Recently I stumbled upon a video from a guy called NOBOX7. Many videos from him are so-so, but this one I want to give a try. He makes a 2500 C refractory from simple ingredients: * NaOH drain cleaner * silica sand * garden lime (probably mostly CaO) and some water to make a peanut butter like substance which can be coated on items for heat protection or used as a glue. Any ideas on this ?
Waterglass is made from sodium hydroxide and silica sand heated up in a kiln and then crushed and mixed with water. It tends to flux refractory materials which melt at around 1000 degrees C or so into a greenish glass if you use the homemade refractory recipe based on silica sand, fireclay, waterglass and cement from personal experience. Most lime is calcium hydroxide (slaked lime) as calcium oxide reacts with atmospheric water over time and it melts at 580 degrees C on it's own but with waterglass fluxing it, it may melt at even lower temps. At any rate the waterglass and the lime are both going to be dried out by heat (calcined) before absorbing moisture again, it's probably not going to have much endurance even if it does withstand a few heat cycles. Using ciment fondu (calcium aluminate cement CAC) would probably be a better basis for home made refractory as it's what's used in commercial products and there's some proven recipes out there: https://www.azom.com/article.aspx?ArticleID=1411
Ca(OH)2 does not melt at 580 C, but it decomposes to CaO and water vapor. But the 'ciment fondu' ('molten cement'), how can one make this ? Better recipe than that of Nobox7 ?
You are right about the calcium hydroxide decomposing rather than melting at 580 degrees C, my understanding is that the resulting calcium oxide will reabsorb water over time, this cycling is probably not good for long lasting refractory. You can make calcium aluminate cement by reacting lime and aluminium oxide at high temperatures to get a "clinker" product that need further crushing. The brown cement is made by reacting lime with bauxite, the white is made with pure aluminium oxide and the grey is available commercially as "ciment fondu" branded product. The higher the alumina content, the higher the temperature it will withstand. I'm only able to buy the white version pre-blended with what looks like fired clay aggregate as 1650 degree C rated refractory cement.
