The concept of geopolymer can be addressed in different setp.

First the type of aluminosilicate you used: it should be low calcium content and have reactive silica and alumina. I mean the glassy phase must be rich in silica and alumina as they dissolve easily and rapidly than crystalline ones.

Second the type of alkali used: even if add NaOH first  is benefit for high dissolution, it's always good to mix  NaOH+sodium silicate before use. This is because sodium silicate need to be depolymerized first by NaOH to be more reactive. Another thing is the rate of reaction is more influenced by polycondensation step than dissolution.

Third, the geopolymer structure is different from the one of  alkali activated slag or fly ash. geopolymer have a 3-dimensional structure (Q4) while the other ones are not 3-dimensional and their structure is mostly Q3. So even if alkali activated slag or fly ash generally have higher strength than geopolymers they have very poor durability properties than geopolymer.

Hi Mr. Kan,

Firstly, alkali activated cement is a broad class of material and geopolymer is one of those cements. 

Since you are using low-calcium fly ash which  essentially means that content of silicon and aluminium are higher. The higher content of Si and Al are essential for geopolymer formation. But the problem probably for you is that you are using sodium silicate as an activator. In geopolymer, sodium silicate act as a source material contributing silica for the geopolymer rather than activator. It is shown that sodium hydroxide at low concentration is the most recommended activator for fly ash with low calcium. 

The concept of geopolymer can be addressed in different setp.

First the type of aluminosilicate you used: it should be low calcium content and have reactive silica and alumina. I mean the glassy phase must be rich in silica and alumina as they dissolve easily and rapidly than crystalline ones.

Second the type of alkali used: even if add NaOH first  is benefit for high dissolution, it's always good to mix  NaOH+sodium silicate before use. This is because sodium silicate need to be depolymerized first by NaOH to be more reactive. Another thing is the rate of reaction is more influenced by polycondensation step than dissolution.

Third, the geopolymer structure is different from the one of  alkali activated slag or fly ash. geopolymer have a 3-dimensional structure (Q4) while the other ones are not 3-dimensional and their structure is mostly Q3. So even if alkali activated slag or fly ash generally have higher strength than geopolymers they have very poor durability properties than geopolymer.

You will be able to get high strength with low calcium fly ash if 10% silica fume is also added. This will enhance the workability as well. 

 what is the value of the Alkali modulus (Na2o/Sio2) of your activator (sodium silicate)

???

Can I obtain fly ash based geopolymers using small amounts of sodium silicate?

What is the mean permeability value of fly ash  based geopolymers?

Thanks.

Geopolymer and alkali activated ,material are two different concept. see the difference here

https://www.youtube.com/watch?v=cHwaa_S4L3Q

Can anyone please refer a paper where it is mentioned that alkali activated binders have Q3 structure? I got many papers presenting the Q4 polymerization of geopolymer binders.

There is a vast difference and it all depends on what happens to the calcium silicate hydrate level . The object of forward progress in developing commercially viable alternatives over the existing way of looking at things is to optimize the bonds that lead to CSH, the good stuff. The chemistry involved is outside that of the typical "cement" perspective and that is why you need to understand the difference between "hydration" and "polymer" pathways. There is a huge difference in outcome from the material scientist or engineer's perspective.

When it contants a lot amount of calcium materials, it is an alkali activated material, and you can do that with activated slag and get CASH phase. but activated low calcium content fly ash will give geopolymer with more NASH, you can see the figure above, which stated by John Provis, could make it easy to understand the difference

Following book provides good information on geopolymer and alkali activated cement ;

C. Shi, P. Krivenko and D.M. Roy, Alkali-Activated Cements and Concretes, Taylor and Francis, Abingdon, UK

https://www.geopolymer.org/faq/alkali-activated-materials-geopolymers/

Hey all, check this out. It seems a different result

The geopolymer concrete is different material than alkaline activatior cmement like slag or fly ash cement. Basically, alkaline activatior like slag or fly ash generally have higher strength than Geopolymers. Durability properties like Cl penetration, depth of water penetration may be higher than alkaline activator cement concrete.Also shrinkage may be higher.

For more Please read following Journal paper.

https://www.sciencedirect.com/topics/engineering/alkaline-activator

Hi! I'm sharing withy you some of article info abut your questions. Here is some of important spots:

"It should be noted that alkali activated is not considered to be a geopolymer as slag is Ca rich material. In a Ca rich system another type of reaction products is formed, namely calcium silicate hydrates. In this type of phases, silicate tetrahedra are found in one- dimensional chains rather than in 3D structures as observed in geopolymers (see figure as named "figure 2"). Another difference is that water is bound in the structure of calcium silicate hydrates, but not in geopolymers, resulting in superior chemical and thermal resistance of the geopolymers." (quoted from: Geopolymers – State of the art, Klaartje De Weerdt,2011)

"Geopolymers and alkali-activated binders are hardened com- pounds which attain their strength and other characteristics by chemical reaction between alkaline solutions and aluminate- silica rich source materials. Geopolymer is inorganic alumina- silicate polymer structured around tetrahedral coordinated Si4+ and Al3+, forming a polymer chain. These geopolymer precursors chemically bond and form oligomers leading to the formation of aluminosilicate polymers. Whereas, alkali-activated binders are formed by activation of calcium-rich source materials under high alkaline conditions. After the dissolution of the precursors in alkaline solution, a binder paste with hydraulic potential is build leading to simultaneous formation of C-S-H gels and aluminosilicate polymers. Figure 1 shows general difference between alkali activated binders and geopolymers." (quoted from: Comparative study of geopolymer and alkali activated slag concrete comprising waste foundry sand, Bavita Bhardwa, Pardeep Kumar, Construction and Building Mat., 2019)

Difinitely agree.

How can geopolymers and AAM be similar when you can use acidic solutions to make Geopolymers?

and regarding the fly ash samples or any other precursors we have to realize that the oxide analysis is not enough. More importantly, the mineralogy of the precursors should also be considered. Crystalline material are not reactive to be used in geopolymer applications. So if your fly ash does not have enough glassy phase, problems may arise during geopolymer production. The temperature at which the fly ash has been burnt at the electricity plants directly affect the reactivity of fly ash.