When I am replacing fine aggregates with slag in geopolymer concrete, after some time of the casting work, bubbles develop at the surface of the geopolymer concrete, due to which lubricant comes out and slow down the setting time, along with that it also makes the surface of specimens rough at the time of demoulding. Please suggest some idea.
Dear Mayank,
I don’t know your system but for sure fine aggregates can’t be replaced by fine slag. This because fine (what size?) slag is super reactive , completely amorphous and easy to react in alkaline medium, precipitating and accelerating condensation step. While fine aggregates if crystalline like silica or quartz sand are completely inert, so they can’t accelerate hardening time.
Air Bubbles are generated because problems about mix design (added in wrong way , at wrong time or mixed too fast ) . From color of your samples I think you used fly ash (I hope only F type) .
Other key point mandatory to well consider is quality (Na or K based ) and its molar ratio (consequently its pH) of your liquid part , the hardener for Geopolymer technology.
What is finally setting time that you have obtained and what about mechanical resistances developed ?
Have a nice work
Alex Reggiani
Other point , you must cover with plastic the samples after casting for at least 24h , this to avoid too fast drying of surface , assuring better aesthetics.
Thank you Alex Reggiani sir for your reply. I shall consider your suggestions.
Hello,
The air bubbles which you mentioned on the surface of geopolymer cubes during casting is a common phenomenon in geopolymers and generated due to lack of proper compaction and necessary vibrations. However, this is seen more in the case of Fly-ash based GPC.
Having said that, I would like to agree that yes demoulding is an important issue in case of geopolymers in certain temperature conditions and also even lubrication of cubes are required and proper demoulding methods should be applied. Thank you
Thank you Jyotirmoy Mishra sir for providing me with the information.
As you told that, bubbles formation is a common phenomenon in geopolymers and generated due to lack of proper compaction and necessary vibrations if I increased the time of compaction and vibration then can the formation of bubbles reduce?
Sir, if possible please provide any research paper for supporting your statement or any other solution for reducing the formation of the bubbles.
1. I would like to suggest you to experiment yourself and find out as there is no other way. But yes, proper compaction and vibration can minimize bubbles up to certain extent.
2. Worldwide different researchers are producing results out of their own understanding. Geopolymers having no such standard codes enables us to experiment and understand the behavior as it depends upon factors like choice of aluminosilicate materials, the viscosity of silicate solution, mixing time too.
I can speak from practical experience: It could depend on how carefully you pour your concrete in. Sometimes when you are casting materials certain mixtures are more prone to air bubbles.
Is the mix smoother or runnier than previousley? Because that could make a difference when you pour it into the mould. Air bubbles can get trapped more easily in a low vicosity mixture and likeley after puring they will rise to the top. This has happened to me many times with plaster and traditional Gesso (as my Job in a past life as a carver and Guilder). Of course you may have already tried pouring the mix in really carefully and slowly making sure that all detail is covered first and filled up slowly. Art/ and science can be a good mix too! Let me know how you get on.