Dear,

I warmly suggest to have a check at the Geopolymer Institute site (www.geopolymer.org) because geopolymerization and alkali activation are completely different processes. In particular on the site the paper 25 is completely dedicated on technical explanation about “Why AAM aren’t Geopolymers “. All the technical terminology is different and so you able to understand because the answer at your question can be extremely different considering each case.

To have a rough idea, for AAM activator is a strong alkaline solution (very high pH, almost or already 14, so classified corrosive. DON’T TOUCH WITH FINGERS AND USE ASPIRATION TO AVOID RISKS ABOUT BREATHING) that works as the main actor for the hardening dissolving the raw materials used as precursor (when the material dries, after air curing or much better heat curing, it develops its proper strength). This reaction is not balanced, to be simple more alkalinity = more strength BUT that reaction is still reversible so in presence of humidity or water immersion, the material leaches out alkalis to water till failing (expanding or shrinking till be destroyed).

For Geopolymers instead doesn’t exist any activator, because what is set to be already the main super reactive character is the precursor in amorphous Aluminosilicate powder, for this reason the acidic or alkaline solution is called GP reagent or hardener. During mixing to prepare the Geopolymer binder (precursor plus reagent ) there isn’t any dissolution but only depolymerization to obtain the bricks (tetrahedron) that will polymerize together with the aggregates creating the final hardened material. These reagents are user friendly, so you can touch by fingers being only irritant never corrosive (however take care for your eyes like normally done when using Portland and lime based materials ). The hardened material hasn’t leaching in water so there’s no problem to let cure even under water immersion after demolding. The stability depends on the correct mix design but you can note that technologies are different. Geopolymers are mineral polymers anhydrous while AAM are still hydraulic materials but unstable under humid conditions and at worst under water immersion.

Wrong scientific papers are creating big misunderstandings about these materials just because both can have alkaline pH, even though very different (pH has logarithmic scale) and both may use similar raw materials as precursor (but for GP raw materials are much more selected and balanced with the reagent to avoid side effects like efflorescence and leaching, that instead are usually the weakest points of AAM, because alkalis are every time too much and too concentrated). GPs have the alkalis inside their matrix that become part of the newborn minerals obtained with the hardening. AAM have the alkalis outside the structure, keeping all together. So can be easily leached out in humid conditions.

AAM for sure seem more close to Portland products (mix design similar, similar hydrated reaction and so on). GP have different mix design and you must forget all rules learned from Portland ,because even though applications can be similar (GP are more hi tech materials: fireproof and chemically more stable) water is just a plasticizer not the key point for the hardening.

I’m sorry for the prolonged speech it’s difficult for me to explain at best in English.

Check at the site and enjoy.

Best regards

Dr. Alex Reggiani

Alex Reggiani thank you for your answer. However, my question still lingers. In the use of both materials in concrete applications, for instance preparing geopolymer concrete, is there a limit in practice for alkali content That is Na2O and K2O in the product. Recall this limits exist in traditional concrete with cement binders.

Ibunkun technology is different so limits cannot be compared. OPC is hydraulic binder based on Calcium, while GP aren’t hydraulic because are mineral polymers (based on amorphous allumino-silicates, so calcium can be inside but never as key material like for Portland and AAM that are not polymers too so can be mixed like Portland. GP reaction depends on the selection of raw materials used for the precursor that is the most reactive ingredient in the system. AAM haven’t this need because the real binder is the strong alkaline activator that dissolve the precursor so isn’t possible an organized polymerization but hydraulic precipitates mainly.

lbukun, it will be better if you first calculate the molar ratios of your mix and manufacture some concrete cubes to check the strength parameters. You can also perform the dissolution experiments on your precursors for various Na2O ratios and decide accordingly. However, as discussed in the technical papers by Prof. J. Davidovits, the pH of Geopolymers after polycondensation reduces to about 8.5 or something. Read the book titled "Geopolymers Chemistry and Applications" by Prof. J.Davidovits, and you will find answers to almost all of your queries.

Ibukun Erunkulu to evaluate the amount of Na (or K) in alkali-activated binders, normally a binder bulk Na/Al = 1 is set as a target. Why is it so? Because that corresponds to the stoichiometric ratio in the reaction product (N-A-S-H) forming, therefore any extra Na present in your system would remain in solution, available to react with CO2 to form alkaline carbonates (potentially leading to efflorescence) or with silica from aggregates to induce ASR. If you want to draw a parallel with Portland cement, when you hydrate OPC you are in excess of Ca (Ca/Si >2 in the binder) with respect to the C-S-H forming (Ca/Si