Trends toward lower-carbon ultra-high performance concrete (UHPC) - A review

The development of ultra-high performance concrete (UHPC) with lower carbon emission has conducted by many researchers. However, there is a lack of quantitative comparison of carbon reduction efficiency, such as carbon emission and energy consumption values, of UHPC materials with various components. This paper reviews the trends toward lower-carbon UHPC based on Life Cycle Assessment related to the used materials, especially different binders, aggregates, and steel fibers. The environmental indicators, including embodied carbon emissions (EC), embodied energy consumption (EE), and embodied CO2 index (CI) value, of UHPC mixtures in published paper are discussed. Since compressive strength is an important indicator, the minimum value of compressive strength of UHPC should be greater than 120 MPa in the assessment process. Moreover, the effectiveness of using various low-carbon alternatives is discussed and then ranked according to the reduction degrees of EC and EE. Results found that ultra-high performance geopolymer concrete (UHPGC) shows the lowest EC and EE and the highest CI. This indicates that UHPGC has decreased carbon emissions without compromising compressive strength. The use of supplementary cementitious materials and recycled powder obtained from waste aggregates to substitute cement is another effective way to reduce EE and EC. However, using waste aggregates or waste fibers to replace conventional aggregates or steel fibers is less efficient than cement replacement with supplementary cementitious materials (SCMs). Especially, the use of waste aggregate leads to negligible reductions in EC and EE. The study seeks to find out the most promising route for low-carbon UHPC in the circular economy through identifying low-carbon alternatives, which has great significance in designing lowcarbon UHPC.