Heavy durable firebricks can be made with waterglass and playsand. It can be co2 cured initially and then heat cured to completion.co2 can be from the exhaust of your engine or from a fire extinguisher or from throwing sodium bicarbonate into vinegar or fermenting sugar into alcohol or spend a very long time breathing on it. Whatever.
Waterglass is not caustic. How could we eat eggs from a jar of the stuff if it were?
It is a high temp bonding agent of excellent strength, adhesion and durability.
It can be made cheaply using silica gel and sodium hydroxide. Sodium hydroxide IS VERY CAUSTIC. Silica gel is made from sodium silicate and can be converted back to a very pure form by reaction with sodium hydroxide. Silica gell is free in dessicant drier packs you find in all kinds of things. Or buy a cheap bag of silica gel kitty litter. Sodium hydroxide is drain cleaner or what you call lye, we just call it caustic. It must be the 99% pure with no additives type; usually a pearl consistency.
Once you have your waterglass you can mix it with playsand and mould it into your desired shape; case harden it with co2 then, slowly at first, heat fire it in stages up to full temp. Be patient, it's worth it.
You can experiment with pearlite and or vermiculite admixtures but the above is a domestic version of how commercial hot face firebricks are made.
Do a little research for the weights, measures and procedures. You will enjoy that more and get a broader range of opinions than if I just tell you. The utube videos convey more than words. People use this method to cast furnaces. It's industrial, cheap and easy to DIY.
Here is a link to start you off. It's straightforward and direct and he has a similar one for the waterglass.
