Potential of Bamboo Leaf Ash for Soil Stabilization - Literature Review

Soil stabilization is an important topic in geotechnical engineering as many problematic soils were found during construction. One of the commonly used soil stabilization is cement, but it is widely known as a non-sustainable material. Much research provided the usage of industrial waste as a substitute for cement, which is then known as a supplementary cementitious material (SCM). One of the alternatives is bamboo, which has the second highest silica content after rice husk. This paper conducted a literature review on the potential of using bamboo leaf ash (BLA), which is the main composition of silica in bamboo. Various studies indicated the usage of BLA, ranging from its initial usage in concrete to its development in soil stabilization. Five studies were pointed out as the extensive research of BLA usage in Nigeria, focusing on lateritic soils. The results indicated that stabilizing lateritic soils with BLA increases the CBR values and the bearing capacity, whereas it decreases plasticity index values. Later studies followed by using other types of soils. The results are all consistent, where the shear strength of soil and CBR values increases with the usage of BLA as SCM.


Introduction
Research on road construction showed that several factors could cause road damage.These causes are poor drainage systems, unsuitable properties of pavement construction materials, unstable soil conditions, differences in climatic conditions, improper pavement layer design, and mistakes in fulfilling the road design rules during construction [1].From these factors, one of the focuses of road repair solutions is soil improvement.
Most of the time, cement is used for soil improvement as it can be procured easily.However, Portland cement has high carbon dioxide emissions which leads to serious environmental impacts.The high emission of procuring cement is contradictive with the sustainable development goals, specifically for SGD Goal 9 and 11.A sustainable industry with lower carbon emission is highly recommended, including reducing the production of cement.Therefore, a replacement for cement usage was proposed by many studies and various industrial wastes were tested as supplementary cementitious material (SCM).The usage of industrial waste was due to its inexpensive prices and abundant nature.One of the wastes that can be considered as SCM is bamboo leaf ash (BLA).The life cycle analysis (LCA) research 1324 (2024) 012044 IOP Publishing doi:10.1088/1755-1315/1324/1/012044 2 on bamboo has shown that using BLA as a partial replacement for cement would be cost-effective and eco-efficient [2].
The usage of bamboo itself is due to its abundant amount in nature with approximately 1200 species around the world, where 160 species can be found in Indonesia.Alternatively, with around 22 million hectares of bamboo land in more than 70 countries, Indonesia is known as the third-largest bambooproducing country in the world.Nowadays, bamboo is used as a craft and construction material that replaces wood due to its advantages.Some of the advantages of using bamboo are its fast-growing time, short harvest time, continuity of growing new shoots, and resilience in growing under different soil conditions [3].
In soil improvement, Portland cement is mainly used due to its pozzolanic traits which react with calcium hydroxide to form calcium silicate hydrate.Ongoing research indicated that the replacement for Portland cement needs to have the same pozzolanic.Bamboo leaf ash, which is the industrial waste of the bamboo industry, has the same pozzolanic activity that can increase with time and temperature [4].The main part of the pozzolanic activity is the silica material, which can be found in around 60-70% of bamboo leaf ash [3].
To be exact, BLA is an N-class pozzolan with high silica content.However, it is not completely amorphous because calcium hydroxide is required in the liquefaction process.During the hydration process, calcium hydroxide reacts with silica to form silica hydrated.As an example, a study in 2021 used a combination of 90% bamboo leaf ash and 10% calcium hydroxide [5].From the referred study, it was known that the optimum composition to achieve the highest compressive strength was to use the 8% additive.The study also concluded that a higher percentage of BLA used in the mixture would give a lighter mass of the mortar.For the cement, it would then result in lower porosity and absorption capacity [5].
It is clear that BLA can contribute in overcoming environmental problems caused by the usage of cement in the civil engineering industry.The similar substances with cement and the abundant existence of bamboo could support the industry to achieve its SGD goals.In tackling more area of the civil engineering industry, the BLA usage has evolved from being an admixture in concrete to being used for soil stabilization in recent years.To understand its mechanism, it is important to understand the trend toward using BLA specifically as a soil stabilization material.Thus, this study summarized the trend of BLA as a soil stabilization material and also its future application in geotechnical engineering.

