UK oven build - help with fire cement

[crisp]

Yes apologies to Sam,

Interesting about sticky rice, I have heard about this before but know little about it.

Some tennis courts are made with calcined clay and can be bought cheaply. In India, "shukri" which is burned clay and then crushed, is still produced traditionally as a pozzolanic additive. Brick dust are available in some countries and is a cheap alternative. There have been some fairly recent studies such as the “Smeaton Project” which have suggested that brick dust with a range of particle sizes between 38 and 600 microns more successful, and that the smaller particles, probably those less than 75 microns, tended to be pozzolanic. A brick dust can also aid carbonation of a lime mortar due to its porous structure.
The cement industry is amongst the worlds largest pollutants, the energy used to produce a pozzolan is far less. Recent research within the cement industry have suggested that by replacing a percentage of cement in the manufacture of concrete with a pozzolan will make a worldwide difference to emissions. With the millions of tons of concrete produced daily worldwide, this is an environmentally friendlier option. Pozzolans are added to cement today for this reason.

Of course if cement is available then use it. However, a lime, sand, clay mortar with an added pozzolan is a good option, and due to brick dust being a clay make for a good oven mortar.

[underpendle]

I have read with interest this discussion, am about to embark on my build and have been pontificating on which mortar to use. I was erring toward a grog/fondue mix, but now I think I will be going for a home brew. Whilst of course money is a consideration, the difference in the big scheme is negligible, if using a manufactured dry fire cement. Would the advice still be home brew?

Could I ask an additional question that doesn't seem to be answered here. Unless Ive missed it. Has anyone considered the effect of plastisiers? Most cements in the UK, if not all, are now multi chem, i.e. contain plastisier, and i would normally add liquid FEB to the water as well. this immensely aids the workability of a normal sand/cement motar. but I'm wondering about the effects in the case. Any Thoughts?


S

[crisp]

I have little experience with manufactured fire cement mortars, but they are standardised, which means we know what they do. I should imagine that they are expensive in most countries, they certainly are in Norway. On the other hand there is nothing wrong with mixing your own mortar. By mixing your own mortar you can control the workability of the mix to how you like it, I'm not sure but perhaps some of these pre-mixed fire cement mortars have poor workability? I don't know.

Plasticisers work as air entrainment and improve the workability of the mortar. As an air entrainment they change the microstructure of the mortar; when hardened the mortar has micro air voids which can be a good thing with thermal expansion. However, I'm not sure how these chemical plasticisers tolerate heat, perhaps the mortar becomes friable or crumbles when subjected to repeated heat? Too much plasticiser added to a masonry mortar weakens it and reduce its durability. Maybe someone here has some experience with using these?
Adding a lime "builders lime" enhances workability because the lime withholds the water content longer and allows the work to dry at a slower rate, lime also tolerates heat well.

[david s]

I have read with interest this discussion, am about to embark on my build and have been pontificating on which mortar to use. I was erring toward a grog/fondue mix, but now I think I will be going for a home brew. Whilst of course money is a consideration, the difference in the big scheme is negligible, if using a manufactured dry fire cement. Would the advice still be home brew?

Could I ask an additional question that doesn't seem to be answered here. Unless Ive missed it. Has anyone considered the effect of plastisiers? Most cements in the UK, if not all, are now multi chem, i.e. contain plastisier, and i would normally add liquid FEB to the water as well. this immensely aids the workability of a normal sand/cement motar. but I'm wondering about the effects in the case. Any Thoughts?


S

Most proprietary refractory mortars are designed for temperatures more than double that of which you'd be firing to in a WFO, so the added expense of high temperature aggregates and calcium aluminate cement is a waste when lime, clay and sand are perfectly adequate for the temperature range to which we fire. In addition calcium aluminate cement hardens very quickly making working time really short which means you have to be continually mixing small batches using chilled water if it's a hot day. Whereas a lime mortar or lime / Portland cement mortar has an extended working time. Strangely enough while lime increases working time of a Portland cement mortar, it has the opposite effect with calcium aluminate cements.

The lime in the mortar acts as a plasticiser so you wouldn't need to add any to the home brew although with a sand and Portland mortar it is common to do so.
I use super plasticiser in other applications to reduce the water volume of a concrete which then increases strength and reduces shrinkage, but you need to be careful to add the correct amount as too much creates separation. If used with calcium aluminate concretes its effect is far stronger and around only half the normal addition is required.

[underpendle]

Thanks for the info. So the advice would be home brew with no Plastiser, and to clarify you would use hydraulic rather than hydrated lime, both readily available here. I assume a fine kiln dried sand for constancy would be the norm?


Cheers S

[crisp]

If you use a hydraulic lime, clay and sand, you don't necessarily need to add cement. A hydraulic lime hardens both by carbonation (intake of Co˛) and a chemical reaction with water like a cement, but slower. Moderns cements have such a rapid chemical hardening that this process nearly cancels out carbonation. A lime mortar should always be allowed to dry slowly to gain strength though carbonation, whether it is hydraulic or non-hydraulic. I should imagine that most oven builders here would want to build slowly.
The choice is yours. I have used a hydraulic lime, clay and sand with my oven. Depending on the aggregate you use this will be a very workable mix. I'm not anti cement, but why use it when you already have a good hydraulic flexible binder in the mix? One thing to mention is that lime mortars need to be thoroughly mixed, that means a minimum of 20 min ca, it will not be fatty enough if it is not mixed correctly. Another positive thing with using lime is that you can "knock up your previous days mortar which means less waste. A cement mortar has such a rapid hardening that this is not possible, mix up too much then you have to throw it away.

[underpendle]

Cheers Chris,

Sold, I've never built with Lime but I'll give it a go. I've read your comments regarding sand particle size, will try your suggestions. Would I be correct in assuming that keeping in sealed container will help retard setting?

S

[crisp]

If using a NHL hydraulic lime, you can "knock it up" the day after depending on the strength hydraulic lime you are using. Knocking up the day after actually improves the workability of the mortar. The strongest NHL is a 5, so I wouldn't advise using this the day after because the chemical set is faster, similar to a cement. The other weaker grades of NHL 2 and 3.5 can be knocked up the day after because they have a slower chemical reaction. If you keep your mortar from air (intake of Co˛) in a sealed container it will not carbonate, but it will start to chemically harden. However, I wouldn't knock up a hydraulic lime mix more than one day after. I would advise using either an NHL 2 or 3.5, the 3.5 sets quicker than the 2 but either are good. The NHL 2 gives a fatter mix but slower hardening.

[underpendle]

Thanks for the info. checked with our local merchant and they stock 3.5, so decision made. Ordering Materials next week so If all arrive in time will hope to start playing in the next week or so. Will post some pics as things develop.

[Alomran]

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?

Allow me to invite you to an interesting link on https://community.fornobravo.com/for...e-bricks/page2
By peterbennett