Cement Dust and Timber

Way back in May 2016, Preschem was asked to give a presentation to the Victorian Woodworkers Association. The topic was Why Timber Changes Colour, which we turned into a news article for the Preschem website. We decided to revisit this topic as we were recently contacted by Shane, who had developed an issue with Aussie Clear Light exterior timber oil on his new Silvertop Ash deck. He noticed that 2 weeks after he oiled and it rained and that the Aussie Clear Light had pretty much disappeared. What was alarming, was the timber had gone quite grey where the rain had fallen.

Thankfully, Shane was observant and had noted at the deck was covered with cement dust prior to it raining, as he had landscape gardeners on site doing work. This bit of information was crucial. Cement dust when mixed with water is highly alkaline (with an online reported pH range of 12 to 13.8). This was the likely cause of the problem.

As you can see from the after photos that Shane provided, there is a significant issue with discolouration on the weather exposed portion of his deck. Preschem has seen this this before on two or three rare occasions but was never able to determine the cause. Shane’s attention to detail has solved this baffling riddle.

WHY DOES TIMBER CHANGE COLOUR?

Firstly, lets recap on issues that causes timber to change colour. In broad terms, these are;

·        Exposure to the sun and rain;

·        Biological staining from bluestain and sapstain moulds;

·        Discolouration due to the onset of brown or white rot;

·        Chemical staining due to chemical agents, such as iron staining (from metal work near or on exterior timber) or ammonia, or pH changes from alkali or acids that oxidise the natural pigments in wood;

·        Leeching of the natural pigments out of the timber from solvents including water;

THE EXPERIMENT

What Preschem decided to do was to confirm that cement dust on timber caused it to change colour and break down the Aussie Clear Light. The method was very straight forward.

We made two panels approximately 350 x 900mm and applied 2 coats of Aussie Clear Light to 3 of the 5 boards on both panels. We then spread a small amount of cement dust onto one of the panels and wet it with water to simulate it raining, and didn’t treat the other panel so we had a control. The difference here is that that we decided to film the experiment as we were doing it.

CONCLUSION

While you can see that there were a number of scenes or takes in the video, it took about an hour in total to film. The discolouration came up very quickly, within 2-3 minutes on raw timber. Some of the other observations, like the pigment and oil from the Aussie Clear Light being stripped out, took more time to be noticeable.

One thing is very clear, cover your deck before conducting landscaping when using cement. Should you get cement dust on the deck, brush or blow off as much as possible first while dry. Then use Grey Deck Cleaner, which is a mild acid known as Oxalic Acid, to nuetralise any remaining cement dust. Then use a lot of water to wash the deck down. This should prevent this type of discolouration and should keep your deck in tip top condition.