Utilizing Rice Hull Ash as Partial Replacement for Cement in Producing Soil-Cement: Materials for Deep Mix Soil Stabilization

. Stabilization of residual soils is studied chemically using cement and Rice Hull Ash (RHA). The investigation includes the evaluation of the physical properties of the soil and its classification. Furthermore, changes in the unit weight, the specific gravity of non-treated and treated with 5-25% RHA partially replacing soil-cement samples, and the moisture content of non-treated and treated soil before and after curing (21 days) were also examined. Moreover, ultimately, an investigation as to what percentage of RHA in partially replacing cement would produce the largest compressive strength and shear strength has also been done in this study. Tests showed that the soil was CL (Low Plasticity Clay) and that it has been found out the unit weight increases as the treatment of percentage increases. The specific gravity also increases with the further addition of cement but decreases when the amount of RHA is noticeable in the mix. As to moisture content, it has been found that there were always changes in the water content before and after curing for hydration. Moreover, the Unconsolidated Undrained Triaxial test was also employed to determine the changes in the strength of non-treated and untreated soil. It has been found that the maximum deviator stress peaked at 20 % RHA replacement, as do the internal friction angle and cohesion value, hence the maximum shear strength, for all confining pressures. These parameters have proved that RHA 20% partial replacement has the potential to Deep Mix Method (DMM) in strength and economics.


Introduction
Rapid urbanization and population growth require various civil engineering infrastructures and facility services.Because of the scarcity of suitable lands, selecting an appropriate location for the infrastructure has become difficult.At present, various types of construction engineers are forced to lay the foundation on soft ground.Ground improvement by treating soft soil like clayey soils with various binders is an attractive alternative and often economical compared to other ground improvement methods.This is called a chemical way of stabilization wherein there is an addition of binder that alters the engineering properties of the existing soil to create a new site material capable of meeting certain soil requirements, like having a reliable bearing capacity [1].The Deep Mixing Method (DMM) is a soil/ground improvement that was first started in the 1960s and was put into practice in Japan.DMM involves reagents like cement mixed into the soil at a certain depth to improve the soil's onsite properties without excavating or removing soiled [2].Due to its many benefits, cement is frequently used to stabilize soil.When cement and water interact, calcium silicate hydrate (C-S-H), a strong and resilient binder, is created.This binder creates a stable and cohesive matrix by filling the spaces between soil particles.As the cement hardens, it gains strong compressive strength, increasing the stabilized soil's stability and load-bearing capability [3].Cement stabilization makes rapid strength gain possible, allowing for quick construction or project turnaround times.However, while ground improvement paves the way for solving the weak soil problem, it is undeniable that the cement's saving element can invite yet another predicament.In the context of the low availability of non-renewable energy resources coupled with the requirements of large quantities of energy for building materials like cement, it is undeniable that cement can be so much expensive [4].With this, pozzolans (pozzolanic materials) can be considered.Pozzolanic materials are silicate-based materials that react with the calcium hydroxide generated by hydrating cement to form additional cementitious materials [5].Rice Hull Ash (RHA) has been known to possess a large amount of silica content that can be compared to pozzolanic materials [6].Based on the literature, using RHA for soil stabilization has several advantages.RHA is a pozzolan that reacts quickly.It contains a sizable amount of silica, which produces cementitious compounds when combined with calcium hydroxide and moisture.RHA can aid in creating a strong and stable binder in soil because of this characteristic.When combined with the soil, RHA can increase the soil's compressive strength, shear strength, and stiffness.The creation of extra cementitious compounds, which fill the spaces and bind the soil particles together due to the pozzolanic reaction between RHA and calcium hydroxide, increases the strength [7].Furthermore, rice hulls, used to make RHA, are a common agricultural byproduct in the locality [8].This makes RHA a cost-effective option for soil stabilization, especially in areas where rice production is prevalent.Using a waste material, RHA contributes to sustainable, eco-friendly soil stabilization practices.Hence, given the expensiveness of the cement, the wide availability of rice hulls, and the problem of soil stabilization, it is proper to research the effect of alternatives of cement, such as rice hull ash, to ascertain its effectiveness should we incorporate it in the soil-cement mix.Investigations shall be undertaken to see whether these rice hull ashes can greatly help improve the bearing capacity of soft soil should we make it a partial replacement in soilcement.This research generally aims to determine the effect of the usage of Rice Hull Ash (RHA) as a partial replacement for cement in producing low-cast soil-cement for Deep Mix Method soil stabilization by subjecting these soils to different confining pressure in the triaxial chamber for the Unconsolidated-Undrained (UU) Test.Specifically, this study aims to describe the changes in physical properties of soil-cement at various mix proportions (treatment) such as unit weight, specific gravity, and moisture content before and after curing of samples at different percentages of treatment; describe the changes of deviator stress at each percentage of treatment when confining pressure in UU Test is increased; describe the changes of the internal angle of friction and cohesion value of the graphs at each percentage of treatment; determine the effectiveness of RHA in partially replacing cement in each percentage at different confining pressure by determining the optimum ideal proportion of rice hull to cement and ultimately, determine the potential of RHA for Deep Mix method of soil stabilization.

