Physico-chemical characterisation of residues from thermal waste treatments (high temperature incineration and pyrolysis) and their impact on mechanical performance of cement-based mortars

  • Kande Kapuge Wijesekara

Student thesis: Doctoral Thesis


Ordinary Portland cement (OPC) is the most frequently utilised substance on the planet after water, whose manufacture is responsible for around 8% of global carbon dioxide emissions. Therefore, research on waste derived materials which can partially or fully replace cement is very timely. Residues derived from thermal treatment of waste, municipal solid waste (MSW) and post-consumer mixed plastic waste (PC-MPW) has encouraging prospects of being able to replace OPC in concretes and mortars. Pyrolysis provides a sustainable solution for managing mixed/contaminated PC-MPW streams by producing valuable mixtures of liquids and gases, which have potential for use as fuels. This reduces the environmental footprint compared with landfilling, gasification and incineration processes. There are no studies reported in literature where PC-MPW pyrolysis residue chars and sintered/vitrified residues from high temperature incineration (1200oC) derived from MSW have been used as additives in cement mixtures. This study aimed to characterise these residues for their bulk properties and physio-chemical properties and perform the mechanical strength performance testing on the mortars with partial replacement of these residues and assess their microstructure, mineralogy and chemical bonds.
Attributed to their higher processing temperature vitrified ash (VA) and sintered bottom ash (SBA) had <0.5% of organics, showed positive pozzolanic activity by Frattini test and saturated lime (SL) method (51% and 55% for VA and SBA respectively) and therefore produced mortars replacing 'type 2 OPC (CEM-II)’ at 10, 25 and 50%. SBA and VA were successfully employed to replace CEM-II in a mortar at dosages of 10% and 25%, respectively, without impairing its mechanical strength qualities. SBA had higher levels of alkalinity (pH=10.7) than VA (pH=8.8), and the lower pH values measured for VA are owing to vitrification of the ash in VA as opposed to reactive ash, SBA. VA acted as a micro-filler whereas SBA could be used as a supplementary cementitious material (SCM) in cement mortars. Additional water added in 2-5% improved the strength in SBA but not in VA replaced mortars at 25% which was attributed to possible water retention capacities in SBA contributing to cement hydration mechanisms.
Pyrolysis residues were highly carbonaceous where biochar (BC) processed at 800oC had ~30% and plastic pyrolysis chars (PPCs) processed at 450oC had 55-71% organic matter that they had neutral pH and showed no pozzolanic activity by Frattini test. They replaced up to 5 wt% of 'type 1 OPC (CEM-I)’ in mortar mixes to maintain mortar consistency and without impacting mechanical performance of the mortar. They did not show any water holding capacity before and after impregnation with potassium hydroxide (KOH). The strength gain mechanism in these carbonaceous residues solely depend on micro-filler effects related to particle size distribution with the view of improving their porous structures via thermal activation.
Date of Award8 Dec 2023
Original languageEnglish
Awarding Institution
  • Teesside University
SupervisorPaul Sargent (Supervisor), David Hughes (Supervisor) & Chris Ennis (Supervisor)

Cite this