Discoloration refers to gross color changes amongst adjacent slabs caused by changes to the concrete mix or efflorescence. Discoloration exhibits dark blemishes or mottled surface appearances. Some causes for discoloration might be: significant changes to the mix design related to raw materials (cement, aggregates, admixtures), changes in the rate of hydration over the concrete slab usually as a result of inconsistent curing periods/operations, significant variations in the water/cement ratio of the concrete, admixtures like calcium chloride are used, finisher improperly estimates the timing of the finishing operations which results in a hard troweled surface. Below are some ways to reduce the likelihood of such problems:
-Eliminate the use of calcium chloride if possible
-Finish concrete at correct time. If one finishes too early it can elevate the water/cement ratio at the top surface and lighten the color.
-Uniform curing system for the necessary time period. Uneven hydration affects cement hydration and in turn color of the concrete.
-If plastic sheet curing, ensure it does not come in contact with surface. It can leave color streaks on the surface
-Ensure raw materials sources aren’t varied throughout life of project