How to rebuild a small removed section of a aggregate sidewalk?

Frame the section with wood. Then you can replace with whatever type aggregates you want in the area. Once it is set you can remove the frame.

https://homeguides.sfgate.com/replace-broken-sidewalk-slab-47814.html

https://www.doityourself.com/stry/how-to-repair-exposed-aggregate-concrete

https://www.bing.com/videos/search?q=How+to+rebuild+a+small+removed+section+of+a+aggregate+sidewalk%3f&qpvt=How+to+rebuild+a+small+removed+section+of+a+aggregate+sidewalk%3f&FORM=VDRE

I hope these are helpful to you on your project!

Aggregate concrete is done by rinsing off the top layer after the concrete has set up slightly. Pour concrete into form, trowel it smooth and then come back later and wash it off using a hose. The top layer will wash off and the stones will be exposed. I inadvertently got exposed aggregate when it started to rain shortly after pouring a mow strip.

Matching the existing aggregate will be tough. Good luck.