I prepared geopolymer concrete specimens using metakaolin as the precursor, and the compressive strength generally reached 50 MPa at the 7-day age. However, severe strength retrogression occurred at the 28-day age, with the strength dropping to just nearly 40 MPa. I am wondering why the strength retrogressed?
Based on my preliminary analysis, I hypothesize that the observed strength retrogression may be attributed to concrete drying shrinkage. Geopolymers are known to exhibit significant drying shrinkage characteristics. Furthermore, the specimens were demolded 48 hours after casting and received only five days of subsequent sprinkler curing. During this curing period, the specimens were covered with moisture-retaining wet cotton quilts. The ambient temperature at the test site was approximately 28°C.
If the retrogression were actually caused by drying shrinkage, would it be necessary for me to continue sprinkler curing beyond 28 days? I am preparing to fabricate multiple 2-meter-long reinforced geopolymer concrete beams for long-term testing. What I need to determine is whether continuous water curing is required throughout the experimental period.
Could you please share the type of activator used and its molarity?
The significant strength regression between 7 and 28 days may be related to factors such as high alkali content, moisture loss during curing, or carbonation of the geopolymer gel. I recommend preparing additional mixtures with varying activator molarities or total alkali content. For curing, wrap all specimens tightly with plastic film to minimise moisture loss and store them in a standard curing room at about 90% relative humidity. This will help reduce drying shrinkage, carbonation, and other degradation mechanisms.
Thank you for the answer, Mahmoud Elmaasrawy ! I used sodium silicate as the activator with the modulus of 1.2~1.4, the water/solid ratio is 0.45~0.55, and the alkali/solid ratio is 0.12~0.14. I thought it would the safe alkali content, but all of the retrogressed.
You're welcome.
For effective activation, I suggest you to use a mix of sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃) with a total modulus (SiO₂/Na₂O) of 1.3 and 6% Na₂O content (by weight of solids). This balances reactivity and strength while avoiding excessive alkalinity.
Dear Liujingyuan Su ,
following your question the issue is that you probably have not followed the correct mix design for Geopolymerization.
You are using too much akalinity on the reagent (Molar Ratio is in the corrosive range, while for GP the ambient may be alkaline, but irritant or even acidic, but not for concrete applications), remember that Geopolymerization isn’t a hydraulic process, there is nothing to be activated (Metakaolin is thermal activated by calcining when transformed from Kaolin in kiln) so, if you proceed like Alkali Activation method you will have problems. You must know better your precursor Metakaolin, from mineralogy, chemistry and particle size, because you must know how to balance its reactive alumina. It’s not a question of 0,55 ratio, it isn’t water/cement ratio like OPC concrete. Mixing is also a key part, that must be learned because every GP materials are mixed at the same, following precise step by step method.
I work with GP technology industrially and trust me, shrinkage is every time almost nothing. Of course you have to cure at ambient temperature with some cares like keeping protected by accelerated evaporation the surface just to allow the Geopolymerization can occur from the bottom to the surface, avoiding creeps.
To be sure GP are not AAM put your hardened samples in water after demoulding and let rest for a month then dry by air (4-5h at least) and if the results will be comparable or slightly lower than the ones obtained after air conditioning your material is a GP 👍🏻. While if your sample fail or expand during the wet curing , means that you have realized an AAM that is not stable in water because the leaching of the over saturated salts you put inside.
Best regards
Your response is highly precise and effective, and I am very grateful for your detailed guidance, Alex Reggiani ! I have revised my mix design and mixing process based on your suggestions. Furthermore, you mentioned distinguishing GP from AAM through water curing methods—I would like to ask whether all AAMs are unstable in water?
Dear Liujingyuan Su ,
the differences between AAMs and GP are many, but because of my NDA I cannot explain too much.
Mainly:
1. the chemical reactions are different (hydraulic for AAMs while polymeric for GP);
2. the raw materials selection (I cannot go deeper I’m sorry): mineralogy, particle size and structure.
3. the mixing process and the mix design in general.
4. the pH during and after both reactions. AAMs are usually caustic, labeled as corrosive, user hostile while GPs are just irritant so user friendly.
5. the temperature of curing. Geopolymerization occurs every time at room temperature while the most of AAMs need heating (a part where is used GGBS).
6. GP are stable under water curing and after boiling test immediately after the demolding. Polymers are chains of mineral units covalent bonded by Aluminum ions. AAMs are kept together by NASH or KASH or other salt, that when dried by air shows mechanical properties but being still water soluble, will disappear when wet again (immersion) even though this process isn’t immediate.
7. GPs are balanced so after the geopolymerization all the alkalis or acid ions are inside the GP matrix so even if put in humid environments they don’t produce efflorescences or blooming. AAMs instead, for the same reason because are unstable under water curing, are subjected to develop efflorescences because the alkali salts are outside the matrix and can be carbonated by CO2 in the air. Lot of beards can grow in relative short time.
Best
Thank you once again for your comprehensive and thoughtful response, Alex Reggiani! I sincerely appreciate your kind assistance. Although our perspectives may differ on certain points, your answer has provided valuable directions for solving the problem. I will conduct further investigations specifically targeting the aspects you mentioned. I extend my sincere thanks once more for your guidance!
You are welcome Liujingyuan Su , the important for me is that you may decide what type of technology using, understanding how reactions happen and what are the correct technical terms to adopt.
Have a good work
- Badges
- Science topic
- Similar topics
- Organizational Psychology
- Training