Mortar Mix for Fire Bricks

Hi, I’m trying to build a small masonry heater in Britain, products are hard to come by and so is the knowledge to build them, I’m thinking I will use fire bricks with a 3mm or 1/8 inch joints, is it best to use a high aluminous cement? Here we have something called Ciment Fondu, which has a high resistance to temperature, the problem do I mix neat with water or do I add sand or a normal cement and in what ratios? Information about high aluminous cement say it sets very quickly so only small patches at a time. Any information would be gladly received. Kind regards Roy

@weyval, thanks for posting and welcome to the forum! Apologies for the slow reply.

Ciment Fondu is basically pure calcium aluminate cement, as far as I know. As you point out, it is going to act a lot like more common cement which means that it will be very unforgiving to use. with thin mortar joints and porous firebrick units.

Both traditional and modern commercial refractory mortars use clay as their primary binder and appropriately fine silica sands or grog for their aggregate. Commercial mortars tend to also have sodium silicate, sometimes referred to as “water glass” in the mix to provide an air setting quality and hardness that produces similar final results to cement while providing for a lot more workability in the process while maintaining its refractory quality.

While we have mixed our own refractory mortars in the past, we have generally come around to the convenience of pre-mixed commercial options. We would still make a clay-based mortar without the sodium silicate for experimental and educational heaters because they will be a lot easier to modify or take apart.

Hope that helps!