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Red Sand or Crushed stone and other considerations...
Red Sand or Crushed stone and other considerations...
Cheshire Red Sandstone, a valuable historical building material, suffers from significant deterioration due to environmental factors and inappropriate construction practices. This research aims to develop a suitable mortar for both new and existing Red Sandstone masonry in the North West of England, particularly focusing on the stone's sensitivity to drying out and lime-induced damage.
Red Sandstone's natural binders vary, including clay, iron oxide (hematite), calcite, and silica. These binders are susceptible to degradation by various agents: calcite by strong alkalis, silica by calcium, clay by gypsum, and iron oxide by strong alkalis. This issue is prevalent in many iron-rich sandstones.
Lime leaching poses a significant threat, often leading to rapid deterioration after installation. A crucial challenge lies in developing an "idiot-proof" mortar system, comparable to established practices like chalk/lime for timber frames or traditional mason's mortar for limestone. This may involve addressing fundamental issues beyond mortar composition, such as improper stone selection, seasoning, docking, laying, bedding, binder formulation, aggregate composition, water gauging, and curing techniques.
Observations of numerous Red Sandstone mortar samples in Chester suggest that lime-lean mason's mortars or those incorporating local red sand may be less detrimental.
Role of Pollution:
In some cases, despite exposure to dampness, Medieval sandstone structures with traditional lime-rich mortars exhibit excellent condition. This may be attributed to:
Clay in Aggregates: Potentially acting as a barrier to lime and pollution.
Lime Binding: The natural inclusion of clay and other reactive components within the mortar might bind with the lime as it travels.
Delayed Mineralization: While carbonation and gypsum formation are evident, other mineral reactions may occur later, potentially involving hydraulic processes.
Hot Mixing:
Hot-mixed mortars, despite seemingly increasing the risk of lime leaching, are often observed to be more stable. This may be due to:
Reduced Moisture Movement: The hot mixing process may induce a pore structure that restricts the movement of moisture within the mortar matrix, minimizing lime leaching.
Rapid Carbonation: Hot mixing may induce rapid carbonation, reducing the availability of free lime.
pH Considerations:
Hot-mixed mortars exhibit higher initial pH (13.5) compared to cold-applied mortars (12), which can contribute to the passivation of iron-containing compounds in the stone.
Upon carbonation, mortars ideally reach a pH of 10, minimizing further iron oxide degradation.
The use of lime inclusions helps maintain a higher pH compared to mortars relying solely on NHLs, which may fully utilize their free lime over time.
Working Practices:
Proper Mixing: Consistent and thorough mixing is crucial, particularly for NHLs.
Hot Application: Applying hot-mixed mortars while hot and allowing them to cool in place is essential for achieving optimal performance.
Density and Porosity: The density of the mortar plays a role in resisting pollution penetration. Dense mortars with appropriate porosity can act as a physical barrier against pollutants.
Organic Additives:
The use of organic additives containing chelating agents could mitigate the impact of heavy metal pollutants. These agents can bind with heavy metals, reducing their detrimental effects on the stone.
Damp Locations:
In damp locations, it is crucial to prevent the complete drying out of Red Sandstone. This minimizes salt expansion and subsequent spalling.
Conclusion:
The preservation of Red Sandstone requires a multi-faceted approach, addressing issues ranging from stone selection and preparation to the development of suitable mortars. This research emphasizes the importance of:
Understanding the complex interactions between stone, mortar, and environment.
Developing mortars with optimized pore structures and binder compositions.
Implementing best practices in stone selection, preparation, and installation.
Addressing the limitations of current construction practices and fostering a greater understanding of traditional building techniques.
Masonry repair mortars
Masonry repair mortars
I have never once seen what I would consider to be a successfully functioning patch repair using a mortar, be it resin based NHL or anything else.
They adhere and endure but they do not match porosity, not even close and because of this it is almost a certainty that they will drive all their salt and sulphur into the surrounding stone. It is very common for the surrounding stone to be compromised as with an unsuitable stone indent.
I would only recommend this type of repair either internally in a dry environment where the stone will undergo less stress or in very small amounts. This patch is on a long run of well dressed stone outside a prestigious building so perhaps in this scenario it makes sense?
The best matching repairs with this and other sandstones that I have observed are using Natural Cement.
But and its a massive Elephant sized but...ANY stonemason can install an indent for a very similar price and anyone who says otherwise is just a BSer. Again this comes with its own set of problems which I'll detail below.
Stone Indents
Stone Indents
The main issue with specifying stone indents with Cheshire Red sandstone is the abject lack of availability. This indent was completed in house by the Cathedral Workshop at Chester who have a limited supply of pebble bed that matches a lot of the original stone. This is a pretty poor example of masonry affixing, the mortar was far too wet and its not laid flush. Its classed as an honest repair, meaning it should be clearly discernible from the original. The finish is also wrong IMO, see below in the dressing section.
