Utilization of Oil Shale Ash As a Concrete Constituent and Carbon Sink

The combustion of oil shale (OS) in electric power plants, based on circulating fluidized bed (CFB) combustion and pulverized firing (PF), is accompanied by the generation of vast amounts of waste ash (45–48% of the OS dry mass). Only a small percentage of oil shale ash (OSA) is utilized, either in the building materials industry, in agriculture as a liming agent or in road construction. Most of the OSA is still deposited in ash sediment fields near the power plants (5-7 Mt annually). Using oil shale ash based Portland cement free concrete for backfilling OS mines to maximize the recovery of mineral resource or as a building material, is a promising idea from the waste utilization point of view, but raises also concerns about the risks of pollution of surface water and groundwater.

The solid wastes of cementitious nature can be remediated by solidification whilst binding CO₂ in the process, because the reaction products can cause rapid hardening. The hydrous transformation of free lime (CaO), anhydrite (CaSO₄), secondary Ca(Mg)-silicate minerals and amorphous Al–Si glass phases control the solidification/cementation of OSAs, mainly by the formation of secondary Ca-rich hydrate phases and by the carbonation of portlandite.

In current study, the OS fire plant ashes were used as binders and the mining residue, lime stone splinters (4/16 mm), were used as an aggregate for concrete mixes. Several types of PF and CFB ashes (collected from the Estonian Power Plant) were tested because they differ noticeably by the chemical and phase composition as well as structure and surface characteristics. The solid residues from direct aqueous carbonation of ash were also included, because OSA could be used as a readily available and cheap Ca-source for CO2 sequestration [1].

The concrete constituents (50% of OSA and 50% of pre-dried aggregate) were mixed in a laboratory concrete mixer. After 48 hours of hardening, concrete cubes were demoulded and curing was carried out at 95±5 %RH and 20±2oC. The OSA based composites hardened for 7, 28 and 91 days were tested before and after leaching for compressive strength (EN 196), water resistance (EVS-EN 12390-8), pore size distribution (mercury intrusion porosimetry), mineral composition (quantitative X-ray diffraction) and microstructural properties (scanning electron microscopy). A batch leaching test according to CEN/TS 15862:2012 was carried out. Electrical conductivity (EC), pH (H2O) and the concentrations of Ca2+, K+, SO42-, Na+, Mg2+, Zn2+ and Cd2+ were determined from the leachates.

Hydration, setting and hardening processes of the OSA binders have specific characteristics influenced by type of combustion aggregate temperature (up to 1400^oC in PF and 800^oC in CFB boilers) and dust collector [2]. Pre-hardening period in humid conditions was needed before water exposure, because the hydraulic properties of OSA binders tend to be much weaker as compared to ordinary Portland cement. The compressive strength of 1 – 5 MPa was achieved after 7 days of curing. In most cases the strength development of OSA concretes was promoted by the 24 h leaching period, because in the curing conditions, the belated pozzolanic reactions might have been inhibited due to lack of reactive water inside concrete cube. The binding capacity of the finer fractions of PF ash, as a hydraulic binder, was mainly controlled by free CaO, secondary Ca-silicate phases and pozzolanic properties of amorphous silicate glass phase that result the fastest strength development and highest compressive strength (up to 26 MPa) among tested ashes. The cementation of coarser cyclone ash, characterized by very high free CaO content, resulted from forming CaCO3 bridges between the particles and as a typical air-binder, it had low compressive strength (2 MPa) and no durability in water. The cementation of pozzolanic CFB ash was controlled by the CaO/SiO2 ratio and the formation of gel-like hydro-silicates (compressive strength up to 14 MPa). The residues from direct aqueous carbonation of OSA might be utilized as neutral binder constituents mixed in combination of specific ash types, while their individual binding properties were very low.

The quantitative X-ray analysis indicated that OSA based concretes contained 30-50% of amorphous phase, but also calcite (up to 30%) and ettringite (up to 13%) as the main reaction products and lesser amount of portlandite, quartz, C2S and K-feldspar originating from the initial ash. The X-ray diffraction method does not allow for the direct determination of amorphous phases such as glass and calcium silicate hydrate gels.

The hardening dynamics of different types of OSA based concretes also reflected on the leaching dynamics. In most cases the pH and EC of the leachates crossed the limits set to general wastes in the earlier stage of curing but decreased considerably after 28-91 days (pH<11,5; EC<1000µs/cm). As expected, the leachates of coarse PF cyclone ash based concretes, characterized by the low water resistance and considerable portlandite content, remained deeply alkaline throughout the hardening process, but decreased to a much lower level (pH<10) in case the concrete mixes contained CFB ash or carbonated ashes. Thus, the carbonated OSA could be used as a pH decreasing binder constituent in combination with pulverized firing ashes.

The results indicated that although the strength development and water durability of different oil shale ash based concretes tend to vary, several types of ashes could be utilized as concrete constituents for mine backfilling. Carbon dioxide is bound in the process.

[1] M. Uibu, O. Velts, R. Kuusik, Developments in CO2 mineral carbonation of oil shale ash, Journal of Hazardous Materials, 174 (2010) 209-214.

[2] L.-M. Raado, R. Kuusik, T. Hain, M. Uibu, P. Somelar, Oil shale based stone formation - hydration, hardening dynamics and phase transformations, Oil Shale, 31 (2014) 91-101.