Dear Hasan Altawil , the file doesn’t work, could you verify or add again the article?

Regards

Hi dear Alex Reggiani,

The best thing, you can download it from my profile.

I’ll try thanks

https://doi.org/10.1016/j.cscm.2025.e04332

Dear Dr. Alex Reggiani,

I sincerely appreciate the time you have taken to review my article and provide detailed feedback. Your insights on geopolymerization mechanisms versus alkali-activated systems are valuable, and I recognize the importance of precise terminologies when discussing the chemistry behind these materials.

Regarding your concerns, I would like to clarify a few key aspects of my study:

1. Nature of the Binding Mechanism

I acknowledge that traditional geopolymerization follows a polymeric mechanism distinct from hydraulic reactions. However, the classification of fly ash Class C-based binders remains an area of debate, as their behavior is influenced by both alkali activation and calcium-induced hydration. Some studies suggest that in systems where both geopolymeric and pozzolanic/hydraulic phases coexist, a hybrid mechanism occurs rather than a purely polymeric or hydraulic one (Provis & van Deventer, 2014).

2. Role of Calcium in the System

You raised an important point regarding the high CaO content in Class C fly ash, which contributes to hydration rather than polymerization. However, recent research has shown that in systems where Si/Al ratios are carefully controlled—such as through silica fume addition—polymeric gel phases (N-A-S-H or C-A-S-H) can form, albeit with different structural characteristics than in traditional metakaolin-based geopolymers (Temuujin et al., 2009). The presence of calcium indeed affects the reaction kinetics, but it does not necessarily preclude polymerization under optimized conditions.

3. Alkaline Activation and Strength Development

My study demonstrated that increasing the NaOH molarity enhanced mechanical properties, which aligns with findings that stronger alkaline media improve dissolution and reactivity of the aluminosilicate source. While I agree that the system exhibits pozzolanic characteristics, there is evidence that geopolymer-like binding phases can still form in such matrices, as reported by several researchers working with high-calcium fly ash-based systems (Garcia-Lodeiro et al., 2011).

4. Definition of a Geopolymer

The definition of a geopolymer is still evolving, and while the classical approach follows Davidovits’ model, alternative pathways for polymerization in calcium-rich systems have been proposed. Studies on hybrid binders incorporating both N-A-S-H and C-A-S-H gels suggest that while these may not be “pure” geopolymers, they still exhibit distinct characteristics from conventional Portland cement hydration products (Bernal et al., 2011).

I greatly appreciate your recommendation to explore Professor Davidovits’ work further, and I will also consider attending the GP Camp in July 2025, as this would be an excellent opportunity to engage with leading experts in the field. I would be interested in discussing this topic further, as there is still room for scientific discourse on defining and differentiating alkali-activated and geopolymeric materials in high-CaO systems.

Once again, thank you for your constructive feedback. I look forward to continuing this discussion and exploring new ways to advance research in this area.

Best regards,

Dr. Hasan A. H. Altawil,

PhD in Civil Engineering

Dear Hasan Altawil , sorry for the late, but I’m following several industrial testing and traveling a lot, I didn’t find the time to answer you before.

I would like to answer point after point, but please read my comments as expert of raw materials for industrial applications (not only Geopolymer technology but more in general). It’s just my point of view that comes from more than 20 years of experience, working from laboratory scaling up till industrialization of the materials designed.

I hope this exchange of opinions could be useful for your academic studies or for your professional career if you have already started your post university work.

1. I know that type of studies (industrially is infeasible) and I know Prof. Provis and his valuable work … but I’m geologist specialized on the industrial applications of minerals and the fly ash C class ha not the necessary mineralogy to promote mineral polymerization that is Geopolymerization process. Lignite coal, in fact, is too much rich of calcium carbonate so when burned the amount of CaO is simply too much and flash setting is very easy when you try to work in alkaline environment. Other problem of class C is the lack of amorphous silicon aluminate source. The last issue is that, it’s very inconstant source, because is pure waste so the mineralogy and chemistry but also the temperature of calcining (inside the power plant) can change a lot depending on the type of lignite coal used and equipment. Too much variables to use untouched for industrial materials that need to be stable or , at least, to vary within more strict range. Definitely Geopolymerization cannot be obtained stably so that raw material cannot be thought as GP precursor. About hydraulic reactions like happen for OPC and AAMs maybe could be more useful (reaction can be managed with admixtures while in GP system are not working at all), but again the lack of stable mineralogy can create big problems.

