Novel hybrid additive for low-carbon concrete

Supplementary Cementitious Materials (SCMs) can be effective levers to reduce the environmental impact of concrete. One of the major limitations for high substitution of clinker by SCMs in Portland cement is the loss of strength of the resulting concrete, particularly at early age, compared with similar concrete made with equivalent dosage of pure clinker Portland cement. Furthermore, the possibility to use larger fractions of SCMs requires strongly alkaline activators which pose environmental and safety problems and are not compatible with commercial PCE superplasticizers. A new hybrid additive was synthesized which works, at the same time, as seed for the clinker phase and as an activator for alkali-activated binders. The new product consists of an alkaline suspension of micro-sized particles containing nano-structured CSH seeds, amorphous portlandite and AFm phases embedded in a polymeric matrix. The structure and the mechanism of action of the new product on slag cements have been investigated by Environmental Scanning Electron Microscopy, X-ray Powder Diffraction, Thermal Analysis and Isothermal Calorimetry. The new product is fully compatible with PCE superplasticizers and can be used to produce alkali-activated concretes characterized by low-carbon impact.