Geopolymer vs. Alkali-Activated Materials: Are they same?

Cement is the widely used binding material in the construction sector and has a negative impact on the environment. So, researchers are trying to develop an alternative to OPC that can be used in the construction industry that has a lower environmental impact and reduces the CO₂ emissions while promoting long-term durability and higher strength compared to OPC.

Geopolymers or alkali-activated materials have emerged as a promising sustainable alternative to conventional cement. However, a common misconception is that geopolymers and alkali-activated materials are the same or synonyms. Actually, they are different from each other and have different chemistry or chemical mechanisms.

When I started studying about geopolymer concrete and AAMs, I thought they were the same things, as both are activated with chemical activators such as NaOH, Na₂SiO₃, etc. Not even me; there are lots of researchers who think both are the same. They are demonstrated in their paper, as the terms "geopolymer" and "AAM" are the same, and there is no significant difference between them. But the founder of geopolymer, Davidovits, cleared the misunderstanding and explained that both are different and have different chemistry. Although some research states that geopolymer is a subclass of alkali-activated materials.

When I first began studying geopolymer concrete and AAMs, I also assumed they were identical, as both systems are activated using alkaline solutions such as sodium hydroxide and sodium silicate. Not even me; many researchers have used the terms interchangeably in their studies, suggesting there is no significant difference between them. 

Let's discuss the concepts of "geopolymer" and “alkali-activated materials" and explain why AAMs are not geopolymers, based on Davidovits’ interpretation. I acknowledge and respect the other researchers' opinions on what they said about AAM or geopolymer.

What is a geopolymer?

Geopolymer is an inorganic alumino-silicate polymer produced by the dissolution of an aluminosilicate oxide in an alkaline solution. The alkaline solution consists of NaOH, a combination of sodium hydroxide and sodium silicate, or KOH with K₂SiO₃ that leads to the formation of 3D polymeric chains of Si-O-Al known as the poly-sialates. Remember, geopolymer is formed through the polymerization (polycondensation).

What is AAM?

Alkali-activated materials are based on low molecular chemistry with a small molecule and are not polymers. These molecules formed cyclic structures, and alkali cations are located on the edge of the network. AAM is formed through the hydration process, according to Davidovits.

Main Differences:

Point 1:

When you activate slag (glassy substance) with alkali, it is subjected to alkalination (not activation) and depolymerizes under the action of alkali. These molecules form a cyclic structure, and the alkali cations Na⁺ or K⁺ are located on the edge of the cycle. Water can easily migrate to them and wash it away from the structure, making the structure less durable and weaker. If you are thinking this alkali activation slag is the same as the geopolymer concept, you are wrong.

To make a "geopolymer," you must add a networking element (like metakaolin) that can interact with the free cations (Na⁺ or K⁺). The process transforms into a stable 3D network, and the alkali cations (Na⁺ or K⁺) are trapped inside the 3D network and provide long-term stability. Fly ash can be used instead of metakaolin as a networking element.  

In conclusion from the above discussion, AAM is a 2D network (cyclic structures), while the geopolymer provides stable 3D structures.

Point 2:

To understand the context fully, we need to know about the terms “polymer” and "gel." A polymer is a strong, long chain made with a definite size and molecular weight, while a gel is an indefinite amorphous compound with unresolved dimensions.

It has been reported that the geopolymer is made of nanoparticles, as they have a definite size ranging from 5 to 15 nm and molecular weight. The core of these geopolymers is made of an aluminosilicate framework. Alkali-activated materials are not polymers, but the geopolymer is. The geopolymer cement is not a C-S-H derivative, and applying CSH terminology from Portland cement is not only inaccurate but also referring to it as NASH or KASH is inaccurate.

Alkali-activated cement is hardened through the hydration process, but geopolymer cement is formed through polycondensation (squeezing out water to form a bond).

According to some researchers, geopolymer hydrates, and the concepts of NASH or KASH are widely accepted for geopolymer, and it is a low-calcium-based binder (like N-A-S-H or K-A-S-H-based), while AAM is a high-calcium-activated binder (likely C-A-S-H), but Davidovits already claimed that geopolymer is not a hydrate and the terms NASH or KASH are not correct in terms of geopolymer.

Note: The above discussion is based on Davidovits's interpretation ^1,2.

-Ariful Islam, Bangladesh
Author, Utopia Educators

References:
1. J. Davidovits, (2018), Why Alkali-Activated Materials (AAM) are Not Geopolymers.
2. Why Alkali-Activated Materials are NOT Geopolymers ?
3. Provis, J. L. (2014). Geopolymers and other alkali activated materials: why, how, and what?. Materials and structures, 47(1), 11-25.