Porosity, capillary action and breathing

This is a topic which is still under investigation so it could well change. But that is, contrary to what social media says, the nature of science. 

Masonry needs to stay dry but it gets wet. We use other materials like lime and cement to manage this. Lime sucks moisture out, stores it and them releases it when the air is dry enough. Cement prevents moisture from getting in, in the majority of cases and can resist the deteriorating effects of moisture. The former can be utilised above ground with nearly all materials, the latter can only be used with materials that have similar function. However, there is a limit to how high you can build solely with lime, where as cement with steel can build very high buildings. 

The porosity of lime mortars is approx. 30% and the pores are a mix of micro, mesa and macro in an interconnected network. Ideally with most falling in the mesa range. 

The porosity of cement/NHL mortars is 20-30% with cement nearly always being 20% and lower fired, binder rich NHL mortars coming closer to 30%. However the range of porosity is skewed towards the micro range and the pores are often unconnected with more high fired binders like cement and strong NHL's or binder lean mortars.

The optimal range for breathability is 30%, interconnected, predominately mesa pores. Which are also irregular and open mouthed.(the relevance of this will be explained later).

So that all seems fairly simple; No, I'm just easing you in gently.

Capillary action; this is the effect of water being draw towards a narrow opening from a larger space, like a tapered tube.  But the shape of this space is important.  In lime mortar the shapes are irregular and connected, so water moves from one section to another, towards the face when correct. When incorrect it can go in the wrong direction or get stuck. Eg. cement mortar is micro porous and often unconnected. So vapour gets in, collects until its gained enough mass to become water then can't get out. Chalk pores can be closed mouth which can inhibit the passage of moisture, retaining too much moisture. I imagine there are other aggregates which also do this which we don't use without knowing why we don't. But tbf hot mixing can often alleviate some of these issues due to it's far superior properties.

Don't turn the strength up to solve problems as you would with modern construction, hot mix instead. This may sound reductive but there aren't many cases above ground where this doesn't apply. 

Lime mortar needs to have a certain level of moisture in it all the time for the vapour molecules to attach too, if it dries out fully, it doesn't work properly. 

Now to confuse the matter... 

The size of aggregates you use and type of lime also affect 'breathability' and to be more specific capillary action. So the reason for using several coats of different mortars is to take advantage of this. 1:3 with 4mm sand has a higher porosity with a greater range of mesa/macro pores than a 1:1 with 1mm sand, which is predominately micro porous. The 1:2 float coat with 2mm sand sits betwixt the two. So the capillary action pulls the moisture towards the wall face, where it dries off through convection as the more microporous the stronger the pull, but it is only suitable for thin coats. eg. top coat is thin and Venetian plaster even thinner, as it's even more microporous, or less, depending upon how you look at it. 

No let me further confuse this issue by saying that the pores can be lined with some materials and the size can be reduced by the use of certain pozzolans. But not all. It also works the other way and gets more confusing. Eg. Iron oxide with increase the porosity of a mortar whilst also 'catching' salt. So the mortar can then be subject to salt cementing too; which can also fill the pores.

So what we have is a material which has 'porosity' at different sizes, within the materials themselves and within the matrix of the mortar itself. All of which can be affected very easily by a number of factors such as the type of aggregate or the use of additives.  

Ok so now no of us have clarity on this issue lets move along to something much more confusing; how water moves in an above ground non-retaining wall which is part of a house. I say that specifically because floors, garden, below ground and retaining walls work differently. 

So you have an interconnected network of mesa and macro pores. Surely that makes it susceptible to water ingress? Nope. The air in the pores works as one whole 'gas mass' which gets compressed when rainwater hits it and prevents water ingress at depth. Tres clever, as they say in French France. But alas this is not all that happens, why would it be, its lime so it's almost needlessly confusing. Vapour enters the wall and condenses each night, then gets released when the atmospheric humidity drops. If the wall does get a lot of water in it, then water also drains downwards into the ground. 

At this point you're probably thinking of selling up and moving into a Redrow... Well all is not lost because of the nature of historic masonry. 10,000 years of observation has, not unsurprisingly, yielded some excellent data. People have lived in the same places, using the same materials in an almost unbroken chain, in enough instances for us to know which materials are good. The issue is laziness and downright stupidity. Eg. there is a prevalence for putty mortars amongst the old guard due to the Well's Report. The front page of which says; this is probably wrong. WTAF? People have been running around with spray bottles and hessian  trying to use a material which should only be used in thin coats because it cures too quickly for a thicker coat to be cured fully at depth. Ie. you cannot get water to the back once the front has carbonated, for the above reasons, which is very rapid due to maturing the lime and how it's slaked. Again, WTAF is wrong with these people? 

So...hot mix properly with local aggregates which have precedent, to historic proportions and you will nearly always have success. Or you know, do a deep dive into physics and chemistry, learn the subjects involved comprehensively, then test your mortars extensively in a wide number of environments and applications for decades. If not centuries or millennia.   

DO NOT use materials from other areas, eg. chalk is fine in East England but would turn into a soggy mess in West Scotland. Red sand plaster is fine with brick houses in the Midlands but could be too strong for a timber frame in the South. Cob works in Cornwall but could suffer in Wales. Etc, etc.

