Sahar, in my opinion a good topic for a PhD consists 90% on your personal interest about the subject, therefore I don't think I can point you exactly where to look at. I can, however, help you understand a little bit more what is going on the science on terms of sustainability and concrete. As far as I'm informed, the properties of concrete (it's composition, durability and consequences) have already been widely studied. The hotspot relies on finding sustainable and cost-efficient alternatives for it to be used.

Here in Brazil there is a researcher called Felipe Lima da Costa that is currently studying sustainable alternatives for concrete and he came upon a mixture of coal and cinder (among other ingredients) that worked similarly and it was less harmful for the environment. Maybe you could look up his work as a starting point. He was the brazilian winner of Fame Lab 2017, where he provides a fairly simple, yet interesting explaination about his research. I'm not sure if this content is available in english, but you should defnitely give it a try.

Consultar sobre el Concreto Celular sus beneficios ambientales

There are many interesting areas to focus on. Here are some examples:

- Life-Cycle Sustainability Assessment (LCSA) of concrete structures; challenges and new perspectives

- Ultra High-Performance Concrete; Strength vs Sustainability

- A decision making framework to choose the type of concrete and reinforcement in various locations for high performance and sustainability

3Dprinting, recycled components on concrete, and the sustainability of additive manufacturing