Case Study: Algal Staining in Grout

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As part of our after sales support, we recently revisited a renovation client whose pool we plastered in Blue Lagoon a month earlier. The owner had noticed two small brown stains in the grout along the fence-side long wall and asked if we could inspect. Although not regularly required, providing guidance and reassurance post completion is simply part of how we support our customers.

The staining was confined to the grout rather than the plaster finish itself. Upon on-site inspection, the marks were consistent with algae that had penetrated the porous grout surface.

Algae can appear brown or green depending on type, but it is always water-borne. Different systems adopt different means of sanitising the water to prevent this (chorine, copper etc). On this occasion, readings revealed that sanitation levels (in this case, chlorine) had been nil to low for a number of weeks following plastering. That timing aligns with how this type of staining can develop.

Importantly, there were no signs of algae establishing on the plaster finish and pH and water balance were within acceptable range at the time of inspection.

Why This Can Occur:

Both plaster and grout are slightly porous materials, with grout being more absorbent. If sanitation levels drop in the early stages after filling, algae present in the water can settle and penetrate slightly below the surface.

The Treatment:

After checking water balances and the client’s historical readings, we assessed how embedded the stain was on a progressive basis:

Nylon brush – minimal effect;
Wire brush – minimal effect;
Pumice stone – successfully removed the affected surface layer of grout

The reachable corner stain was cleared using the pumice stone, effectively sanding off the surface layer.

Preventing Recurrence

Surface removal* alone though is not enough if algae remains active in the water. The priority is consistent water chemistry management. Pool owners should work closely with an experienced pool care professional to ensure:

Chlorine remains within the recommended range at all times;
Shock treatment (temporarily dialed up chlorine) is applied promptly if warranted; and
Regular brushing is maintained, particularly along edges and corners.

*If algae ever adheres to plaster and does not brush off easily, a very light pass with a metal brush suitable for pool finishes can help break the surface layer and allow chlorine to penetrate effectively.

Outcome

Our customer was pleased that we were able to identify the cause and provide clear guidance on managing it going forward. With correct water balance and regular maintenance overseen by an experienced professional, the issue should not recur. Overall, the pool was looking great and settling as expected. Aside from addressing the chlorine level early on, the owner had been very diligent with brushing, water chemistry maintenance (including retaining records) and keeping their pool clean of debris.

If you have a newly plastered pool, maintaining proper water chemistry with the assistance of a knowledgeable pool care specialist who is familiar with your system goes a long way to protecting the surface and ensuring long term performance.

Rainbow Quartz

Your pool, Your colour, Your choice

The above content is of a general nature. Different pools offer different variables and Rainbow Quartz encourages you to seek the advice of your professional pool builder or technician in relation to your specific circumstances. If you’re in the market for a new pool and would like Rainbow Quartz for the finish on your concrete pool, please reach out – we are happy to answer queries and can put you in contact with vetted pool builders in your area.

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