It looks like it might be sand and cement. Has a chimney breast or something been removed? Is it a party wall that was built from the other side, against an obstruction?
Stand back and take a wider pic. Include something to show scale.
What is on the other side of the wall?
In some districts a cement or lime render is applied before finish plaster, but IME it it not very hard or strong.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local