Geopolymeric Potential of the Estonian Oil Shale Solid Residues: Petroter Solid Heat Carrier Retorting Ash

Abstract

In this paper, the geopolymerization of the solid heat carrier (SHC) ash waste produced at Petroter shale oil plants, Viru Keemia Grupp (VKG), Estonia was studied. Different mixtures were prepared to study and evaluate the potential use of this SHC ash for geopolymer-type mortar and cement production and to compare alkali-activated black ash with the material having self-cemented upon hydration with plain water. Mixtures prepared with plain water and NaOH solution show comparable compressive strength development, but the mixture with NaOH affords significantly lower compressive strength values, which can be explained by the absence of an ettringite/monosulphate phase in the NaOH-activated samples. Hydrocalumite precipitated instead of ettringite in the NaOH-activated mixture does not provide the interlocking structure that is found in water mixtures, though the formation of an amorphous geopolymer phase is possibly observed in the NaOH-activated sample after 90 days of curing. Sodium silicate-and Na-silicate/NaOH-activated samples show a strong geopolymerization and development of Ca-Na-Al-silicate gel formed in the pore space of the ash aggregate. However, due to strong shrinkage upon drying, the compressive strength obtained after 7 days of curing is lost in the prolonged curing process, and further research into the causes and prevention of Ca-Na-Al-silicate gel shrinkage is needed.

Self-Cementing Properties and Alkali Activation of Enefit280 Solid Heat Carrier Retorting Ash

“…However, this gel and the platy crystals of the dominant cementitious secondary mineral hydrocalumite do not provide a similar interlocking and an overall strength as the ettringite meshes. Similar effects were observed by Paaver et al [15] in Petroter ashes treated with the NaOH solution.…”

“…This prediction of limited self-cementing properties of the Enefit280 WHB ash is further supported by the uniaxial compression tests of the WHB ash mixtures with plain water, showing an average compressive strength value of about 3.5 MPa after 7 days of curing under ambient conditions and an average of 3.8 MPa after 28 days. In comparison, the ash from pulverized firing boilers reaches a compressive strength value > 10 MPa, the CFB ash > 7 MPa [19] and the Petroter SHC ash > 5 MPa [15] in the same period of time.…”

“…However, the mineral characteristics of the WHB ash indicate that the content of several reactive phases (e.g. lime, belite-C 2 S) that can potentially react with water and are therefore responsible for hydraulic self-cementing, is low when compared to the Petroter ash or any other TPP ashes [15,17,19]. For example, the content of belite, which is a typical phase in a cement clinker with a final compressive strength of about 40 MPa for a pure compound [20], is rather low (< 4 wt%) in the WHB ash.…”

“…The composition of the Enefit280 ash is significantly different from that of the other retorting residues produced either in the Kiviter-type retorts (semicoke) or other SHC Galoter-type retorts, including Petroter and Enefit140 (black ash). Firstly, the residues from the latter are black/dark grey in colour due to the presence of unburnt organic carbon (C org ) whose content varies between 1.5 and 2.2 wt% in the Petroter ash [15] and from 8 to 10 wt% in semi-coke [4], whereas the Enefit280 ash is grey-beige and does not contain any significant amount of residual organic carbon. This is due to the effective carbon burnout during the waste processing in the CFB furnace in the Enefit280 retorting system.…”

“…In comparison, the hydration of oil shale ash from thermal power plants and the semi-coke waste from shale oil retorting typically results in a high ettringite content, reaching 30 wt% of crystalline phases [4]. A similar low content of ettringite upon hydration with water was observed in the Petroter ash where ettringite was nearly absent [16] or was found to occur in a low content, < 5 wt% [15]. Ettringite plays an important role in the self-cementing properties of oil shale ash and semi-coke deposits by forming interlocking meshes of needle-like crystals filling the pores space [22].…”