Renewable biomass-derived lignin in transportation infrastructure strengthening applications

Abstract

Lignin is considered as nature's most abundant aromatic polymer co-generated during papermaking and biomass fractionation. There are different types of lignins depending on the source (hardwood, softwood, annual crops, etc.) and recovery process. Recently, an emerging class of lignin products, namely sulphur-free lignins, from biomass conversion processes, solvent pulping and soda pulping, have generated interesting new applications owing to their versatility. As the renewable energy industry is expanding into developing the next generation of biofuels based on cellulosic biomass (e.g. corn stover, forest products waste, switch grass), abundant supply of sulphur-free lignin will become available as co-products for which value-added engineering applications are being sought. This paper discusses the potential for utilising lignin-containing biofuel co-products for stabilisation of geo-foundation beneath road pavements. Laboratory test results indicate that the biofuel co-products were effective in stabilising the Iowa class 10 soil (CL or A-6(8) soil classification). Utilisation of cellulosic biomass-derived lignin in transportation infrastructure strengthening applications appears to be one of the many viable answers to the profitability of the bio-based products and the bioenergy business from the perspectives of sustainable infrastructure systems.

Keywords::

• biofuel
• soil stabilisation
• lignin
• sustainability
• co-product
• pavement

Introduction

Sustainable development has been globally recognised in the context of depleting non-renewable resources (petroleum, natural gas, coal, minerals, etc.), regulations on using synthetic materials, growing environmental awareness and economic considerations (Kamm and Kamm 2004). Terminologies associated with life-cycle analysis, sustainable infrastructure, industrial ecology, green energy and technology, eco-efficiency, eco-labelling, green rating, etc. are becoming more and more common in research literature and product marketing. Sustainable development requires safe, sustainable resources to replace fossil-based energy for various industrial applications (Kamm and Kamm 2004). The bioenergy cycle, at least conceptually, does not have the negative environmental impact associated with fossil fuel-based energy sources and is, therefore, considered an attractive alternative.

Agricultural biomass is one of the sustainable resources having cost-effectiveness and can be transformed into bio-based energy such as biofuel and ethanol. Agricultural biomass such as corn products can be converted into biofuels or ethanol by hydrolysis and subsequent fermentation (Hamelinck et al.2005). Biofuel production also produces many different co-products that have many unexplored potential uses (Bothast and Schlicher 2005).

Among many different co-products, lignin, which represents the third largest fraction of agricultural biomass, has been considered as a waste material or a low-value co-product with its utilisation predominantly limited to use as a fuel in the production of octane boosters, and in bio-based products and chemical productions (Stewart 2008). However, the amount of lignin as a biofuel co-product will become abundantly available with the growing biofuel production industry. New uses for biomass-derived lignin need to be developed to provide additional revenue streams to improve the economics of the bio-based products and the bioenergy business. This paper proposes the application of biomass-derived lignin for stabilising soils to provide good foundation for roads.

A good road (paved or unpaved) requires a suitable foundation which in turn requires stability. Unfortunately, many of the soil deposits do not naturally possess the requisite engineering properties to serve as a good foundation material for roads and highways. As a result, soil-stabilising additives or admixtures are used to improve the properties of less-desirable road soils (ARBA 1976). Lignin has been implicated as having a positive role in soil stabilisation (Kozan 1955, Nicholls and Davidson 1958, Lane et al.1984, Palmer et al.1995, Puppala and Hanchanloet 1999, Tingle and Santoni 2003). Adding lignin to clay soils increases the soil stability by causing dispersion of the clay fraction (Davidson and Handy 1960, Gow et al.1961).

Previous studies on the use of lignin-based products in transportation infrastructure have focused on sulphite lignin (lignosulphonates or lignin-sulphonates) which is derived from the paper industry, whereas the lignins obtained from biofuel or ethanol production are sulphur-free lignins. Even though sulphur-free lignins have been known for many years, the use of sulphur-free lignins has recently gained interest as a result of diversification of biomass processing schemes (Lora and Glasser 2002). Value-added engineering applications of lignocellulosic residues from biofuel production methods are being actively researched and promoted in an effort to maintain economic competitiveness of cellulosic-ethanol processes (Gopalakrishnan et al.2012).

The primary objective of this study was to examine the potential of lignin-containing biofuel co-product, as a material for stabilising soil. The procedure and the results of testing are presented in this paper, highlighting the important findings regarding the utilisation of biofuel co-product for roadway soil stabilisation. A brief overview of the historical uses of sulphite lignin in transportation infrastructure applications is first provided.

