Dear Hao,

I think you have understood that alkali Activation is hydraulic process not far from Portland hydration reaction.

I’m agree with you that pozzolanic raw materials like slag ,that you mentioned, can be used as supplementary cementitious materials because and, in different ratio, also fly ash and metakaolin can work to reduce Portland cement clinker.

Traditionally Alkali Activation born with soil cements of Dr. Glukowsky in Ukraine. He used GGBS micronized slag and corrosive alkaline medium (Sodium Hydroxide) to obtain within few hours of curing at high temperature very high strength concretes. This technology was used for precasting and even for civil building (there is a palace in Russian Federation done with this technology in the 80’, that is still existent...but with some problems due to collateral effects of Alkali Activation: strong efflorescence (still growing after almost 40 years!) and variation of surface color due to surface migration of ions like iron and sulphur that oxidation became colored carbonates). Actually Alkali Activation uses also fly ash as powder precursor because if heat cured can assure more stable performances (you can find tons of papers here in Research Gate). Of course to have room temperature setting is already common to blend fly ash with slag , but the process still is hydraulic and alkaline solution very caustic. Even in this case more stable results have been found using heat curing (from 60°C to even 90° C or more for 24h to 7days!).

I understand your doubt, but Portland cement , even with supplementary cementitious materials (pozzolans) is mixed with water (better if decalcified), while Alkali activated materials (AAM) are mixed with Sodium or Potassium hydroxide usually and latest recipes started to use also sodium or potassium silicate, to increase performances.

Pozzolanic reaction is well known and occurs when hydrolysis lime, generated from Portland hydration , reacts with pozzolanas (slag and other) depending on their pozzolanic index (that depends on their mineralogy and size distribution).

Alkali activation is instead forced reaction between strong alkali as sodium hydroxide that dissolves the silicate powder used instead of Portland cement, then throw precipitation of hydrated calcium silicates minerals there is growing of new phases till complete hardening. There aren’t mineral stable polymers. The real binder is the hydroxide that keeps together the hardened materia. This process is quite stable under air. The problem is that alkaline ions aren’t blocked in the bulk, on contrary they are free to move to the Surface following evaporation channels or are already on surface due to over saturation. At that point these alkalis can carbonate absorbing CO2 from air, producing sodium carbonate as beard of efflorescence.

AAM for this reason aren‘t stable under water because alkali ions are still very soluble so they are leached out from surface and the process can go further till collapsing of the AAM. Practically the binders salts are removed from water and the strength of material decreases till failure. Migration of ions can promote ASR (Alkali Silica reaction).

Last class of materials you touched are Geopolymers and are definitely different materials from Portland cement and from AAM too.

Prof. Davidovits called geopolymerization the chemical process that happens ,at room temperature, between mineral amorphous alumino silicates and precise user friendly medium that can be acid or alkaline.

Usually acid geopolymers are mainly for Hi-tech technologies and applications (dental repair, ceramic or to create bone-like materials based on hydroxiapatite and so on).

Alkaline medium instead is used for low-tech technologies and applications like building materials (concrete is one of the cheapest product and of course can be done, but there are lot of niece applications where is possible to develop and use this technology at best).

Geopolymer is a concept , can exist Geopolymer concrete, GP mortars (GP plasters, GP finishings GP repair mortars and so on) , GP sprays, GP paints, GP foams or GP mortars for 3D printing for example (check at our site: www.renca.org), GP lightweight/thermal insulated panels, GP cold ceramics and so on.

This technology is based on selected amorphous alumina silicates like metakaolins (but not calcined kaolins or chamotte), ferrosialates (like red meta clays, burned at specific temperature to obtain the necessary super reactive mineralogy, that is mandatory to have correct geopolymerization reaction) called precursors. I can’t go more in deep because my NDA. The reagents are silicates (sodium or potassium depending on field of application and target price to respect) with precise Molar ratio and pH that isn’t corrosive but classified only irritant like lime and Portland (for this reason there are no problems about transportation and GP can be used as green materials). Geopolymerization is promoted by the powder precursor, that is already super reactive ,not from silicate that is only the best balanced hardener (never activator , that is proper term for AAM technology where the main actor is the liquid part) to obtain the reaction.

This reaction is anhydrous and follows steps : depolymerization, nucleation, 3D polymerization and condensation mainly. It’s non destructive reaction , thanks to precursor‘s mineralogy and hardener alkalis will be created a new ordered, stable and 3D network that creates new minerals phases. Inside these new minerals will be engaged the most of alkaline ions so carbonation due surface migration is very low, no efflorescence occurs and no leaching is possible even under water curing. A proof of genuine geopolymerization is properly the under water curing. Covalent bonding assures very long durability even under acid or base attack, after freeze /thaw cycles or thermal shocks and the material results fireproof and if used refractory aggregates can be also heat reasistant till 1700°C.

This technology doesn’t promote ASR because Alkali are inside the matrix . We are touching just the top of the iceberg about this technology.

Check at the Geopolymer Institute‘s site (www.geopolymer.org) the book of prof. Davidovits that can explain better than me all these important concepts and finally you will have more clear ideas about these different materials.

Check also at our site in Italy (www.geomits.com) if you need more informations about also the mixing systems because even mix design of GP is really different from AAM and Portland.

Best

Dr. Alex Reggiani

CTO GeoMITS

CTO Renca Rus

Supplementary cementitious materials are those materials which react with lime ( produced during hydration of cements) in presence of moisture and forms compounds which are similar to compounds produced during hydration of cements like C-S-H.

Whereas in alkaliactivated concrete Alkalis like caustic alkalis or alkaline salts are used as alkaline activators with cementitious compounds.

In cement chemistry, hydration refers to the chemical reactions between

cement and mixing water.In alkaliactivated concrete , hydration refers to the chemical reaction between Cementitious components and mixing water in the presence of activator.

Dear Hao Chen ,

Supplementary cementitious materials are Pozzolan which defined as “a siliceous or siliceous and aluminous material which chemically reacts with calcium hydroxide produced from cement hydration at ordinary temperatures to form compounds possessing cementitious properties” according to ASTM C618.

But, alkali-activated materials are used to substitute Ca with Na or k to form N-A-S-H or K-A-S-H instead of extra C-S-H in case of using Pozzolan. I think the attached pictures will simplify the difference