Study of using Coal Fly Ash (CFA) and Rice Husk Ash (RHA) on the Compressive Strength of Geopolymer Concrete

. Concrete is a material that is widely used in the construction world. The production of Portland cement in concrete leads to CO 2 emissions that have an impact on global warming. Geopolymer Concrete is an eco-friendly material because it does not use Portland cement. Geopolymer cement is made from waste materials such as fly ash (FA) by alkali activation. In geopolymer, sodium silicate is a commonly used activator that is produced commercially. In this study, rice husk ash (RHA) from agricultural waste was used as an activator for geopolymer cement. The objective of this study is to review the chemical component of FCA and RHA and, to examine the effect of RHA in geopolymer concrete mixed design on the compressive strength. The geopolymer concrete binder is using 12 M NaOH and Na 2 SiO 3 , with variations for RHA 0%, 5%, 7.5%, and 10%. The specimens were treated for 28 days, curing in a 70 o C oven for 24 hours then curing at ambient temperature. The result shows that geopolymer concrete has a higher compressive strength compared to ordinary Portland cement (OPC) as much as a 5.9 MPa difference with geopolymer concrete of RHA10% of 25 MPa.


Introduction
The increasing level of cement production has an effect on the environment with the potential for pollution that will disturb the people around the industrial area because it requires relatively large natural resources.Starting with the extraction process which dredges the limestone mountain, the production or combustion process with a temperature of ± 1500 o C, and requires a lot of energy in the production process.In essence, all processes in the use of concrete have many negative impacts.High energy use in the cement industry produces 5-7% of CO2 emissions in the world and is predicted to continue to increase and can affect world climate change [1].Therefore, it is necessary to take anticipatory steps by reducing fuel consumption in the cement production process, one of which is by using geopolymer concrete.Geopolymer concrete is different from conventional concrete, the difference is in the elimination of the use of cement and the use of alkaline solutions for the polymerization process [2].Various studies have been conducted to find alternative uses for cement as a concrete-forming material.Materials that have chemical properties similar to cement are coal fly ash or what is often called fly ash (FA).Fly ash is the residue from burning coal which has a very fine texture [3].
Corresponding author: yureana.wijayanti@binus.ac.idTo obtain geopolymer concrete with good quality, it is necessary to have the type of activator in accordance with the compounds contained in the fly ash.The activator used in this concrete is rice husk ash (RHA).Rice husk ash is used because it has high sodium silicate.The right composition is needed for a perfect chemical reaction to occur [4].In this research, the use of fly ash and rice husk ash waste is used as a substitute material for concrete constituents in the form of fly ash as a substitute for cement and rice husk ash as an alkali activator.This way, the waste can become alternative materials for making sustainable concrete.The objective of this study is to review the chemical component of FA and RHA and to present a case study of FA and RHA geopolymer concrete compressive strength performance.

Research Review
Geopolymer was first proposed by Joseph Davidovits [5].Geopolymer is an inorganic material synthesized through a polymerization process.The form of geopolymer is made from an inorganic alumina-silica material taken from materials containing Silica (Si) and Alumina (Al) which can come from nature or industrial byproduct materials such as fly ash and rice husk ash which can be reacted with an alkaline solution.Geopolymer concrete is a type of concrete that does not use Portland cement as a concrete binder.The difference between geopolymer concrete and other conventional concrete is that polymer concrete is not formed from a hydration process but is formed from a chemical reaction.Geopolymer concrete is suitable for use in earthquake-prone areas because it has high compressive strength and durability [6].Geopolymer concrete was created to protect the environment from future hazards such as the dangers of gas emissions produced by burning cement because the material that is composed of geopolymer concrete is composed of the synthesis of non-organic natural materials through a polymerization process, which replaces the ordinary portland cement (OPC) which functions as a binder with environmentally friendly materials [7].In this study, the binding material for geopolymer concrete is a 2-component inorganic system consisting of solid components containing Silica (Si) and Alumina (Al), namely fly ash, rice husk ash, and alkali activator solution components such as sodium silicate and sodium hydroxide.The compounds and crystalline structure of the FA and RHA determine by XRD (X-Ray Diffractometer) and the chemical composition by XRF (X-Ray Fluorescence).The chemical composition of fly ash from Coal Fire Power Plant Suralaya from other studies is presented in Table 1.The fly ash obtained from the coal-fired power plant Suralaya was classified as class F fly ash according to the ASTM C618-15 standard specification for coal fly ash and raw or calcined natural pozzolan for use in concrete [11], with SO3 content < 5% and SiO2+Al2O3+Fe2O2 content > 70% (Table 2).