Research materials
To control this possible variation from source to source, own RHA is produced in this research.After it had been burned for several hours, RHA was passed through the #200 Standard US sieve to remove unburned materials and maintain a relatively higher silica volume, as shown in Fig 1 .The soil is from the excavated foundation of a three-story building to be constructed in USEP Obrero, Davao City.The cement used is Ordinary Portland cement labeled as Holcim Excel, manufactured by the Holcim company.Potable water free from any inorganic matter and deleterious substances were used.

Research procedure
After the soil's moisture content was determined the next day, the samples were prepared for use in the UU Test.As for the non-treated soil, extrusion of the sample is impossible since the molder of the equipment does not permit extrusion but rather remolding of the soil.Although remolding would greatly affect the undisturbed strength of the soil, it is still permissible since the remolded soils were not tested right then but were allowed to rest for some time or cured.The soil experiences thixotropy when it gradually gains strength when it remains at rest and without loss of moisture content [9].Other variables, such as mixing time, optimum remolding water content, the water-cement ratio of 0.6, and the amount of wet soil in the mix, were kept constant to see the unbiased effects of partially replacing cement by the RHA.Researchers employed Asturias's findings, as shown in Table 1 [ Where: P = Percentage of RHA in the total amount of cement in the mix WT = wet weight of the soil in (g); Wc = weight of cement in (g); Ww = weight of water already in the mix (g); Ww = additional weight of water for remolding of treated specimens in (g) as defined by the equation: Wcf = Weight final of the cement after RHA had been incorporated in the mix (g).WRHA = Weight of RHA (g) After the design mix had been determined, the specimens were prepared by mixing the sample for 10 minutes using an electric mixer with occasional hand mixing, compacted in 25 lifts with poking and light tamping to exclude the air voids [1].The sample was then put into the mold with an average ratio of height to diameter 2:1.Fig. 2 shows the soil with RHA and cement mixed into it.Curing is important since it controls the rate and extent of moisture loss from concrete to ensure the uninterrupted hydration of Portland cement and RHA after mixing.Curing also ensures the maintenance of an adequate temperature of concrete in its early ages, as this directly affects the rate of hydration of cement and, eventually, the strength gain of concrete or mortars [1].To achieve this, the prepared samples were cured when the temperature did not exceed 20°C.The samples were also wrapped in sealant plastic non-biodegradable bags to preserve the moisture content of the samples.Curing of concrete must begin as soon as possible after placement & finishing and continue for a reasonable period per the relevant standards for the concrete to achieve its desired strength and durability.The samples have undergone 21 days of curing.Adhering to the standard, the confining pressure the soil samples were subjected to was 45 kPa, 60 kPa, and 75 kPa.These values of confining pressure were derived following what was stated in the standard that the soil must be subjected to, its unit weight multiplied by the depth these were extracted.The value of the unit weight of the soil was found to be 15.11 kN/m 3 .Listed in Table 2 are the values of pressure at depth.To see how well it behaves, the soil is subjected to different confining pressures using a triaxial apparatus, as shown in Fig. 3. Determining this engineering property of the soil would be a great help in various geotechnical designs and analyses.Having plotted the maximum deviator stress as seen on each stress vs. strain graph, it shall be added to the minor stress σ3 to get σ1.These coordinates at different confining pressure shall be plotted in a τ vs. σ graph to get a failure line.These lines shall give the angle of internal friction (∅) and the cohesion of a soil sample being tested.

Fig. 3. Test set-up
As previously stressed, once the soil properties and the optimum amount of binder material's gain of strength have been determined after treatment, these shall be examined whether there could be potential in it for the Deep Mix Method of soil stabilization by looking at the optimum increase of strength.

Soil classification
As per the Unified Soil Classification System or the USCS, the soil classification is CL (Low Plasticity Clay).