Sulphite lignin in transportation infrastructure

The first utilisation of lignin in industry began in the 1880s when lignosulphonates were used in leather tanning and dye baths. Since then, a number of studies have been conducted to expand the use of lignin in many applications including the production of dyes, vanilla, plastics, base-exchange material for water softening and the cleavage products of lignin from nitration, chlorinate and caustic fusion (Cooper 1942). Conventional sulphite lignin (lignosulphonates) is the most mature product among all types of lignin. The International Lignin Institute (ILI 1991) lists the following traditional applications that sulphite lignin (lignosulphonates) recovered from sulphite pulping can serve: binder, dispersant, emulsifier and sequestrant.

Sulphite lignin or lignosulphonate has been used standalone or in combination with other chemicals to achieve soil improvement for supporting pavement infrastructure (Nicholls and Davidson 1958). Lignin as a soil additive causes dispersion of the clay fraction of some soils resulting in the increase in shear strength of the soil due to particle rearrangement (Addo et al.2004). Various studies on lignin as a soil additive have concluded that lignin is primarily a cementing agent (Woods 1960, Ingles and Metcalf 1973, Landon and Williamson 1983). Nicholls and Davidson (Puppala and Hanchanloet 1999) confirmed that lignin admixtures indeed do improve some engineering properties related to stability of soils. They also reported that the strength of lignin-treated soil increases rapidly with an increase in the length of air curing.

Ligninsulphonates were first utilised to control duct on unpaved roads in Sweden in the 1910s (Arnfelt 1939). The Institute of Road Research in Sweden in their dust control experiment with ligninsulphonate reported that it reacted well with dust and bound particles together if the road surface was rich in clay. Several states in the USA also made much use of ligninsulphonate as a dust suppressant on road surface with impervious wearing courses in 1930s (Sinha et al.1957). Field observation of the lignin-treated test sections indicated that the lignin acted like cement, binding the soil particles together into a hard surface that show strength gains over time (Addo et al.2004).

Lignin has been used as an emulsifier in asphalt emulsions due to its sequestering and dispersing properties (ILI 1991). Several laboratory experiments have been conducted to examine the use of lignin from wood pulping as a substitute or an extender for asphalt in paving mixtures (Terrel and Rimstritong 1979, Sundstrom et al.1983, Kandhal 1992). Recently, attempts have been made to investigate the use of lignin from wood pulping as an antioxidant in asphalt (Bishara et al.2005, Guffey et al.2005a, 2005b). These studies imply that lignin-modified asphalt can decrease the rate of oxidation without adverse effects on the other asphalt performance properties.

The use of ligninsulphonate as an admixture in concrete has been known for more than 60 years (Zhor and Bremner 1999). Ligninsulphonate has been used as a water-reducing and a set-retarding admixture to reduce water and offset the effects of high temperature without losing workability (Mindess et al.2002). The hydrophilic modified aromatic structures of lignin can reduce the amount of water necessary in a concrete to reach a certain fluidity, resulting in the improvement of the concrete's final strength (Nadif et al.2002). However, the dosage of ligninsulphonate should be controlled to prevent retarding the development of strength. Zhor and Bremner (1999) investigated the effect of ligninsulphonate dosage rate on fresh concrete properties and concluded that the highest dosage rates will always cause set retardation. Ligninsulphonate has also been used in ready-mix and pre-cast concrete to produce concrete with an improved rheology at the job-site and to obtain high-strength concrete (Plank 2004).

Materials

Soils

The natural soil used in this study conformed to class 10 soil as described in the Iowa Department of Transportation (DOT) specification (Iowa DOT 2008). The class 10 soil was obtained from a new construction site prepared for Highway US-20 in Calhoun County, Iowa (STA. 706 to STA.712, Project Number NHSX-20-3(102)-3H-13). The class 10 soil is the typical excavated soil including all normal earth materials such as loam, silt, clay, sand and gravel. Table 1 summarises the engineering properties of Iowa class 10 soil used in this study. Based on characterised engineering properties, the soil could be classified to CL and A-6(8) in accordance with the unified soil classification system (USCS) and American Association of State Highway and Transportation Officials (AASHTO) soil classification system, respectively.

Table 1 Summary of Iowa class 10 soil engineering properties.

