The Effect of Bottom Ash as Partial Replacement of Fine Aggregate in Different NaOH Molarities on the Compressive Strength of Geopolymer Mortar

. This study compared the compressive strength of the geopolymer mortar with different percentages of bottom ash as a replacement for fine aggregate. Fly ash from two sources was blended as raw materials. Sodium silicate (Na 2 SiO 3 ) and sodium hydroxide (NaOH) were used as alkaline solutions, with a ratio of 2.5 and two NaOH concentrations of 8M and 10M. The percentage of bottom ash was substituted up to 30%. The result indicated that the setting time was longer due to the higher molarity of NaOH. The compressive strength of mortar geopolymer at 10M of NaOH was higher than 8M. Higher percentages of bottom ash significantly decreased the compressive strength. However, at 10M NaOH, the compressive strength was decreased by only 11% (20% of substituting). Increasing the molarity of NaOH decreased the absorption of mortar geopolymer and increased the restrain to sodium sulfate.


Introduction
Concrete is a common building material, and its extensive use has increased global cement consumption.Cement is crucial in producing concrete, the second most consumed substance on Earth, following water [1].The infrastructure enhancements led to a 4.75% rise in electricity demand in 2021 compared to 2020 [2].Coal is the primary energy source in Steam Power Plants.Coal combustion generates roughly 5-10% of solid pollutants in ash, of which 10-20% is bottom ash and 80-90% is fly ash [3,4].Consequently, the amount of fly ash and bottom ash waste increases yearly.
An effective strategy to address environmental pollution resulting from cement production and use waste generated from coal combustion in Steam Power Plants is to substitute a portion or even the entirety of the cement content in concrete with fly ash.Geopolymers are a solution to reduce the use of cement and utilize 100% fly ash waste.Many studies use fly ash as a raw material in geopolymer [5][6][7].Bottom ash, however, has attracted less research attention in geopolymers [8].Bottom ash has a chemical composition comparable to fly ash but with various particle sizes and shapes.Chotetanorm et al. [9] used ground bottom ash as raw material on geopolymer.Fine bottom had a higher compressive strength than medium and coarse bottom ash due to increased reactivity and a larger surface area.
Meanwhile, bottom ash rarely partially replaces fine aggregate in geopolymer.Notably, the bottom ash has not found a suitable application in civil construction and is usually discarded in sedimentation basins and must be supplemented to enhance the mechanical and durability qualities [10].This may be due to the characteristics of bottom ash, specifically its porous particles.The polymerization degree of bottom ash geopolymer and dissolving in NaOH solution is lower than that of fly ash [11].
This study prepared the mixture of fly ash as 100% cement replacement and bottom ash as a fine aggregate replacement for a geopolymer mortar mix.Raw materials were blended with fly ash from two sources.This mixture was then applied to the manufacture of geopolymer paving blocks.The setting time and compressive strength of geopolymer mortar and paving block were studied.

Material and Composition
In this study, the raw materials were obtained from waste generated by the Asam-Asam (AA) and PT.Tanjung Power Indonesia (TPI) steam power plants.The raw material composition consisted of 75% AA fly ash and 25% TPI fly ash.Raw material from both steam power plants underwent a 24-hour drying in an oven and was subsequently filtered through a No. 200 sieve.Material passing through the No. 200 sieve was categorized as fly ash.The XRF test was conducted on AA and TPI fly ash to obtain the chemical composition of fly ash, as listed in Table 1.According to the chemical composition provided in Table 1, the combined percentage of SiO2, Al2O3, and Fe2O3 in AA fly ash was 74.71%, and the CaO content was 12.86%.TPI fly ash featured a total SiO2, Al2O3, and Fe2O3 content of 68.13%, and the CaO content exceeded 10%.Following ASTM C 618-05 [12], AA fly ash was categorized as class F, and TPI fly ash was classified as class C.
Specifically, for the TPI raw material, the portion retained on sieve No. 200 underwent additional filtration using a No. 4 sieve with a 4.75 mm opening.Particles passing through the No. 4 sieve were identified as bottom ash.Consequently, the bottom ash passed through the No. 4 sieve and was retained on the No. 200 sieve.Bottom ash was employed as a substitute for fine aggregate, while the fine aggregate was collected from a nearby quarry.Bottom ash substituted fine aggregate up to 0%, 20%, and 30%.Table 2 lists the properties of fine aggregate, fly ash, and bottom ash.The alkaline solution employed in this study was a mixture of sodium silicate (Na2SiO3) and sodium hydroxide (NaOH).The liquid sodium silicate comprised 30.62%SiO2, 9.42% Na2O, and 59.96% H2O.It had a SiO2/Na2O ratio of 3.25, a specific gravity of 1.42, and a Baume temperature of 43°Bé at 20°C.Since preparing the NaOH solution produces heat, it was done one day before mixing with Na2SiO3.To create one liter of 8M and one liter of 10M NaOH solution, 320 grams and 400 grams of NaOH flakes were combined with tap water.The Na2SiO3/NaOH ratio was set at 2.5.The ratio of fly ash to alkaline solution was 1.86.The mix composition is displayed in Table 3.

