5. Binder Chemistry – Blended Systems and Intermediate Ca Content

Following the discussion in the two preceding chapters, which addressed high-calcium and low-calcium alkali-activated binder systems respectively, this chapter will provide a brief discussion of the progress which has been made in the development and characterisation of hybrid binders derived from intermediate-Ca precursors and mixtures of precursors. The need for durable, high-performance, low-CO₂ alternative binder systems, along with the good existing understanding of the chemical mechanisms of mechanical strength development and durability of high-calcium and low-calcium alkali-activated materials (AAMs) as outlined in Chaps. 3 and 4, has given motivation for an increasing focus on hybrid systems over the past years. These binders are expected to provide a good synergy between mechanical strength and durability, making use of the stable coexistence of the hydration-reaction products characteristic of hydration of Portland clinker or alkali-activated BFS (mainly C-S-H gels) and alkali-activated aluminosilicates (geopolymeric gel) [1–3]. Blending of aluminosilicate-rich materials with more reactive calcium sources (including Portland cement clinker) and with the use of a source of alkalis also opens the possibility for the use of aluminosilicate wastes or by-products which may be insufficiently reactive to provide good strength development when activated alone, providing a pathway to valorisation for these materials.