Developed by the ancient Romans, concrete is the material of choice for footing design in Passive Solar Designed homes, spreading mass evenly throughout the house, and locking it to the mass of the earth below to stabilize internal temperatures. It also acts as an energy well; absorbing winter sunlight and re-raditiating the heat once the sun goes down. And in summer it is a coolness store, taking many days of hot weather to heat up. In a house with good solar access, well designed windows and good insulation, a concrete slab will usually add around 1 star in thermal improvement, compared to a timber subfloor. Unfortuantely, mainly due to the cement and steel inside, concrete can be very high in embodied energy. But there are things you can do to significantly reduce the embodied energy while retaining the operational benefit on concrete.
The cement in concete is very energy intensive to make, with 1 tonne of cement releasing around 1 tonne of CO2. To reduce the embodied energy, up to 60% of cement can be replaced with slag and flyash, which are both waste products, (slag from iron smelting, and flyash from coal power plants). These products have a similar bonding mechanism to cement, and most concrete companies will be willing to to do some cement replacement. When buying bagged cement, look out for ones advertising they are a triple blend.
Steel is also very high in embodied energy, and there is a lot of steel to provide tension strength in our modern slabs and tilt up panels. Conveniently there are 100% recycled steel alternatives on the market. Look out for steel made from scrap recycled and produced in ARC furnaces.
Lastly the plastic underlay membrane and bar chairs can also be sourced with recycled content.
Through such specification you could quite easily save around 5 tonnes of CO2 in a standard pour.
We have also had a lot of success seeding such slabs with recycled glass and polishing it to give that hard performing eco slab the 'bling' it deserves.