In Croatia, 52% of the total territory is agricultural land, with ~80% of the arable surface under maize, wheat, soybean, sunflower, rapeseed, grapevine, olive, apple and plum cultivation [1
]. This presents great potential for agricultural biomass usage in energy production but there is also a need for sustainable waste management, since significant amounts of bio-ash will be generated.
Agricultural biomass is a residue in the fields after harvesting crops that is then used to produce energy in biomass power plants. Although a significantly lower quantity of ash is generated during agricultural biomass combustion compared to coal, this ash is not being suitably managed and new applications for it need to be found. Currently, waste from biomass incineration is being disposed of at landfills, on farmland or in forests, most often without any control, which can cause environmental pollution and potential human health risks [2
Road construction is the branch of civil engineering that is most dependent on natural material availability. Simultaneously, increases in traffic loads and the need for building roads on less accessible and suitable terrain result in a need to find new ways of locally available material usage, as well as improving their mechanical characteristics. Consequently, the ash generated from agricultural biomass incineration is being studied as a material for all pavement layers. Depending on its characteristics, ash can be utilised as a filler, as a replacement for small aggregate fractions, as a binder itself when it contains active minerals (e.g., lime, calcium and magnesium silicate or alumina silicates), resulting in hydraulic binding, or as a binder supplement or addition when it contains pozzolanic minerals (e.g., glass, Portland, gypsum or clay minerals), which in combination with other materials leads to a pozzolanic reaction [3
]. For wearing course construction, biomass ash has been investigated for both asphalt [4
] and concrete [7
] pavements. However, for both pavement systems, a good quality subgrade is of high importance. Locally available soil is often not suitable for subgrade or embankment construction, so different ways of stabilisation are used, most often by lime or cement.
Due to its potential pozzolanic and hydraulic characteristics, there is a high potential of using bio-ash in soil stabilisation for the specified purposes. The use of bottom ash from biomass (olive) combustion reduces the expansion of expansive soils to the same extent as from treatment with lime, as presented in [9
]. Research results indicate that 6–8% cement and 10–15% rice husk ash are optimal additions for residual soils from the viewpoints of plasticity, compaction, strength characteristics and cost [10
]. The stabilisation of alluvial soil by biomass ashes from rice husk and sugar cane bagasse results in a plasticity index decrease with an increase in the proportion of ash from 2.5% to 12.5%, with the optimal ash content for stabilisation reported to be 7.5% [11
]. Admixing of rice husk ash, bagasse ash and rice straw ash with soil results in a higher optimal moisture content as the dosages of stabilisers increase [12
]. The addition of the same ash to clayey soil at a concentration of 20–25% also increased the California Bearing Ratio (CBR) values.
Rice husk and sugarcane bagasse-based mixed biomass ash combined with hydrated lime as an activator in clay resulted in an increase in compressive strength, as described in [13
]. Similar results are presented in [14
], where the addition of bagasse ash to expansive soils results in CBR, compressive strength and maximum dry density increases, as well as a swelling decrease [15
]. Sugarcane straw ash can also be an effective stabiliser for improving the geotechnical properties of lateritic soil samples [16
]. The combination of wheat husk and sugarcane straw ash also positively influences the geotechnical properties of soil [17
]. Soil admixtures with coal fly ash and rice husk ash have the potential to improve soil resistance to permanent deformation [18
]. Biomass furnace ash from agricultural olive residues can also be used as a filler material in road embankments [19
]. In contrast, biomass fly ash of olive waste used in [20
] was found to be the least effective additive in the stabilisation of marl soil, which indicates that its effectiveness could depend on the type of soil to be treated.
Thus, the aim of this study is to identify possible applications of biomass ashes in order to promote sustainable energy production from which it originates and to preserve natural resources and energy needed for lime production. Namely, energy generated from the biomass production is currently the fourth most common energy source in the European Union [1
] and large quantities of waste biomass ash are generated. Before recycling of biomass ashes as construction materials, detailed investigation need to be conducted, demonstrating its acceptable level of performance and economical comparability to traditional materials. Therefore, the purpose of this research is to define basic characteristics of agricultural biomass ash for its potential earthwork application in road construction. This article reports an experimental study of the properties of three biomass ashes used as additives to lime stabilised low bearing soil for embankment and subgrade purposes.
