Hi @Evelyn_y,
Welcome to the Bunnings Workshop community, it is wonderful to have you with us.
It looks like you have a concrete plinth or column, and the surface has weakened and is crumbling away. This is known as dusting or surface powdering. This happens when the top layer of the concrete is weak and didn’t harden properly during curing. It’s often caused by finishing the surface too early while bleed water was still present, using too much water in the mix, or allowing the concrete to dry out too quickly. Over time, this weak surface breaks down into fine dust when walked on or brushed, giving the appearance that the concrete is slowly crumbling away. In your case, it looks like the electrical cable may have rubbed against the surface, exposing an already existing weakness.
To fix this issue, you should start by brushing down the surface with a slightly stiff brush, such as this Mr Clean Handheld Iron Scrub Brush. The aim is to remove as much powdered concrete as possible. You want to make the surface as stable as possible before moving on.
You can then use a concrete sealer to seal the surface and better bind the crumbling concrete together. It can be applied with a paintbrush or paint roller.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
