Chemical, mineralogical, and microstructural interactions between lime-pozzolan binders and miscanthus particles in lightweight concretes

The incorporation of bio-aggregates such as miscanthus in mineral binder matrices can produce lightweight insulation materials. Bio-aggregate lightweight concretes show peculiar microstructure compared to standard mineral aggregate concretes. This paper evaluates the chemical and mineralogical interactions between miscanthus particles and blends of mineral binders that include hydrated lime (CL90s), natural hydraulic lime (NHL3.5), formulated lime (FLA3.5) and mineral additions such as ground granulated blast furnace slag (GGBS), fly ash (FA) and Portland cement (Ce). The properties and the influence of ternary binder blends made of 75%CL90s, 15%NHL3.5 and 10% mineral additions (GGBS/FA/Ce) on the morphology of the interfacial zone were investigated. It was observed that carbonation of samples proceeds from the outer shell to the inner core of samples leaving a physically identifiable hardened shell (external 1–2 cm) and a softer inner core. In all samples, the X-ray diffraction results show that there is a reduction in peak intensities and broadening of Ca(OH)₂ peaks whereas peaks of CaCO₃ sharpen and increase in intensity. The mineralogical changes in the binding matrix depend on the type of mineral addition. Thermogravimetric analysis shows that the addition of 10% Portland cement results in the highest levels of carbonation (1.1–1.4%) and hydration (8.6–17.1%). However, also fly ash incorporation was found to be beneficial in terms of reaction processes, and its use can have positive environmental impacts due to its low embodied carbon. FTIR results show that the chemical composition of miscanthus surface changes in contact with all binders, with effects on the structures of both hemicellulose and lignin.