Good day all.
Can someone guide me in what to look at (possible contributing factors) to investigate a false setting cement please?
Thanks.
Silastman
Ted Krapkat told about it in this report:
http://www.cemnet.com/Forum/thread/150725?page=2
I can explain it. When grinding of clinker temperature can rise up to more than 100 oC. In such conditions, gypsum dihydrate CaSO4 2H2O added to clinker when grinding in about 4-5%, loses water and goes into the gypsum hemihydrate CaSO4 0,5H2O. Gypsum hemihydrate in water hardening for 6-10 minutes. This is a false setting. If the cement dough mix, it again starts to harden properly.
Stephenb
Thank you Mr. Silastman.
One question though, other than cement milling temperature, does things such as PSD, clinker free lime etc a factor to consider for false setting cement?
Thanks
Ted Krapkat
Hello Stephen,
Yes, as Silastman has pointed out, the predominant cause of false set is the precipitation of gypsum crystals from supersaturated solution due to the presence of too much hemihydrate in the cement. However there are other cement and concrete properties that can also influence false set.
With cements that contain a high percentage of soluble K2O (>1% NaEq.), the mineral syngenite (K2SO4.CaSO4.H2O) can be formed. Rapid precipitation of syngenite can also cause false set and if the quantity of syngenite precipitated is large enough this can even cause flash set, due to the removal of CaSO4 from the system.
Researchers have found that the intensity of false set is governed by the ionic strength of the pore solution surrounding hydrating cement particles. Soluble alkalis increase the ionic strength of this liquid and hence the rate and intensity of CaSO4 nucleation and crystallisation. So that, all else being equal, cements that are low in alkalis are less prone to false set than those that are high in alkalis.
In addition to cement properties, some concrete admixtures such as lignin sulphonate water reducers negatively affect the solubility of gypsum, increasing the tendency towards false set.
Admixtures which contain strong electrolytes not only accelerate the hydration of calcium silicates but also the rate of gypsum nucleation and precipitation, so increasing the chances of false setting.
On the other hand admixtures containing weak electrolytes, or organic molecules with a retarding effect, interfere with the nucleation of gypsum crystals and so reduce the tendency towards false set.
Hope this helps...
Regards,
Ted.