“Greening” the Construction Industry: Sustainable Alternatives to Conventional Cements Made Possible Through New PCE Superplasticizer Design

Concrete is the most commonly used man-made material globally. Regretably, cement - its major ingredient – exhibits an extremely high CO2 footprint. However, substituting cement clinker with supplementary cementitious materials (SCMs) that possess a low carbon footprint can substantially decrease CO2 emission. The two main alternatives to clinker which are currently under investigation include calcined clay and slag (GGBFS). A prominent example of a low carbon binder based on thermally activated (calcined) clay is LC3 which contains 50% clinker, 30% calcined clay, 15% limestone and 5% gypsum and exhibits a CO2 footprint of 550 - 600 kg CO2/ton. Advanced chemical admixtures are indispensable to achieve sufficient workability (especially slump retention) and – depending on the composition of the raw clay – high early strength. The second concept involves clinker substitution by incorporating slag from the iron industry and – at high clinker replacement rates – an alkali activator. An extreme example of such alkali-activated slag cement comprises 15% clinker, 82% slag and 3% alkali activator. This binder allows to reduce CO2 emission from ~825 kg CO2/ton for OPC to ~220 kg CO2/ton for this slag cement. Despite the remarkable progress achieved in reducing CO2 emissions through the use of low carbon substitutes for cement clinker, the problem emerged that these binders often exhibit much inferior rheological properties and low early (1d) strength. Consequently, substantial research efforts have been made to develop suitable superplasticizers and accelerators. This article presents about novel polycarboxylate admixtures (PCEs) for low carbon binders and innovative new materials which can greatly promote early strength. Finally, some still unresolved problems are addressed and the decisive role of chemical admixtures in the transition process to “green” binders is evidenced.