As I know the pH value of the Geopolymer concrete is about 13 on 28 days but the alkali silica reaction and expansion on the Geopolymer concrete is very less as compare to OPC. Am I right? If I am correct then how could it happen?
Thank you in advance
Hello Shree,
As you have seen in previous comment, studies about alkali activated materials (slag, fly ash or other, also opc cement could be used for that systems) have already done and Dr. Shi has given a very complete sommary and articles.
But if I have correctly understood you are asking what's the difference about alkali aggregate reaction between geopolymer concrete and Portland concrete, so things are quite different, because first of all geopolymers aren't alkali activated materials, at all!
This is a big mistake that often creates confusion...the most important reason because in geopolymer concrete alkali aggregate reaction doesn't occur is because all alkali content is inside, involved chemically in geopolymer matrix, that is a very organized reticule, so there isn't free alkali on surface of concrete that may react with silica.
Search on geopolymer institute site (geopolymerinstitute.org) the paper dedicated to this topic and please take a look at the videos that Prof. Davidovitz dedicated to clarify this big difference (these video are also in YouTube) . Remember geopolymer isn't alkali activated materials, I created geopolymer products using acids, or also with neutral pH...look also at geopolymer camp field you will find me and my russian collegues speaking on geopolymer concrete for 3d printer, if are you interested to know more.
I usually found pH value of hardened geopolymer concrete but also of geopolymer mortar, of about 8,5-9 (hardening at room temperature), so If you use correct mix design (other topic that creates misunderstandings with alkali activated materials) and if you reduce as possible also amount of extra water, you will be sure that shrinkage will be very little or nothing and your surface will be safe about alkali aggregate reaction.
Hoping to be useful.
Regards
Alex Reggiani
Hi,
In the published studies, ASR experiments are usually performed under accelerated condition, i.e., very high alkali concentration of the exposure solution. In this case, the initial alkalinity of the pore solution of mortars is highly affected by the the exposure solution. Thus, the pH of the pore solution can be higher for the PC mortar than for the geopolymer mortar.
Therefore, in the attached paper, we have compared the impact of different exposure conditions (water, steam, NaOH) on ASR expansion of PC and alkali activated slag mortars. You will see in Figure 8 that ASR expansion is higher when the hydroxide ion concentration in the extraction solution (index of pore solution) is higher.
https://www.researchgate.net/publication/271774022_Comparison_of_alkali-silica_reactions_in_alkali-activated_slag_and_Portland_cement_mortars
Simply, the question you point out reflect the limitation of the current standard for evaluating ASR in geopolymer concrete. More details about the applicability of testing methods for ASR in geopolymer can be found in the following review article.
https://www.researchgate.net/publication/270516596_A_review_on_alkali-aggregate_reactions_in_alkali-activated_mortarsconcretes_made_with_alkali-reactive_aggregates
Regards,
Zhenguo
Article Comparison of alkali–silica reactions in alkali-activated sl...
Article A review on alkali-aggregate reactions in alkali-activated m...
Hello Shree,
As you have seen in previous comment, studies about alkali activated materials (slag, fly ash or other, also opc cement could be used for that systems) have already done and Dr. Shi has given a very complete sommary and articles.
But if I have correctly understood you are asking what's the difference about alkali aggregate reaction between geopolymer concrete and Portland concrete, so things are quite different, because first of all geopolymers aren't alkali activated materials, at all!
This is a big mistake that often creates confusion...the most important reason because in geopolymer concrete alkali aggregate reaction doesn't occur is because all alkali content is inside, involved chemically in geopolymer matrix, that is a very organized reticule, so there isn't free alkali on surface of concrete that may react with silica.
Search on geopolymer institute site (geopolymerinstitute.org) the paper dedicated to this topic and please take a look at the videos that Prof. Davidovitz dedicated to clarify this big difference (these video are also in YouTube) . Remember geopolymer isn't alkali activated materials, I created geopolymer products using acids, or also with neutral pH...look also at geopolymer camp field you will find me and my russian collegues speaking on geopolymer concrete for 3d printer, if are you interested to know more.
I usually found pH value of hardened geopolymer concrete but also of geopolymer mortar, of about 8,5-9 (hardening at room temperature), so If you use correct mix design (other topic that creates misunderstandings with alkali activated materials) and if you reduce as possible also amount of extra water, you will be sure that shrinkage will be very little or nothing and your surface will be safe about alkali aggregate reaction.
Hoping to be useful.
Regards
Alex Reggiani
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