Since a winter flatwork mix was used and considering the severity of the discolorations most likely your concrete contained calcium chloride.
Calcium chloride concrete discoloration.
Calcium chloride in concrete is a primary cause of concrete discoloration.
However concrete discoloration could occur if there are sudden changes to the cement material if the finishing operations are rushed or if calcium chloride is added.
Uneven curing will result in uneven coloring.
Inadequate mixing after addition of calcium chloride makes the concrete color even more irregular or blotchy.
The slab should be alternately flushed and brushed and dried overnight.
Top rated concrete pros and industry association s we consulted say that calcium chloride especially if added in amounts that approach 2 percent of the cement weight can cause a mottled blotchy surface.
This can occur in any of the following forms.
The chances for discoloration are much less if calcium chloride or chloride bearing chemical admixtures are not used.
Lastly there s the issue of poor workmanship.
Air cured concrete which is to say lack of curing exhibits the greatest amount of discoloration.
This is especially true in concrete containing calcium chloride.
Discoloration caused by calcium chloride admixtures and some finishing and curing methods can be reduced by repeated washing with hot water and a scrub brush.
There are also other complicated chemical reactions that can happen with the portland cement and other additives fly ash slag etc that might be in the concrete.
You can get trowel burns which produces a darkening when you use calcium chloride.
A frequent reason behind a darkened concrete appearance is the addition of calcium chloride to accelerate the setting time of concrete see photo 1 especially when calcium chloride dosages approach 2 by weight of cement.
The type kind and condition of formwork can influence surface color.
Thoroughly moist cured concrete exhibits the least.
Secondly the usage of calcium chloride in an attempt to speed up the setting process of the concrete can lead to discoloration.
They say the discoloration is probably just an aesthetic issue not a sign of weakness in the slab.
The use of calcium chloride as a chemical accelerator can exacerbate the problem by increasing the magnitude and permanence of the surface discolorations.
Discoloration issues on new concrete discoloration on the surface of new concrete can be from several factors including inconsistent mixtures too much or not enough water low quality materials poor workmanship the use of calcium chloride environmental issues or issues created during the pore or during the curing process.
Concrete should be of one color if you are using the correct batch and materials from the ready mix.