Unit weight for treated and untreated soils
A graphical presentation of the comparison among the unit weights of the treated and untreated soil samples subjected to different confining pressure is shown in Fig. 4. It can be observed that the unit weight has soared as the amount of RHA increases.This is because cement, being a hydraulic one, undeniably demands water for hydration.So, when the water is consumed, voids in the soil decrease, yielding a larger unit weight, especially when the RHA is noticeably present.RHA has a large porosity value and thus demands much water [10].The decrease in voids eventually leads to an increase in solids per unit volume [1].Confining pressures 45, 60, and 75 kPa do not influence the unit weight for each sample since the measurements were conducted before the samples were even tested.Further, it was also made sure that during the loading in the triaxial, none of the water would enter the soil sample as it was being tested.

Specific gravity
Fig. 5 shows that the specific gravity of the soil samples rose at some point but lowered.The rise can be attributed to the cement's dominating presence, which brought up the gravity of now-treated soil.However, then it went down because the RHA took over.RHA has lower specific gravity than cement and the base soil, bringing down the value of specific gravity.Thus, adding RHA decreases the specific gravity of the soil [4].

Moisture content
It can be deduced that the moisture content decreased over time during the cementing process by silicate-based additives; there was an increase in hydration.Some water comes out of the sample though efforts to seal it were made.The water evaporated was just minimal; thus, this is still acceptable.

Stress-strain graphs
Fig. 6 shows the average stress vs. average strain graph of untreated (0%) and treated soil with 5%-25% RHA at different confining pressures (45 kPa, 60 kPa, and 75 kPa).At 0%, it can be observed that the soil has experienced a ductile type of failure, for the failure does not happen instantly, but the increments rise bit by bit hence a not-so-steep graph of stress and strain.Meanwhile, at 5%-25% treated soil, each graph's failure seems brittle; the graph is steep, and there was an instant drop in values.The brittle kind of failure is a kind of failure that has relatively little plastic deformation and propagates rapidly without an increase in applied stress [11].Moreover, it can be observed that the highest value of strength is found at 20% RHA replacement in the soil-cement mix.

Failure
Fig. 7 shows the type of failure of the untreated soil.It is a bulging failure in which there is an upward expansion or displacement of soil due to loading.Expansive soils-soils that significantly alter their volume in response to variations in moisture content-typically experience this phenomenon.These soils contain clay minerals with the capacity to absorb and release water.These soils both expand and contract because of water absorption.This natural cycle of expansion and contraction can pose serious issues for structures erected on or near expansive soils.One of the main causes of soil bulging is fluctuations in moisture content.Expansive soils absorb moisture and expand due to becoming saturated with it.The arrangement of particles within a clayey-like soil must have exhibited an extensive plastic deformation ahead of the crack since it resists further extension unless applied stress is increased [12].The consequences of soil bulging failure can be significant.It can cause cracks in foundations, walls, and floors, leading to structural instability.On the other hand, the failure exhibited by the 5% to 25% treated soil, as shown in Fig. 8, seems to be the opposite, as evidenced by 45° shear cracks in the samples.Soils possess shear strength, which resists the sliding or failure of soil masses.The angle of internal friction, denoted as ∅ (phi), measures the soil's resistance to shear.In some cases, a 45-degree angle of cracking or failure aligns with the angle of internal friction for certain soils.When a slope fails along this angle, it suggests that the shear strength of the soil has been mobilized.Fig. 8 Failure photographs of 5%,10%, 15%,20%, and 25% rha-treated soil (l-r).

Deviator stress
The Unconsolidated Undrained Test (UU) is a quick test to obtain the shear strength parameters of both fine-and coarse-grained soils in undisturbed or remolded states.The stress that would be computed from UU is total since water could not dissipate, thus providing resistance to the load applied [9].Since the drainage of the water valve is closed in the UU Test, it is assumed that the volume of the sample remains constant and that the area of the sample increases uniformly as the length decreases [9].The maximum deviator stress produced by the UU Test is also the maximum compressive strength exhibited by the sample [9].As the confining pressure increases, the value of the peak or the deviator stress also increases.Ideally, the maximum deviator stress should remain the same for UU Test for clayey soils even though the confining pressure increases.However, that is emphasized so long as the clayey sample is 100% saturated [9].It was previously mentioned at the beginning of the findings that the soil saturation is not 100%, with the presence of foreign additives (cement and RHA).As observed in Fig. 6, the maximum deviator stress is found when there is a 20% RHA replacement in the soil-cement mix.

