@ Ara: If you dissolve portlandite in deionized water, pH will remain constant (around 12.6) as long as you have solid portlandite present. If all portlandite is dissolved, further dilution will lead to a decrease of pH.

Thus the decrease of pH in cement pore solutions with increasing w/c is related to the dilution of the alkalis, as long as you have solid portlandite present.

Nevertheless, your statement above regarding change of solubility of portlandite is almost right. If you increase w/c, the alkali concentrations go down and pH is decreased. As the solubility product (or more exactly the activity product) of portlandite needs to be maintained, the calcium concentration in the pore solution increases.

Hi

You can find some useful details in the following article.

Article The pore solution of blended cements: a review

The pH of the pore solution decreases with increasing water/cement ratio as the alkalis present in the pore solution are diluted.

Just adding to the above statement from Frank; The given alkali content in "cement" being in contact with more or less water ought to result in different pH sounds reasonable. Also, mind that the pH scale is logarithmic, so a few percentage in w/b may only affect the second or third decimal of the pH value.

High cement water ratio disfavors the mechanical strength of the concrete, as it increases the pore system (greater than 0.27). Therefore, the leaching condition of calcium hydroxide increases, and is therefore likely to reduce pH.

http://www.concretescience.com/wp-content/uploads/2013/02/ph-of-Concrete.pdf

This article directly answers your question as it provides pH data for concrete mixtures prepared at several w/c. The published data indicates a slight decrease in pH as w/c increases from 0.40 to 0.55. But I also wonder, that with increasing w/c, we are also increasing the solubility of Ca(OH)2, which apparently is so limited that it does not overcome the dilution effect of the alkali content.

the ph of water should be seven a.k.a potable water or drinking water ..the mixing of concrete or cement mortar should happen in a clean place to prevent effect of other materials or chemicals to the mix.

@ Ara: If you dissolve portlandite in deionized water, pH will remain constant (around 12.6) as long as you have solid portlandite present. If all portlandite is dissolved, further dilution will lead to a decrease of pH.

Thus the decrease of pH in cement pore solutions with increasing w/c is related to the dilution of the alkalis, as long as you have solid portlandite present.

Nevertheless, your statement above regarding change of solubility of portlandite is almost right. If you increase w/c, the alkali concentrations go down and pH is decreased. As the solubility product (or more exactly the activity product) of portlandite needs to be maintained, the calcium concentration in the pore solution increases.

Due to the low solubility of portlandite (and due to the presence of sodium and potassium, which dissolve much more easily and suppress portlandite solubility even further, a phenomenon called the common-ion effect), portlandite is practically always saturated, so it wouldn't modify the value of pH. At saturation, Ca(OH)2 would establish a pH of 12.5 at normal temperature, in practice the pH is higher in wet cement or in the pore fluid as the alkali's push it up.

All in all, yes, more water would allow more portlandite to be dissolved in the pore fluid, but there still be plenty left undissolved.

Lowering of pH by dilution (i.e. using more water, higher w/c) when the alkali's are dissolved in more water hence their concentrations become lower and the pH is expected to follow, is correct, yet the effect is rather 'academic'.

Using more water affects the chemistry of the system in many ways, it is a complex scenario with transport and thermodynamic effects also counting one way, with lower concentrations (and hence lower chemical potential differences) acting the other way.