Carbon footprint and water use of alkali-activated and hybrid cement mortars

Cement industry consumes large amounts of water, energy and it is one of the main sources of anthropogenic greenhouse gas emissions. Hence, addressing these environmental constraints leads to a search for alternative mortars to ordinary Portland cement (OPC) replacing the clinker by industrial by-products, such as blast furnace slag and/or fly ash. In this context, alkaline activated materials (AAM) show good mechanical properties comparable to OPC. However, AAM present durability problems against leaching phenomena, and workability problems because of the use of alkaline solutions. Therefore, other alternatives as the hybrid materials (HM) are receiving greater attention. In order to determine if the mechanical properties of hybrid and alkali-activated mortars are indeed comparable to those from Ordinary Portland Cements (OPC), this paper first examines the compressive strength and porosity of an AAM case study from slag and two HM systems, both from slag and fly ash. Moreover, since very little is known about their environmental impacts in terms on water use and carbon footprint they were calculated through the Life Cycle Assessment methodology and compared to those of OPC. Results revealed that the assessed AAM and two HM case studies obtained mechanical properties comparable to those for OPC. The AAM obtained −406.02 kg CO2/m3 while for the hybrids the carbon footprints were −253.26 kg CO2/m3 (HM from slag) and −253.27 kg CO2/m3 (HM from fly ash), proving that partial or total substitution of cement by waste in mortars reduces the carbon footprint of OPC more than 100%. In terms on water footprint, more than a 40% reduction in water use is obtained if HM are used instead of OPC supporting the idea that HM achieve good balance between carbon footprint and water use reductions.This study was funded by the regional project HORATSO-CM-UC3M. In addition, its authors would like to thank Noelia Casiano for her help during the tests carried out in the laboratory