UK oven build - help with fire cement

[crisp]

Hi I posted this in a another thread earlier,

If you can get your hands on hydraulic lime http://www.limes.us/distributors/ , you don't necessarily need to add cement, and thus a cheaper alternative. Cement was first patented in 1824, so all ovens prior to this had no cement only clay mortars with perhaps lime added as a binder to assist hardening. A lime which is not hydraulic (common builders lime) will harden only by intake of coČ (air) which can take some time to harden, whereas a hydraulic lime hardens both with coČ intake and a chemical reaction with the water in the mix. A lime / clay / sand mix is more flexible than one with added cement (which becomes hard and brittle), and, a lime based mortar allows for thermal expansion and movement, which is arguably a better mix. Depending on the type of clay used, once the oven is in use and fired, the clay in the mortar will turn to ceramic and harden. An addition of either cement or hydraulic lime helps to harden the mortar whilst building. It is also feasible to use just a clay/sand mix without cement or lime but this mortar will only harden with firing. Using a mortar which doesn't harden to some degree overnight, can present difficulties under building because it remains wet, but not impossible.

There are three types of hydraulic lime available NHL2; NHL3.5 and NHL 5. The NHL5 is the strongest compressively and the NHL2 the weakest. I personally would be happy to use the weakest which is NHL2, but you can also use a NHL3.5 which sets quicker; the NHL5 i think is unnecessarily strong. The hydraulic lime most available in the UK is made in France by St Astier, you can read about it, and where to buy it here http://www.stastier.co.uk
What grade of sand you use really depends on the size of your joints. The largest grain in your sand should generally be no larger than one third of your intended joint size, so if you are intending on building with 3mm joints, your sand should have no larger particles than 1mm. If your joints are 10mm then you can increase the largest particle size to 3-4mm etc.. Sand is very important in any mortar and the correct graded sand combats shrinkage. When I built my dome I tapered all my bed joints which meant these were full joints of only 2-3mm. However, I did not taper all perpendicular joints which meant although the joints were tight on the inside of the dome they were large to the exterior. I used two mortar mixes for this, one with finer sand and the other with coarse sand. Both were the same ratio but differently graded sand. The coarser sand mix was used only on the exterior larger joints.
My mix ratio was by volume; 1 hydraulic lime, 1 clay, 3 sand

Another thought is that all solid walled masonry which is in compression is stable; it is only unstable where tensile forces are present. A drystone vaulted bridge without mortar is stable because all the masonry units are in compression, in theory, you should be able to build your dome without mortar as long as the dome is the correct shape and there are no tensile forces it will be stable. Likewise, mortar between solid masonry does not have to have tensile strength only compressive strength with is provided by the sand or aggregate. There are many who believe that it is the mortar that 'glues' everything together; mortar actually needs only spread the compressive load forces.

Chris

[david s]

Hydrated lime is that which is readily available in Australia, also called here "builders lime" Hydraulic lime, which is a different animal is hard to obtain and extremely expensive presumably because it's imported rather than manufactured locally. If I had the choice and the price was the same I'd use hydraulic lime.
http://nmnikhilmishra.blogspot.com.a...-hydrated.html

Regarding the clay content in the mortar, it will harden, but the firing will not render it permanent (sintered) because that change, for all clays, only begins at 573 C, which is hotter than you'll achieve in a WFO, except maybe at the top of the dome. Above this temperature the structure of the clay particles changes, making the clay permanent ie. will not return to mud.
What this means is that the clay component of the mortar can only be considered as a fine aggregate.

[crisp]

Hi David,

Its a shame that Hydraulic lime is so expensive in Australia, I personally wouldn't be using cement at all.

I agree with everything that you have said apart from the last bit where you say "What this means is that the clay component of the mortar can only be considered as a fine aggregate". Do you mean that a clay / sand mix is all aggregate? Agreed that clay can be a fine aggregate, but the clay was normally added as a binder to bind the aggregate. There are historic buildings all over the world which are built with just simple clay mortars with adequate compressive strength. The only thing about clay mortars is that they don't do well in wet conditions. There are historic high status buildings built entirely with clay mortar where lime at that time was expensive, they simply pointed or rendered a protective finish in lime after. Regarding a pizza oven you wouldn't want it to get wet anyway.

[Alomran]

Hi I posted this in a another thread earlier,

If you can get your hands on hydraulic lime http://www.limes.us/distributors/ , you don't necessarily need to add cement, and thus a cheaper alternative. Cement was first patented in 1824, so all ovens prior to this had no cement only clay mortars with perhaps lime added as a binder to assist hardening. ............
What grade of sand you use really depends on the size of your joints.
My mix ratio was by volume; 1 hydraulic lime, 1 clay, 3 sand

Another thought is that all solid walled masonry which is in compression is stable; .......... everything together; mortar actually needs only spread the compressive load forces.

