Here is a try for procedural concrete, basically it's only *Noise Texture* nodes mixed together (with *Math/Add* nodes or with *MixRGB/Overlay* nodes) to have various sizes and shapes of 3D bumps and grey spots:

[![enter image description here][1]][1]
[![enter image description here][2]][2]

File: [<img src="https://blend-exchange.giantcowfilms.com/embedImage.png?bid=NB3kMO4G" />](https://blend-exchange.giantcowfilms.com/b/NB3kMO4G/)


  [1]: https://i.sstatic.net/FTnNA.jpg
  [2]: https://i.sstatic.net/pCW1W.jpg