But does anyone know if it is really fireproof.
Nothing is really fireproof , only fire resistant. Boric acid is used in cellulose insulation to increase its fire resistance, but even cement and sand mortar without paper fiber begins to break down around 700 degrees F.
Fire-resistant concrete.
Publication: Concrete Construction
Date: Wednesday, October 1 2003
Q. I have a customer building a fire pit with a poured floor and walls, for burning pallets and such. How can I make the concrete more fire-resistant?
A. Since you are planning to burn pallets (we assume they are wood), the combustion temperature is below 400[degrees] F (200[degrees] C). At this temperature, only the free water in the concrete will be lost, and the strength loss is minimal. Mineralogical changes in hydrated cement occur at higher temperatures. The most important factor for you to consider is the selection of aggregate. The differential thermal movement between the cement paste and the aggregate is what can cause damage. Quartzite aggregate is the most prone to fire damage by cracking through the quartzite aggregate and bond failure between the cement paste and the aggregate. Limestone aggregate exhibits better fire resistance when exposed to low temperature fire. Lightweight aggregate also performs well. For this situation, a 6 1/2-sack lightweight concrete should provide all the heat resistance you need.
At higher temperatures (above about 800[degrees]F), high-heat-resistant concrete is generally needed. Light-weight aggregates have proven to be more heat resistant and so that is the first step--use lightweight. When temperatures get extremely high, above 1000[degrees]F, calcium aluminate cement produces a fire-resistant concrete.
http://www.allbusiness.com/manufacturing/nonmetallic-mineral-product-manufacturing/681948-1.html
Janosh
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