MOISTURE AND SALT DECAY IN MASONRY

I’ve always been fascinated by the mechanism of decay in building materials and as a surveyor specialising in damp diagnosis, decay to both timber and masonry is a familiar sight.

Decay I consider a natural process an inevitable, inherent risk to all building materials however, presented with the right conditions decay can be prolonged for centuries. Materials exposed to excess moisture will almost certainly decay at some stage, the question is when? Exposure to unfavorable wet and salty conditions (sea spray), excess ground moisture or lateral dampness will all promote moisture and salt decay in buildings. The extent and duration of exposure will dictate to what degree and how quickly. We must also consider the harmful effects of alterations which may un-intentionally contribute to decay to explain why.

Unsightly efflorescent salts in high concentrations observed in new construction

Salt decay I find particularly mysterious and fascinating. How salts emerge from nowhere after being dissolved in water only to appear upon the surface of masonry like fairy dust. Salt damage isn’t just a phenomenon which occurs to old historic buildings, it a problem that affects all porous building materials old and new, constructed from stone, brick and concrete. New buildings can be problematic with surface salts usually caused by water added during new construction or salts within the masonry materials themselves. These are efflorescent salts, a white powdery crystal which aren’t particularly harmful although can be unsightly.

Accumulation of surface salts due to long term cyclic dampness

Salts present in older construction, however, often occur through contamination or the unwanted movement of moisture after construction. When masonry materials become wet dependent upon the origin of water, salts present may be drawn into the fabric of the structure. These soluble salts are responsible for a large proportion of decay in porous building materials. This damage occurs during the process of moisture evaporation as salts crystallise upon the surface of the material. The salts are merely the deposit left behind from the contaminated water. Salts which arise from a single incident such as; water ingress via materials contaminated with salt are rarely a worry, however, salts which are present through long term cyclic dampness can be much more damaging.

Salt crystallisation can also occur within or between layers of different materials. We often refer to this as crypto-efflorescence. This occurs when salts develop within the pores of the material a process often caused by barriers to the drying process or by extensive crystallisation on the surface. The result is that salts which can no longer reach the surface, crystallise within the pores. The stress this exerts can often be stronger than the masonry causing it to shale and delaminate. In this circumstance crypto-efflorescence can be very damaging to masonry.

crypto-efflorescent salts causing the face plaster to delaminate

Inevitably the biggest influencing factor will be cyclic weather conditions which promote the wet / dry climate changes encouraging moisture evaporation and salt crystallisation however, internal masonry can also be affected simply due to internal atmospheric conditions which affect the hygroscopic properties of salt.

THE EXPERIMENT

Recently I attempted an experiment to observe salt decay in masonry caused simply by subsurface efflorescence which would exclude the influence of varying external atmospheric conditions. I attempted to demonstrate decay and surface loss caused by the crystallisation of salt within the pores of masonry. Now obviously I didn’t have years to conduct the experiment or own a period damp property, so I settled for a series of limestone pillars which stood in a solution of saline water. I set the pillars up to stand in a water proof container with an inch of water incorporating dissolved sodium chloride salt. I used a camera on a tripod mount to time-lapse the experiment and record a new image each morning. The atmospheric conditions within our office are pretty stable therefore I envisaged little influence to the experiment from atmospheric changes.

Unfortunately, our lab doesn’t extend into expensive sophisticated techniques to measure the erosion of the stonework such as; laser scanning of micro erosion meters, therefore this was merely a visual observation. I should also mention that our experiment was undertaken using a single salt (sodium chloride) as oppose to most circumstances that could occur naturally where several different types of salt would be present.

To reduce the overall timescale of the experiment I increased the salinity of the water to 3.5% the same as seawater with 35 grams of salt per 1000ml. This allowed the me to witness the crystallisation of salts in the experiment much quicker than had I used normal tap water or a lighter saline solution. In total I ran the experiment for just over two months from the 23rd October – 30th December 2019.

