clay-sand mortar

When I built my Matt Walker stove Matt Walker build in Portugal I built the lower part of the body using refractory mortar.  The top part I built using clay-sand mortar as recommended by many folks, on the grounds that it's possible to get it apart if need be.

Now, the problem I'm seeing is that it tends to come apart by itself.  The clay-sand mortar adheres poorly to the bricks, so poorly that some of the one on the top came loose and had to be stuck back with alternative stuff to hand such as high temperature silicone or some fire cement stuff intended for sealing chimney flue joints.

Now, it might be that the sand I used was an issue.  It's sold as "yellow" sand and is what you would normally use for rendering, as distinct from building a wall.  I guess in the UK I'd call it soft sand, rather than sharp sand.  It's also possible I should've had more, or less, clay in the mix.

As I'm soon going to embark on building the bench, and I would like the top of that to be removable in case of need, can I ask what sand and what mix ratio other people are using in their clay-based mortar?   The clay I have is dry powder in a sack.

Hi Austin;
I suspect you are correct that your sand is not sharp enough.
I tried using different sands over the years, and I had cracking or popping issues every time.
For a straight mortar, I use one scoop of dry clay and three scoops of medium-graded sand, pure white and completely uniform.

I have had top bricks that got bumped, and they pop loose. I scrape off the dry clay to rehydrate, mix some new up, and reattach.

Nothing wrong with using refractory if it comes free; try to clean the refractory off, or use a new  brick.

i used fireclay and builders mortar sand for my stove build in the ratio recommended by Matt ---and others --with a dash of  fire ash ---which made the mortar a bit stickier----i  did have problems with the sand being too coarse --lots of small  stone---and the fireclay had a lot of hard biggish crumbs in it ---had to sieve it for a nice smooth mortar mix---should have bought plasterers sand ---could not source a fine- er fireclay though---there s only one supplier  over here---i did find that the mortar was slow to dry and gave a weak bond---which greatly improved once the stove s bricks got really heated up ---then the mortar colour changed from a dark brown to a light beige /white grey and it set much harder ---which now takes some effort with a steel tool to remove or scratch out a joint ---as compared to when its in the dark brown state and your finger/finger nails can rub it out of the joint----i still have the lowest course of  bricks with a light brown colour mortar joint ---the stove base i can never get above 110 C even after several firings on a warm day---but thats no problem as the above layers of weight keep it all together----but even the hard set joints are easy to erode if a damp cloth or squirt of water is used on them---i have started lately to brush/wipe the stove bricks with a lightly -- raw linseed oiled  paint brush ---picks up the colour of the bricks and dulls down the mortar joint a bit ---which i prefer---might give some water proofing as well----  i dont think the fireclay in the mortar sets up from the stove s heat as this is well below its rated firing temp  -----so any fine clay does work as a component of the mortar

Austin.

The aggregate should be sharp.
The largest grain size should be two times smaller than the joint. So for 2 mm joint, the particle size should be 1 mm (mesh 18). For smaller 1 mm joints 0.5 mm aggregate should be used (mesh 35).
You can use sharp sand or ground fire bricks (sold by refractory stores) or calcined clay used as grog in ceramics. From all these three sand will be most expanding resulting in less stable mortar.

The proportions have to be determined empirically by making test balls of various ratios, flattening them, drying completely and testing the strength. Sometimes the strongest one may be very lean in clay and difficult to work with when laying bricks, because it will not want to stick  The rich mix may result in excessive shrinkage when drying. For local clay (but not from my land) that I used for my bread oven firebox I have used 1.5 (mesh 18 sand):1 (clay) ratio. For outside walls I have used 2 (plaster sand): 1. For my adobe house I have used the same clay but with 2.8 parts of coarse sand (up to 5 mm grain) to 1 part of clay. The larger aggregate has smaller total surface area of the particles when compared to finer ones, so it worked better with lower clay.

Clays with lower particle size (ball clays, fire clays or terracota) will need higher amount of aggregate than for example kaolins or some local impure clays.

You can add 10% (of aggregate amount) of Portland cement to the mix. It will not increase strength, but will make the mortar set faster and be more water resistant (usefull when building outside).

Also sodium silicate (water glass) or kitchen salt may be added to the mortar, so it works as flux (Na2O) lowering the sintering temperature of the firebox mortar.

So called homebrew can be used: 3:1:1:1 (sand:clay:lime:cement). It's strong and is popular among bread/pizza oven builders. I have used a leaner version of 6:1:1:1 to build my oven chimney.
Mixes that are too strong will crack easier and will be also more difficult to remove if the stove is being rebuilt.