I am attempting to come up with a solution to the ongoing deterioration of Cheshire Red Sandstone due to environmental conditons and incorrect materials. Or more precisely; I am attempting to come up with a mortar which is suitable for use with new and existing Red Sandstone masonry in the North West. It it absolutely needs drying out but is susceptible to damage by lime. Catch 22...  

Red sandstone is most often bound with clay or Ferris Oxide Hematite. Clay is broken down by gypsum (calcium sulfate) and Iron Oxide is broken down by strong alkalines. Have fun specifying that... 

The issue being that lime leaching is atrocious to the stone, often resulting in rapid deterioration of the stone after installation. There does not seem to be a suitable binder or mortar which is idiotproof, as one would normally find with building systems eg. chalk/pure lime for timber frame or a proper Mason's mortar with limestone building stones. Again, as mentioned in other sections, is this more a case of so much being done wrong that one thing within the discipline of the person studying it, seems worse than it is. EG. is the stone not selected or seasoned correctly, not docked, not laid correctly, not bedded correctly, wrong binder form, incorrect aggregate compostion, incorrect water gauging, not either well tempered or used hot, poor curing.  

Having observed hundreds if not thousands of Red Sandstone mortar samples in situ, predominately in Chester but also the surrounding area, I have come to the conclusion that a lime lean mason's mortar or one which utilises local red sand is likely the least detrimental with regards to damage caused by lime. What role does pollution play as I am currently sitting at a coffee shop table, in a Medieval vaulted chamber constructed from a combination of brick and sandstone, which has clearly been subject to damp, yet the sandstone is in excellent condition. It has what is arguably the standard mortar of the time period, off pink, with lime inclusions, high concentration of fines. Does the natural inclusion of clay in the aggregates act as a barrier to the lime and/or pollution? Are there still that many reactive components left that the lime gets bound by them as it travels through the mortar?

Is it even possible for the same mineral forming reactions to take place at a later date? Obviously carbonation occurs, as does Gypsum formation but are other minerals formed, hydraulic? Seems likely if hydrated limes can be utilised as binders, then surely the extant unbound components in the mortar are no different? 

Hot mixed in most examples, which seems contrary as those inclusions should increase the chances of lime leaching and therefore further deterioration of the stone. But it seems rough and unscreened which again seems contrary to the quality of the masonry work. I believe that in mixing and using a mortar immediately, one is drastically reducing the potential for it to become compromised. EG. it must cure to a certain degree in the manner in which feebly hydraulic limes do, but then can be rejuvenated. Does this weaken the bond and increase the chances of unhelpful moisture transfer within the matrix of the mortar? Due to damage to already cured material, you're breaking it apart and recompacting it, not normally an issue as the autogenous properties of lime will repair that damage almost immediately. Does colloidal shrinkage play any part in this? 

Hot mixed mortars are less alkaline than cold mortars...who knew? This is just enough to not break down the ferrous oxide hematite that naturally binds the stone. 

I have to wonder whether hot mixing, inducing greater porosity restricts the movement of moisture throughout its matrix. And therefore lime leaching., Is the goal to have enough lime inclusions to repair a smaller area with less capacity to transfer moisture. This is after all the goal of OPC mortars that are to be resistant to salt and frost through a increased number of larger pores which can handle the expansion of both salt and water. This same effect is witnessed on a lesser scale with aerial lime mortars, especially hot mixed and traditionally aerated mortar. 

It would seem so, yes. Clare Torney's article on exposed mortars indicates that the pore structure in hot mixes is such that it prevents deep penetration of water by creating a break in the capillary flow with larger pores, albeit not so large as to cause other problems. Hence the complicated nature of lime mortars... And the necessity of removing salts and replacing mortar regularly to prevent clogging of these beneificial big boys. Although clogging is a little misleading as it's more a case of micro coating the internal surface of each pore and salt isn't required, it can be purely calite and still prevent movement. It's all so simple, I'm surprised any stone is damaged...lol. 

Is this the reason for the time period of workability of NHL's, is it really just a recognition of the low levels of free lime present in the stronger NHL's being incapable of repairing the aforementioned damage. Yes they stiffen and cure hydraulically but, in theory should that matter? Are you just binding small lumps of cured material to one another through carbonation/hydration. Yes, not entirely sure what my point was here...got it. The demonisation of NHL is more a case of poor mixing, surprise surprise. It is very likely to me that it would be far superior to use fresh hot mixed hydraulic limes and that steam slaking has further confused the issue. I am in no way advocating the use of lean NHL mortars over the top of lime rich aerial lime mortars but there is a place for them, when used correctly. And there is a strong argument for their specification with red sandstone due to the poor mixing skills of most 'lime experts'. 

To summarise; Red sandstone is a dick about loose liquids and unbound minerals.

But does the density of the mortar then become a consideration? As a means of preventing damage caused by pollution by the shear physical action of not letting it in? Not 100% convinced of that in regards to historical mortar but definitely the reason for modern installations failing. People are using washed sand with no clay, no fines, little to no reactive material and then making it cold and curing it badly with bad working practices during mortar preparation, tempering and installation. Is it really a surprise that it doesn't work? 

I think a good solution could also come in the form of organic additives, these can have chelates in them which will bond with the heavy metals from pollution. So if the majority of the mineralisation is from sulfates, this could be mitigated. This leaves the leaching minerals causing damage to the stones binder, which can definitely be controlled with better mixing practices. Red sand, crushed sandstone or local subsoil, lean, clay and chelates mixed hot and stiff. Induce rapid carbonation with mixing practices. If it damages rust; it will likely damage red sandstone.

Bad working practices or lost craft skills..? Depends what mood I'm in and who I'm talking to...haha. In all seriousness its both a graduate problem in not acknowledging the differences between craftspeople and the workers themselves for assuming that the skillset they possess is suitable or the best it could be. How do we get them to actually listen? Both camps. The graduates who give a shit already know and the ones that don't, don't. give a shit anyway And lets be honest, they're dicks too. The Masons are generally gobshites who think very highly of themselves already. Plastic shims and sloppy shite...skillz As with most problems in the world, those that need to listen are the least likely to. 

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. 

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

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. 

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 CWR are generally very poor at masonry affixing. 

Or it could be something else entirely...alien lizards messing with us probably. 

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.