Dear Alaa,

the reason of this big difference depends on hardening process , that follows completely different way.

1. concrete based on ordinary Portland cement (OPC) is hydraulic process and adhesion is obtained mechanically and thanks to porosity on substrate. If substrate is a metal, there isn‘t porosity that allows good binding , but only the weight of concrete can obtain weak adhesion after curing. To increase this adhesion you need a promoter like a primer , a latex or a resin in general that creates some chemical link (by the way this procedure is more used for mortars, like coating, elastomers membrane, waterproof membrane very high in resin).

2. Geopolymer concrete (GPC) isn’t hydraulic material, as OPC concrete and Alkali Activated concrete (AAC) are. So AAC isn’t the same product than GPC . Geopolymerization process follows precise steps and final hardened material is condensed product not hydrate. GPC has different way to create binding , due to pH of reaction higher than Portland concrete (about 13,2-13,4 versus 12-12,5 , logarithmic scale) so geopolymer ambient is more reactive and better adhesives. Geopolymer binders used for GPC have lower size about particles than Portland and generate chemical bonding with surface (in particular covalent bond ) so elements of substrate are shared with Geopolymer materials (Geopolymers are mineral polymers) . This creates a very high and stable link even without open porosity (Geopolymers can be attached on metals and glasses too) .

Alkali activated instead, cannot have this covalent bond with substrate because they aren’t polymers, but Portland like ones. On other hand, due to their super high alkalinity (13,8-14 pH) they can affect metals (corrosive materials, dangerous for skin and breathing) creating that irregular substrate necessary to obtain better adhesion than OPC concrete , but lower If compared with GP .

All best

Alex Reggiani

Thank you Alex for your respond,

can you give a references for that? It's better if it include equations or expressions to measure or calculate this increasing of bonding theoretically.

Appreciation

I’m sorry Alaa, I work with Geopolymer technology in Italy and Russia. I’m private worker I’m not a professor or a student, I wanted to help you understanding the different hardening technologies but mathematically you have to ask directly to prof. Davidovits . For free this is the best that I can do ;D.

Cordially

Alex Reggiani