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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 '20 at 17:53
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The article referenced in the comment above (Ref 1) is not relevant to concrete. It analyzes solutions with a water to solids (cement plus $NaOH$) ratio of 1, whereas ordinary concrete seldom goes above 0.5. This is a paste, not a structural material. The $NaOH$ speeded up early hydration at 1 M, and retarded it at 2 M. It was apparently affecting the aluminates more than the silicates.

The composition of typical portland cements varies, but is usually considered to be mostly $C_3S$, a faster reactor and $C_2S$, a slower, but eventually stronger component. Ref 2

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Many admixtures are used in portland cement concrete (Ref 3), but sodium hydroxide is not one of them. $NaOH$ is used as a promoter with silica-rich additives like flyash in highly modified cements, but portland cement is already silica-poor. When portland cement hydrates, $Ca(OH)_2$ is formed, which is not a really good strengthener, except that over long periods of time, it will become incorporated into the silicate gel, strengthening the concrete somewhat.

However, there are adverse reactions noted with silica-rich aggregates and the high alkalinity of sodium hydroxide (Ref 4), so it is always advisable to keep the sodium level low, whether from an alkali, an admixture, or external applications, like salt.

Ref 1: https://www.sciencedirect.com/science/article/abs/pii/000888468990046X

Ref 2: https://en.wikipedia.org/wiki/Portland_cement

Ref 3: https://www.cement.org/cement-concrete-applications/concrete-materials/chemical-admixtures

Ref 4: https://www.concreteconstruction.net/how-to/repair/chemical-attack-on-hardened-concrete_o

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