This still causes net CO2 emission to the atmosphere (cement does end up absorbing some CO2 later, but it cannot absorb all of it.)
Eliminating CO2 emission from cement production is actually a considerable problem. It's either going to require complete sequestration of all this unavoidable CO2 production, or it's going to require production of calcium oxide from silicates. For example:
CaSiO3 + 2 HCl --> CaCl2 + H2O + SiO2
CaCl2 + H2O + heat --> CaO + 2 HCl
The last step occurs at high temperature, driving the reaction to the right.
Eliminating CO2 emission from cement production is actually a considerable problem. It's either going to require complete sequestration of all this unavoidable CO2 production, or it's going to require production of calcium oxide from silicates. For example:
CaSiO3 + 2 HCl --> CaCl2 + H2O + SiO2
CaCl2 + H2O + heat --> CaO + 2 HCl
The last step occurs at high temperature, driving the reaction to the right.