Concrete Mixes - Strength & Insulation Value

[SableSprings]

I've attached a pdf table that shows the relative compressive strength and insulation value for several standard component mixes. Note the recommended mix for oven floors is 5:1 and although not on this table, 10:1 is the ratio mix used most often by oven builders on this forum. If not using ceramic board (below cooking floor) or ceramic batting (over the dome)...both are more expensive than using insulating cement. Many builders choosing to save money, opt for 4" of the insulating cement mixes for their ovens. Note the superior ceramic products only need about half of the thickness required by the cement mixes.

[david s]

I've attached a pdf table that shows the relative compressive strength and insulation value for several standard component mixes. Note the recommended mix for oven floors is 5:1 and although not on this table, 10:1 is the ratio mix used most often by oven builders on this forum. If not using ceramic board (below cooking floor) or ceramic batting (over the dome)...both are more expensive than using insulating cement. Many builders choosing to save money, opt for 4" of the insulating cement mixes for their ovens. Note the superior ceramic products only need about half of the thickness required by the cement mixes.

The testing of the various mixes does not give figures for the 10:1 mix. I believe the insulation value of a 10:1 mix is quite close to that of blanket, but it is quite low in strength, although strong enough to provide a firm enough substrate on which to render against. Even at 10:1 there is more mass in the cement than the aggregate used. A little powdered clay (a handful for every litre of cement added assists greatly in making this lean mix more workable, as does a mix of perlite and vermiculite rather than either alone. I use fine vermiculite and medium perlite at 50/50 which provides a good variation of grain size which also helps make the mix workable. The finer the grade of aggregate, the more water is required. Too much water washes the cement off the grains while too little leaves the mixx too dry, not so workable and prone to collapse. I use 4 litres of water for every 10 litres of aggregate. An alternative method is to keep adding water, mixing until water just begins to pool in the bottom of the barrow. Hand mixing this way is superior to using a mixer because the mix can be inspected, texture felt as you mix. This mix sticks to the sides and blades of a mixer and it is not possible to see or feel the mix during the mixing process. Even with plenty of blanket layers, a vermicrete/perlite mix will even out the lumps and bumps on the blanket surface, producing a nice form and surface for a final, thinner cement rendered layer outer shell. If you don't require this insulating layer to be particularly insulating (lots of blanket to provide that) a richer and therefore stronger mix can be used.
Because the aggregate is so hydroscopic the water elimination will take longer, particularly if a thick layer is used. I find a 37mm (1.5") layer with a week of drying in the sun and wind works well. If a thicker layer is required wait the week before applying the next layer, otherwise you run the risk of trapping water deep in the inner layer.