A personal observation of mine would be "for whom are we doing this?" No mason would struggle to discern an indent no matter how well someone tried and graduate professionals would be similar and they could certainly ascertain it through testing and records. So why bother: it looks awful and no-one enjoys doing it? Just a case of people repeating more competent people from a point of ignorance methinks...I wonder how many times I've done this?
Flush Pointed with worn arises
Flush Pointed with worn arises
This is a good example of how it should be done with this type of stone, Good selection of sand, lime and finish. See the Repointing page for more details.
Wet, wet, wet...
Wet, wet, wet...
This mortar was far too wet when it went in or the docking was excessive, either way it has leached all down the face of the stone. The missing sections are not a good idea, that moulding is designed to shed water NOT allow it to run down in lines which will erode, fast. This is almost always down to someone wanting it to look a certain way for the aesthetics rather than for function or a lack of skill as getting a sloppy wet mortar to hold well in those joints is challenging.
Job and clock...
Job and clock...
This cured too fast for the person to finish, which should be tooled and/or then beaten with a churn brush. It could be more flush with the face of the stone too. However this will still last a long time as is due to how well the mortar has been made. The range for success with air limes is very limited because the strength is so close to being too weak.
This mortar was very well made with the correct amount of water. It is likely a hot mix which has been remixed, my assumption is based on my experience of how different mix methods alter the carbonation rate. And in my experience this is well made hot mix which has been made with minimal water, allowed to temper and then been remixed on the day of use. This method in conjunction with a few other things will produce a mortar which can carbonate almost immediately. The only other method I'm aware of is when hot mixing using quicklime flour and traditional aggregates which also carbonates very quickly when well produced and applied promptly.
Good enough...
Good enough...
This is laziness and a lack of attention to detail. The mortar was made too wet and will dust. It also has a greater potential to lime leach. It will likely be weaker than it could be too. If the water gets into the gaps then it will increase the likelihood of leaching and wash it out. Then the washed out material will cause further damage.
Not cool.
Gypsum formation
Gypsum formation
This is a combination of lime leaching and pollution, sources of calcium and sulfates. These combine and deposit on the drying face forming an unbreathable coating of gypsum. This deteriorates the stone immediately behind the seasoned surface and often causes delamination. Once this has happened the stone will then rapidly erode. This type of stone seasons on the surface rather than at full depth meaning it has a hardened protective layer created by the evaporation of the quarry sap. Calcium sulfate also breaks down clay binders in stone, so if it's Cheshire Pebble Bed Sandstone, for example, it will likely cause damage at the point of adhesion. St Bees is iron oxide so would be damaged by the water trapped behind the gypsum.
On a personal note: modern life has seriously compromised the quality of a lot of sedimentary stones due to demand at short notice. This type of stone and others would benefit greatly from being cut wet to its final shape and THEN seasoned. However this would require specifiers to plan 2 or 3 years ahead and stick to the plans....so that's a hard no.
https://www.youtube.com/watch?v=6VubHpChRg8&list=PLmJDknXr4_-JbgqQA5-kFdFkD4DL34XM4&index=11
Mineralisation of lime mortar
Mineralisation of lime mortar
I have to verify this...which is virtually impossible as the Cathedral Fellowship Workshop is one of the most arrogant and ignorant organisations I've ever come across. I think because I'm 'uneducated' I can't get past their gatekeeping POC. Which I find intensely amusing and is the main reason for choosing their terrible craftsmanship as an example of what not to do. All the pictures of poor quality work on this page are from Chester Cathedral undertaken by the CWF.
I'm pretty sure what has happened here is a rare clear example of one of the other reasons lime mortar is often called sacrificial. I believe that it has absorbed material from the wall in a poulticing manner and has formed a new type of potential harmful mineral on the face.
My only reluctance to make this claim definitively is that it could be stone dust dashed onto the mortar joint whilst the mortar was still wet. Which is a traditional stonemasons method of finishing pointing to blend it better with the building stone. But I doubt it tbh as the standard of work on the rest of the project was terrible and the CWF are generally very poor at masonry affixing. Fine banker masons though. Although they are using reclaimed stone which is already seasoned but as it's very difficult to find the correct type of stone that is the best choice, I say reluctantly.
Or it could be something else entirely...alien lizards messing with us probably.
Specialist Dressing
Specialist Dressing
I have seen numerous examples of different methods of dressing or finishing Red sandstone. I am almost certain this has been done to mitigate the effects of wind driven erosion. It is almost always some form of punch dressing, meaning it has been struck with a punch or point chisel to remove large volumes of stone. This is can be in a variety of styles from a single large punch hole to the entire surface being carefully finished for a specific look.
