Discolouration is a common and often unexpected issue that can arise with concrete floors, resulting in uneven colour variations in the finished product. Any mistake in the concrete floor creation process can contribute to this issue, making it challenging to determine the exact cause.

Polishing concrete removes many surface defects, but fixing some discolouration requires extra care. In fact, issues that begin deep into a slab’s surface can’t be simply ground away, and need more invasive repairs.

It’s easy to see when concrete is discoloured, but understanding the cause is another matter. This blog post will describe why your concrete might have strange colouring so you can get started on having it fixed.


Discoloration From Concrete Mix Issues

One possible cause of discoloration is inconsistency in the mix or the quality of aggregates used. When porous aggregates emerge on the surface, they can absorb moisture, resulting in darker areas in the surrounding paste

Contractors sometimes adjust concrete batches on-site by adding water to achieve better workability during the finishing operation. While slight variations in moisture content are generally acceptable, noticeable differences in coloration can occur when comparing the edges of two separate concrete placements.

Another factor that can lead to blotchy concrete coloration is the addition of calcium chloride to accelerate the setting time of the concrete or poor mixing practices. Additionally, incorporating supplemental cementitious materials like fly ash, blast-furnace slag, or silica fume can produce different colour shades compared with pure cement.


Concrete Segregation Discolouration

Discolouration can also stem from the separation of cement paste and aggregates. This problem is caused by excessive vibration or compaction, mixing mistakes or transportation damage.

Often, segregation discolouration can be corrected by deeper grinding to achieve a uniform colour. However, it’s worth noting that this procedure requires more aggressive tools and equipment, making it a more expensive option.


Trowling Errors Discolouloration

Trowling is a delicate step in the concrete finishing process that must happen at the precise moment when the surface is dry but still workable. Trowling at the wrong time, with the wrong tools, or other errors can cause discolouration.

Metal trowels used with a power trowling machine can result in a dark or almost black appearance on the concrete’s surface. To prevent this, use plastic blades on walk-behind and ride-on power trowels and limit the number of passes over the floor.


Discolouration From Curing Compounds

Resin-based curing compounds can penetrate the concrete and cause deep dark areas within the slab. These compounds are challenging to remove, especially if over-applied, as they load up the concrete polishing diamonds, similar to glue on sandpaper.


Uneven Hydration Discolouration

Uneven hydration, which can be caused by factors like a blown-off curing blanket or materials left on the slab that slows down hydration, also contributes to discolouration.

To mitigate the risks of discolouration, use ASTM-approved curing compounds and ensure proper hydration by avoiding obstacles or penetrations that may create impressions or marks on the slab.

By taking these precautions, the cost of concrete polishing can be minimized, and a more consistent colour result can be achieved