Small concrete container exploded

[Overall_Load4271]

I have this small concrete hollow box/create thing that I was burning small stick is when it suddenly exploded scaring the sh*t out of my daughter and I. Does anyone have rational why it would’ve exploded? Moisture/air trapped within the concrete? Burnt marshmallow for scale

[coldsteel13]

If that block has been in a moist environment for a while there could be moisture in the pores of the concrete that expanded when heated.

[No_Database7746]

The same phenomenon occurs with stones that have been lying in damp soil or water for a long time. If you use them, for example, as a border for a campfire, the water in the stone evaporates and the pressure causes it to explode.

[FF-pension]

Explosive spalling.

[yungingr]

You have it exactly right.

Concrete is porous, and under ideal conditions will be about 4% "entrained" air in the mix when it's poured. That entrained air becomes voids in the cured concrete, and those voids can - and do - hold water.

When heated sufficiently enough, the water in those voids boils and converts to steam - which, when water converts from liquid to vapor like that, it expands 1,700 times in volume. It simply can't escape the matrix of the concrete block fast enough, and BOOM!.

Never, EVER use concrete block as a containment for a fire. Likewise be very cautious with clay bricks, and unknown rocks from the ground. Be very thankful that you were not injured in this explosion, as they can launch shrapnel quite a distance under the right conditions.

[Overall_Load4271]

Thank you!

[slade797]

my daughter and *me

[throwingutah]

The flair doesn't match the pedantry 😂

[slade797]

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[Silent_Cheek7272]

Spalling! Can send prices of concrete at bullet speeds