A lot of fossil fuels are burned in the gathering and transportation of the components for making concrete.
Here in Jakarta, 30-40 000 tonnes of sand and aggregate are trucked into the city every day. Good quality sand with sufficient angularity is becoming more scarce, and has to be hauled longer distances. The price of good sand has tripled in the last 5 years.
I do not know the consumption figures for Singapore, but they are now transporting some of their sand all the way from Thailand!