Interest in using biomass conversion to produce biofuels, such as ethanol, has been a rising trend in the industry. A major source of biomass, for cellulosic ethanol in particular, is corn stover, the residue of corn harvesting. It was estimated that approximately 75 million dry tons of corn stover were produced annually . This amount would go to waste if there were no other practical industrial use for it. It is estimated that 11.4% of this amount contains lignin. Thus, corn stover could provide 8.5 million tons of lignin per year . Lignin is an essential component of all plant life that bonds the cellulose fibers together in plant cell walls, whereby the latter provide the tensile strength. Lignin is also the second most abundant biological material on Earth, exceeded only by cellulose and hemicellulose and comprising 15–25% of the dry weight of woody plants .
The impact on the environment from applying lignosulfonates to roads has been found to be negligible . In addition, lignin has been used as an ingredient in animal feeds and even used in many human foods for decades without an incident. Lignosulfonates are nontoxic to animals; however, minor irritation could occur if eyes or skin is directly exposed to lignosulfonate powder due to its extremely small particle size. Data indicate that there is minimal risk of groundwater contamination at concentrations of <10 kg/m2 .
The corrosion effects from lignosulfonate have considerably smaller consequences than those of other chemical treatments typically used for dust suppression such as calcium chloride or magnesium chloride . It is because of the nature of their bonding mechanism that lignosulfonates mix with soils to form cohesive bonds. Thus, lignin, as a treatment, is not as easily transferable as, for example, calcium chloride, where the chemical is loosely concentrated on the surface and is more readily passed to vehicles.
Although lignin materials have long been known as a coproduct of the paper industry, it has only recently become a coproduct of the biofuel industry. The most common use of lignosulfonate today is as a dust palliative. However, in the future the use of lignin materials could prove to be an effective and sustainable method for preventing slope erosion and possibly stabilization of paved road beds and slopes. The importance of lignin could also increase substantially in the future due to its status of a highly sustainable and rapidly renewable material. Presently, the sustainability of soil additives is a topic of increasing relevance. For example, replacing traditional road bed stabilizers with lignin materials could reduce man-made carbon dioxide emissions to the atmosphere, thus decreasing the overall greenhouse gas emissions .
Lignin is also used for a wide variety of organic and inorganic industrial binding applications such as the agglomeration of limestone, coal, ceramics, and fertilizer. Other uses include, but are not limited to, dust control of unpaved roads and stockpiles and as a low-cost dispersant of various substrates including gypsum and concrete .
Few laboratory and field investigations related to the evaluation of soil stabilizing potential of lignosulfonates have been reported in the literature [7–13]. Wang et al.  used lignin sand-stabilizing material (LSSM) extracted from spent liquor of straw pulping mills. They found that LSSM was effective in stabilizing mobile sand dunes and enabling the growth and survival of arenaceous plants. Surdahl et al. [8, 12] and Woll et al.  reported that the potential of lignosulfonate for soil stabilization and as a dust palliative material was demonstrated effectively for unpaved roads. In these cases, lignosulfonates were applied to wildlife preserves in mostly dry climates. Ceylan et al.  investigated the utilization of lignin-containing biofuel coproducts (BCPs) for pavement subgrade stabilization. Kestler  stated that nontraditional stabilizers are typically grouped into seven categories: chlorides, clay additives, electrolyte emulsion, enzymatic emulsions, synthetic-polymer emulsions, tree-resin emulsions, and lignosulfonates. Currently available research results indicate that innovative uses of BCP in pavement-related applications could not only provide additional revenue streams to improve the economics of bio refineries but could also serve to establish green road infrastructures.
The main goal of this research was to assess the strength benefits of S-CaL-W mixes. For this reason, a series of experiments were performed on S-CaL-W mixes including Atterberg limits, laboratory compaction tests and direct shear tests.
2 Phase relationships
The existing phase relationships for soil were revised by including CaL as an additional constituent as shown in Figure 1. It is noted that this phase diagram represents the individual constituents as placed into the mix. The traditional definition of water content (w) remains unchanged. Standard basic definitions of void ratio (e), dry mass density (ρd), and degrees of water (Sw) and air saturation (Sa) remain unchanged when using the quantities shown in Figure 1. New basic definitions are introduced for gravimetric CaL content (χl), gravimetric water to CaL ratio (ww/l), modified water content (w*), and degree of CaL saturation (Sl) as follows:
It is noted that subscripts s, l, and w refer to sand, CaL, and water, respectively. In addition to specific gravity of sand solids (Gs), a specific gravity of CaL solids (Gl) is introduced. The masses of all individual constituents are shown in Figure 1, thus giving the following expression for the mass density of the S-CaL-W mix:
3 Materials and methods
3.1 Material properties
The material tested is a mix of a dry masonry sand, CaL powder, and water. The masonry sand is medium-sized clean, uniformly graded sand with D50 of 0.3 mm . The sand was obtained from a quarry in Manhattan, Kansas, and is typical for north central United States.
