What is a sustainable or low impact concrete?

This paper compares a range of new and proposed 'greener' concretes and evaluates their environmental impact via quantification of their embodied energy. These new concretes are further compared with bio based concretes so as to develop a broad picture of the relative environmental impact of the increasing array of concretes now available to building designers. Some uses, advantages and disadvantages of each type are discussed. Particularly the quantity and volume of concrete material for each specific use case is considered for comparison of the embodied energy for a square meter of building envelope structure. Results show that bio based concretes have considerably lower impact than standard concretes, as exhibited by much lower embodied energies per kilogram of material. However, those values documented in only a few studies, and further repeatedly referenced in the wider literature, are approximate at best and sometimes inaccurate. Ultra high performance and geopolymer concretes have higher embodied energies but due to their high strengths less material is used, giving them a low environmental impact advantage over standard concrete materials. However, claims that these concretes are many multiples less impactful is widely inaccurate, and misleading promotion. In a similar vein, this work also questions the claims of carbon negativity of popular bio based concretes, such as hemp-lime. Investigation of the means of carbon sequestration and the difficulties in its quantification are discussed. More realistic estimates of the energy embodied of hemp-lime are used for calculation of the embodied energy, and carbon, for walls sized appropriate to low energy architecture.