If small stones or chippings are thrown onto the sand and cement mix already applied to the
wall surface, the finish is known as a peppledash surface. Pebbledash is a popular surface coat, consisting of a thick base coat covered with a thin coat of render and small stones.
Small cracks should be filled as with cracks in render.
It could be the thin layer has separated from the thick base coat or the whole surface has separated from the wall behind. Careully break away the loose area and determine the extent of the damage.
If the thin top coat has separated, use a stabiliser to seal the surface and repair the surface. If the damage is through to the wall, replace both surfaces in two stages, allowing the thick base coat to dry before applying the thin pebbled layer.
If the pebbledash is to remain unpainted, try to use some of the original pebbles as replacements. If the patch is too obvious and in a very public area, it may be worth considering painting the wall for an alternative attractive finish.