Angle of friction and cohesion
The modified failure-plane envelope is an equivalent graph for  vs.  graph where Mohr circles are plotted.It is a graph that is defined by p= (1 + 3)/2 on the x-axis while q= (1 -3)/2 on the y-axis.As previously mentioned,  1 is the sum of cell confining pressure 3, plus the maximum deviator stress d.The angle of friction values could be used in determining the shear strength, , of the sample when subjected to a certain value of .At 20% RHA treatment, the internal angle of friction and cohesion stopped increasing, then started decreasing when the RHA was 25%.As the binder content increases, there would also be an improvement in the internal angle of friction and cohesion properties of the soil [12].However, since the binder used was two different materials (cement and RHA), a percentage of RHA could be optimum since its cementitious properties are much less inferior to cement.

Effectiveness of rha as partial replacement for cement
It can be deduced that partially replacing cement with RHA at an optimum percentage of RHA is effective.The problem lies as to which percentage it would materialize.The performance of RHA for stabilizing increases enormously at a certain point, yet the value drops after that [4].From the recent discussion, the maximum value peaked at 20% at all treatments and dropped when the treatment reached 25%.Moreover, in terms of shear strength, as the internal angle of friction and cohesion is high, then the shear strength is also high since the relationship is defined as  = tan∅ + C;  as shear strength,  as confining pressure, ∅ for the internal angle of friction and C as the cohesion value of the soil.Thus, from the results, it can be construed that at 20%, the RHA-treated soil can perform at its best.
Due to the reaction between RHA, cement, and soil, there was an increase in strength.The exchange of cat ions and formation of cementing products must have bonded the particles altogether, and thus the strength gets bigger.However, when the RHA's presence noticeably dominates, the strength decreases because its capacity is much inferior to the cement's.

Potential to deep mix method
Hence, looking at the improvement of the performance of soil when added with cement and 20% RHA, with the incorporation of other factors such as water-cement ratio, and best mixing time, among others, it is recommended that it be used for stabilization techniques such as the Deep Mix Method.Moreover, not only does RHA help in terms of gained strength, but it also helps economically.DMM expenses can be lessened up to a considerable percentage by incorporating RHA in the mix, for RHA is much cheaper than cement.Further, the knowledge of the behavior of the soil in terms of unit weight, specific gravity, and moisture content would also be crucial in different analyses, such as stability and settlement, among others employed in DMM.

Conclusion
The following conclusions are drawn from this study based on the objectives: The unit weight rises as the amount of RHA increases.The larger unit weight is due to the hydration wherein the cement consumes water for the process, and RHA has a large porosity value and demands much water.The decrease in voids eventually leads to an increase in solids per unit volume.The specific gravity rises due to the high presence of cement but lowers when the presence of RHA dominates.RHA has lower specific gravity than cement and the base soil, bringing down the value of specific gravity before and after curing to show a decrease in moisture content.This is because as hydration takes place, the water evaporates.However, this is just tolerable since the amount was just minimal.
It can be inferred that the soil at 0% treatment has experienced a ductile type of failure, for the failure does not happen instantly, but the increments rise bit by bit hence a not-sosteep graph of stress, while the graphs from 5% RHA Treated Soil to 25% Treated Soil shows that the failure exhibited by each of the graphs seems to be a brittle kind of failure; the graph is steep, and there was like an instant drop of values.The brittle kind of failure is a kind of failure that has relatively little plastic deformation and propagates rapidly without an increase in applied stress.At 20 % RHA replacement, maximum compressive strength and largest shear strength are found based on the internal friction and cohesion angle values.Values beyond 20% at 5% increment lower down the value.It can be attributed to the binding of the particles altogether, so the strength gets bigger unless RHA's presence noticeably dominates.
From the results, it can be construed that at 20%, the RHA-treated soil can perform at its best for maximum compressive strength and shear strength.
Hence, proving that incorporating RHA can still increase the gained strength of the soil up to 20% leads to the belief that it has potential on DMM.Not only does it help in strength but also the economy.Knowledge of the behavior of the soil in terms of unit weight, specific gravity, and moisture content would also be crucial in different analyses, such as stability and settlement, among others employed in DMM.

Table 1 .
1]. Best Mixing Time = 10 minutes, excluding remolding the specimens.Optimum Remolding Water Content for Treated Samples = 1.15WoAmount of Cement in the Total Mix AC = 20% or 0.2 Original Moisture Content of Soil, Wo = 35.11%W/C (Water-Cement) ratio = 0.6 Design mix for each percentage

Table 2 .
Schedule of confining pressure