Cement is a major component of concrete, and that’s really the major CO2 issue:

Cement manufacturing releases CO2 in the atmosphere both directly when calcium carbonate is heated, producing lime and carbon dioxide, and also indirectly through the use of energy if its production involves the emission of CO2. The cement industry produces about 10% of global man-made CO2 emissions, of which 60% is from the chemical process, and 40% from burning fuel. A Chatham House study from 2018 estimates that the 4 billion tonnes of cement produced annually account for 8% of worldwide CO2 emissions.

Of course, concrete has many benefits, too, so it’s not as simple as “concrete bad.”

If we’re talking about existing buildings, maintaining those existing buildings (regardless of what they’re made of) is always going to have less of an environmental impact than tearing them down and building a new one. If we’re talking about new buildings, then yeah, minimizing the use of concrete is probably good. But if concrete is used to build public transit or sustainable power generation it might be a net gain.