Mixing Process
Before mixing with other components, Na2SiO3 and NaOH solutions were blended.The alkali solution was put in the mixer bowl after adding the fly ash.After thoroughly mixing, the fine aggregate was added, and the mixture was stirred once more.The geopolymer mortar was molded into a 555 cm 3 iron mold in the first stage.Geopolymer mortar was molded into a 61020 cm 3 paving block mold in the second stage.The mold was opened once the geopolymer mortar had hardened, and the specimens were cured by moistening.

Setting Time
The Vicat penetration test based on the ASTM C191-82 [13] determined the setting time of geopolymer paste.The setting time of the geopolymer mixture was evaluated using corresponding paste mixes.The composition of these pastes resembled that of the mortar mixtures outlined in Table 3, except for the omission of fine aggregate.When a 25 mm penetration was achieved, the initial setting time was determined, and the final setting time was attained when the needle could no longer penetrate the paste.

Compressive Strength
Compressive strength testing was conducted under ASTM C191 [14] in two stages, starting with 5 cm-sided cubes.Compressive strength tests were evaluated at 7 and 28 days using 8M and 10M NaOH.Paving block test specimens of 61020 cm 3 were manufactured after determining the molarity of NaOH that produced the greatest compressive strength in the next stage.This size was chosen to represent the dimensions of regularly available paving blocks on the market.The paving blocks were subjected to compressive strength tests at 28 days.A paving block compressive strength test in full-sized of 61020 cm 3 was carried out in compliance with British Standards [14], and 666 cm 3 cubes were cut from 61020 cm 3 paving blocks in compliance with Indonesian Standards [15].

Absorption Test
The water absorption test was carried out after the specimens had been curing for 28 days.
Absorption evaluation was conducted based on Indonesian Standard [15].The water absorption value is a percentage (%) measurement of the extent water may infiltrate or permeate porous concrete.The water absorption test process for paving blocks was done as follows.The specimens were soaked in water for 24 hours before being weighed in their wet state.The test pieces were then dried in an oven for approximately 24 hours at 105°C until their weight stabilized with two successive weighs that deviated by 0.2% from the previous weigh.This test was initially carried out on 555 cm³ cubes during the first stage.

Sodium Sulfate Test
The resistance to sodium sulfate test was conducted after the specimens were cured for 28 days.The sodium sulfate resistance test was performed as follows.Two clean specimens were dried in an oven at (1052)°C until they reached a consistent weight, then cooled and weighed.The specimens were immersed for 8 to 16 hours in a sodium sulfate solution and drained.Next, the specimens were put in the oven at 105 2°C for 2 hours before cooling at room temperature.This cycle could be performed up to five times in a row.The test specimens were washed after the final drying procedure.Then, the test specimens were dried in the oven for 2-4 hours until a steady weight was attained and weighed again.The weight difference was calculated before and after soaking in a sodium sulfate solution.
Visual examinations of the test specimens were also carried out after submerging them in sodium sulfate.During the first stage, this test was performed using 555 cm³ cubes.It involved full-sized specimens measuring 61020 cm³ and 666 cm³ cubes cut from the 61020 cm³ specimens in the second stage.

Setting Time
According to ASTM C150 [16], the initial and final setting times of cement paste are 45 and 375, respectively.Figure 1 depicts the initial and final setting times of geopolymer paste.The initial setting time with 8M NaOH was less than 45 minutes, faster than the 10M NaOH.The initial setting time of 10M NaOH was 90 min.The final setting time for both morality of NaOH was no more than 375 min.Higher molarity of NaOH longer initial and final setting time.This is consistent with the results obtained in the investigation by Mariamah et al. [17].

