I created a walkway with treated 4x4 as a perimeter. Post are anchored to the ground and divided every 4 feet with 4x4 horizontals. I filled walkway 3/4 full with concrete sand and compacted. The last inch I mixed concrete sand with mortar mix. Once leveled I wetted and let set up. After a month top layer is starting to chip apart. My friend didn't want concrete or stone because walkway is on a downhill slope. Can anyone help?