Property	Class 10
Gravel (%)	7.6
Sand (%)	40.4
Silt and clay (%)	51.9
Liquid limit (%)	39.3
Plasticity limit (%)	16.0
Plasticity index (%)	23.3
USCS group symbol	CL
USCS group name	Sandy lean clay
AASHTO (group index)	A-6(8)
Optimum moisture content (%)	17.7
Maximum dry unit weight (kg/m^3) pound per cubic foot, pcf	1693 (105.7)

BioOil

A commercially available BioOil was used as the experimental lignin-containing biofuel co-product for this study. BioOil is a dark brown, free-flowing liquid fuel with a smoky odour reminiscent of the plant from which it was derived. BioOil is formed in a process called pyrolysis wherein plant materials (biomass) such as forest residues (bark, sawdust, etc.) and agricultural residues (sugar cane, cornhusks, bagasse, wheat straw, etc.) are exposed to 400–500°C in an oxygen-free environment (Dynamotive Energy Systems Corporation 2007). Recently, several qualification trial tests have been conducted for using this commercial BioOil to heat the Iowa Capitol Complex (Iowa DAS 2008).

The raw BioOil contains about 25% lignin and up to 25% water with a pH of 2.2. Table 2 presents a summary of constituent materials present in raw BioOil. The water component in raw BioOil for use of liquid fuel is not a separate phase because it lowers the viscosity of the fuel. However, the water content is significantly removed by heating the raw BioOil in the oven for a specified period of time. This water-removed BioOil is defined as the evaporated BioOil in this study.

Table 2 Component materials in BioOil.

Components	Percent by weight
Gases	5–10
Water	Up to 25
Lignin	25
Char	4
Aldehydes	35–41

This study used raw BioOil as well as evaporated BioOil. Since raw BioOil already contains water in it, it was directly mixed with soil during compaction without the addition of further water. With evaporated BioOil, water was added while mixing it with soil to investigate the effect of different moisture contents on strength properties.

Fly ash

The relative performance of biofuel co-product was assessed with respect to a traditional soil stabilising agent, Ottumwa class C fly ash. Ottumwa class C fly ash is a coal combustion by-product from Ottumwa Generating Station located near Chillicothe, Iowa. This fly ash is commonly used in soil treatment in Iowa. The chemical composition of Ottumwa fly ash is presented in Table 3.

Table 3 Chemical composition of Ottumwa fly ash.

Components	Percent by weight
Na2O	3.21
MgO	6.81
Al2O3	16.2
SiO2	31.6
P2O5	1.02
SO3	3.13
K2O	0.32
CaO	28.8
TiO2	1.24
Fe2O3	6.03
SrO	0.51
Mn2O3	0.02
BaO	0.89
Loss on ignition (LOI)	0.30

Experimental plan

For comparison purposes, the primary experimental plan encompassed preparation and testing of three broad categories of treatment types: (1) untreated soil sample (control), (2) soil sample treated with the biofuel co-product and (3) soil sample treated with fly ash. The soil was mixed with each additive (biofuel co-product or fly ash) at varying amounts to identify the optimal additive content. The BioOil and fly ash contents evaluated are 1%, 3%, 6%, 12% and 15% by dry soil weight. The untreated soil samples were also tested without any additive.

Similarly, variable moisture contents and curing periods were incorporated into the test factorial. All soil specimens were tested at three different moisture contents: untreated soil optimum moisture content (OMC), OMC+4% and OMC − 4%. For soils mixed with the raw BioOil, the additive concentration levels were adjusted to achieve the same water contents corresponding to OMC, OMC+4% and OMC − 4%. The curing periods primarily investigated were 1 day and 7 days after sample fabrication for strength tests.

Table 4 lists the primary treatment group combinations evaluated during the study. Specimens incorporating 78 different treatment combinations were fabricated in the laboratory which underwent unconfined compression strength (UCS) test programme. In order to obtain quality test results, two specimens were prepared for each treatment which resulted in a total of 156 specimens.

Table 4 Primary treatment group combinations.

		Additives (%)^a
Moisture content level	Curing period (day)	BioOil	Fly ash
Without water	1	13.7^b, 17.7^b, 21.7^b	N/A^c
	7	13.7^b, 17.7^b, 21.7^b	N/A^c
OMC − 4	1	0, 1, 3, 6, 12, 15	0, 1, 3, 6, 12, 15
	7	0, 1, 3, 6, 12, 15	0, 1, 3, 6, 12, 15
OMC	1	0, 1, 3, 6, 12, 15	0, 1, 3, 6, 12, 15
	7	0, 1, 3, 6, 12, 15	0, 1, 3, 6, 12, 15
OMC+4	1	0, 1, 3, 6, 12, 15	0, 1, 3, 6, 12, 15
	7	0, 1, 3, 6, 12, 15	0, 1, 3, 6, 12, 15

a Numbers indicate per cent of additive added by dry soil weight.

b Same water contents as OMC, OMC+4 and OMC − 4 of untreated soil.

c Not available.

