Limestone might not be the only source for Portland cement. A sample of Portland cement made from basalt. A sample of Portland cement made from basalt. Cement production alone currently accounts for about 8 percent of global CO2 emissions, so considerable effort is going into lowering that number. Efficiency can be increased, and energy sources can be swapped for cleaner ones, but a stubborn reality remains: The byproduct of turning limestone into lime during cement production releases CO2 gas.