We have made a geopolymer using slag, NaOH and microsilica and we have obtained, after a post 60ºC curing, a good compressive strength: 40 MPa at the 3th day.
After this test, we tried to eliminate all the humidity in an oven at 105ºC and after 12 h inside the specimens presents many micro and microfissures. Also, the strength of the come down to 5 MPa.
With out extra temperature, at room temperature, the compressive strength decrease to 15 MPa at 30 days. However when the specimens cure immersed in water, no loss of strength is observed.
I show same pictures of the specimens with 1 day after curing process. After 30 min in a over at 105º and after 24 h.
Anybody can suggest us something?
I don't know how were your specimens cured in oven? In my view, the first reason is that heating shocked. Because if you put your specimens directly into the oven with 105ºC then the specimens will be heating shocked. Another reason is: 105ºC is to high, it make water vapouring too fast lead to cracks in specimens. In some my results with Alkali activated slag, we don't need heating curing, even curing in normal temperature (25ºC) our specimens can get 40MPa after 3 days curing and 80 MPa in 28 days.
Hi Carlos not sure if you got your answer, I can only see one, but I can share our experience, When we used Microsilica and the forces generated at higher curing temperatures actually break our plastic moulds. (we not sure what causes it) then we had the same problem when the activater content where on the high side that we get this cracks in the concrete and the strengths drop over a period of time. We now cure at 60 and "DRY " 55 degrees because at 105 degrees you boil the water that is left in the capiliry pores and the steam if it can not escape will cause this damage, This is my opinion I know it is not sceintific but we also experienced it and solve it by removing the silicafume and use lower temperatures. Hope this helps
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