2. You are confirming my vision. If a raw material that should be used as precursor isn’t stable or it needs too many adjustments, controls and even doing all things professionally, it continues to be unstable needing too much care to handle (to use that waste you need, as company, special permission because it’s pure waste, not a certified by product or end of waste, so the cost of the fly ash C can be charged of taxes or extra treatment). I repeat my point of view is professional only. I respect the studies that you added but ,I hope you know , there are lot of other raw materials that can be used better as alternative (not necessarily that kind of waste).

Stabilizing what is naturally unstable can be interesting if ,at least, it gives you back good performance, lower cost of the final product and more durability. In my experience Fly ash Class C has not those features.

3. I’m not agree with those studies (the ones you mentioned ,at least) and the reason starts from the fact that the authors, probably, didn’t know how geopolymerization process works. To obtain a Geopolymer binder that keeps together an aggregate charge (mortars, concretes and so on) you cannot follow a process based on the dissolution, because you cannot condense an ordered material like a polymer (definition of polymer in general not only valid for Geopolymer) from a random mud. Your study confirms that increasing the pH and the NaOH concentration the mechanical properties increase, but this is a question of mechanical strength not connected to the type of reaction. It’s quite logical that considering NaOH as the true binder of your AAM, increasing the amount of NaOH and / or its molarity , you will increase the mechanical resistance 👍🏻. But dissolving the fly ash you will form a mud that is randomly composed and for sure over saturated by NaOH salts so cannot be obtained any mineral polymers (Geopolymer product) because you don‘t have repetitive units to bond together 🤷🏻‍♂️. As side effect of the increasing of NaOH amount and molarity you will have almost surely an unbalanced formula with too many Na ions free to move everywhere in the product, before drying. On the hardened state this fact will produce strong carbonation that becomes quickly efflorescence (capturing CO2 from air) generating Na2CO3 that is sodium carbonate (very soluble in water) it is a salt, if the material has been cured by air (where humidity works as a sucking pump to promote the growth of the beards). But the worse scenario, happens when you put under water the material, as told before, the sodium carbonate is very soluble and will be leached out from the material, weakening hour after hour, day after day, the mechanical strength. Usually the final effect is a collapse or a strength reduction till 90%, if compared with the mechanical strength reached by air curing after 28 days (putting the sample in water at the demoulding following the OPC standard, after 2 days of air hardening). In Geopolymer technology the most reactive element is the precursor so if you want to use fly ash F , even though is the weakest one, you don’t need to dissolve anything but you must depolymerize it, I’m sorry I cannot explain more (NDA 😅), to have the small repetitive units that will be able to condense in polymeric structure, reacting with a hardener or a reagent liquid that could be alkaline silicate (not so corrosive like the high NaOH molarity you told 🤷🏻‍♂️) or an acidic medium (suitable for specific applications).

4. I’m a technician and technical terms are important. When you told that the Geopolymer definition is still evolving…I thought I have red wrong, but it was true. Well a definition can be discussed for sure if the subject is still open, but Professor Davidovits has patented the term Geopolymer explaining why has been called like that and there is not written geo-hydrate or geo-precipitate or alkali activated geo-something. The patent deals with geological source so minerals that hardens trough a polymeric reaction so the technical name is GeoPolymer that gives the idea of that concept in one easy word. It is not an open field is patented 🤷🏻‍♂️. If you or the authors mentioned think to have found a new definition you cannot proceed in this way. You should patent if possible your material that, your words, has inside NASH , CASH (is already in the patent of Portland cement I suppose) or KASH , because H means hydrate , so that reaction product cannot be a polymer. How those materials should be named or re-defined as Geopolymers ?

I repeat my work is untouched by this kind of discussion, because I respect definitions and I know what I have to formulate when I need to create a Geopolymer material or a cementitious material or a pozzolanic lime based material for my clients.

But I respect the technologies I have to use. If the reactions are hydraulic cannot be at the same time polymeric and viceversa.

Using Geopolymer as AAM as synonyms is really wrong, technically speaking first, from the point of view of the chemical mineralogical reaction for second.

Would be really interesting for you to come at the GP Camp to learn more, from the Geopolymer Institute team.

I really trust students and Universities could do much better work about articles and other type of publications, if they could open their mind understanding reactions that from the pure chemical mechanism are quite simple. Of course the geopolymerization process must be understood to learn how to formulate materials fitting different fields of application, because it deals with chemical reactions that happen between minerals at room temperature… but this is my work 👍🏻.

I don‘t want to offend anyone so please trust this. But it is difficult to “digest“ every day the same misunderstandings the same wrong concepts , trying to transform a polymer material in a hydrate, just because seems more easy or closer to Portland technology.

With my best regards

Dr. Alex Reggiani

CTO GeoMITS