[Engineering] What's the white crap on my wall...?!

Tomorrow is my last day at work before vacation but I recently decided to systematize what I've learnt in the field of historic preservation and building diagnostics, and that I'm going to share a summary of it with you. It gets pretty exciting when you start digging into it. Luckily we have @hrissm here who is a specialist in the field and hopefully will intervene if I start abusing the topic too much. :) So without further ado, let's start with my current favorite - efflorescence.

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What is efflorescence?

It is the white soiling, or as it's sometimes called - white scum - that may appear on either new or old masonry wall. Although it is often misdiagnosed by laymen as white mold, a vast majority of efflorescence is caused by salts or calcites collected on the surface of the wall.

How does it work? If there is a source of water soluble salts, water and routes for moisture to travel through masonry - salts will dissolve in water and travel to the surface of the wall, and then crystallize as white deposit as the water evaporates. Thus efflorescence will appear. If any of these factors do not occur, efflorescence cannot appear on the surface of a masonry wall.

What are the most common salts and what could be the sources of them in masonry?

According to the Masonry Institute of America[1], sodium sulfates and potassium sulfates are responsible for the majority of cases of efflorescence. These salts are highly soluble in water and very common in products such as mortars and grouts. Thus the major sources of salts in masonry are:

• mortars - portland cement and sands used in the production of most common mortars,
• grouts,
• clay brick,
• water,
• external sources such as for example road salt.
  When we look at the list above it's easy to realize that in most cases we will have a source of water soluble salts. So, how do we prevent this unwanted ornamentation on our wall?

Prevention

To prevent efflorescence it is essential to provide excellent workmanship during construction and preservation works. Improper workmanship may cause a variety of issues from cracking or debonding of mortar, cracking and spalling of masonry units (bricks, stones, CMUs...), insufficient flashing installation to debonding and failing sealant joints, and many more.
Other means of prevention against efflorescence:

• correct selection of mortars,
• proper bricklaying technique,
• using temporary waterproofing during construction,
• avoiding mortar additives which aerate it,
• providing well tooled, concave joints, etc.

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Is white soiling on masonry always salt?

No. The most commonly used mortars consists of lime, cement and sand. The precise ingredients and ratios depend on the specific blend and are assigned a letter (M, S, N, O, K), where the most common one is N, and maybe also M and O, depending on the use. According to ASTM C 270-511 the letters are derived from the phrase "MaSoN wOrK", which seems pretty funny. Anyway - lime being part of mortar has the smallest particles, which may cause calcites to leak out with moisture coming out of masonry. On a side note, when the masonry is well maintained and regularly repointed with good quality mortar, the small particles of lime will seal pores in mortar, not allowing water to enter masonry and thus preventing the capillary action of moisture.
In other words, the so-called "lime run" is often caused by rainwater dripping off the surface of the wall and not entering masonry, in which case it's not a big issue. I have read that in most cases the next rainfalls will wash it off. However, it is not always easy to determine what the character of the soiling is, especially because it is often more than one reason.

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When efflorescence occurs, it is necessary to find the components of the wall system that are failing and letting moisture in. We can assume that when heavy efflorescence occurs, the source of water infiltration is not horizontal capillary action through the natural pores of brick or mortar, but a larger issue in the moisture management system of the wall.
It is also not a good practice to seal the wall with coatings without investigating the cause of efflorescence, and may result in the appearance of cryptoflorescence (I thought you may like this term...) within the masonry, causing it to crack and spall.

In my opinion efflorescence on its own is not really a condition - but more like a symptom, which should not be neglected. As it indicates water infiltration into the masonry in sufficient amounts to dissolve salts present in the wall components. Such masonry should be further evaluated for any cracks, leaks, defects in sealants, flashings, copings etc., depending on the type of masonry walls.

Sources:
[1] Masonry Institute of America, Efflorescence: Cause and Control
[2] ASTM C 270-511 Standard Specification for Mortar for Unit Masonry
[3] https://en.wikipedia.org/wiki/Efflorescence