Apart from the primary treatment group combinations listed in Table 4, several other treatment group combinations were also considered to evaluate the effect of variability in specimen preparation methodology on the strength testing results. Also, soils were mixed with water before and after the addition of biofuel co-product to identify the effect of mixing procedure on strength.

Strength property testing

The UCS test result was used as an index of specimen performance. The performance of test specimens relative to control specimen performance provided a means of evaluating the effects of specimen preparation methods, and the additive types and concentration levels. The control specimens consisted of untreated soil prepared at the desired moisture contents without any stabiliser.

The ASTM D2166 (ASTM International 2006) specification describes a general test procedure for determining the UCS of soil samples, but does not specify the sample geometry. Portland Cement Association (PCA) recommends three types of sample geometry for compression test of soil–cement mixture: 102 mm (4 in.) in diameter by 117 mm (4.6 in.) in height, 51 × 51 mm (2 × 2 in.), and 71 × 142 mm (2.8 × 5.6 in.) (PCA 1971). The compaction method for producing 51 × 51 mm specimens was developed by previous researchers at Iowa State University (ISU; Chu and Davidson 1960). The compression strengths of 51 × 51 mm specimens were correlated with those of the other geometry specimens for the soil and the soil–fly ash mixtures (ASTM International 2005, White et al.2005). The use of 51 × 51 mm specimens can save time and materials. Because of these and other advantages reported in the literature (White et al.2005), specimens of these dimensions were prepared in this study for UCS testing.

Sample preparation

Each sample for UCS testing was prepared following five steps: soil preparation, soil–water-additive mixing, moulding, compaction and curing. The soil was dried in an oven at 60°C before mixing with water and additives. Fly ash was dried in air before mixing with soil and water. As mentioned previously, both raw BioOil (which contains moisture) and evaporated BioOil were used in the preparation of soil samples.

Once the soil and additive were prepared, the soil was mixed with water and additives to obtain the desired moisture content and additive content. The materials were mixed together to produce a uniform, homogenous mixture. A sample of the mixture was used to determine the initial moisture content of the soils according to ASTM D2216 (ASTM International 2005).

The effect of mixing procedures on compression strength was evaluated by testing samples mixed through two types of mixing methods. Type I method involves mixing the soil with water before the addition of biofuel co-product and type II method involves mixing the soil with water after the addition of biofuel co-product. Figure 1 compares the strengths of soil prepared by these two methods with 6% BioOil. No significant difference can be observed between these two methods. However, all samples in primary treatment group combinations (See Table 4) were prepared by the Type I method for further investigation, since this method better represents actual field practice of soil stabilisation.

Figure 1 Effect of mixing procedures on UCS.

A quantity of loose material was measured for each sample that would produce a 51 mm (2 in.) high compacted sample. The ISU 51 mm (2 in.) by 51 mm (2 in.) specimen preparation method specifies that loose materials are compacted in 51 mm (2 in.) diameter mould with removable collar by dynamic loading. The term ‘dynamic loading’ herein refers to five blows of 22 N (5 lb) hammer falling from a height of 305 mm (12 in.) on each end of the single layer of material (White et al.2005). However, it was found that this compaction approach produces compacted samples with higher variations of density and strength. To reduce these variations of samples, a static compaction approach was employed which is similar to the approach used in soil specimen preparation for resilient modulus test in accordance with AASHTO T307 (AASHTO 1999).

A specially designed mould apparatus was fabricated and used to compact loose materials by static compaction. A 25 mm (1 in.) high spacer plug was inserted into the specimen mould with removable collar. Measured amounts of loose material were placed in the specimen mould and then the 102 mm (4 in.) high spacer plugs were inserted on loose materials in the specimen mould. A static load was applied to 102 mm (4 in.) high spacer plugs until the plug rested firmly against the mould end. After compaction was completed, the compacted specimen, as shown in Figure 2, was extracted from the mould using an extrusion ram. Tables 5 and 6 list the average of dry density and wet density for the compacted samples, respectively.

Figure 2 Prepared samples for USC test.

