Concrete Casting: How to Stop White Powder Forming in Silicone Moulds
- Astrid van Essen
- 2 days ago
- 4 min read
I cast a lot of concrete and try to look after my moulds the best I can—but now and then, I still get that frustrating white powder on the surface of a finished piece. If you’ve seen this too, you’re not alone. It’s usually efflorescence, a common issue caused by moisture and salts in the mix or environment.
Over time, I’ve picked up a few reliable tricks to keep it at bay. This blog shares what’s worked for me—simple mould prep steps, moisture control tips, and sealing advice that can help keep your castings clean and smooth.
What Is Efflorescence?
Efflorescence is a white, chalky powder that appears on the surface of concrete when moisture carries salts from within the mix to the outer layer. As the water evaporates, salt deposits are left behind. This often happens when moulds are damp or the curing environment is too humid.
Step-by-Step: How To Prevent White Powder in My Castings
1. Prep Your Moulds: Damp or Dry?
I like to give my silicone moulds a quick rinse just before casting. This helps reduce surface bubbles and gives a smoother finish, especially with smaller details or warmer weather.
That said, moisture can sometimes contribute to efflorescence (the white powdery residue), especially if you’re in a humid environment or using too much water. So here’s what I recommend:
If you're trying to reduce air bubbles: A light rinse or mist is helpful. Just tip the mould upside down and let any excess drip out before pouring. If you're trying to avoid efflorescence: Make sure your mould is fully dry and focus on moisture control during curing instead.
It’s all about finding the balance for your environment and the type of concrete pieces you're casting.
🟢 Tip: If you’ve washed your moulds recently, let them air dry overnight or store them with silica gel packets to keep humidity low.
2. Clean Before Every Use
I gently wash my moulds with mild soap and warm water, then dry them with a lint-free cloth. Any leftover concrete dust, oils, or salts can interfere with the next casting.
Avoid using harsh scrubbing pads—they can damage the silicone over time.
3. Use a Mould Release Agent
A good release agent helps the piece pop out cleanly and forms a barrier between the concrete and the silicone. This helps limit moisture transfer and reduces the chance of surface salts.
I use a silicone-safe mould release before every pour—a quick step that makes a big difference.
4. Watch the Water Content
Concrete should not be soupy. I use just enough water to make the mix workable and avoid adding more than needed. Too much water increases the chance of efflorescence and weakens the final piece.
When possible, I use distilled water—especially if I know my tap water is hard or mineral-rich.
5. Cure in the Right Environment
Humidity plays a significant role in how the concrete sets. I cure my pieces in a dry, well-ventilated space, away from direct sunlight or overly damp corners of the house.
It’s a balance: you don’t want it to dry too fast (that can cause cracks), but you also don’t want lingering moisture encouraging salt to surface.
6. Seal Once Dry
Once fully cured and dry, I always seal or wax my pieces. A breathable, water-repellent sealer helps protect the surface and stops moisture from creeping in later.
I use this on planters, trays, incense holders—everything I cast that might come into contact with air or water.
7. Clean and Store Moulds Properly
After each use, I gently clean my moulds with soap and let them dry thoroughly before putting them away. This helps prevent white buildup and prolongs the life of the silicone.
Concrete Mould Care Checklist
Use this to keep your castings clean and your moulds in top shape:
Before Casting
Dry silicone moulds completely
Clean with soap and warm water
Apply a mould release agent
Use minimal, clean water in your mix
During Curing
Cure in a dry, well-ventilated space
Avoid overly humid environments
After Demoulding
Let the piece cure thoroughly before sealing
Seal with breathable water-repellent sealer
Clean and dry the moulds again before storage
Final Thoughts
Casting concrete at home is a hands-on, satisfying process, but keeping things clean and consistent takes a bit of planning. Try tweaking your prep and curing routine if you've struggled with powdery finishes. A few small changes can make a big difference in the quality of your handmade pieces.
Want to learn more about how I care for my concrete accessories?
Check out:
Concrete Casting With Silicone Moulds: Frequently Asked Questions
1. What is the white powder on my concrete? That’s usually efflorescence—a result of salts rising to the surface when moisture evaporates from the concrete.
2. Can I remove it once it appears? Yes. You can gently brush it off or use a 50/50 mix of water and vinegar. Always test on a small area first if the piece is sealed.
3. Is distilled water essential? No, but it's a really good option when you live in a hard water area, as it can help reduce salts and improve the final finish.
4. Should I use a mould release every time? You don't have to, but it does make demolding easier and helps prevent moisture and salts from leaching through the silicone.
5. Does sealing completely stop efflorescence? It helps a lot, but prevention starts with a dry mould, the right mix, and proper curing. Sealing adds an extra layer of protection.
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