Garden border/retaining wall – What is this pointing?

Hi, I need to repair the loose sand stones blocks in this retaining wall/garden border. However I don't have much experience with masonry work and am unsure whether this pointing is made up cement, concrete or mortar. I don't think its concrete since it appears to be quite smooth and therefore is unlikely to contain aggregate. Hoping someone here can tell me what it is so I can use the same stuff.

Thanks!