Pointed too flush. Surface is friable and held together with water so when you repoint it the drying action causes it to deteriorate.
Face of the stone eroded back from the face of the pointing. This is also caused by alkaline damage if the mortar is used cold.
More erosion but in this more extreme example I'd assume that there was salt present in the stone which expanded as it dried, causing this spalling.
Freelime from the pointing running down the face of the stone. As to why: poor specificagtion, poor mixing, poor application or poor aftercare are all possible causes. The result is that the lime will likely bond with the sulphur in the atmosphere creating a gypsum crust, especially considering its inner city location.
On the formation of sandstone...
"Over time, the small quantities of iron-rich minerals in the sand break down and the iron is oxidized into hematite crystals (Fe2O3) that form as very thin paint-like coating on the quartz sand grains. The hematite crystals absorb all light colors except red which they reflect, giving the sandstones their red colour."
On the corrosion and protection of metals...
"There is something of a 'U' shaped curve according to pH where corrosion occurs. For iron and zinc, corrosion occurs either at low pH or high pH. In a moderate alkaline environment ( < pH 12.5), passivisation occurs where a thin layer forms that prevents further corrosion.
The alkalinity of lime depends on various factors including strength and temperature. At 25Cº, lime will be a maximum pH of 12.5, but that increases to pH 13 at 10Cº."
Therefore IF one can make a hot mix which is dry enough not to release too much gauging water into the stone, doesn't lime leach due to having good water retention and has no sulfates in it, then it's suitable for use with Red Sandstone. It is sensible for there to be a minimum of freelime in the final mortar due to calcium sulfate damage to clay binder and mineralisation on the stones surface from rainwater percolation. Or in anticipation of an introduction of excess water from flooding and other external sources.
In the past, prior to the current levels of sulphur in the atmosphere, lime inclusions were less of a problem. This is adapting to modern life. Or attempting to at least. I'm making this observation in response to people thinking that if we just recreate past mortars we will solve our problems but unfortunately that approach is often causing more problems.
Acid rain is also an issue but this is just being pedantic about the delivery system of damage IMO. The graduates can discuss that for another 50 years if they want. It'll give them something new to research as opposed to the ridiculously derivative and repetitive research which accounts for the bulk of what they currently do. I don't mean to be quite so derogatory about degree level education but it is frustrating that their are graduates who could easily solve all of these problems if they could work together and not make assumptions about their breadth of knowledge. Eg. a concrete engineer, conservation surveyor, a chemist, a physicist, a geologist and a mortarman could probably achieve some amazing things working together. The surveyor says this and that are causing damage, the physicist and chemist say why, the engineer says how it could be mitigated, the geologist says where the materials for the solution could be found and the mortarman makes the mortar. The aforementioned graduates could then test the mortar and adapt it in collaboration with the mortarman as required. SPAB's technical panel is predominately made up of Architects who do not have anywhere near the depth of knowledge required to provide complete and effective solutions.
https://backend.orbit.dtu.dk/ws/portalfiles/portal/239596074/Nina_Marie_Sigvardsen_PhD_thesis.pdf
Limewash doesn't like clay based materials like marl sandstone. I would strongly argue that limewash is not good for most sandstone as its too porous and doesn't bond well, often sitting on the surface. Hot would be better but again I'm back to the point that most people are terrible at working with lime, especially the 'lime specialists'. Eg. pre-wetting prior to applying the limewash will likely mean incredibly poor adhesion and in some cases damage.
https://www.mdpi.com/2075-5309/12/4/402
https://www.sciencedirect.com/science/article/pii/S0008884620311194
So to summarise;
Low amount of freelime, allow sand to carry as little lime as possible unless freelime percentages need match or surpass the existing mortar.
Iron content in aggregates, red.
No gypsum content or clay.
Correctly graded aggregates, preferably a red sand like Cheshire sand which is round/angular. Soft red building sand is no good. Plastering sand is often red but a nightmare to reliably source.
Dry as possible.
Hot mixed and used hot not cold.
No lime inclusions in mortar for severely exposed external or persistently damp applications.
No pre-wetting or docking, Vital. Wet on wet poulticing action is faster than wet on dry.
No wood ash or sulfates, no NHL or OPC gauging. No pozzolans at all.
Reduce contamination from substrate through cleaning and poulticing.
Water resistant finish through the use of different tool materials and types.
Avoid application in the rain, winter preferred or low temp and moderate humidity to be precise.
Use fermented organic additive during slaking process, predominately to improve water retention.
Mill correctly for rapid carbonation.
Can we ever solve the road salt problem?
To be continued...
Stop using it and find an alternative material.
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