2. Materials and Methods
2.1. Raw Materials
The size distribution of used soil was determined according to standard EN ISO 17892-4 by combination of sieving and hydrometer methods. The particle size distribution curve is presented in Figure 1
and the density of soil used was 2.74 kg/dm3
. Specific surface area (SSA) of used soil determined by the Brunauer, Emmett and Teller (BET) method according to standard ISO 9277 is 9760 cm2
The liquid and plastic limits of the used soil, determined according to standard EN ISO 17892-12, were 34.5% and 21.9%, respectively, with a plasticity index of 12.5%. It was classified as low plasticity clay-CL according to the Unified Soil Classification System (USCS). The optimal water content and maximal dry density, determined by standard EN 13286-2, were 13% and 1.80 g/cm3, respectively.
For soil stabilisation, CL 80 S hydrated calcium lime was used according to EN 459-1, with a density of 2.65 kg/dm3. SSA of used lime determined by the BET method according to standard ISO 9277 is 16,671 cm2/g.
As a binder substitute, three biomass fly ashes were used. The oil factory Čepin uses sunflower seed shells as a fuel during sunflower oil production. Biomass is burned within the furnace, in a hot-air stream, and the biomass ash produced for the purpose of this research was collected from a specialized landfill within factory premises. In order to test some new energy resources, it has been tried as a replacement for sunflower seed shells by barley and wheat straws. Fly ash from sunflower seed shells (S), barley (B) and wheat straws (W) from this factory was used in this study, with the chemical composition of the used ashes determined in accordance with ISO/TS 16996:2015 presented in Table 1
. The densities of the S, B and W ashes were 2.26, 2.23 and 2.36 kg/dm3
, respectively, tested according to standard EN 1097-7. The SSA for the S, B and W ashes was 33,740, 34,080 and 36,850 cm2
/g, respectively, determined by the BET method according to standard ISO 9277.
2.2. Sample Preparation and Strength Tests
The optimal lime and ash portions were determined by standard ASTM D 6276-99a, measuring the pH values of soil-lime and soil-lime-ash mixtures with various content ratios. It was determined that the optimal lime content is 7% of the total dry mass of soil and the optimal lime/ash ratio is 80%/20%. After defining the optimal stabilised soil composition, the maximum dry density (MDD) and optimal water content (OWC) were determined according to standard EN 13286-2. The specimens were prepared at their respective OWC and maximal dry density (MDD), measured after compaction 100 mm in diameter and 200 mm in height. Prepared specimens were cured for 28 days in a temperature and moisture controlled chamber (20 °C and 60% relative humidity). The compressive strength test (according to standard EN 13286-41) and the 3D digital image correlation (DIC) were determined for these specimens. The CBR and linear swelling were determined according to standard EN 13286-47.
2.3. Elastic Properties-3D Digital Image Correlation
DIC is a non-destructive, non-contact method used for determination of loaded object surface deformation, i.e., it allows us to track displacements in a field of view of applied cameras (Figure 2
). It can be utilised using a single camera (2D) or two camera (3D) setup, and further details on this method and its potential application are presented in [21
]. Spatial DIC measurements for the purpose of this study were implemented using a GOM Aramis 3D optical deformation analysis system, along with its corresponding software package. Specimens were monitored during compressive testing, with the force data being supplied to the DIC system via an output channel of the utilised Shimadzu AG-X universal testing machine, connecting deformation stages to corresponding frames. The system was set to capture images with a frequency of 4 Hz, per camera, which was suitable to obtain more than 50 images (data points) in the elastic range. The analysis of obtained recordings enabled the determination of elastic modulus in accordance with EN 13286-43 and insights into the development of fracture mechanisms.
Although classical, contact-based, instrumentation can provide data regarding the elastic modulus, such data is more prone to errors due to problems with adequate contact, concentration of deformation, gauge length influence, and so on. Additionally, such point-based instrumentation cannot provide adequate information on the fracture mechanism, i.e., deformation concertation and propagation. By tracking the entire field of view, i.e., the entire visible surface, deformation results are more reliable and an adequate insight into the phenomena of deformation distribution and redistribution with load increase can be obtained. These insights can be of great importance when assessing ductility and possible mechanisms of fracture for a certain material type.