Outdoor fireplace issue

[Anthony B]

Hi everyone!

I wonder if you can give me some advice. We had an outdoor fireplace built about 18 years ago and following a total refurb last year, the garden was re paved with Indian sandstone and the same product was used on the hearth within the fireplace. Last week we made a fire outdoors in the fireplace (we have a large cast iron basket to hold the logs etc) and once the fire was going really well, suddenly there was a loud bang and the central sandstone tile completely exploded sending shards of stone all over the place. Luckily neither of us were in the vicinity as it could certainly have proved fatal! 

What is surprising is that the same product had previously been used for the past 18 years without incident, so we are wondering if there might have been a gap beneath the stone which resulted in the air expanding rapidly causing the event.

 

Does anyone have any advice on the best product to use to replace the blown tile and indeed what kind of cement/bonding we should use? Someone suggested using granite or granite chips on sand. Obviously we are now reluctant to use the fire in case the same thing happens. Many thanks in advance for any suggestions!

[Alan Ambrose]

A pic would be useful. I wonder whether this is due to moisture ingress - sandstone is fairly porous. Water expanding into steam might do it. Maybe granite would be more impervious to moisture. Or maybe a metal (cast iron?) or even sand hearth?

[jack]

I'm glad to hear no-one was hurt!

 

I personally wouldn't want sandstone this close to a fire. I lived in Australia when I was younger. There's lots of sandstone around, and I remember being told more than once not to use sandstone around campfires due to the potential for explosion.

 

As @Alan Ambrose says, it's more likely to be moisture than air. Air won't expand aggressively enough to cause an explosion.

 

What you've probably experienced is a small version of what happens in a pressurised hot water cylinder explosion.

• Water gets into a closed space (e.g., water enters space as vapour but condenses to liquid that can no longer get out the way the vapour got in edited: having thought further about this, I think it's more likely that water gets into spaces in the sandestone via capillary action.)
• Water heats up
• Water wants to boil, but if the vapour can't get out fast enough, pressure builds up
• Higher pressure means the boiling point rises
• Pressure and boiling point continue to rise in tandem
• Eventually, the pressure causes something to break
• Superheated water (i.e., above the boiling point at atmospheric pressure) hits the atmosphere and explosively boils, causing a steam explosion due to the rapid increase in volume

[SteamyTea]

Liquid water has a specific heat capacity of 4.18 kJ/kg.K.

Water vaporisation is 2,260 kJ/kg.

At phase change, water expands 1600 times.

Phase change can be virtually instantaneous.

 

So it is easy to see how easily a drop or two of absorbed water can cause mechanical damage when heated.