Carbonation Processing and Characterization of Fly Ash-Portlandite Blends as Cement-Free Binding Agents

Carbonation of fly ash mortars augmented with portlandite (CH) under ambient conditions to produce cement-free binding agents that sequester CO₂ is discussed. Fly ash-CH blends are proportioned to produce shape-stable mixtures that can be conventionally cast or extruded. The strength and CO₂ uptake are noted to strongly depend on specimen saturation. High-Ca fly ash blended with 10% CH (by mass) results in a 7-day compressive strength of more than 35 MPa when carbonated, which is ~40% higher than that of the conventionally cured mixture, and a CO₂ uptake of ~8% by mass of the binder. Low-Ca fly ash blended with 30% CH results in 15 MPa strength, which is 3 times higher than that of the conventionally cured mixture, and a CO₂ uptake of ~16% by mass of the binder. The proportion of initial voids that remain after carbonation or conventional curing, which scales well with compressive strength, is shown to be a convenient measure to quantify the relative efficiency of carbonation. Thermal analysis, electron micrographs and X-ray maps, and FTIR spectroscopy confirm the carbonation efficiency of the mixtures. The beneficial effects of low w/p and particle packing in extrudable mixtures towards providing increased strengths and CO₂ uptake are also brought out. Overall, this study establishes the applicability (in terms of strength) of portlandite-enriched fly ashes to form binding agents with significantly lower CO₂ footprint for several construction applications.