Bottom ash is a by-product generated during the coal combustion of thermal power plant. Bottom ash-based geopolymer has been reported as a promising substitute of cement in concrete.
In this study, bottom ash collected from Honam Coal Power Plant was ground to be used as a soil stabilizer. Sodium hydroxide solution (NaOH) with different molarity concentration and sodium silicate solution (Na2SiO3) were added to enhance the polymerization reaction of ground bottom ash. The effects of water/ground bottom ash, Na2SiO3/NaOH ratio and concentration of NaOH solution on the unconfined compressive strength of sand mixture at the ambient curing conditions were investigated.
The results indicated that ground bottom ash can be utilized to stabilize sand as a main binder at the ambient curing conditions. In particular, with the 4 M concentration of sodium hydroxide solution, the Na2SiO3/NaOH ratio of 1.5 and the water/binder ratio of 0.35, the highest compressive strength obtained at 28 days was around 9 MPa.
The compressive strength of sand mixture increased with an increase of NaOH solution concentration and the ratio of Na2SiO3/NaOH, however, it dramatically decreased with the addition of water to the mixture.
Nowadays, cement plays an important role in the construction industry. According to statistics, the production of cement in the world increased from 3310 million metric tons in 2010 to 4100 million metric tons in 2015 , http://www.statista.com. However, the production of cement creates some environmental issues especially such as CO2 emissions causing the greenhouse effects. It is estimated that corresponds to one ton of cement produced, one ton of CO2 will be emitted into the atmosphere . The cement production contributed about 7 % of the total greenhouse gas emissions to the earth’s atmosphere . In the efforts to solve these issues many studies to find out the alternative material to substitute cement have been published [3, 11, 14, 16, 18]. One of the new alternative materials which can be used to replace cement is geopolymer materials.
The term “geopolymer” was first introduced by Joseph Davidovits in 1978 . Geopolymers are formed by reacting silica-rich and alumina-rich solids with an alkaline solution, resulting in a mixture of gels and crystalline compounds that eventually harden into a new strong matrix . Depending on the raw material characteristics and processing conditions such as curing conditions, alkaline concentration and types of alkali cations, geopolymers can exhibit a wide variety of properties and characteristics, including properties of cementitious materials and adhesives, high compressive strength, fire resistance, and low thermal conductivity . Using geopolymer materials have great significance for the environment. It is not only reduce greenhouse gas emissions but also consume large volumes of industrial wastes . In general, the two main ingredients involve in the formation of geopolymer are the aluminosilicate binder and alkali activating agent. The binders are composed of high aluminosilicates contain such as slag, fly ash, rice-husk ash while the alkaline activating agents are compounds of alkali metals (NaOH, KOH).
Fly ash and bottom ash are waste materials generated from coal-fired thermal power plants. As mentioned above fly ash is being widely used to manufacture the geopolymer materials [4, 13, 20]. Meanwhile, except in some countries where bottom ash is used as a low-cost replacement for more expensive sand in the production of concrete blocks and a base in road construction , most of it is disposed in landfills and caused environmental pollution. However, based on the X-ray fluorescence test results (Table 1), it can be found that bottom ash and fly ash have similar chemical compositions. The XRD patterns of bottom ash (Fig. 1) also indicated that its crystalline phases are predominantly quartz and mullite which represent for silica and alumina—two main gradients to form geopolymer. Therefore, by grinding to reduce particle size, bottom ash can be used as a pozzolanic material that was repoted by Jaturapitakkul and Cheerarot .
Currently, some researchers have used ground bottom ash as a partial replacement of cement or combined it with alkali activating agent to completely replace cement. The most common alkaline activator used in geopolymerisation is a combination of sodium hydroxide (NaOH) or potassium hydroxide (KOH) and sodium silicate or potassium silicate . In general, the geopolymer mortar is prepared with a high concentration of NaOH solution (10 M) and specimens are cured at the high temperature (65 °C for 48 h) to achieve higher compressive strength of over 20 Mpa .
This study focuses on the assessment of the compressive strengths of sand mixtures which were stabilized by alkali-activated ground bottom ash prepared with low concentration of NaOH solution and cured at the ambient conditions. It would lay a foundation for the future utilization of ground bottom ash as a valuable resource of stabilizing soil for sustainable infrastructure constructions instead of treating bottom ash as a waste material.