Chris

Chris: I am most grateful for the immensely valuable information. I have been involved with the design of Eden Project, St Pancras station, houses, museum and large showroom...BUT I have never faced so much intricate details on such a small construction!
As for your question on the spaces between the brick will be 2-3mm. For financial reasons, I intend to use wire cut conventional clay bricks to formulate the 1.7m internal diameter(reduced from 2 m!!).
The dome's wall is one brick length ie. at 215mm thickness. I have read your informative postings on your topic and you have referred to fire clay to clad the dome brick work on the outside, which apparently you have refrained from pursuing. Do you think I should render the internal surface of the oven with fire clay?
Also, do you think I should mix lime with pumice as opposed to using cement?
Do you think it is worth cladding, internally, the first 2 internal courses of vertical (straight) brickwork prior with 1 course of fire tiles (old heating storage bricks) to increase the thermal mass as well as protecting the brick work from being damaged by direct fire??

[Alomran]

Hydrated lime is that which is readily available in Australia,...........

Regarding the clay content in the mortar, it will harden, but the firing will not render it permanent (sintered) because that change, for all clays, only begins at 573 C, which is hotter than you'll achieve in a WFO, except maybe at the top of the dome. Above this temperature the structure of the clay particles changes, making the clay permanent ie. will not return to mud.
What this means is that the clay component of the mortar can only be considered as a fine aggregate.

David: Thank you for the valuable details.
Are you with the idea that the clay is a waste of money and that only sand and lime should be used?
Or do you think the cement will be useful instead of clay?
I would like to exchange thoughts with you and Chris.

[jonv]

I thought I was going large at 1.2M! 1.7M is still a huge oven. It is going to take a lot of wood and a long burn to get up to temperature with walls that thick too. I guess you have something other than pizza in mind... Big bread bakes?

I look forward to following the build. One thing I would be concerned about is using ordinary brick. I know firebricks are a little more expensive but they are the right material for the job. Perhaps you could find some reclaimed firebricks?

[david s]

David: Thank you for the valuable details.
Are you with the idea that the clay is a waste of money and that only sand and lime should be used?
Or do you think the cement will be useful instead of clay?
I would like to exchange thoughts with you and Chris.

The clay content in the mortar does a couple of things. Firstly it makes the mortar really sticky which is an advantage especially when Gravity is working against you when building a dome as it rises further. Secondly, clay is refractory ie imparts a fireproof quality.
The problem with a high clay content in the mortar is that it imparts a higher degree of shrinkage which may create some shrinkage cracks once it's dry. Avoid bentonite as the clay particles are really small and shrinkage is greater.
My advice would be to use both clay and cement as well as lime in the mortar. The 3:1:1:1 sand, lime, cement and clay is a great mix that works really well for the temperature range we fire to, and has proved successful for many builds. It's cheap too and more successful than many expensive proprietary high temperature refractory mortars.

Nothing to do with mortar, but I find a small addition of powdered clay to a lean vermicrete mix makes it way more workable (sticks together) resulting in easier placement.

[david s]

Hi David,

Its a shame that Hydraulic lime is so expensive in Australia, I personally wouldn't be using cement at all.

I agree with everything that you have said apart from the last bit where you say "What this means is that the clay component of the mortar can only be considered as a fine aggregate". Do you mean that a clay / sand mix is all aggregate? Agreed that clay can be a fine aggregate, but the clay was normally added as a binder to bind the aggregate. There are historic buildings all over the world which are built with just simple clay mortars with adequate compressive strength. The only thing about clay mortars is that they don't do well in wet conditions. There are historic high status buildings built entirely with clay mortar where lime at that time was expensive, they simply pointed or rendered a protective finish in lime after. Regarding a pizza oven you wouldn't want it to get wet anyway.

If you wet some sand and allow it to dry it goes hard although you can easily break it up. If you take some clay and add water to it it also goes hard when it dries, but much harder than the sand mix. The difference is in the size of the particles, the smaller the particles the greater the bond between them. My point in the previous post was that the dried clay will not become really hard and permanent unless it is sintered so therefore it's bond is only a mechanical one like the hard sand lump I used in my explanation, it is therefore only a fine aggregate rather than a cementious material. If you want a chemical bond you either need to use some cementious material like calcium silicate cement, lime, or calcium aluminate cement, or fire the mortar to a range where the material wil change (sinter). It is not practical to do this in a WFO with wood because the range where these changes take place (500-650 C) is particularly sensitive where sudden thermal expansion of various materials takes place. The result will be more severe cracking of the mortar. Potters always slow down the temperature increase in this range to prevent cracks in clay wares.
The use of Portland cement in the mortar produces early strength that a lime only mortar will not give you. If the Portland gives out, the lime content takes over. My feeling is that the Portland will still be fine except for those parts of the oven, top of dome and inner surfaces facing the fire, will still be ok. I'll be rebuilding my mobile oven soon, if hire demand drops off, so will be interested to inspect lots of things when I deconstruct it.

