Effect NaOH Concentration on Bagasse Ash Based Geopolymerization

Geopolymer is a natural adhesive material which can be developed as a substitute for cement. The natural ingredients which want to use should contain silica and alumina. This paper uses bagasse ash as a basic material of mortar geopolymer. As an adhesive, the bagasse ash should be mixed with water and another activator alkali such as sodium hydroxide (NaOH) and sodium silicate (Na2SiO3). The NaOH’s molarity variation are 8, 10, 12, 14 and 16 M with Na2SiO3/NaOH = 1,0 sand/bagasse ash = 2,75 and activator/bagasse ash = 0,42. This research use 50 x 50 x 50 mm cube sized specimen and conduct a compressive strength test with 3, 7, 14, 21 and 28 days. The fresh mortar test result showed that the use of NaOH’s molarity variation influences the slump value and time setting. The bigger NaOH molarity variation that been used, the smaller slump value. But, the time setting is increased. While the result for density and compressive strength shown that the bigger NaOH molarity variation, the bigger density and the compressive strength. Maximum compressive strength resulted from the mixture of mortar geopolymer with 16 M concentration.


Introduction
Mortar geopolymer is an environmentally friendly material, which can be developed as a substitute for future construction material. While it can be used from industrial waste, the making process of geopolymer cement did not use as much energy as Portland cement. Portland cement needs 1.000 o C temperature for clinker burning process. Those burning processes produce CO 2 which pollutes the air. The geopolymer making process could reduce the greenhouse gasses emission up to only 20% left.
The main ingredient for geopolymer is a natural ingredient which contains silica (Si) and Aluminum (Al). The silica and aluminium are important for polymerization bond. These basic ingredients then reacted with alkali activator solvents such as NaOH and Na 2 SiO 3 , with NaOH concentration about 8-16 M [1]. The rice husk ash has a huge potential to become the main ingredient of mortar geopolymer. The research has shown that after 28 days, the maximum compressive strength was 45 MPa with NaOH concentration for about 10 M [2].
The basic ingredient used in this geopolymer research is bagasse ash waste, which is the fuel for sugar cane mill. Sugar cane is the raw material to produce sugar. This plant only grows in a tropical territory such as Indonesia. The age of this plant, from the plantation until the harvest, is around 1 year. In Indonesia, sugar cane usually grows in Java and Sumatera island. After harvest, the sugar cane processed into 5% sugar, 5% water and 90% the bagasse (sugar cane waste). The bagasse then used in a burning process within a steam boiler. The steam boiler is the main tools to stir the sugar cane mill.
Previous research on the use of bagasse ashes as a mixture of concrete material shown a satisfying result. [3] shown that the conventional concrete with a bagasse ash mixture could bear a 30-40 MPa compressive strength. [4][5] shown that the compressive, tensile and flexural strength a concrete with bagasse ash mixture, are 10% higher than an original concrete. Besides improving mechanical characteristic, bagasse ash also could improve workability. So, it does not need to use additional superplasticizer material.

Material
The basic material for mortar geopolymer making should contain high silica and alumina. In this research, the basic material is bagasse ash waste, which is the main fuel for sugarcane mill. The bagasse ash comes from PT Perkebunan Nusantara X (Persero) Surabaya, East Java. Figure 1 shows the result of bagasse ash's scanning electron microscope (SEM) test. Based on this test, the bagasse ash shape has a spherical form with ± 20 μm maximum diameter. Figure 2 is the result of XRD analysis which shown that this material contains high silica and alumina. Activator that been used in this research is Na 2 SiO 3 and NaOH. Na 2 SiO 3 contain 58% gel-formed sodium silicate. Meanwhile, the powder-formed NaOH contain 98% purity level. NaOH mixed with water until it formed 8 M, 10 M, 12 M, 14 M, and 16 M concentration. This liquid then mixed with Na 2 SiO 3. The comparison value between NaOH and Na 2 SiO 3 is 1. Before it used to create mortar, this activator liquid bleached for about 1 day until it reaches a normal room temperature. The fine aggregate (sand) that used in this research comes from Tanjung Raja's rivers and OKI, South Sumatera. The ratio between sand/bagasse ash = 2,75 and the ratio between activator/bagasse ash = 0,42.

Laboratory test
This research conducted in Laboratory of Concrete and Material, Civil Engineering Faculty of Engineering, Universitas Sriwijaya. The standard for mortar mixed material refers to American Standard Testing and Material (ASTM C109). The experiment that has been conducted were workability, setting time, density and compressive strength test. The NaOH's molarity variation are 8, 10, 12, 14 and 16 M with Na 2 SiO 3 /NaOH = 1,0. sand/bagasse ash = 2,75 and activator/bagasse ash = 0,42. The workability test conducted by test the flow table to know the diameter distribution. While the compressive strength test conducted using a 50 x 50 x 50 mm cube-sized material for 3, 7, 14, 21 and 28 days period.
NaOH liquid become thicker. Therefore, the use of small NaOH concentration makes the mortar mixed become flow and produced a broader diameter.
The decreasing percentage of slump flow mortar geopolymer value shown in Table 1 which shows that the slump flow percentage decreasing congruent with the increasing of NaOH concentration. The decreasing percentage are 10.93%, 33.72%, 40.82%, and 46.81%. This shown that the increasing concentration of NaOH can decrease mortar geopolymer workability.

Setting time
The result of setting time test can be shown in Figure 4.

Density
The result for density test can be shown in Figure 5. The maximum density which resulted from the mixture of 16 M NaOH is 1,79 gram/cm. The minimum density resulted from the mixture of 8M NaOH is 1,69 gram/cm. Based on this result, the more vary the NaOH, the higher its density. In a long period, the density will decrease until it reached the constant number. This happens because the water that contained in the mortar becomes evaporated. The increasing density percentage of mortar geopolymer can be shown in Table 2. Based on the data in Table 2, the density increaseds along with the increasing of NaOH concentration. The increasing density percentage on 10 M, 12 M, 14 M and 16 M in a row is 1.74%, 2.87%, 4.51% and 5.58%

Compressive strength vs density
Graphic for analysis of mortar geopolymer's compressive strength and density can be shown in Figure 7.

Compressive strength vs NaOH concentration
Analysis of compressive strength regression and NaOH concentration can be shown in Figure 8. The analysis found that coefficient determination value (R 2 ) is 0.9962 with Equation 2: (2) where:

NaOH concentration vs density
Analysis of NaOH concentration regression and density is shown in Figure 9. On the 28 th day, the test found that the coefficient determination value (R 2 ) is 0.9962 with Equation 3: where the: γ = density (gram/cm 3

Slump flow vs NaOH concentration
Analysis for slump flow regression and NaOH concentration is seen in Figure 10.

Conclusion
Conclusions of this research are: • The test result for slump flow mortar geopolymer shows that NaOH concentration influences the workability of the mixture. This happens due to the bigger NaOH