Learn French: Ils ont fait le béton ce matin. - They made the concrete this morning.

You would use être en train de (to be in the process of). Because the action is in the past, you put être in the imperfect tense: Ils étaient en train de faire le béton ce matin. This strongly emphasizes the ongoing process, contrasting with the completed action of Ils ont fait.

We usually 'mix' or 'pour' concrete in English. Is faire the standard verb here?

Yes, faire is extremely versatile in French. While you can use more specific verbs like couler (to pour) or mélanger (to mix), faire le béton is perfectly natural and commonly used for the general act of making or preparing concrete.

Why do we use ce matin instead of cette matinée?

Matin is masculine, so it takes the demonstrative adjective ce (this). We use matin to point to the specific time of day the event happened. You would use matinée (feminine) to emphasize the duration of the morning, for example if you said they spent the whole morning making concrete (toute la matinée).

Could Ils ont fait also be translated as 'They did make' or 'They have made'?