Method
The method of acquiring the trend for BLA soil stabilization in geotechnical engineering is presented in this section.A systematic literature review was conducted on the topic of BLA as SCM for soil stabilization.This study compiled research from various databases with the keyword soil stability using bamboo leaf ash (BLA).The search was conducted using titles, abstracts, and keywords of bamboo leaf ash.The findings were sorted by the oldest year to the present.
Afterward, a separate section to discuss the usage of BLA on lateritic soil is given.A separate section is used to understand the effect of BLA on lateritic soils, which are quite common in Indonesia.

Bamboo Leaf Ash (BLA) and its usage in soil stabilization
Before diving into the research development of BLA as soil stabilization, it is important to understand the process of acquiring bamboo leaf ash from the bamboo industry.Bamboo leaf ash is the leaves of bamboo burned in open air and heated at 600 o C for 2 hours in an oven.As previously mentioned, bamboo leaves are amorphous materials containing amorphous silica.The silica was captured through rigorous testing using chemical analyses, SEM techniques, and powder XRD.According to the results, the ash reacted with calcium hydroxide and has pozzolanic properties.Previous research provided that when 20% of BLA by weight is mixed with Portland cement, the compressive strength at 28 days of hydration is comparable to that without ash addition [4].The reaction can be seen in equation (1): To compare the silica amount of BLA and Portland Cement, the oxides of both materials were compared in Table 1.The table showed that the present composition values of BLA have high amorphous SiO2, which is expected to be a pozzolanic material and can react with calcium hydroxide in solution as well as in paste.As can be seen from the previous research, BLA can be a great candidate for SCM.In 2010, a study used BLA as a soil stabilization material in Nigeria [6].The study investigated the stabilization properties of BLA in lateritic soils for highway construction.Using three soil samples, the BLA was added by weight at 2, 4, 6, 8, and 10% mixture.The results indicated that BLA significantly improves the quality of the soil by lowering the plastic index value, increasing the shear strength properties, and increasing the CBR value.
Following the study, a similar experiment was conducted in 2011 [7].The study determined the effect of bamboo fly ash on laterite soil with the addition of lime on a highway construction project.The study tried BLA to determine its usefulness as an additional stabilizer for lime, which results in reducing the cost of soil stabilization material.Three samples were collected from different locations in Ile-Ife, Nigeria.The study conducted preliminary tests such as moisture content, specific gravity, grain size analysis, and Atterberg limits.Technical characteristics such as compression, California Bearing Ratio (CBR), and three-axis ground sense were also conducted in the study.The optimum stability was achieved when the percentage of lime and BLA combination was 2, 4, and 6%.The strength and undrained CBR values increase with the increase of lime and BLA as the soil stabilizer.
In 2012, another study used the combination of cement and BLA as its binding material in road construction in the Makurdi region [8].Stabilization was performed using the dry weight of soil with a combination of cement-BLA ratios in the mixture ranging from 2% to 14%, whereas the pure BLA ratios ranged from 4% to 20%.The samples were tested with soil classification, compaction, consistency, CBR, and unconfined compression test (UCS).The test results showed that adding 14% of BLA would reduce the plasticity index by 34.7%, increase the maximum dry volume value (MDD), increase the optimal moisture content (OMC) by 18.6%, and reach the maximum soaked CBR value when compared to other BLA mixture.The study concluded that adding BLA from 14%-20% to the sedimentary soil is recommended to strengthen the base material in the construction of curve pavements.
In 2013, the improved Makurdi shale was analyzed [9].The shale was stabilized using BLA at a rate of 4-20% of the dry weight of the soil.Exponential property tests and technical property tests were conducted to understand the effect of BLA on the soil.The results showed a decrease of IP of as much as 25% and a decrease of strength of as much as 39.4% compared to previous mixing results.The study concluded that the BLA rate fell below the lower limit of the road construction strength standard, which led to a recommendation of adding other stabilizers such as cement or lime to reach the desired outcome.
In 2014, the effect of cement with BLA stabilization was analyzed using 2%, 4%, 6%, and 8% cement and BLA addition compared to the weight of soil [10].The natural state for three soil samples showed a maximum dry solidity ranging from 16.3 kN/m 3 to 17.45 kN/m 3 , whereas the optimum moisture content ranges from 12.5% to 17.5%.When the stabilizers were added, the MDD gradually dropped at 4% cement-BLA mixture and abruptly increased at 6% cement-BLA mixture.On the other hand, the 4 OMC of the soil increases with the increased percentage of cement-BLA mixture.The CBR values increased slightly 2.36%-9.61%at an average of 8% cement+ BLA mixtures for the three samples.
In 2015, the impact of BLA addition was studied on its effect on the plasticity index values [11].The addition ranges from 0% to 10% of BLA to the soil.The result of the study showed that the PI values of the stabilized soil decrease with the increase of BLA amount.
In the same year, a study evaluated the effect of fly ash, rice husk ash, and BLA derived from each industrial waste as a stabilizer for clayey soils [12].The study concluded that a 15% addition of the three materials would increase the dry density, unconfined compressive strength, and CBR.On the other hand, the three strengths of the soil would decrease when the stabilizer addition reached 20% of the soil weight.
In 2018, a laboratory study was conducted using BLA content ranging from 2-8% for laterite soils [13].During the study, additional lime was given between 2.5% and 10% for the constant BLA of 5%.Four different mixtures were created with the ratio of laterite-BLA-lime: Mixture 1 (20:1:0.5),Mixture 2 (20:1:1), Mixture 3 (20:1:1.5),and Mixture 4 (20:1:2).The mixtures were evaluated with UCS, CBR, compressive strength, and water absorption test.The UCS showed an increasing value as the BLA increased from 2% to 6% and the laterite-BLA-lime mixture from Mixture 1 to Mixture 4. From the various mixtures, the maximum UCS was found at BLA of 5-6% and the lime was at 7.5%.The CBR values for soaked and unsoaked conditions increased as much as 34% and 49% for the soil, which made the laterite soil meet the minimum requirements of the subgrade soils.