CaL used in this research is a light brown powder, which contains 17.9% high-performance liquid chromatography sugars, a total of 5.8% sulfur, 4.4% of calcium, and 0.4% of sodium. CaL is produced from lignin, which is a biopolymer of subaromatic units characterized by lack of a defined primary structure. Water used in this study was nonpotable water taken from the tap in the laboratory.
The experimental study was carried out in three phases including the preliminary material characterization, compaction, and direct shear tests. The first phase consisted of sieve analysis of sand  and determination of specific gravities of sand and CaL . In addition, liquid limit test was carried out on S-CaL-W mix in accordance with the American Society for Testing and Materials standard .
The second phase included Standard Proctor Test . Laboratory compaction curves were obtained for the following lignin concentrations: 0%, 2%, 4%, 6%, 9%, and 14%. These results provided optimum moisture contents and maximum dry densities.
The third phase consisted of direct shear tests . The test procedure included thoroughly mixing oven-dried sand with oven-dried CaL powder at the desired gravimetric lignin contents. The preselected amounts of water were then added to the dry mix, depending on the desired configuration, and again mixed thoroughly. Each sample was then placed in the shear box and compacted statically to the desired height, thus giving the desired dry mass density. In order to obtain early age strength, the samples were tested immediately after static compaction.
4.1 Preliminary material characterization
According to the sieve analysis results, the coefficients of uniformity and gradation of the sand were found to be equal to 2.75 and 1.45, respectively. Specific gravities of sand and CaL solids were determined to be 2.64 and 1.60, respectively . Liquid limits were also determined for fresh S-CaL-W mixes. It was found that liquid limit increased nearly linearly with increasing gravimetric CaL content ranging from 5.4% for 4% CaL to 10.5% for 14% CaL content . Liquid limit test at 2% CaL proved inconclusive. Plastic limit test could not be performed due to inability of the material to form the threads having a 3 mm diameter.
4.2 Compaction test results
Laboratory compaction tests were carried out in order to determine the maximum dry densities of sand containing different amounts of CaL. A summary of standard Proctor tests results is provided in Figure 2. These results show that the maximum dry density of S-CaL-W mix decreases with the increasing CaL content, thus indicating that the smaller amount of sand can be packed into the standard volume size by using the standard compactive effort as the amount of CaL increases.
Figure 3 depicts experimentally obtained dry mass density versus moisture content for χl=6%. Additional curves shown in Figure 3 are zero air voids curve and constant water and air saturation curves corresponding to the air and water saturations at the optimum moisture content, respectively.
Saturation condition at the optimum water content can be found from the following:
whereby the void ratio is determined from dry mass density. Next, by combining Eqs. (2) and (4) and rearranging, CaL saturation is expressed as
thus enabling determination of CaL saturation at the optimum water content. Air saturation at the optimum water content is then determined simply from Eq. (4).
4.3 Direct shear test results
The program for direct shear testing was selected based on the results of the Proctor tests and in an effort to assess the effects of void ratio, water content, and CaL content. A number of different water contents were selected including optimum, dry, and wet optimum. For each individual CaL content value, five different sample configurations as shown in Figure 2 were tested. Three configurations are at the optimum water content, and they have the relative compactions of 100% (point A), 95% (point E), and 90% (point C). Two additional configurations are at the relative compaction of 95% (point D and point B). As shown in Figure 2, point D is located dry of optimum, while point B is located wet of optimum. These configurations were tested at CaL concentrations of 2%, 4%, 6%, 9%, and 14%. In addition, dry sand was tested at relative compactions of 100%, 95%, and 90%. Furthermore, each single configuration included five direct shear tests at the following normal stresses: 62 kPa, 92.9 kPa, 123.9 kPa, 185.9 kPa, and 247.8 kPa.
All direct shear tests were tested under displacement control whereby a slow displacement rate was selected. Effective friction angle and cohesion values were deduced from the direct shear test program. They are listed for all tested sample configurations in Table 1.
Shear force and horizontal and vertical displacements were continuously recorded during the shear phase of direct shear tests by using a data acquisition system. Failure was defined by the first attainment of the maximum shear stress. Shear stress versus horizontal displacement responses for configuration of 6% (E) at different normal stresses is depicted in Figure 5.
