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KILLER CONCRETE As I have made, in an idle moment, the mistake of actually reading the instructions on a bag of Sakrete, can any of the more biochemically inclined tell me if there's any rational reason to not want it on one's bare hands? --andrew It's basic. It'll take your fingerprints right off after an afternoon handling it. No idea what happens if you handle it day in, day out. -russ nelson I got mortar mix on my hands fairly regularly over the past couple of years, and it seems to have an increasing caustic effect causing the skin on my hands to crack and bleed. This lasts for days. Wear heavy rubber work gloves. WMMV, --Liz 1) Concrete mix contains a goodly percentage of lime. 2) Lime is Calcium Oxide, CaO. 3) When lime is mixed with water, the following occurs: CaO(s) + H2O(l) --> Ca(OH)2(s) 4) Ca(OH)2 is calcium hydroxide, which, is rather basic. How basic? (my chemistry is layered with ~14 years of rust. chem weenies, point out anything wrong with the following...) We make the simplifiying assumption that when you mix the cement, the water forms a solution which is saturated w.r.t to the Ca(OH)2. (we're not adding much water to the mix, after all, we want a "concrete" result) Ca(OH)2 has a Ksp (solubility product constant) of 5.5x10^-6 (*) Let the concentration of Ca(+2) ion be "x". Then [Ca+2] = x, [OH-] = 2x
Now, pH = -log10 [H30], but we have [OH-] here, so we need to figure out [H30] using the ion-product constant of water:
5) So, your hands are playing with pretty caustic stuff. My hands dont seem to have any problem working with concrete mix, but Liz's get all dry and cracked if she doesnt use rubber gloves when working with it. YMMV. -phil |
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