To form concrete, 3 components are required: Cement + Sand + Water = Concrete
It is a common misconception that water is used simply to mix the concrete and when it dries, the concrete should be hard. Water is actually required along with the cement and sand for the concrete to fully cure. During the mixing process, the addition of water causes a chemical reaction to occur. This is referred to as ‘hydration’. The rest of the water is then lost to the air via drying. This occurs quickly in dry (low humidity) temperatures and if the water evaporates before the cement has fully hydrated, it can cause dry, soft and powdery render.
The formation of concrete takes time, with a period of around 28 days for concrete to fully harden. Assuming it has appropriate moisture, concrete takes about 7 days to reach 80% of strength at 25°C. At low temperatures the initial setting of concrete is considerably slower: at 5°C it takes 5 days to achieve the same strength as 1 day at 40°C.
So what happens in dry weather?
- Walls draw moisture from the render more quickly.
- The air will dry the render quickly, especially when combined with windy conditions (a heavy dew overnight will assist in hydration, but in dry and windy conditions this does not occur).
In cold, dry conditions, the cement curing process is much slower. As a result, it typically runs out of water before it hardens even if the same amount of time was suitable in warmer conditions.
How do I fix dry, soft and powdery render?
Gently saturate the rendered wall with water, ensuring the render is completely wet and leave for 24 hours. After 24 hours, check the render and it should be harder than it was. If necessary, repeat the process of saturating with water to continue hardening.
How can I overcome these issues?
Wet down masonry walls with low moisture or high suction and wait for the wet look to recede off the wall prior to application of render. A coat of Rockcote Keycote can be applied prior to the application of cement render basecoat(s) to further reduce suction. If the cement render looks dry upon completion, give it a heavy mist of water (may have to repeat the following day).