he has the necessary comprehension and skills to apply the mortars and follow the necessary site practice.

It is the responsibility of the mortar manufacturer, meanwhile, to give architects, engineers and specifiers all the information necessary so that the most suitable mortars are used. This information should start with the chemical and mineralogical composition of the raw material and derived products, the performance of mortars on multiple parameters* and technical back-up to Specifiers and Contractors whenever requested.

* Saint-Astier® limes provide 2 year multiple parameter performance figures of NHL mortars at various dosages and straight comparison with blended mortars (NHL + putty and cementicious mixes as 1:1:6 and 1:2:9).

See 24 months test results