Figure 6 shows vertical displacement versus horizontal displacement for different confining stress levels for configuration of 6% (E). It should be noted that an increase in the sample thickness indicates dilation, which is negative herein.
Addition of CaL decreases dilatancy whereby the highest rate of dilation occurs usually at point A, which is followed by B, E, D, and C. Among D, E, and B, the configurations which have equal initial void ratios, the material at point B is often the most dilatant, and it produces the highest shear stress at failure.
In order to achieve better understanding of the interactions between sand and bonding material, a further analysis is necessary. To this end, additional equations have been derived starting from the basic definitions of phase relationships given in Eqs. (1)–(7). The derived equations provide the basis for determination of the portion of the total cross-sectional area, which is occupied by CaL-water paste. A schematic of the load-bearing cross-sectional area, as well as the entire cross-sectional area, is depicted in Figure 7.
It follows from Eq. (6) that
Similarly for CaL area ratio, the following is obtained:
And adding Eqs. (8) and (9) results in the following:
which gives a normalized area ratio as follows:
Equation (11) shows that water content, gravimetric lignin content, and void ratio affect the normalized area ratio. The latter is a ratio of the cross-sectional areas occupied by CaL and water paste and a total cross-sectional area, which is further divided by gravimetric lignin content. Thus, there are two different constituents carrying a load: CaL-water paste and solid skeleton. Upon mixing CaL and water, they form a paste, which acts as a bonding agent. This concept is illustrated in Figure 7.
From the equilibrium of forces depicted in Figure 7, it follows that the external or total normal force that is carried by these two constituents is given by the following:
where σc is the interparticle contact stress and σl+w denotes a stress in the CaL-water paste.
By dividing Eq. (12) by the entire cross-sectional area, A, the following is obtained:
Thus, there is a portion of the external or total stress that is carried by the contacts of sand particles, which is also known as the effective stress σ′. Adding the CaL-water bonding agent leads to an increase in the load-bearing cross-sectional area, as indicated in Figure 7. Thus, the external load is now also carried by the bonding agent, within which it induces a stress denoted as σl+w.
By applying the Mohr-Coulomb criterion, which holds based on the experimental results obtained in this study, the following is obtained:
where ϕ is the friction angle of the S-CaL-W mix in terms of total stress. Next, Eq. (14) is combined with Eq. (13) resulting in
Setting c′=0 implies that Φ=Φ′. Thus, the friction angle in terms of total stress is equal to the effective friction angle. Also, it follows from Eq. (15) that the effective cohesion (c′) is given by
Thus, the cohesion is provided by the bonding agent. According to Eq. (16), the amount of cohesion depends on the ratio of the portion of a total cross-sectional area that is occupied by CaL-W mix, and the total cross-sectional area. The cohesion also depends on the stress in the CaL-W mix (σl+w) and the effective friction angle of the S-CaL-W mix (Φ′). In addition, Eq. (15) shows that S-CaL-W mix can sustain a shear stress in the absence of normal stress. This further implies that it also possesses a tensile strength, which is clearly derived from the presence of the bonding agent.
Eq. (16) also gives the following expression for a stress in the CaL-W paste:
Next, the expression for the limiting normal stress (σl), which is the maximum normal stress that can be applied to the S-CaL-W mix while still producing a larger peak shear stress at failure as compared to dry sand, is given as follows:
For normal stresses larger than the limiting normal stress, the S-CaL-W mix is superseded in ranking of peak shear stress by the dry sand. The limiting normal stress provided by each percent of CaL content decreases with the increasing CaL content (Figure 8). It remains the largest in configuration B, followed by C, A, E, and D. Configuration C is positioned high because of the lowest value of the friction angle in dry sand at relative compaction of 90%. In addition, the range of normalized limiting stress for a given χl decreases with increasing χl.
Figure 9 shows an increase in a normalized cohesion with an increase in the normalized area ratio. All sample configurations except A, C, D, and E at 2% of CaL appear to follow a unique trend.
Sample configurations in Figure 10 were selected by excluding 2A, 2C, 2D, 2E, 9B, and all configurations at χl=14%. This improved the regression coefficient significantly (from 0.47 to 0.89). Furthermore, the depicted graph shows that the peak experimentally obtained value of a normalized cohesion is about 4 (kPa/%). To provide a rationale for excluding the aforementioned sample configurations from Figure 10, a mass of CaL-water mix is divided by the total mass of the mix. This provides the mass of the bonding agent as a percentage of the total mass of the mix. It is because the bonding agent consists of extremely small CaL particles that the percentage of CaL-W paste resembles the percentage of fines contained in the coarse-grained soils, which is evaluated during the soil classification procedure. The results are shown in Table 2.
