Biomass ash (BA) waste as an activator to produce carbon-negative cement

The use of biomass, as a renewable energy source, to run heating and power plants is propelled by sustainable European policy. Olive is an important resource in Mediterranean countries. The residues from the extraction of olive oil are used as biomass, either to produce the oil or to generate heat or electricity. The disposal of ash residue poses an important burden. This study uses olive pit bottom ash waste (OBA) to produce carbon-negative cement. The OBA is mixed with waste GGBS (GGBS), and neither calcination nor thermal curing are used to lower environmental impact.

The cements produced contain up to 60 %OBA and have a carbon sequestration capacity up to -97.45 kg CO₂e/m³. An optimum mix with 40 %OBA is developed (using auxiliary activator), with compressive strength of 36–44 MPa and a carbon sequestration capacity of 40–45 kg CO₂/m³. A modified loss on ignition test is proposed to evaluate the embodied carbon of biomass ash.

The OBA's main chemical constituents: K₂O and CaO, afford outstanding activation and alkalinity to release Ca²⁺ Si⁴⁺ and Al³⁺ from GGBS to form calcite, hydrotalcite, C-(A)-S-H and amorphous cements. Using sodium carbonate (NC) and lime as supplementary activators enhanced the mechanical properties of the cements and slightly changed their composition and microstructure. NC is the most efficient activator, it increased dissolution, and produced a denser and stronger cement with higher Si and K concentration that includes gaylussite, N-A-S-H and C(K)-A-S-H. Pre-dissolving the NC prior to mixing increases the activator's efficiency, producing less calcite cement for the same amount of NC. By adding 4 % pre-dissolved NC, the compressive strength increased by 138.76 % (compared to OBA-GGBS mortar without auxiliary activators) and 113.94 % compared to the material with NC in powder form.