How to Treat Mold and Algae on UHPC Surfaces? How to Prevent Recurrence?

How to Deal with Mold and Algae Growth on UHPC Surfaces? How to Prevent Recurrence?

As an engineering service provider with nearly 30 years of expertise in high-performance decorative building materials such as UHPC, we have frequently encountered mold and algae growth on UHPC facades in rainy, humid southern regions and waterfront environments while serving over a thousand projects at home and abroad. Many projects develop localized black or green microbial spots within 3-5 years of delivery, which not only ruins the original smooth and beautiful fair-faced finish but, in severe cases, penetrates and erodes the surface structure, shortening the service life of components. Many operations teams initially rely solely on high-pressure water jets, but the problem often recurs within six months, and repeated washing actually makes the surface more prone to trapping dirt. Today, drawing on our hands-on project experience, we will thoroughly explain this practical treatment and prevention solution.

I. First, Understand: Why Does Mold and Algae Grow on UHPC Surfaces?

Many people assume that UHPC's high strength and density make it immune to mold, but that is not the case. Having analyzed nearly a hundred problematic projects, we have found the root causes center on three factors: First, inadequate surface pore sealing during UHPC component production. If the pore-sealing process is skimped on before the components leave the factory, moisture and airborne spores will penetrate through micropores into the interior, naturally breeding microorganisms under suitable temperature and humidity conditions. Second, the project environment happens to meet mold growth conditions—for example, in South China where annual rainfall exceeds 1500mm and air humidity stays above 70% year-round, or where buildings are surrounded by greenery and sit long-term in shaded positions without direct sunlight for sterilization, microbial spots easily recur. Third, inadequate daily maintenance. Dust and decaying fallen leaves accumulated on the facade over time attach to the surface, essentially providing nutrients for mold—even with qualified pore sealing, they will gradually grow outward from gaps over time.

II. Correct Steps for Treating Mold and Algae on UHPC—Stop Washing Recklessly

Many people's first reaction is to blast it with a high-pressure water jet, but this is actually the most damaging approach: high-pressure water loosens the original surface sealing layer and even enlarges the micropores, making it easier for moisture to penetrate later and speeding up recurrence. The standard treatment process we have verified in multiple renovation projects is as follows: Step one, pretreatment—use a soft-bristle brush to first clean off surface dust and decaying debris; do not scrape directly with hard steel wool to avoid scratching UHPC's original finish. Step two, targeted mold removal—spray a neutral special algae remover evenly over the spotted areas, let it sit for 10-15 minutes so the agent fully penetrates and decomposes the microbial spots; do not use strong acid or alkaline cleaners to avoid corroding the cement-based components of the UHPC surface and causing discoloration. Step three, low-pressure rinsing—use low-pressure water at a pressure no higher than 5MPa to rinse off residual agents and decomposed spots, then air-dry naturally in a ventilated location; never bake at high temperature to avoid stress cracking of components. Step four, the most easily overlooked step: after drying, the entire treated area must be re-sealed and re-coated to restore the damaged sealing layer—otherwise all the previous work is wasted.

III. Preventing UHPC Mold and Algae Recurrence at the Root: Focus on Three Key Stages

To solve this problem once and for all, you cannot rely solely on later-stage maintenance; you must plan ahead across the production, construction, and maintenance stages: First is upfront treatment at the production stage. Before leaving the factory, UHPC components should undergo a double-layer pore sealing process—first fill pores to block internal capillary channels, then apply a weather-resistant fluorocarbon or silicone-based topcoat to completely seal off the substrate for microbial attachment. In multiple coastal South China projects we completed using this process, no large-scale mold growth has occurred in the 7 years since delivery. Second is joint treatment during the construction stage. The joints between UHPC panels must be sealed with weather-resistant neutral silicone sealant, avoiding cracked or sunken sealant joints that allow water and dust to accumulate in the gaps. Finally, routine maintenance should be normalized: clean surface dust once before and once after each rainy season each year, and do not let fallen leaves and dust adhere to the surface long-term. Components in shaded areas can have the surface topcoat reapplied every 2-3 years—low in cost but greatly extending service life.

FAQ:

1. Does mold and algae growth affect the structural strength of UHPC?

Mild surface microbial spots do not affect internal structural strength, but if penetration into the component has caused surface powdering or flaking, timely surface reinforcement treatment is needed to prevent further erosion.

2. Can ordinary household mold removers be used on UHPC facades?

Not recommended. Ordinary household mold removers mostly contain high concentrations of bleach, which can easily cause uneven white spots on the UHPC surface and ruin the original finish. Neutral algae removal products designed specifically for cement-based building materials must be used.

3. Can UHPC that has undergone pore sealing treatment still grow mold?

If the sealing layer is intact and daily maintenance is properly performed, large-scale mold and algae growth will basically not occur within 10 years. If the components are long-term exposed to special environments with high humidity and shade, simply reapply the topcoat periodically.