[crisp]

Interesting points David,

A clay mortar where the clay is definitely defined as the binder, can become very hard and very permanent without sintering, buildings built with clay mortars which have survived for hundreds of years are proof of this. A clay mortar with the correctly graded aggregate can be extremely durable and load bearing once it has had time to harden. A clay mortar does not harden in the same way as a cement or hydraulic lime mortar; both hydraulic lime and cement have a chemical reaction in the presence of water or solution; a clay mortar hardens slowly by only losing its water content (drying). Adding cement or lime to a clay mortar enhances an early set by adding this chemical reaction. It is of course an advantage to have an early set enabling a quicker pace of work, but my point was that although it makes work easier, and there is nothing wrong with using a cement to achieve this, it is not a necessity. All masonry structures move to some degree with thermal expansion. Cement mortars are also renown for their inflexibly hence the need for expansion joints with modern construction. Historic buildings built with lime and clay mortars did not need expansion joints due to the mortars flexibility. I do not know enough about mortars for ovens, but i would assume flexibility, heat tolerance, and load bearing qualities are important.
It would be interesting to see when you inspect your oven when you deconstruct it.

[david s]

Yeah, I'd agree with all of that. I don't dispute that clay is a binder, but it is not cementious, so unless it can be kept totally dry it can return to mud. Most mud structures are built in dry regions for that reason.We live in the tropics where consistent humidity can be over 90% even if it's not raining. Building in mud would be unsuitable here. In our wet season my oven gets wet even when there's no rain during those months. In really hot humid weather here pottery takes longer to dry than in our cooler but drier months.
Thanks, good discussion.

[crisp]

Thanks for a good discussion, hopefully our points discussed will help others understand what mortar is, and their options. Material choices effect a budget, like you said hydraulic lime in Australia is very expensive, but there are always other options. There is a guy here in Trondheim who specialises in oven building and wood fired masonry heating systems. He has been working for 30 years or more, and he told me that he only used clay and sand as his refractory mortar. I guess there is not just one option depending on traditions and materials available where you live, all can work well depending on skill levels. There are always alternatives when it is difficult to get your hands on either hydrated lime (builders lime), hydraulic lime, cement etc. We haven't even addressed the use of pozzolans either https://en.wikipedia.org/wiki/Pozzolan , which if added to a non-hydraulic lime (builders lime or putty) will react with lime solution giving the mortar hydraulic properties. For example, by simply adding a proportion of low fired brick dust to a builders lime, enables the mortar to harden under water i.e. hydraulically. Hence, no cement needed.

I visited Townsville 30 years ago, and worked in Cairns for a while, so I understand the humidity levels there. Some of my best memories in Australia.

[david s]

If you're interested in pozzolans this may be useful. Wood ash is easily obtained, but I've done quite a bit of experimenting with it as a basis for stoneware glazes. The main problem is its inconsistency depending on the wood type used to create the ash. The same would no doubt apply to its use as a pozzolans.

https://archive.org/stream/effectsof...0good_djvu.txt

[crisp]

I have done quite a bit of research with pozzolans, and yes wood ash does have a small reaction with lime solution, interesting to hear about stoneware. Most clays contain silicates and aluminates amongst other things, if these are heated to under 800 degrees c and then crushed, they react with lime solution. Volcanic ash or earth is a natural pozzolan (basically a fired clay or earth) which the romans and greeks used. Clays that are fired to a high temperature like modern bricks and crushed have little or no pozzolanic properties, because the clay becomes vitrified. However, historic bricks and tiles were fired at lower temperatures and if crushed to small particles react with lime solution. There are hundreds of different types of pozzolans, which today are used mainly in the production of cement. Adding a pozzolan, particularly a clay based one, to a non-hydraulic lime gives a hydraulic set and can give an alternative for oven mortar. You might want to try crushed or powdered shell, which is basically calcium carbonate, with your clay stoneware, it would be interesting to see if this also does something? It certainly assists hardening with lime mortar.

[david s]

Apologies to Sam, we appear to have high jacked this thread. Firing a material to 800C (called calcining) and then crushing it is a lot of energy intensive work. You may as well just use Portland cement. How fine do you have to crush it? I presume pretty fine otherwise it won't react. The beauty of ash is that it is already fine. The normal prodeure is to other dry seive it through an 80 mesh seive or soak it in a bucket and discard floating bits, then wet seive it and allow it to dry. Fly ash is added to some Portland cements to improve flow of concrete. We have "Builders cement" commonly available here which has a little in it.

[david s]

Here's another, probably easier solution, but may not stand up to heat if used as a fire mortar.
http://www.telegraph.co.uk/news/worl...icky-rice.html