Bottom ash was collected from Honam thermal power plant and dried to completely loss of moisture content. To grind bottom ash, a ball-grinding machine was used. The velocity of roller was kept at 120 round per minute and the diameter of 40 mm of ball was selected. After grinding, bottom ash fineness was checked complying standard ASTM C204 —the test methods for fineness of hydraulic cement by air-permeability apparatus. The test result showed that after 3 h of grinding the fineness of ground bottom ash was 2000 cm2/g. Even though it is still coarse in comparing with Portland cement (3600 cm2/g) and fly ash (3000 cm2/g) , a fineness of 2000 cm2/g is enough to get a significant strength which was reported by Sathonsaowaphak et al. . The effectiveness of grinding can be visualized in Fig. 2 and expressed in detailed via grain size distributions in Fig. 3. A scanning electron microscopy (SEM) test was conducted on raw and ground bottom ash to study surfaces and particles. At the same magnification, raw bottom ash exposed as large and very irregular particle shapes, containing high pores and cavities (Fig. 4a), meanwhile those of ground bottom ash were smaller and relatively uniform (Fig. 4b). Grinding process not only reduced the particle size and porosity but also destructed the grain structure of the material that facilitate the alkaline-activators to contact with aluminosilicate ingredients.
The alkaline activators used in this study were sodium hydroxide solution (NaOH) and sodium silicate solution (Na2SiO3). The sodium hydroxide is available in the form of pellets with 98 % purity. The sodium hydroxide solution was prepared by dissolving the pellets in distilled water. The mass of sodium hydroxide solid in a solution depends on the concentration of solution and it is expressed in terms of molarities. Sodium silicate solution was purchased directly from the company with a SiO2/Na2O molar ratio of 3.1 and the percentages by weight are 10 % of Na2O, 30 % of SiO2 and 60 % of water. Well-graded sand was used as a fine aggregate. Particle size distribution of sand was shown in Fig. 3 and other properties were given in Table 2.
At first, in order to evaluate the effect of NaOH solution concentration on compressive strength of stabilized sand, various molar concentrations of 2, 4, 6 and 8 M were employed. While the liquid alkali activator/ground bottom ash ratio and the Na2SiO3/NaOH ratio of were fixed as 0.5 and 1.5, respectively.
Secondly, 4 M of concentration of NaOH solution was selected basing on the results of the first experiment. To assess the effect of water/ground bottom ash ratio on compressive strength of stabilized sand, the various amounts of water were added to the mixture. Other parameters as the liquid alkali activator/ground bottom ash ratio and the Na2SiO3/NaOH ratio were still kept as 0.5 and 1.5, respectively.
Finally, the Na2SiO3/NaOH ratio was varied from 0.5 to 2.5 to assess the effects of Na2SiO3/NaOH ratio on compressive strength and to find out the optimum ratio. Meanwhile, the liquid alkali activator/ground bottom ash ratio of 0.5 were kept.
Ground bottom ash was first mixed with NaOH solution and Na2SiO3 solution for 10 min to allow the leaching of ions. Sand was mixed with the ground bottom ash with a ratio of 3:1 and extra water was added if needed. Thereafter, fresh mixtures were cast in the cylindrical molds (ϕ = 50 mm, h = 100 mm). Based on the test method for laboratory compaction characteristics of soil , fresh mixture in the mold was divided into three layers and compacted with 25 blows of rammer for each layer. Finally, molds were immediately capped to prevent moisture loss. All specimens were demolded after 24 h and then cured at the ambient condition (50 % humidity and 15 °C). Compressive strengths of specimens were measured at the ages of 7, 14, 28 days.
Results and discussion
Effect of NaOH solution concentration
Figure 5 shows the effect of concentration of NaOH solution on the compressive strength of sand mixture stabilized with the alkali-activated ground bottom ash. It can be observed that the setting process has been delayed with the increase of concentrations of NaOH solution. At 7 days, sand mixture prepared with lowest concentration of NaOH solution (2 M) achieved 2.6 MPa of compressive strength meanwhile that of highest concentration case (8 M) just achieved 1.2 MPa. However, the trend was reversed shortly thereafter, sand mixture improved with higher concentration of NaOH showed a faster rate of gain of strength. The final results indicated that the compressive strength slightly increase with the increase of NaOH solution concentration. The highest compressive strength obtained at 28 days by using the highest concentration of sodium hydroxide was approximately 10 Mpa.
