Utilization of Industrial Wastes and CO2 in Construction Materials - an Estonian Perspective

Using oil shale (OS) as a primary source for energy production in Estonia is the cause of vast amounts of mineral wastes, which currently are almost entirely deposited in the ash fields. OS wastes constitute about 80% of all waste produced in Estonia. The industrial ashes from cement industry and municipal solid waste incineration (MSWI) make up another significant waste stream. The MSWI fly ash and APC residues are hazardous wastes that require pre-treatment before landfilling. In addition, these wastes from power and cement plants are accompanied with CO₂ emissions. Closing the waste loop requires applications for millions of tons of ashes. By using the agglomeration process, these wastes could become products either in construction or agricultural industry. Previous practice has shown that OS ashes and cement kiln dust (CKD) could be utilized as liming agents for acidic soils, but the ashes need to be granulated to meet the standards of modern agriculture. About 200,000 ha of agricultural land in Estonia requires annual liming. Alternatively, the wastes could be utilized to produce construction aggregates by accelerated carbonation-based solidification [1].

The aim of the current study was to investigate the variety of industrial wastes (OS ashes from Narva Power Plants (PP) and shale oil production (Enefit280), CKD from AS Kunda Nordic Tsement and MSWI APC and fly ashes from IRU PP) in Estonia for production of constructional aggregates, focusing on the cementitious properties, CO₂ binding ability and leaching behaviour. The experiments were carried out in a mixer-type granulation. The wastes were treated individually as well as in composition of mixtures (using 0…20% of cement as additive) under different operating regimes (by varying solid to liquid ratio, rotation speed, CO₂ content in model gas, residence time, curing conditions etc.) to maximize the CO₂ uptake and produce construction aggregates with respective mechanical and environmental characteristics.

The results indicated the CO₂ uptake as well as the mechanical properties of OS ashes, CKD and MSWI residues differ in very wide range. The CO₂ uptake is mainly attributed by the free lime content, which is relatively high (10…15%) in Narva PP ashes and Iru MSWI APC ash and negligible in CKD, Iru fly ash and Enefit-280. Tests indicated that the free lime content was exhausted with 30min in the conditions of low range water solid ratio (0,2…0,5w/w) and 18% CO₂ in air. The mixtures of different types of wastes with the addition of cement (5…15%) resulted aggregates with satisfactory mechanical and leaching properties by capsuling the heavy metals, chlorides and sulphates.

The results of this study could be used as an important step in closing the waste cycle in Estonian energy and cement sectors.

[1] P.J. Gunning, C.D. Hills, P.J. Carey, Production of lightweight aggregate from industrial waste and carbon dioxide, Waste Management, 29 (2009) 2722-2728.