we do have efflorescence problem with our GPC concrete after demolding 4-5 days as air moisture content is high to leach the salt probably Na2CO3. They looked like the needle crystals. How can we avoid this occurrence, anybody can help? Decreasing Sodium Hydrocyde or raise sodium silicate contents will help?
Hello Hung , efflorescence are due essentially to : too much amounts of soluble salts into the system, bad mixing , too free water (extra water) out of the system, so not already involved into reactant liquid you use.
If you need to use only Na based solution, check to reduce at minimum amount ,that results enough to your system, your liquid and possibly work on Molar Ratio to have harmful condition (not corrosive). In this way soluble salts inside your recipe will be reduced and also movement to surface , then efflorescence may be almost totally avoid. If you could change from Na based to K based solution you could solve better and with improved properties, because K molecule weight 58 compared to 40 of Na , so results more stable and difficult to move. Other advantages are : K liquid has very low viscosity that permits better wettability of the concrete, so you could reduce liquid (I found till 45% save liquid, using K liquid compared to Na liquid) amount maintaining the same workability or improving workability and strength ; potassium salt are transparent crystal so when it arrive in surface creates efflorescence difficult to see, compared to well known white beard that you find (sodium carbonate) .
Other parameter that you have to master is extra water amount for workability , it must be lower as possible to don't affect system reducing strength and creating migration of Na ion to surface.
Regards
That is a very good answer by Alex. I don't think you can get rid of efflorescence entirely but only can limit it to a lower amount by playing with the variables (i.e. water content, alkali solution type and amount). I wonder what would happen if you have some calcium as well?
Efflorescence in geopolymer materials arises from the presence of unreacted or excess alkaline solution in geopolymer product, which when exposed to air while curing, reacts with Carbon dioxide to form a carbonate salt, on the surface of the material. My findings from experience, in other to reduce this phenomenon, is to reduce the concentration of alkaline content in the mixture. This, however should be done only to a level that sufficient alkaline concentration needed for reaction is used.
Use of KOH instead of sodium silicate is recommended to minimize Na+ content in the geopolymer mix.
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