Information on Curing Render During Dry Weather
During dry weather, particularly when combined with warm or windy conditions, Rockcote often receives feedback that render can dry soft and powdery. Before concluding that something is wrong with the render, we would like to offer some information to our customers.
To form concrete, three components are required:
Cement + Sand + Water = Concrete
It is a common misconception that water is simply used to mix concrete and that once it dries, the concrete should be hard. In fact, water is required alongside cement and sand to form concrete.
During the mixing process, the addition of water causes a chemical reaction known as hydration. This process allows the cement to develop strength over time. Any excess water is gradually lost through drying.
The formation of concrete takes time.
Concrete typically takes around 28 days to develop its specified compressive strength under standard curing conditions. Assuming it has sufficient moisture, concrete can achieve approximately 80% of its strength within seven days at 25°C, which helps explain why you can’t drive on a new concrete driveway straight away.
At lower temperatures, the initial setting and strength development of concrete are considerably slower. For example, at 5°C, it can take five days to achieve the same strength as one day at 40°C.
So, what happens in dry weather?
Walls can draw moisture from the render more quickly, while dry air and windy conditions can accelerate moisture loss. Warm temperatures can further increase the rate of drying, while cooler temperatures can slow the cement hydration process.
A heavy dew overnight may assist with hydration by providing additional moisture. However, in dry and windy conditions, this additional moisture may not be available.
In dry conditions, cement render may lose the moisture required for proper hydration before it has developed sufficient strength. This can result in render remaining soft or powdery, regardless of whether conditions are warm or cool.
Frequently Asked Questions:
How do I tell if I have an issue with hydration?
Spray a heavy mist of water over most of the affected wall, ensuring the render is thoroughly dampened, and leave it overnight.
Check the render after 24 hours. If insufficient hydration is causing the issue, the dampened area should be harder than the area that was not sprayed.
Repeat the process of misting with water as required to assist with continued hydration and hardening.
Another way to identify potential hydration issues is to compare different areas of the building. For example, exposed northern or western walls may be softer than southern walls due to increased exposure to sun and wind.
Why can’t Rockcote make a product to cater for these conditions?
Rockcote uses technology that helps cement render retain moisture during the curing process. However, adding too much water-retaining technology can create other challenges, including extended setting times and reduced workability.
Rockcote does not change its cement render formulations between summer and winter. This is because our products are used across a wide variety of geographical locations and climatic conditions, making seasonal formulation adjustments difficult to control effectively.
How can I overcome these issues?
- Wet down masonry walls with low moisture content or high suction before applying render. For walls with very high or variable suction, consider using a Rockcote Keycote slurry to help manage moisture absorption.
- Assess substrate absorption: To determine whether the wall or substrate has a high or low absorption rate, lightly spray the surface with water and observe how quickly it is absorbed. If the water is absorbed quickly, consider sealing the wall first with Rockcote Rockbond 27 or a mixture of Rockcote Acrybond and water at a ratio of 1 part Acrybond to 4 parts water. A coarse slurry mix of Rockcote Keycote is suitable for both high and low absorption rates. This helps control suction across the wall, supports cement hydration and slows moisture evaporation from the render, providing a well-bonded, stable surface ready for the next stage of the Rockcote coating system.
- Mixing: Mix the render for 3–5 minutes, then allow the mixture to stand for approximately 5 minutes before remixing and adjusting the consistency as required.
- Curing: In hot, dry and/or windy conditions, re-wet the walls twice daily for two days after application to support proper curing.