Dear colleagues,

please use the correct terminology and decide what you are researching and what you want to do.

Geopolymer concrete isn’t an alkali activated material, mixing together fly ash and slag doesn‘t assure that you will obtain Geopolymer reaction .

If you want to realize alkali activated concrete you can use sodium carbonate as activator and you can use superplasticizer like Portland cement technology, for sure. But you will have inside hydrate minerals (NASH or KASH depending on the alkali that you will choose) so you will have the same problem of Portland cement based concrete when you check fire resistance so spallation and explosion of the material. This isn’t Geopolymer at all. No 3D network, no covalent bonding. Alkali activated materials have also big problems when you cure under water your concrete, because alkali aren’t part of the matrix , but they are outside close to surface of material, still soluble salts , (they are only dried , not linked in stable matrix) so will be leached out and concrete will start losing resistance and consistency day by day, to arrive at 28 days check with cracks or in the better cases losing 40-50% of mechanical strength compared to air curing. Also efflorescence can start on surface due to migration of Na+ or K+ during plastic phase.

Geopolymer concrete may be made of fly ash (only F class and necessarily calcined at more than 1300°C , or sulphur , carbon and other pollutions don’t allow good geopolymerization process) and slag , but mix design is completely different. Super Reactive part are amorphous alumino-silicates powders and liquid part (alkali or acidic medium ) is named hardener or reagent , never activator because what has been already activated by heating process are fly ash F and slag . In this case , after correct mixing , alkali become parts of the matrix of Geopolymer cement so stable , never will be possible to leach them out under water or after fire treatment . There aren’t hydroxides in the 3D network formed starting from base unit tetrahedron with 3 Si and 1 Al , covalent bonded. No spallation, nor explosion under heating , only fusion when will be passed the melting point . The final structure is organized and strong and MR of hardener isn’t necessarily so high alkaline, because Geopolymer materials are user friendly not corrosives but only irritant , like classification of lime and Portland cement .

To increase workability in real Geopolymer concrete you must understand that aren’t necessary superplasticizers , because they can be engaged in the product, you have to find balance between aggregate curve and extra water , that is used only as rheology modifier.

Naftalen sulphonate can be used at low dosage just because creates a layer of bubbles in alkali pH , that promotes flowing .

To adjust setting time is mandatory to work on geopolymer cement that is the binder. Admixtures like traditional retarders don’t work and often they can accelerate the reaction .

Please give a look at geopolymer.org the site of Geopolymer Institute where prof. Davidovits worked ,since 1979 , on this technology. You will find all videos and dedicated technical paper that explain better , not only the differences between Geopolymers and alkali activated materials , but also how to solve the problems.

I’m sorry for the length , but this is research gate, we must diffuse correctly the informations or we will create more doubts instead of promote innovations.

Best regards to all

Alex Reggiani

CTO GeoMITS (Italy) and Renca Rus (Russia)

Queensland's University GCI building with 3 suspended floors made from structural geopolymer concrete. (Credit: Hassel Architect and Wagners) – [17] This master thesis presents the effects of fly ash on the properties of alkali activated slag concrete, commonly referred as Geopolymer concrete (GPC).

Queenland's University is made up of flyash. it can help you to understand the phenomena regarding it.

The quick set is possibly from the use of sodium silicate as an activator. The workability and set time can be extended with the use of alternative activators such as sodium carbonate. The use of superplasticizer can also help to increase the slump of your mixture

Following the answers

Dear colleagues,

please use the correct terminology and decide what you are researching and what you want to do.

Geopolymer concrete isn’t an alkali activated material, mixing together fly ash and slag doesn‘t assure that you will obtain Geopolymer reaction .

If you want to realize alkali activated concrete you can use sodium carbonate as activator and you can use superplasticizer like Portland cement technology, for sure. But you will have inside hydrate minerals (NASH or KASH depending on the alkali that you will choose) so you will have the same problem of Portland cement based concrete when you check fire resistance so spallation and explosion of the material. This isn’t Geopolymer at all. No 3D network, no covalent bonding. Alkali activated materials have also big problems when you cure under water your concrete, because alkali aren’t part of the matrix , but they are outside close to surface of material, still soluble salts , (they are only dried , not linked in stable matrix) so will be leached out and concrete will start losing resistance and consistency day by day, to arrive at 28 days check with cracks or in the better cases losing 40-50% of mechanical strength compared to air curing. Also efflorescence can start on surface due to migration of Na+ or K+ during plastic phase.

Geopolymer concrete may be made of fly ash (only F class and necessarily calcined at more than 1300°C , or sulphur , carbon and other pollutions don’t allow good geopolymerization process) and slag , but mix design is completely different. Super Reactive part are amorphous alumino-silicates powders and liquid part (alkali or acidic medium ) is named hardener or reagent , never activator because what has been already activated by heating process are fly ash F and slag . In this case , after correct mixing , alkali become parts of the matrix of Geopolymer cement so stable , never will be possible to leach them out under water or after fire treatment . There aren’t hydroxides in the 3D network formed starting from base unit tetrahedron with 3 Si and 1 Al , covalent bonded. No spallation, nor explosion under heating , only fusion when will be passed the melting point . The final structure is organized and strong and MR of hardener isn’t necessarily so high alkaline, because Geopolymer materials are user friendly not corrosives but only irritant , like classification of lime and Portland cement .

To increase workability in real Geopolymer concrete you must understand that aren’t necessary superplasticizers , because they can be engaged in the product, you have to find balance between aggregate curve and extra water , that is used only as rheology modifier.

Naftalen sulphonate can be used at low dosage just because creates a layer of bubbles in alkali pH , that promotes flowing .

To adjust setting time is mandatory to work on geopolymer cement that is the binder. Admixtures like traditional retarders don’t work and often they can accelerate the reaction .

Please give a look at geopolymer.org the site of Geopolymer Institute where prof. Davidovits worked ,since 1979 , on this technology. You will find all videos and dedicated technical paper that explain better , not only the differences between Geopolymers and alkali activated materials , but also how to solve the problems.

I’m sorry for the length , but this is research gate, we must diffuse correctly the informations or we will create more doubts instead of promote innovations.

Best regards to all

Alex Reggiani

CTO GeoMITS (Italy) and Renca Rus (Russia)

Dear Adeyemi,

Thanks for your suggestion. As sugegsted let me try an alternate activator

The best option is what you have decided to do. However, for more knowledge in this area please obtain the thesis from Queensland University and stand tall instead of guessing at anytime or using trial and error method.