A common question on my radio show is how to clean oil stains off driveways.
Before describing how to clean it, the first thing to look at is what the stain actually is.
When you first use it, engine oil is essentially clear. But after being in your engine for a while, it darkens, particularly in the case of a diesel where it becomes a very dark sooty black.
This black stuff in your oil is simply carbon that comes from unburnt fuel (I’ll put up a separate blog post at some stage explaining the chemical process that occurs when fuel burns.).
Now the actual oil is very easy to get rid of. If you spill some new oil onto your driveway, it could be easily removed with any number of degreasers, with no residue left behind. But with engine oil residue is left behind, due to the carbon, so that’s what we must address when we are considering how to clean the driveway.
The other problem with carbon, is that it sticks very strongly to many things. This is why it makes such a popular filtration medium (activated carbon or charcoal). When used as a filter medium, the carbon effectively removes many of the impurities in your water stream, and while that is good when you are using as a filter, it is not so good on your driveway, as it adsorbs quite strongly to the cement.
So how to get it off?
Remove the oil. They are there are any number of very good degreasers on the market just pour some on and rub it with the stiff broom, leave it for a few minutes and hose off. Do that a couple of times until you are seeing no more colour removal.
Get some plaster of Paris, make it up according to the instructions, with the addition of a small amount of dishwashing detergent (to help it wet into the cement) pour it onto the stain and let it dry for a day or two. It should then brush off with a stiff brush (or perhaps a wire brush) taking most of the stain with it.
What is happening here is that the plaster is acting as a poultice. That is, it is providing an alternative medium for the carbon to stick to. The chemistry of cement and the chemistry of the plaster, as far as carbon is concerned, are quite similar: they are both inorganic composites that the carbon will happily adsorb to. The good thing about the plaster is that because it is very fine it has a very high surface area and therefore provides a good alternative for the carbon to adsorb to it.
Paint some whitewash on top.
Whitewash? What’s that? Stay tuned…..with a bit of help from Tom Sawyer we’ll revive one of the oldest and simplest ways of restoring cement….