Peatlands cover vast areas of the land surface, especially in the temperate and cold climate zones of the Northern Hemisphere, but they are also found in the hot regions. Peatlands cover 22% of the land area of Estonia [1
]. Peat is highly compressible under high loading, which makes it one of the most difficult soils on which to construct buildings, roads, railroads, or other structures. Construction of large infrastructure in areas with loose peat soils often relies upon sophisticated building techniques.
To achieve an adequate load-bearing capacity of the roadbed, soil stabilization is usually required when a road or railway embankment will be constructed over a wetland area containing a peaty soil. The most common approach to the construction of road and rail track beds over peatlands is soil replacement [3
]. This involves the excavation and displacement of peat and sapropel, removal of the peat to reveal the bedrock layer, and then filling up the cavities with a material with a higher load-bearing capacity. Soil replacement is usually an expensive task since, in addition to the excavation work, it requires transportation of the displaced soil away from the site and the provision of a new soil and its transport to the site. The displaced material also requires disposal, which needs to be ecologically safe [4
Excavation works typically require drainage of the surrounding wetland, leading to rapid decomposition of the peat deposits. As a result, the wetland switches from a carbon sink to become a net source of carbon dioxide emissions [1
]. This excavation–displacement–replacement technique is considered feasible for peat deposits that are up to 5 m in peat depth. If the peat depth exceeds 3 m, the excavation and replacement costs will increase substantially [7
]. In these deep excavations, the sideslopes become unstable and collapse into the excavations before they can be backfilled. To avoid this problem, large volumes of soil need to be removed from the sideslopes to ensure that the slope angle does not exceed the wet soil’s critical angle of repose, thus increasing the total volume of the excavation work. Moreover, the excavation area requires installation of its own drainage system, thus further damaging the drainage regime and overall hydrology of the area [8
] and potentially further increasing CO2
]. Moreover, removed soil needs be transported from the site for use in landscaping in the adjacent areas, further increasing the costs. These problems highlight the need to develop functional, cost-effective, and environmentally sound methods for stabilizing embankments over peatland areas.
Economic and environmental impact can be met when an in situ soil reinforcement technique, such as mass stabilization, is applied. To achieve mass stabilization, the peat soil is mixed with a dry or a wet binder and additives (CaCl2
, or SiO2
) throughout the volume of the treated soil column. Common binders include Portland cement, quicklime, and gypsum [3
]. Environmentally sustainable waste-based binders applied for stabilization of various soils (peaty, clayey or sandy soils) may include calcareous fly ashes [11
], carbide lime [12
], or phosphogypsum [10
]. In order to facilitate the hardening of the soil-binder mixture, pozzolanic agents like silica fume, waterglass, alumino-siliceous fly ash, granulated blast furnace slag, ground glass, bentonite, kaolin, or other siliceous and alumino-siliceous materials can be used [3
]. Soluble calcium or aluminum salts such as CaCl2
can be added as supplementary Ca and Al sources [9
]. Peat soils contain a high percentage of organic material and few to no solid mineral particles. Therefore, within the mass stabilization process the addition of granular filler materials, such as sand or inert construction wastes (e.g., recycled asphalt pavement or crushed bricks), to the mixture allows cementitious compounds to carry out a pozzolanic reaction, or from the hydration of cement to bind the particles together, enhancing the strength of the stabilized peat [12
Humic and fulvic acids in the peat hinder the setting and hardening of cement pastes, which occurs for several reasons [7
]. Adsorption of insoluble calcium and magnesium humates onto the binder particle surfaces, and additive materials interfere with the hydration of calcium silicates and aluminates, which slows down pozzolanic reactions. Moreover, fulvic acids selectively associate with aluminum-bearing mineral particles, inhibiting the hydration of aluminates and impeding crystallization hydration products [14
]. Leaching of organic acids from peat may reduce the pH of the mixture, decreasing the solubility of siliceous species and therefore hindering formation of calcium (aluminum) silicate hydrate (C(A)SH) gels [7
]. If the pH of the pore water is below 9, secondary cementation products cannot form [17
]. A high ratio of organic to mineral in peat implies a high water-retention capacity and a high moisture content, which results in a high water-to-binder ratio that, in turn, limits any increase in the strength of the mixture. In the case of peat soils, this combination of a high organic content, a lack of pozzolanic and solid inert particles, an acidic media (primarily from humic and fulvic acids), and a high water-to-solids ratio impedes the efficient hydration of cement and hinders any subsequent increase in the compressive strength of the soils [14
]. The compressive strength and content of hydration products in the soil–cement mixture decreases until a certain threshold value of humic matter is achieved, beyond which the additional humic matter has a limited effect on the mixture’s compressive strength [18
The cost-efficiency of the mass stabilization technology can be improved using a cheap substitute for cement or lime. In Estonia, kukersite oil shale ash can be used for this purpose. Oil shale ash has been used in a mixture with Portland cement in Estonia for road construction using the mass stabilization technique and could also be used in peaty areas [11
]. Earlier studies have concluded that ash from a circulating fluidized bed (CFB) firing process would need a sophisticated approach for use as a binder agent for the soil stabilization purposes, because of its low free lime content and low pozzolanic activity [16
]. The CFB firing is a newer technology for incineration of kukersite oil-shale in Estonian thermal power plants. Therefore, different soil mass stabilization and other approaches are needed to make CFB ash recovery more efficient. Mass stabilization technology would be successfully applied by using CFB ash and peat. This hypothesis is also based on the experience of development of an environmentally friendly peat–ash composite building material [16
In this study, peat soil samples from south Estonia were mixed with different oil shale samples, pozzolanic additives and potassium hydroxide. Subsequently, X-ray diffraction analyses (XRD), X-ray fluorescence spectroscopy (XRF), and Fourier-transform infrared spectroscopy (FTIR) analyses were conducted, and stress–strain diagrams for compression were created to prove the suitable chemical content and hardness of the material. We aim to show that the hardened peat composite is a ductile material and that peat fibers behave like steel reinforcement. In this study, we hypothesized that all forms of oil shale ashes can be used as binders for organic soils in a cost-effective and environmentally sound way, based on addition of pozzolanic agents with high chemical activities, and an increase in the pH of the pore water in the organic soil-binder mixture using addition alkali metal hydroxides. Mass stabilization technology has the potential to be applied by using hazardous waste, CFB ash, and peat in composite material.