One day I'll have someone edit this site so it actually makes sense, that day has not come...

Understanding the different pore sizes in lime mortars is crucial because they significantly influence the mortar's properties, especially its strength, permeability, and durability. Let's break down micro-, meso-, and macroporosity in lime mortars and how they relate to the overall porosity percentage.

Categorizing Pore Sizes (According to IUPAC):

The International Union of Pure and Applied Chemistry (IUPAC) provides a standard classification for pore sizes:  

• Micropores: Width less than 2 nm (<2×10−9 m)  

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• Mesopores: Width between 2 nm and 50 nm (2×10−9 m to 50×10−9 m)  

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• Macropores: Width greater than 50 nm (>50×10−9 m)  

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Porosity in Lime Mortars and Pore Size Distribution:

Lime mortars are inherently porous materials. Their porosity develops during the carbonation process, where calcium hydroxide (the main component of lime putty) reacts with carbon dioxide from the air to form calcium carbonate (the binder) and water. The way this carbonation occurs and the initial mix design (lime to aggregate ratio, water content) significantly influence the total porosity and the distribution of these pore sizes.  

Micro-, Meso-, and Macroporosity in Lime Mortars:

• Microporosity:
  
  

  • Formation: These very fine pores primarily arise within the newly formed calcium carbonate matrix itself, as well as in the interfaces between the calcium carbonate crystals. They can also be associated with the very fine particles of the aggregate.

  • Impact on Properties:

    • High Surface Area: Micropores contribute significantly to the overall surface area of the mortar. This high surface area can influence water retention and the potential for chemical reactions within the mortar.

    • Limited Permeability: Due to their small size, micropores restrict the movement of water and air through the mortar. A high proportion of micropores can lead to lower permeability.

    • Strength Contribution: While not the primary factor, a well-formed, dense matrix with some microporosity can contribute to the cohesive strength of the mortar.

    • Water Retention: The capillary forces within micropores are strong, leading to good water retention, which is crucial for proper carbonation.

• Mesoporosity:
  
  

  • Formation: Mesopores develop at the interfaces between the calcium carbonate matrix and the aggregate particles. They can also be formed by the arrangement of finer aggregate particles and the spaces left after the evaporation of some mixing water.

  • Impact on Properties:

    • Capillary Action: Mesopores are the most significant contributors to capillary action in the mortar. They allow for the transport of water into and out of the material.

    • Breathability: Mesopores allow for the movement of water vapor, contributing to the "breathability" of lime mortars, which is essential for managing moisture in historic buildings.

    • Strength: A good distribution of mesopores can contribute to the overall strength and flexibility of the mortar.

    • Salt Transport: Mesopores are also the pathways through which dissolved salts can migrate, potentially leading to salt damage.

• Macroporosity:
  
  

  • Formation: Macropores are the larger voids within the mortar structure. They are primarily created by:

    • The packing of the aggregate particles: The size, shape, and grading of the aggregate significantly influence the size and distribution of macropores.  

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    • Entrapped air during mixing: Poor mixing techniques can lead to larger air voids.

    • Evaporation of excess mixing water: Larger water-filled spaces become macropores upon drying.

    • Cracking: Micro-cracking can propagate and become larger macropores.

  • Impact on Properties:

    • High Permeability: Macropores allow for relatively free movement of water and air, leading to higher permeability.

    • Reduced Strength: A high proportion of large macropores generally reduces the overall strength of the mortar as they represent areas of weakness in the binding matrix.

    • Frost Susceptibility: Large, interconnected macropores can become saturated with water, making the mortar more susceptible to frost damage.

    • Limited Water Retention: Capillary forces in macropores are weak, so they contribute less to water retention.

Relationship to Total Porosity Percentage:

The total porosity percentage of a lime mortar is the sum of the volumes of all the pores (micro, meso, and macro) divided by the total volume of the mortar. While the total porosity gives an overall indication of how much void space exists, it's the distribution of these pore sizes that is most critical for understanding the mortar's behavior.

• High Total Porosity with Predominantly Micropores: This would likely result in a mortar with good water retention but low permeability and potentially higher susceptibility to salt crystallization within the fine pore network.

• High Total Porosity with Predominantly Macropores: This would lead to a very permeable but weak mortar with poor water retention and increased risk of frost damage.

• A Well-Distributed Porosity (with a significant proportion of mesopores): This is generally considered ideal for lime mortars in historic applications. It allows for breathability (moisture movement), good capillary action, and a reasonable balance between strength and flexibility.

In essence:

• The total porosity percentage tells you how much void space is present.

• The micro-, meso-, and macropore distribution tells you the size and type of that void space, which dictates how the mortar will behave in terms of water transport, strength development, and durability.

Therefore, when analyzing lime mortars, it's not enough to just know the total porosity. Understanding the relative proportions of micro-, meso-, and macropores provides a much more comprehensive picture of the material's characteristics and its suitability for specific applications. Techniques like mercury intrusion porosimetry (MIP) and nitrogen adsorption can be used to determine the pore size distribution in porous materials like lime mortars.