Reducing energy demand and CO2 emissions from industrial ceramic manufacture using novel raw materials and additives
Authors:   , Wie-Addo Gloria 
Published: November 2023

This study of reducing energy demand and CO2 emissions from industrial ceramic manufacture using novel raw materials and additives was undertaken to contribute to the industry's requirement to decrease energy inputs and CO2 emissions and increase material efficiency when manufacturing structural ceramic products whilst maintaining product quality. Weald clay was replaced with additive levels 1-4wt% alkali and alkaline earth carbonates, industrial and inorganic mineral additives and shaped into cylindrical pellets. The admixtures were fired to 850°C, 900°C, 1000°C and 1040°C. Ten characterisation techniques (XRF, XRD, TG/MS, MIP, dilatometry, volumetric shrinkage testing, boiling water absorption testing, compressive strength testing, Mössbauer spectroscopy and Raman spectroscopy) were used to determine the chemistry, microstructure and changes to the resulting ceramics in comparison to a reference standard and existing products. Mineralogically, the additive K2CO3 and Na2CO3 admixtures do not promote new firing phases not already present in the Weald clay ceramics fired to 900°C, 1000°C and 1040°C, as opposed to the effects of the other 12 additives studied. Alkali additives, however, promoted stronger bonds on firing, so the mechanical strength obtained for K2CO3 doped samples was over 85MPa compared to 81.5MPa for the control sample. Of the industrial waste additives, 2wt% container glass and 4wt% mineral wool were promising candidate for other products with resulting ceramic compressive strengths of 58MPa and ⁓54MPa, respectively. Only 4wt% wollastonite and talc additions for inorganic additives increased ceramic strength to 72MPa and 60MPa for ceramics fired to 1040°C, respectively. Nepheline syenite and colemanite additives functioned as pore formers, resulting in ceramics with lower compressive strengths of below 50MPa. Technologically, boiling water absorption, strength, mercury intrusion porosity (MIP), pore size, and shrinkage depended on firing temperature, additive type and proportion of additive. Volatiles in all cases were found to be CO2, re-absorbed water and chemical water. Pore sizes moved towards larger pores (> 1μm) with increased firing temperature (⁓1040°C). The alkaline earth additives controlled excessive expansion caused by carbonate decomposition and promoted reduced sintering temperatures of between 4- 41°C. Meanwhile, adding alkali carbonate additives (Li, K) at levels ⁓ >2wt% caused the clay body to expand again above the quartz transition temperature. The surface scum observed was found to be CaSO4 on 12 samples except for the samples doped with BaCO3 and greater than 2wt% MgCO3, both of which prevented the formation of the scum (whitish stains) on fired ceramic bodies. Fired ceramics with alkali carbonate additives added at 1wt% (Li, Na, and K) were free from surface scum. Surface defects such as patches and darker rims on ceramic surfaces were noticed in the >2wt% Li, Na, and K carbonate ceramic samples. Changes in fired ceramic colour were found due to differences in iron in the paramagnetic and magnetic iron sites. Iron is present in all admixtures as Fe3+ hematite (Fe2O3) except for the unfired Weald clay, which contained iron as Fe2+ and Fe3+. By firing ceramics at 1000°C and 1040°C, 90°C and 50°C temperature reductions have been obtained compared to industrial firing of bricks at 1090°C. This implies that ceramics fired to 1000°C and 1040°C resulted in reduced energy of 64kWh and 23kWh, respectively. Moreover, the additive 1wt% K2CO3 gives better CO2 savings than 1wt% Li2CO3 fired at a lower temperature yet retains the quality of a class B engineering brick. The replaced additives can save about 80,000-320,000 tonnes of clay. It is recommended that direct durability testing is conducted on all samples, but mainly on 2wt% MgCO3, 4wt% Wollastonite, 4wt% Talc, 1wt% K2CO3 samples fired to 1040°C and 1wt% Li2CO3 samples fired to 1000°C, to validate the modified frost resistance for heavy clay ceramic manufacturers. Pilot-scale testing for dopants to prevent efflorescence and scumming formed on fired ceramic products of MgCO3 as an alternative to BaCO3 is also recommended.