2. Materials and Methods
We tested different mixtures of peat soil from south Estonia, and oil shale ashes with various siliceous additives, and selected three mixtures with substantially different properties for further detailed studies of the concrete compositions and compressive strength.
For the preparation of laboratory samples, 200 g of peat soil was mixed with different additives. Additives from pulverized firing (PF; T10 and T17) or circulating fluidized bed (CFB) firing (T11) of kukersite oil shale in the power plant of Eesti Energia were used. As pozzolanic agents, silica fume (Efaco, Egypt), and Na2
(20% solution) were used, and KOH was added to some samples to increase the pH value of the mixture. The compositions of the mixtures from the analyzed test pieces are given in Table 1
The XRD analysis was performed on a diffractometer SmartLabTM (Rigaku, Tokyo, Japan) using a Cu rotating anode working at 45 kV and 180 mA, a coordinate-sensitive 1D detector D/teX Ultra, and a Bragg–Brentano optical setup. The diffraction pattern was measured between diffraction angles of 8° and 90°, with a step size of 0.01° (2θ) and a scan speed of 4° min−1. PDXL (Rigaku) software and the PDF-2 (International Centre of Diffraction Data, 2018) database were used for phase identification, and TOPAS 6 Academic software was used for the Rietveld analysis. The Rietveld analysis involves the fitting of a complete experimental diffraction pattern with calculated profiles and background.
The XRF analysis was performed on spectrometer ZSX 400 (Rigaku, Japan) using an Rh X-ray tube working at 3 kW.
One-axis compressive tests were carried out on an Instron Universal Testing Machine (Model 3369; max load 50 kN). Samples were weighed with a Kern 578 electronic balance (d = 0.05 g) before the tests. The surfaces of loaded test samples were flattened and ground, so that the difference in the surface smoothness did not exceed 0.1 mm per 100 mm side length, and were then supported on the loading plates evenly. The loading on the test pieces was applied by continuously increasing force until breaking or exact displacement of 16 mm. Subsequently, the maximum force was recorded. The compressive strength fc (N mm−2) was calculated as a quotient of Fc (N), maximum compression force, and the gross area of loaded surface (mm2). The mean compressive strength was calculated as the arithmetic mean of the sum of the single specimen with the accuracy of 0.1 N mm−2. The FTIR spectra were measured by Spectrum GXII (Perkin Elmer) spectrometer with an ATR (ZnSe) device (Interspectrum OÜ, Tartu, Estonia) to assess the chemical bonds within the composition of the samples.
It could be proposed that fibers naturally present in peat act mechanically in the same way as the steel or glass fiber in ordinary concrete, with the difference under compression showing the differences in strength (Table 3
). The reinforcement and non-loosing plasticity provided by peat fiber is better when the load is higher on the peat-ash composite and the displacement is lower. Figure 1
shows that the composite compressive strength increases again, unlike the ordinary concrete. This may be because increasing heat from pressure acts together with lignin from the fibers naturally present in peat, providing an additional gluing effect inside the composite. This phenomenon needs to be further explored.
As shown in Table 6
, the content of the semi-amorphous CSH phase almost quadrupled in the sequence T17 > T11 > T10, whereas non-hydrated quartz followed the diametrically opposite trend. Quartz content showed decrease from 33.6 to 10.3 and 6.8% between tests T10 > T11 > T17, respectively. The high SiO2
content in T10 also shows that SiO2
was added in excess and its hydration and inclusion into CSH gel was Ca-limited, resulting in lower plasticity compared with the other samples. For T10 and T11, which had a smaller CSH paste content, the crystallization of alkali feldspars progressed more rapidly, whereas it was delayed in T17. The crystallization of alkali feldspars has also been observed in our previous work aiming to produce 3D-printable peat-ash composite construction material [16
]. The full-amorphous phase (which includes both organic matter as well as a fraction of CSH paste) was relatively evenly presented among all samples. Belite was present in T17 only. The crystalline phases of T11, which contains CFB ash, also showed a higher content of Mg-and Fe-bearing minerals. Calcium silicate hydrate (CSH) is the main and most important constituent of cement paste (and pastes formed in the reaction of lime with siliceous agents). Its hydration forms most of the new solid phases that give hardened cement paste its strength. It occupies approximately 50% of the paste volume and responds to nearly all the engineering properties of the cement paste [18
]. Related equilibrium studies have been performed assessing the effect of pH and other factors on the material performance [19