Compressive Strength of Mortar
Figure 2 shows the effect of NaOH concentration and the proportion of fine aggregate replaced by bottom ash on compressive strength after 7 and 28 days.The compressive strength of the specimens increased with age.The compressive strength is the average of three specimens.The compressive strength of the specimens without bottom ash at 7 days was 21.73 and 23.25 MPa at 8M and 10M, respectively.The compressive strength increased to 26.11 MPa and 37.30 MPa after 28 days.This corresponded to a 20% and 60% increase in compressive strength.Similarly, when fine aggregate was substituted with bottom ash, compressive strength increased by 60% and 131% at 20% bottom ash and 14% and 38% at 30% bottom ash.It is worth noticing that the higher the NaOH molarity, the more significant the improvement in compressive strength at 28 days compared to 7 days.
Replacing fine aggregate with bottom ash led to reduced compressive strength at 7 and 28 days.When applying 8M NaOH for 28 days, the compressive strength with 20% bottom ash was reduced to 8.68 MPa and 3.04 MPa with 30% bottom ash, resulting in a 67% and 88% loss in compressive strength, respectively.The decline in compressive strength was more pronounced with higher percentages of fine aggregate replacement by bottom ash.This outcome aligns with the findings of Hardjito and Fung [18].However, the reduction in compressive strength was less noticeable when using higher NaOH molarities (10M) compared to 8M.With 20% bottom ash, the compressive strength was 33.15 MPa, and with 30% bottom ash, it was 18.41 MPa.Compared to compressive strength without bottom ash, there was an 11% and 51% loss in compressive strength, respectively.Consequently, the compressive strength of geopolymer mortar with the replacement of fine aggregate with bottom ash 30% maintained relatively high compressive strength, particularly with higher NaOH molarity.The increased leaching of silica and alumina from fly ash and mortar with higher NaOH concentrations exhibited elevated compressive strengths due to heightened geopolymerization and the formation of NASH gel [19].Data in Figure 3 represents the subsequent phase involving the fabrication of 61020 cm 3 paving block specimens using a 10M NaOH molarity.The compression strength test was carried out at 28 days.When the compressive strength of the 61020 cm 3 paving blocks was evaluated by cutting them into 666 cm 3 cubes, the findings revealed a decreased compressive strength, ranging from 7% to 22% lower than that of the uncut paving blocks.Compared to the compressive strength of 555 cm 3 cubes of mortar, the compressive strength of uncut and cut paving blocks at 0% and 30% bottom ash was similar.However, at 20% bottom ash, the compressive strength of uncut and cut paving blocks decreased by 26% and 31%, respectively.

Absorption
Figure 4 depicts the water absorption characteristics of geopolymer mortar.The absorption percentage represents the average of three individual specimens.It is clear that as the percentage of fine aggregate substituted with bottom ash increased, the percentage of water absorption increased, regardless of NaOH molarity.Porous bottom ash causes high water absorption [11].In contrast, increasing the NaOH molarity from 8M to 10M reduced the absorption percentage.Figure 5 shows how cutting the paving blocks affects the absorption percentage.In the uncut and cut paving block tests, the greatest water absorption in 30% bottom ash was 0.15% and 0.16%, respectively.While paving blocks without bottom ash had the lowest water absorption of 0.04% and 0.05%, respectively.These results comply with Indonesian Standards for paving blocks [15], indicating that water absorption should not exceed 0.2%.When 30% bottom ash was used, cutting the paving blocks into 555 cm 3 cubes did not affect the absorption percentage.Significant changes occurred in the specimens with 20% bottom ash, where the cut paving blocks displayed a higher absorption percentage than those tested for absorption in the uncut condition.

Sodium Sulfate
After the specimens were 28 days old, the test object was removed from the curing place, and the sodium sulfate resistance test was carried out.If the weight difference before and after immersing is less than 1% and no cracks or other faults are visible, the test specimens are regarded as excellent.Figure 6 shows the results of tests evaluating the resistance of geopolymer pavement to sodium sulfate on geopolymer mortar.The higher bottom ash replacement to fine aggregate increased the weight loss; the weight loss decreased as the molarity of NaOH increased.
The phenomenon occurred with both types of uncut and cut paving blocks as seen in Figure 7.The lowest weight loss was 0.64% in uncut paving blocks in a 0% bottom ash mixture.The weight difference grew to 4.02% as the bottom ash content increased to 20%.Additionally, at 30% bottom ash, the weight difference reached 7.11%.Bottom ash cut paving had the smallest weight loss (0.47%) at 0%. Weight loss was 3.23% when 20% bottom ash was added.
Similarly, adding 30% bottom ash reduced the weight to 5.98%.The weight loss of cut paving was smaller than that of uncut paving.Visual observations were undertaken to assess the degradation of the specimens caused by sodium sulfate.Only the paving blocks with 0% bottom ash were free of defects and in acceptable condition.This was further substantiated by the weight decrease of less than 1%.Paving blocks containing 20% and 30% bottom ash, on the other hand, exhibited cracking and brittleness.Weight loss was successful in satisfying the Indonesian Standard for paving blocks.Based on the analysis of compressive strength, absorption percentages, and sodium sulfate resistance, it is concluded that geopolymer paving blocks with no substitution of fine aggregate with bottom ash fall into class A. Class A paving blocks are suitable for roads.Paving blocks with 20% and 30% bottom ash fall under class D, rendering them appropriate for gardens and other purposes but not recommended for extreme situations such as acid rain exposure.

Conclusion
1.The higher the molarity of NaOH, the higher the compressive strength of the geopolymer mortar.2. The amount of fine aggregate substituted by bottom ash influences the compressive strength of the geopolymer mortar and paving block.The higher the percentage of bottom ash, the lower the compressive strength of mortar and paving block geopolymer.

Fig. 6 .
Fig. 6.Effect of bottom ash replacement on loss of weight.

Table 1 .
Chemical composition of fly ash.

Table 2 .
The properties of materials.