In the same year, the deviator stress and shear strength of BLA-gray clay-cement samples were evaluated using free compression tests (UCT) [14].The samples were divided into a mixture of 0%, 5%, and 10% of BLA.All samples were taken within 0, 7, and 28 days.The optimal curing time for BLA, Portland cement, and the soil mixture was found to be 7 days, where the maximum stress was around 292.7 kPa.At the same curing time, the Mohr-Coulomb criterion showed the highest shear strength value when the 5% BLA was mixed with cement.
In 2019, the combination of 5% lime added to the proportion of 10%, 15%, and 20% BLA from the weight of the soil sample was tested to evaluate its performance in soil stabilization [15].The study showed that adding BLA and lime improved the overall strength of the soil.The optimum content of BLA and lime addition was found at 15% BLA and 5% lime.The test results showed an 8.5% increase in moisture content, a 0.7 gram/cm 3 decrease in maximum dry content value, and a 25.4% increase in soaked CBR value.The study concluded that the 15% BLA and 5% lime have a significant impact on improving road construction.
In the same year, the effect of curing and an additional percentage of BLA was analyzed [16].The BLA was varied at 0%, 2%, 6%, and 10%, whereas the time of curing was varied at 0 days, 7 days, 14 days, and 28 days.The original soil in the study belongs to the category of organic clay soil with a plasticity value of 20.11%.After the soil was stabilized with BLA, the optimum value was found at 2% BLA and 28 days of curing for the soil, where the compression test reached a value of 4.01 kg/cm.
In 2020, the CBR value was investigated for a BLA addition and reduced cement percentage of soft clayey soil [17].The overall content of both cement and BLA is 12% of the dry soil weight.The BLA content was varied at 0%, 25%, 50%, 75%, and 100% from the total stabilizer content.The study showed that replacing 25% of the cement with BLA increases the CBR values at both its unsoaked and soaked conditions.The study noted that to ensure the binding of the mixtures, the soil should be cured at a longer aging time.
In 2021, the evaluation of BLA and rice husk was conducted to improve the properties of lateritic soil [18].The study investigated the microstructure of the cement-treated laterite and its geotechnical properties at both conditions of being stabilized with rice husk ash and BLA.The test ranges from compaction, CBR, and preliminary tests such as specific gravity, grain size distribution, and Atterberg limit.The results showed that the addition of 6% and 8% BLA was enough to increase the soil quality suitable as a subgrade.According to the SEM and XRD results, were new connections formed, and microstructural arrangements were seen for the stabilized soil.The study concluded that there was a hydration reaction of pozzolan and cement that occurred during the stabilization process of the soil with BLA.
In the same year, a study analyzed the effect of combining 5%, 10%, 15%, and 20% of the soil weight with BLA and 5% cement to the natural soil sample provided [19].From the various BLA combination, the 5% BLA and 5% cement was proved to be its optimum content which can increase the maximum dry content by 1.24 g/cm 3 and the optimum moisture content by 35.22%.Furthermore, the CBR value increases by as much as 31.84%,and its swelling potential decreases to 0.303%.
Another study in 2021 investigated the geotechnical and microstructural properties of cement-treated laterite soil stabilized using rice husk ash and BLA [20].The soil was stabilized with cement at a ratio of 0 to 12% and was separately mixed with BLA and rice husk ash (RHA) at a ratio of 0% to 16%.The study conducted compaction, CBR, and Atterberg limits at each soil mixture.Both the BLA-cement and RHA-cement mixtures increased the CBR values of the soil, which makes it suitable for the subgrade of a road.Additionally, the optimum mixture was found to be at 6% cement + 8% BLA and 8% cement + 8% RHA. the same percentage of BLA and RHA, an optimum result can be seen at a lower cement percentage for BLA.
A study about expansive soils with improvement by RHA, Polypropylene Fiber (PPF), and BLA was conducted in 2021 [21].The results showed that the soil permeability decreases with the increase of RHA, PPF, and BLA in the soil.On the contrary, the OMC decreased and the MDD of the soil increased during the compaction test of soil with increasing RHA and BLA stabilizers.On the other hand, the PPF showed the opposite behavior of increasing OMC and decreasing MDD with increasing PPF content.Finally, the study also found that the UCS of soil increases rapidly with the increase of RHA, BLA, and PPF content as the soil stabilizers.
Another study in 2021 evaluated the varying percentages of BLA of 0%, 2%, 4%, 6%, and 8% with laterite soils [22].Results showed that increasing BLA content would lead to a decreasing trend of Atterberg limits, increasing maximum dry density (MDD) values, and decreasing optimum moisture content (OMC) values.For the soaked CBR, the CBR values increase until 6% of the BLA mixture but decrease at 8% BLA mixture.Generally, the study concluded that the increase of BLA as the soil stabilizer would give an increase in compressive strength as it decreases the hydration process.
The last study included in this paper is a study in 2023 that investigated the geotechnical properties of mixed laterite and bentonite soils treated with BLA [23].The study included sodium bentonite and added it to laterite soil with an amount of 5%, 10%, 15%, and 20% from the weight of laterite soil.The mixture was then treated with BLA with an increasing weight of 4%, 8%, 12%, and 16%.Generally, the plasticity index of the treated soil decreases by as much as 10.94%, and the UCS value increases by as much as 141.29 kN/m 2 .Thus, BLA gave an increase in soil strength for the bentonite-laterite mixture.
From the extensive literature review, it can be seen that the highest amount of BLA research was conducted in 2021.At the time, many types of soils and additives alongside BLA was analyzed to understand its behavior as SCM.

