web analytics
Chimney Mortar - Brick Grog, Fireclay or Both? - Forno Bravo Forum: The Wood-Fired Oven Community


No announcement yet.

Chimney Mortar - Brick Grog, Fireclay or Both?

  • Filter
  • Time
  • Show
Clear All
new posts

  • Chimney Mortar - Brick Grog, Fireclay or Both?

    Dear friends,

    Thank you for the chance to ask my question at this forum!

    I am investigating the use of fireclays and grogs in making a portland/hydrated lime based mortar for building an unlined chimney stack (inside structure, no freeze/thaw concerns).

    My desire is to have a mortar with the best possible mix of properties - compressive strength between type O and type N, low shrinkage, weather resistance, good bond strength, adhesion, workability and excellent FIRE RESISTANCE.

    I couldn't find the answer anywhere to my questions, so am trying your forum where more is known about refractories!

    BACKGROUND: All standard construction mortar mixes use a ratio of 3:1, aggregate to cement (portland & lime). For example, Type N is 6:1:1, giving a 3:1 ratio aggregate to portland and lime. Type S is 9:2:1 - same 3:1 ratio. Pizza oven guys often use a 'homebrew' of 3-5:1:1:1 (sand, fireclay, lime, portland). I'm not sure why they ofter use low sand ratios. I'm leaning toward using the traditional 3:1 ratio, as I think the additional aggregate will make a mortar with better characteristics for my application. I have read that silicone sands may be the worst to use due to expansion and inversion (perhaps one reason for less sand in the pizza 'homebrew'). For that reason, I'm looking at using black beauty (coal slag) or brick grog.


    1: Which is preferred, coal slag or brick grog?

    2: If brick grog is used as an aggregate in place of sand to improve thermal stability, should fireclay still be added to the mix? Why?

    Curious, what role does fireclay play in the formula? I am told 'heat resistance', but how so? Fireclay is much too fine to replace the aggregate and would seem to interact with or replace the cementitious portions of the mix (i.e. portland, hydrated lyme, etc.) Is this desirable? If so, in what ratio or percent?

    3: I am considering the use of an acrylic admix that greatly increases abrasion and chemical resistance in traditional lime/portland cements, as well as flexural and tensile strengths and bonding. I've used the product before (Silpro C21) and had amazing results, albeit with no regard to thermal loads. I know that 'plastic' melts, but I'm not sure in this context if that fact matters. Perhaps the latex does it's job during the curing phase and can be driven off during use?

    Any light you can shed on this subject would be greatly appreciated. Thank you.


  • #2
    The 3:1:1:1 homebrew mix has proved to be adequate for the temperature range that we fire to and has quite good strength. Portland cement does not like temps over 300 C and that is where the lime does its job as itíís good for up to around 500C. The clay in the mix plays a double role, that of improving workability of the wet mix as well as increasing the refractory quality. Itís downside is, if unfired, is that it increases shrinkage if itís proportion is too high, this is where brick grog which is fired is superior, but more expensive. I donít know about the qualities of coal slag, but probably the most important factor with the sand is to use something with a variety of grain size. The most important thing if using a mix to create a chimney is to make sure it is insulated. You can use the most refractory material able to withstand extreme temperatures, but if it is uninsulated it will surely crack. This is because the interior face is receiving a big temperature increase while the outside face is in ambient temperature and the resulting thermal expansion difference is too much for the material to withstand. A fired clay flue liner will crack if used uninsulated. If you are building a brick chimney the inside face of the brick, particularly at the bottom of the chimney should be rendered or parted with the mix to protect the bricks from spalling. Commercial brick chimneys are often lined with a mix of calcium aluminate cement (cement fondu) and sand. I believe that the homebrew is superior.
    Regarding the acrylic addition Iíím not sure. While it does a great job in concrete in normal conditions I suspect temperature to cause it to fail probably north of 300 C or so. I donít use it in areas that would see these temperatures, but it probably wouldnít hurt to throw it in anyway.
    Last edited by david s; 03-20-2018, 01:47 PM.
    Kindled with zeal and fired with passion.


    • #3
      Thanks David. Appreciate your input.