This phenomenon is similar with other previous studies of Chindaprasirt et al. , Sathonsaowaphak et al. , and Hanjitsuwan et al. . Chindaprasirt et al.  showed that the strength increased with the increase of NaOH concentration mainly because of the amount of leaching of silica and alumina ions and resulted in a high degree of geopolymerization. In another study performed by Chindaprasirt and Chalee  involving the chloride penetration of geopolymer concrete, the authors have pointed out that an increase NaOH solution concentration in geopolymer concrete resulted in a low porosity of paste and an increase in the compressive strength.
Based on this case study, alkaline-activated ground bottom ash prepared with low concentration of NaOH solution has demonstrated the ability to stabilize sand at the ambient temperature curing condition. In addition, it can also be observed from Fig. 5 that with 2 M of NaOH solution concentration, the compressive strength of stabilized sand is relatively high. However, a concentration of 4 M was selected for the further experiments to ensure the attainment of high strength when concentration of solution will be significantly reduced by adding water.
Effect of water/ground bottom ash ratio
In this case study, the ratio between the amount of water contained in alkali solution and ground bottom ash was 0.35. By adding various amounts of water to the mixture, the effect of water/ground bottom ash ratio was studied. It can be seen from Fig. 6 that the compressive strength rapidly decreases with the increase of water amounts. In the case of absence of added water the compressive strength is 8.7 MPa while the lowest compressive strength is 4 MPa corresponding to the water/ground bottom ash ratio of 0.65.
The main reason of the decrease of strength is excess water, which is diluted the liquid and thus slowed down the dissolution and reaction of the geopolymerization process. Another reason is that extra water reduces the efficiency of compaction, which cause high porosity in the specimens.
The variation of compressive strength corresponding to the ratio of water and ground bottom ash is one of the key parameters in design of sand stabilization. From the value of water content of sand material collected in the field, engineers can estimate the amount of binder needed to produce a desired strength by comparing with the results indicated in Fig. 6.
Effect of Na2SiO3/NaOH ratio
The effect of Na2SiO3/NaOH ratio on compressive strength of sand mixture is shown in Fig. 7. The experimental results showed that the compressive strength increased with the increase of Na2SiO3/NaOH ratio from 0.5 to 2. The highest compressive strength was 9.1 MPa corresponding with the Na2SiO3/NaOH ratio of 2. The development of compressive strength is the result of the increase of SiO2 content leached from Na2SiO3 solution. Similar explanations found in previous study of Xu and Van Deventer  indicated that the higher amount of sodium silicate improves the reaction rate and thereby develops long chain polymerization when compared to sodium hydroxide. However, when the Na2SiO3/NaOH ratio was 2.5, the strength started to decrease. This may be due to the greater amount of soluble silica in sodium silicate solution reduced the geopolymerisation reaction. In a similar phenomenon, Sathonsaowaphak et al.  explained that it is due to the difficulty in compaction.
At the ambient curing conditions, alkali-activated ground bottom ash can be effectively used as a binder and it achieves sufficient strength to apply for sustainable infrastructure constructions. The factors which can affect to the compressive strength of stabilized sand were investigated. Based on the obtained data, the following conclusions could be derived.
By grinding the bottom ash and activating it with optimized amount of NaOH and Na2SiO3, sand can be stabilized sufficiently in early age and compressive strength of sand mixture was increased up to around 10 MPa within 28 days even though cured in ambient condition.
The higher the concentration of NaOH, the higher the compressive strength of sand mixture. However, 2–4 M of NaOH is appropriate not only for the activating ground bottom ash for the geotechnical infrastructure but also for the economic benefit.
The increment of amount of water causes the rapid decrease of compressive strength of sand mixture. However, the compressive strength of 28 days was obtained as 4 MPa even though the amount of water in sand mixture is increased nearly double.
The compressive strength of sand mixture increases to a certain limit with the increase of Na2SiO3/NaOH ratio. The optimum of Na2SiO3/NaOH ratio is 2.
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YSK gave ideas and participated in amending the manuscript. MQD and TMD participated in performing experiments, analyzing data, and drafting the manuscript. All authors read and approved the final manuscript.
This research was supported by a Grant (No 16-RDRP-B076564-03) from Regional Development Research Program funded by Ministry of Land, Infrastructure and Transport of Korean government.
The authors declare that they have no competing interests.
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Kim, Y., Dang, M.Q. & Do, T.M. Studies on compressive strength of sand stabilized by alkali-activated ground bottom ash and cured at the ambient conditions. Geo-Engineering 7, 15 (2016). https://doi.org/10.1186/s40703-016-0029-4
- Compressive strength
- Ground bottom ash
- Alkaline activator
- Ambient condition