Bamboo Leaf Ash (BLA) initial study on lateritic soil
The availability of lateritic soil is especially abundant on developing countries, especially its usage in road construction and brick manufacturing [2].On the other hand, studies had shown that laterite soils generally have poor compaction and tend to absorb excess water [7].Due to these properties, lateritic soils need stabilization to improve its usage.The BLA was used for lateritic soils as it can substitute cement due to its pozzolanic traits.The early studies of BLA were mainly done in Nigeria where the average soil condition was found to have lateritic soil properties.The details can be seen in Table 2.The literature review of BLA showed that soils stabilized with BLA would change the soil properties such as moisture content, specific gravity, plastic limits, liquid limits, and grain size distribution.It can also increase the value of bearing capacity of a soil and its shear strength.The soil mixture can also reduce the optimum water content, increase the maximum dry volume, increase its CBR values both at unsoaked and soaked conditions, and lower its plasticity index.

Conclusion
Based on the results of literature studies that have been conducted by researchers, the following conclusions can be drawn.
• The addition of BLA can significantly improve the quality of soil samples by reducing the value of the plasticity index on soil stabilization.• The addition of BLA can significantly improve the quality of soil samples by increasing the value of CBR on soil stabilization.• The addition of BLA can significantly improve the quality of soil samples by increasing the value of shear strength against soil stabilization

Table 2 .
Variable of BLA