Table 5 Average dry density of compacted samples (unit: kg/m³).

Moisture content level			OMC − 4		OMC		OMC+4
Curing period			1 day	7 days	1 day	7 days	1 day	7 days
Type	Untreated soil		1625	1628	1627	1636	1653	1642
	BioOil (raw)		1620	1618	1620	1622	1637	1627
	BioOil (evap.)	1%	1630	1630	1624	1631	1641	1645
		3%	1615	1621	1629	1627	1629	1633
		6%	1622	1617	1623	1615	1606	1611
		12%	1613	1621	1574	1604	1549	1571
		15%	1614	1625	1529	1576	1538	1549
	Fly ash	1%	1642	1635	1655	1649	1647	1641
		3%	1633	1635	1653	1641	1646	1650
		6%	1628	1626	1639	1635	1637	1638
		12%	1625	1621	1618	1614	1610	1609
		15%	1625	1627	1618	1625	1611	1611

Table 6 Average wet density of compacted samples (unit: kg/m³).

Moisture content level			OMC − 4		OMC		OMC+4
Curing period			1 day	7 days	1 day	7 days	1 day	7 days
Type	Untreated soil		1831	1831	1898	1904	1972	1967
	BioOil (raw)		1829	2147	1897	1890	1981	1963
	BioOil (evap.)	1%	1848	1837	1905	1929	1975	1982
		3%	1854	1860	1931	1928	1997	1996
		6%	1900	1892	1965	1955	1985	1986
		12%	1954	1957	1961	1999	1933	1964
		15%	1942	1974	1905	1977	1912	1938
	Fly Ash	1%	1864	1933	1940	1883	1997	2055
		3%	1881	1874	1959	1935	2036	2027
		6%	1826	1810	1905	1898	1964	1968
		12%	2040	2002	2050	2027	2076	2093
		15%	2064	2039	2110	2105	2107	2095

The compacted sample was sealed in a plastic wrap and then placed in a temperature-controlled room where it was allowed to cure at 25°C and 40% relative humidity for various cure times. The curing process could be considered as the hardening or cementation of the additive–soil matrix. The air-curing process was selected to represent field condition.

UCS test

The UCS test was conducted following ASTM D2166 (ASTM International 2006). The cured sample was positioned in the test equipment and a compression load was applied at a constant rate of 1.3 mm per minute (0.05 in. per minute). The magnitude of compression load and the corresponding sample deformation were monitored and recorded. Each sample was compressed until a peak load was reached and either decreased or remained constant, or until deformation of sample exceeded past 20% strain before reaching the peak. A sample of the broken material was taken to determine the moisture content of the materials according to ASTM D2216 (ASTM International 2005).

Results and discussion

The effect of additive types and contents on strength was evaluated under different moisture conditions: OMC represented moisture condition providing maximum dry density of soil and used for construction quality control, OMC − 4 representing more dry side of soil condition, and OMC+4 representing more wet side of soil condition. The evaluations were also made under different curing periods. The results are shown graphically in Figure 3 through Figure 5. The strength values at 0% additive content on these figures indicate those corresponding to untreated soil. The strengths of the soil and raw BioOil samples without the addition of water are also depicted by dashed lines in these figures to provide comparisons.

Figure 3 Effect of additive types and contents on UCS under OMC − 4 condition.

Optimum moisture content−4

As shown in Figure 3, fly ash (a traditional soil stabiliser) seems to be a very effective additive in enhancing the strength of tested soils under dry condition of soil (OMC − 4). The BioOil-treated soil test results also show improved UCS similar to fly ash-treated soil. Under the dry condition of the soil, the increase in the amount of additives and the curing periods seems to improve the strength. Changes in the additive amount for BioOil revealed a definite optimum additive quantity near 12%. These results indicate that BioOil can be as effective as fly ash in stabilising the natural soil for strength improvement under dry condition.

Optimum moisture content

Similar to dry condition of soil (OMC − 4), both the fly ash and the BioOil-treated soil test results in Figure 4 show improved strength at the OMC condition of soil. Overall, an increase in the amount of additives and the curing periods improves strength with 12% as the optimum additive quantity for BioOil. These results indicate that BioOil can still be effective, but not better than fly ash, to stabilise pure soil with a target moisture condition for construction.

Figure 4 Effect of additive types and contents on UCS under OMC condition.