To add another dimension to the experiment I also coated 4 out of 5 of the limestone pillars in a number of different coatings that have been used commonly in the damp proofing industry throughout the years. The purpose was to highlight the influences these coatings may have on moisture evaporation and on where the salts will finally evade.

For those interested the coatings are as follows: From left to right

1 – Felted damp proof membrane, 2 – Water resilient cementitous membrane (tank) 3 – Natural limestone pillar, 4 – Surface water repellent coating, 5 – Latex based damp proof membrane.

Although the experiment worked well and I was able to observe the early signs of cohesion failure the extent of salt erosion was minimal. The reality is, I really needed to run the experiment for longer to witness the damaging effects in greater detail or perhaps use a water with a higher salinity. This however was my first attempt, so go easy.

Here’s a time lapse of the experiment.

At the beginning of the experiment as you would expect there was little change in the limestone pillars with just visible discoloration observed to the base in contact with the water as moisture began to wick up the capillaries. Within just three days however light salting upon the surface could be seen.

Within seven days the effects of capillary dampness was clearly visible with salts drawn into the pillars being clearly deposited on the surface of the untreated stones.

Within fifteen days we were able to observed surface salting on the top of the tanked and latex coated pillars despite no visual evidence of dampness within the pillar column.

I also noted some apparent failing of the latex coated pillar where blisters had started to emerge.

After 21 days a clear and distinct salt tidemark was visible across the untreated pillar and the pillar treated with just a surface water repellent.

At this stage the pillar with the latex membrane coating which started to show blisters earlier also revealed evidence of salts emanating through tiny imperfections within the coating.

Finally after just five weeks I was able to observe the first signs of distortion to the stonework where the limestone just above the band of salt deposits started to fracture and blister.

As the degree of surface salting increased as too did the extent of decay where the pillars started to reveal large blisters and spawling of the face.

The end result was some pretty poorly looking moisture and salt damaged limestone pillars.

Extensive salt crystals were observed upon the face of the untreated limestone pillar.

Extensive salt crystals were also observed emerging through the minute imperfections in the latex membrane treated pillar. Their appearance resembling that of burst popcorn.

Over the years through training and experience I feel I’ve gained a good understanding of salts, their movement, their effects and the damage they cause. The study of salt damage to building materials however, is widely misunderstood which when examined in detail can be extremely scientific, far beyond my level of understanding. In reality I only attempt to understand the effects of salts from a basic grasp of science, what some may call a practitioners level of understanding without having a scientific background. 

Over the past few years I’ve began to take more note of salt damage in buildings and the effects that prior poor building practices have had upon a materials condition. It’s widely understood that soft porous building materials are damaged and decay as a result of moisture and salt, the process of cyclic wetting and drying and the freeze / thaw cycle.

As a surveyor and practitioner in repair however, I see little evidence despite this understanding that buildings are repaired and maintained sympathetically. I suspect in our industry this is often influenced by a combination of commercial incentives and available skills of the surveyor and workforce. All companies have a commercial incentive to sell their business and the services they offer which more often than not means providing a financially acceptable resolution to a problem which can be guaranteed. Unfortunately for the building this does not always mean a sympathetic resolution without consequential effects.

Myself I take the design of our repairs seriously ensuring not only do they provide the desired result for our clients but equally just as important, that our involvement does not have a damaging or negative impact to the building in it’s current condition or future.

Over the past few years I’ve adopted a much more holistic approach at repair with monitoring upon completion. It’s not an easy path to take particularly as this often involves re-wiring our clients thought process due to the poor advice they’ve received prior or read online. Most people believe that a conservative approach to damp and decay is a method reserved only for listed and historic buildings when the reality is it’s not, it’s for every building. Highlighting the benefits and adopting these principles should be in everyone’s interest.

If you’ve made it this far, I hope you found this article useful and somewhat informative and if you have any questions or queries about this subject please don’t hesitate to get in touch.

Happy surveying

Russ