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What happens if you’re mixing concrete and you add some lye to it? Does it make it weaker, stronger or does nothing at all? I’m guessing concrete is basic so adding lye to it might expedite the reaction, causing it harden faster? Alternatively, it may react to with the water and starve the mixture?

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Cement is basically made of impure $Ca_2SiO_4$, which is made of independent $Ca^{2+}$ and $SiO_4^{4-}$ ions. When mixed with water, the following reaction happens : $$Ca_2SiO_4 + H_2O \unicode{x2192}Ca(OH)_2 + CaSiO_3$$ The useful part of this final mixture is $CaSiO_3$ which has the structure of a polymer made of a long chain $H-O-(-Si(O^-)_2-O-Si(O^-)_2 -O-)_n-H$. And of course there are $Ca^{2+}$ ions near each Si atom to compensate the charges. This makes a long chain of covalences $-Si-O-Si-O-$ and this is why the concrete is so hard and so resistant.

However to be correct, one should not write $CaSiO_3$ but rather $Ca_n(SiO_3)_n$ and the previous equation should be written : $$n Ca_2SiO_4 + nH_2O \unicode{x2192} nCa(OH)_2 + Ca_n(SiO_3)_n$$

To go back to the question of User148298, one sees that $Ca(OH)_2$ is a by-product in making cement. So it no use adding it to the original cement. It does not help.

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  • $\begingroup$ Isn’t lye Na(OH)2? Your point still stands. $\endgroup$ – user148298 Jan 28 at 17:53

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