Why and how can concrete burn someone?

[flipflop104]

Was following a cement mixer lorry the other day and noticed a wanring on the back that said the concrete could cause serious burns.

How? I'm assuming it's because it's hot but WHY is it hot? If you mix it yourself at home it's not hot??????

Thanx in advance

[Doc Spock]

Its the lime in the sand.

[nightmare]

doc spock
The lime is in the cement !

[almcd007]

A lot of people think concrete "goes off" with the air but its the chemical reaction - there's no heat as such - but the burn is a chemical one.

[sddsddean]

As said, the burns indicated by the sign on the truck (BTW its a concrete mixer, not a cement mixer) are caused by the lime. I knew a guy on site who once (and only once) knelt in wet concrete to 'float it off' and had serious burns to both knees. He was off work for over a month. However the other point is that concrete undergoes an exothermic reaction. This means it gives off heat during the first 10 hours or so of its curing. I did an experiment at Uni when doing my Civ Eng degree that involved mixing some concrete, putting it into a containment vessel (thats a milk bottle to you and me!) and inserting an electronic thermometer. About 5 hours after mixing, it was over 100C, but dropped back to normal about 5 hours after that. This is why it is hard to make thick concrete sections, because you have to allow for the thermal contraction after curing.

[Vanilla Face]

May I ask what the difference is exactly between concrete and cement?

[nightmare]

Cement is made from lime and clay plus a few other things.
Cement is mixed with sand, aggregate and water to make concrete.