An easy way to obtain a solution with similar properties and pH than concrete is to mix the paste you want to simulate and try to filter the water from that fresh paste.

I use the following composition when I need to simulate a pore solution without cement, in grams per litre of water :

* CaSO4, 2H2O (a.k.a. gypsum) powder : 2.10

* K2SO4 powder : 8.70

* NaOH dry pellets: 7.05 (careful with those !)

This will get you close to the real thing except for the aluminate/silicate ions whose concentration is 1000 times lower than the ones above anyway. Increase K2SO4 to 17.4 or 34.8 g/l in order to simulate better the change in alkali amount from one cement to the other.

This lies around the solubility of gypsum and Portlandite so filter thouroughly on a büchner with a 1µm filter paper/membrane.

It carbonates readily in contact with air so a fresh solution should be prepared every day unless you store it under nitrogen. Bubbling nitrogen in water before adding the salts will help reduce calcite precipitation.

All this was computed out of data from :

[1]D. Rothstein, J. J. Thomas, B. J. Christensen, and H. M. Jennings, “Solubility behavior of Ca-, S-, Al-, and Si-bearing solid phases in Portland cement pore solutions as a function of hydration time,” Cement and Concrete Research, vol. 32, no. 10, pp. 1663–1671, 2002.

[2]B. Lothenbach, F. Winnefeld, C. Alder, E. Wieland, and P. Lunk, “Effect of temperature on the pore solution, microstructure and hydration products of Portland cement pastes,” Cement and Concrete Research, vol. 37, no. 4, pp. 483–491, 2007.

Thank You very much Pascal,

just to make sure I understood it well - I put: 2.10g gypsum + 8.40 K2SO4 + 7.05 g NaOH in liter of water and then filter it on büchner and then solution is ready for testing? So even though You do not add cement to solution it truly simulates concrete?

You got it right - it is as close as possible to a concrete pore solution by having the pH and the main ions present in solution (Ca2+, Na+, K+, SO4--, OH-) usually in the order of hundreds of mmol/l. It just lacks the aluminate and silicate which are present in the order of hundreds of µmol/l (1000 times lower).

Scanning the influence of sulfate by increasing K2SO4 will help you determine the influence of such ions (crucial in my application) as well as the influence of ionic strength. From one cement plant to the other this is the main parameter that changes.

For the best results use MilliQ-grade water (high resistivity, very low ions to avoid salt combination).

Thank You very much again for your time and thorough explanation. This is very helpful, I am very new in this field.

So, You recommend to prepare one solution with 8.70 g K2SO4 and another with 17.4 or 34.8 g/l to observe influence of K2SO4 or to increase it gradually in the same (first) solution? Will plain distilled water also give good results? And for what period of time do You bubble water with nitrogen before adding the salts?

I would rather prepare different solutions with increasing K2SO4 - as I said it carbonates quickly in contact with CO2 in air so you cannot keep it for very long.

Distilled water should be fine, nitrogen bubbling depends on the N2 flow rate, total volume of solution and flask free volume. As a rule of thumb I would bubble as much N2 as the volume of solution (I.E. 1 litre of N2 per litre of solution). CO2 (or more precisely CO3-- ions) is the problem here.

You asked for same pH and same properties. This is not possible.

You have to choose. A solution with the same pH value will have a much more aggresive chemical ingress on the specimen you test than concrete.

There are different solution published. One of the weakest is saturated CaOH around ph12. In ACI 440 a solution with pH 12.7 is published. We have made tests with pH 13 (Rehm) and pH 13,7 solutions.

Real concrete pore solution won under very high pressure shows a value between 13 and 14 even after years.

Thank You,

what do You suggest, to choose the same properties? Would solution made of 2.10g gypsum + 8.40g K2SO4 + 7.05 g NaOH in liter of water be good for that? Will it's pH be too aggresive?

As I sad I am new in this so every suggestion and help is very welcome!

And yes, can You recommend some online literature where I can read general information related to this issue?

You can research Sanja to ASTM or ACI regarding information and detail about this issue(CONCRETE).

An easy way to obtain a solution with similar properties and pH than concrete is to mix the paste you want to simulate and try to filter the water from that fresh paste.

Thank You!

Sanja, although I like Mr Boustingorry's solution as being repeatable, I often make up a batch of paste in a 20L pail with a water cement ratio of 1 and let the suspension decant (1-2 hrs, 5 hrs max).

Normally the supernatant is clear (if not you can always filter after). This gives you a few litres of true pore solution rather quickly and reasonably reproducible so long as you use the same batch of cement. Change cement and you have a new solution.

Depending on the nature of your work, this is often the best approach, although you should analyse the solution if concentrations are critical.

Distilled water is sufficient for this type of solution. Both distilled and deionised water are very aggressive and will leach out a maximum of ions.

Bubbling of N2 will help you but does not appear to be necessary to your application just try not to breath on your solution and keep it closed.

If you wish to change the ionic strength, you can always add salts; just be aware of the various equilibra: adding K2SO4 will increase K+, increase SO4-- somewhat, decrease Ca++, and therefore increase OH-. Very little can be changed without changing everything else in the solution­.