Replies

1. 
   CombatRescue | Sep 27, 2003 04:59am | #1

   What you're describing sounds like efflorescence.  This occurs somewhat frequently in concrete, and would account for the white splotchyness you described.  Efflorescence happens when calcium, lime and/or salts are wicked by moisture to the surface either during the curing process or after curing in moist conditions.  As far as I know, no one really knows why one batch of concrete will efflorese while another doesn't, but using a minimum amount of water in the batch seems to help.

   Since your concrete is already poured, you'll probably be forced to try some of the products sold to combat efflorescense.  Do a google search on "concrete" and "efflorescence" to get info on them, and more info about the problem.  I've never used any of the products, so I can't vouch for them.

   I assume this floor is slab on grade?  If so, I hope you have some plastic under it.  Moisture from the ground can enter the slab and migrate more stuff to the surface. 

   If none of the efflorescence products work for you, then you could have the slab ground or painted.  Grinding will expose aggregate and change the look of the floor, giving it sort of a terazzo look.  You could paint the whole thing or maybe hire a pro to faux-finish it for you.

   Good luck,

   Andy

2. 
   WayneL5 | Sep 27, 2003 07:19am | #2

   Color in concrete seems to be sensitive to the amount of moisture.  It's a common effect in tile grout.  If two batches of identical material are mixed with different amounts of water, the color comes out different.

   I'm guessing, but don't know, that there were wetter or dryer spots in the mix, perhaps because of uneven moisture takeup by the sand below, or the way the surface was worked, as the concrete cured.  Unfortunately, I don't think it can be "fixed", only covered over or additional color applied.

   Also, you mentioned "lots of sandy dust".  This clearly isn't normal for concrete.  It suggests too much water in the mix or the surface drying too quickly on a hot, sunny day.

   Perhaps a concrete stain might produce good looking results, turning the effect more into stone.  You could create more variation in color rather than trying to even it out, and it might look really nice.

   Since you've sealed the concrete that may limit your options.  Before you commit to tile, be sure the sealer would not prevent the tile adhesive from adhering to the floor.

3. 
   scriber | Sep 27, 2003 07:39am | #3

   I've had a similar problem with "Davis Color' concrete mix with regard to splotchinness that was caused in the curing process. Given that the customer was not 100% with his color choice we avoided using a cure and seal product to protect and seal the feshly poured slab. ( just in case we needed to acid wash or something)Over the next month the slab took on all interesting sorts of blotches that were totally unexplainable. We triple checked our mix, never added water, placed in cool cloudy weather and yet disaster! We used poly under the slab and although it was a two truck mix there was no rime to the discoloration.

   I would highly recomend the use of a cure & seal product to stabilize the cure process, especailly outdoors.