They indicate that there is simply not enough of CaL-W paste at very low water and gravimetric CaL contents corresponding to sample configurations 2A, 2C, 2D, and 2E because these configurations result in the percentage of fines smaller than 5%. This is based on the fact that sands containing <5% of fines are named clean sands according to the Unified Soil Classification System (USCS) . On the other end of the spectrum sands with more than 12% fines contain a significant amount of fines, thus losing the gradation characteristics in their name and ending up only as simply clayey and/or silty sands. In analogy with this USCS  procedure, the sample configurations that contain more than 12% fines are also excluded in Figure 10 as they are likely to form a different material. Furthermore, the results depicted in Figure 9 clearly indicate that there is an overabundance of a bonding agent for sample configurations containing more than 12% fines. Thus, the increase in cohesion becomes not only significantly smaller but also the difference in normalized cohesion values among the sample configurations A, B, C, D, and E becomes significantly smaller at χl=14%. This also indicates that the effect of void ratio and water content becomes less significant at 14% of CaL. In addition, Figure 9 shows that sample configurations 2A, 2B, 2C, and 2D are all located away from all remaining sample configurations. In summary, the results shown in Figure 10 and Table 2 indicate that the mixes containing anywhere from 4% to 9% of CaL (including 2B and excluding 9B) exhibit a common behavior, which differs from the remaining samples (Bold values in Table 2). This behavior corresponds to the range of an effective contribution of the CaL-water paste. Specifically, the lower bound of the range indicates that a sufficient amount of CaL-water is necessary, and the upper bound indicates that too much of CaL-water paste begins to weaken the S-CaL-W mix. The latter probably happens because of the lower strength of CaL-W mix as compared to the sand skeleton and also because of the possibility that sand particles might be pulled away from each other due to the presence of too much CaL-water paste. This can also be observed in Figure 11, which shows optical microscope scans of S-CaL-W mixes at 0, 6, and 14% of CaL. The balance of CaL, water, and sand appears to be nearly optimal in Figure 11B because CaL-W paste appears to coat the sand particles evenly without the presence of too much paste. On the other hand, at 14% of CaL, which is depicted in Figure 11C, the overabundance of lignin is evident, thus pushing the sand particles away from each other.
Figure 10 can serve as a design chart, which is based on the experimental data from this study. Specifically, by first selecting the water content, void ratio, and CaL content the material design curve depicted in Figure 10 provides the normalized cohesion, which can be obtained for the selected parameters.
From the extensive experimental program followed by the data analysis, the following conclusions and recommendations can be drawn.
The range in which the bonding benefit is fully realized falls between χl=2% (2B only) and χl=9% excluding 9B with the maximum cohesion occurring at χl=6%. Figure 10 represents a baseline design chart, which provides the amount of a normalized cohesion gained for selected values of water content, gravimetric CaL content, and void ratio, for masonry sand used in this study.
The bonding agent increases a load-bearing cross-sectional area through a particle bonding mechanism.
Increasing the normalized ratio of the cross-sectional areas occupied by CaL and water paste or bonding agent increases the amount of normalized cohesion gained up to a point (2B).
While the normalized cohesion increases with the normalized area ratio, the cohesion itself reaches the maximum value of 18.3 kPa at χl=6% (point A). The cohesion of S-CaL-W mixes is always larger than the cohesion of dry sand. The latter is nonexistent. Consequently, adding CaL-W paste not only increases the shear stress at failure for normal stresses below the limiting normal stress, but it also adds a tensile strength.
Lignin is a widely available, sustainable, and safe material to work with. This study shows that an early age gain of cohesion can be obtained by adding small amounts of CaL and water to dry sand. This is likely very beneficial for unpaved roads and as a protection from erosion in desert areas. The potential for CaL use as a sand stabilizer and dust suppressant is nearly unlimited.
In summary, the use of widely available sustainable and environmentally safe material such as CaL that is derived from lignin in combination with sand is a very effective and sustainable approach. It is noted that this approach makes effective use of the two nearly most abundant materials on Earth, lignin and soil.
The authors would like to acknowledge the University Transportation Center at Kansas State University, which made this study possible by providing the student support. The authors are also grateful to Ligno Tech USA Inc and Midwest Concrete Materials for donating calcium lignosulfonate and sand.
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About the article
Published Online: 2014-09-20
Published in Print: 2016-03-01