Optimum moisture content+4

All additives-treated soil test results in Figure 5 also show improved strength under wet soil conditions (OMC+4). However, the fly ash provides more strength improvement with the increase in the amount of additive rather than the BioOil. These results indicate that BioOil cannot be used solely under wet soil conditions to achieve a given strength comparable to that of fly ash-treated soil. Further investigation is recommended to improve the strength of BioOil-treated soils in addition to fly ash under wet condition of the soil. A lesser amount of fly ash might be required in this investigation since fly ash is costlier than BioOil.

Figure 5 Effect of additive types and contents on UCS under OMC+4 condition.

Discussion

Multiple comparison tests were carried out to see how the different treatments could be ranked. The Student–Newman–Keuls (SNK) test utilised in this study is one of the multiple comparison tests that can be used to determine which means amongst a set of means differ from the rest. A SNK test result can be expressed in terms of a p-value, which represents the weight of evidence for rejecting the null hypothesis. The null hypothesis is the equality of mean between each pair of comparisons. The null hypothesis can be rejected, i.e. the mean between each pair of comparisons is significantly different, if the p-value is less than the selected significance level (α). A 0.05 of significance level (α) was used in this study.

Table 7 presents multiple comparison test results for the different treatments. Significant difference between each pair of comparisons was noted by p-value with less than 0.5 and levels not connected by the same letter. Fly ash-treated soil has the highest strength, followed by raw BioOil-treated soil, evaporated BioOil-treated soil and natural soil. Especially, raw BioOil-treated soil appears not to provide significant strength difference to fly ash.

Table 7 Multiple comparison test results.

		p-value^a
Type	Mean	Fly ash	BioOil (raw)	BioOil (evap.)	Soil	Level^b
Fly ash	536.8		0.9922	0.0073	0.0017	A
BioOil (raw)	510.7	0.9922		0.3782	0.0432	A B
BioOil (evap.)	362.0	0.0073	0.3782		0.267	B C
Soil	193.2	0.0017	0.0432	0.267		C

a Italic value indicates p-value is less than 0.5, which means significant difference in a pair of comparisons.

b Levels not connected by the same letter are significantly different.

The strength gain mechanism from cellulosic biomass-derived lignin has not been identified. However, Gow et al. (1961) addressed the increase in soil stability from conventional sulphite lignin (lignosulphonates) by several explanations including (a) plugging voids and consequently improving water tightness and reducing frost susceptibility, (b) eliminating soft spots caused by local concentrations of binder soil, (c) filling voids with fines thus increasing density and (d) increasing the effective surface area of the binder fraction which results in greater contribution to strength. It is speculated that some of these mechanisms could also contribute strength gain mechanism from cellulosic biomass-derived lignin.

Since the industry supplied the experimental lignin-containing biofuel co-product investigated in this study, it was difficult to estimate the cost of these materials in construction in this study. However, it is obvious that sustainable development can drive more use of bio-based energy as an alternative energy to fossil fuels. Considering the increase in biofuel production and limited commercial utilisation of these materials, the cost of these materials would be comparable to or even less than traditional stabilisers such as fly ash that is a by-product of fossil fuels (coal) and which might be less produced with a decrease of coal-fired power plants.

Conclusions

This study investigated the utilisation of a lignin-containing biofuel co-product for roadway soil stabilisation. Laboratory tests were conducted to determine strength properties of untreated soil samples, soil samples treated with biofuel co-product and soil samples treated with a traditional soil stabilising agent, fly ash. The analysis of the test data focused on identifying effects of additive types and contents. The following conclusions can be drawn on the basis of test results obtained from this study:

• Both biofuel co-product and fly ash were effective in stabilising the Iowa class 10 soil classified to CL or A-6(8).

• The UCS of soils with Biofuel co-product is comparable to that of soils with fly ash under dry condition.

• A definite optimum additive quantity of biofuel co-product is near 12% for stabilising soils underneath roadways in Iowa.

Utilisation of biofuel co-product as a soil stabilisation agent appears to be one of the many viable answers to the profitability of the bio-based products and the bioenergy business, especially in and around Iowa. Since there is much more biofuel co-product that is disposed of rather than utilised, making more productive use of biofuel co-product would have considerable benefits for sustainable development.

Acknowledgements

The authors gratefully thank the Iowa Highway Research Board (IHRB) and ISU for supporting this study. The contents of this paper reflect the views of the authors who are responsible for the facts and accuracy of the data presented within. The contents do not necessarily reflect the official views and policies of the IHRB and ISU. This paper does not constitute a standard, specification or regulation.