Dear Mohamed,

if you mean Portland cement , as substitute for fly ash , you will not obtain geopolymer binder. Portland cement isn’t amorphous and it doesn’t have the necessary mineralogy to work in geopolymer technology.

For sure you will increase mechanical strength but your system is only alkali activated. You don’t have mineral polymers and network forming but only precipitation of Calcium that will produce CSH (calcium silicate hydrate ) that is well known cementitious hydrate mineral. In geopolymer you haven’t hydrate minerals , at all. If you think that NASH (sodium alumina silicates hydrates) or KASH (potassium alumina silicates hydrates) are minerals of geopolymer technology, you completely wrong . Alkali activation isn’t a sinonim of geopolymerization . Raw materials used can be similar but silicates used are called “reagents or hardeners”, in geopolymer terminology, because these liquids aren’t activators of nothing while powders are very reactive , because amorphous and with good mineralogy .

Fly ash is too general , for obtaining good geopolymer mortar or concrete is mandatory to select only class F (low calcium fly ash , preferably CaO less than 4% ) and you must know calcining temperature of that fly ash. Calcination is very important because fly ash is waste so “the only” quality indicator is temperature during production process.

If you want to produce concrete of course fly ash is a cheap source but if you don’t do quality checks you will not have repeatable results. Coal, sulphur and other pollutions interfere with geopolymerization like the choice of too high alkali silicate (user hostile and corrosive liquid) or too low alkaline silicates (not enough reactive) like those used in cementitious products as accelerants .

You will note the instability, using Portland cement when you will do durability tests (freeze / thaw cycles, dry / wet cycles , acids and sulphate attacks) or Curing in water immersion . After treatments mechanical strength compared to air curing will be very different (even 40-50% od loss).

If you cure using heating strength will be higher but maintaining instability.

Using Portland I confirm that you are not more doing geopolymer concrete . You can use slag but take care about flash set . Only testing can show you the best amounts of raw materials.

Best regards

Alex Reggiani

thanks alot dr. Alex Reggiani

You are welcome

Of course you will achieve an enhanced compressive strength because you introduce more calcium ions in the binder . However, the geopolymerisation process will be partial, you will end up with both hydration and partial geopolymerisation. But by using high calcium fly ash you will achieve high compressive strength than low calcium fly ash particularly at ambient curing condition.

Dear Sani,

Using class C fly ash is not possible to obtain geopolymer matrix but only alkali activation , with problems about workability, because of precipitation of calcium ions . Class C fly ash isn’t regular about mineralogy and chemistry composition, is a waste, so if you need stable results is better to use class F fly ash then add some Slag GGBS (that is already accepted and classified as by product , bringing in the system amorphous calcium minerals ) to accelerate setting time at ambient temperature, maintaining mineral polymer net .

The product that you obtain using class C is rich in hydrate minerals so it isn’t a geopolymer, at all.

Geopolymerization isn’t synonym of alkali activation .

Please follow teaching of Prof Davidovits, you can verify my words on his site in technical papers and videos at geopolymer.org .

Best regards

Alex Reggiani

Dear Mohamed

The compressive strength of the fly ash-based geopopolymer mixture would depend on several factors including, but not limited, to the chemical composition of fly ash, particle size, and amorphous content of the fly ash. However, replacing cement can increase the early age strength (if system is not heat cured).

As Alex mentioned correctly, the cement part does not produce Si-Al network typically known as NASH or KASH network. Instead the cement par produces alkali activated system. These alkali activated systems mainly have 2-D structures, rather than 3D structure (high Si/Al ratio).

The main drawback of the alkali activated systems are they weak durability properties due to the formation of high CH content in the product. If this material to be used in field the durability of it is a point of concern.

Thank you Mittal Patel Mam for sharing this question. Actually the answer has already been given by Alex Reggiani decently. There is no necessity to repeat what he has said. I recommend his answer.

T.hank you Dr K N Sheth for your response. I am sure this will help Mr. Mohmmad

I think Dr. Alex Reggiani provided a very good explanation. Thank you.