Material
The Rice husk fly ash geopolymer concrete use fly ash and rice husk ash as the source material, coarse and fine aggregate as the filler, sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) solution as a binder, and water as workability measure.In this study, fly ash and rice husk were obtained from the Suralaya power plant, Banten, and paddy field in Yogyakarta, Central Java, Indonesia.a b Figure 1a shows the pre-burnt condition of fly ash at 200 o C for 2 hours.Then, the rice husk ash was produced in a

Geopolymer mix design and production
The mix design used in this study is referred to Verma, 2018 [12]18, "Behaviour of fly ash and rice husk ash based geopolymer concrete".The ratio of binder (CFA & RHA): fine aggregates: coarse aggregates is 1 : 1.45 : 3.4.In this study, Three variations of RHA percentage to substitute FHA are 5%, 7.5% and 10% were applied.Table 1 shows the mix design of geopolymer (GPC) in this study.There are 4 types of specimens consist of: 1) cement concrete, 2) GPC-1 (95% CFA and 5% RHA), 3) GPC-2 (92.5% CFA and 7.5% RHA), and 4) GPC-3 (90% CFA and 10% RHA).Table 4 shows the mix design of the 4 specimen types. of aggregates were tested to get the mix design of coarse and fine aggregates to meet the Indonesian standard of concrete SNI 7656-2012.

Material preparation of alkaline solution
The preparation of an alkali activator solution consists of the following process: a.To make 1 kg of 12 M NaOH solution, prepare 358.7 grams of NaOH flakes and 641.4 grams of water; b.Mix NaOH flakes and water for a few minutes until evenly mixed; c.Let the mixture sit for 1 hour so the mixture isn't hot; d.After the NaOH solution has reached room temperature, mix it with sodium silica with a ratio of 0.45:2.5;e. Leave the solution for one hour then the lye is ready to use.
According to [18], the following is the process for mixing geopolymer concrete: a. Prepare casting tools and concrete constituents; b.Turn on the mixer to rotate consistently; c.All materials are weighed beforehand; d.Mix all the dry ingredients into the mixer for 3-4 minutes; e.Once mixed, add the lye to the mixture; f.Then mix it for 4-5 minutes and add water to make it easier to work with.
The production process is shown in Figure 2. Firstly, the high silica and alumina sources such as FA and RHA are combined with fine and coarse aggregates to form the dry mix.Then, the alkali solution and water are added.This mix allows a cure that will undergo polymerization and form a geopolymer concrete.

Moulding and curing
The specimens used were cubic 10 cm x 10 cm x 10 cm for compressive strength test.The geopolymer concrete is left in the mold for 24 hours, and all specimens will be cured at 70 o C in the oven for 1 day, as shown in Figure 3.After that, all specimens will be cured at room temperature until the age of testing is 28 days.

Testing Procedure
The testing procedure is following the Indonesia standard SNI 1974-2011.Place the concrete to be tested in the centre of the testing machine.Operate the testing machine with a constant load increase between 2 kg/cm 2 to 4 kg/cm 2 per second.This load test continues until the concrete is crushed.The maximum load during the test was recorded for data analysis.

Material testing
The purpose of material testing is to find out the specification of the aggregate used.Material test results must be suitable for the material specification requirements that based on ASTM and SNI.Table 5 shows the comparison between the material test result and standards.

Compressive strength
The result shows that all specimens meet the standard of minimum compressive strength for structural concrete of 17 MPa, based on Indonesian standard SNI 6880:2016 for Structural Concrete Specification.Geopolymer concrete has better compressive strength than ordinary Portland cement concrete (Figure 4).This can be attributed to the enhancement of the microstructure, which results in a more robust connection between the polymeric gel and aggregates, leading to a denser and more compact structure [24].The difference in compressive strength of ordinary cement concrete with GPC-1 has a difference of 0.4 MPa, with GPC-2 has a difference of 5.9 MPa, and with GPC-1 has a difference of 4.1 MPa.
Figure 5 shows the comparison of compressive strength in this study with other studies [18,25,26].Compared to the results of research by [25] which substituted fly ash with rice husk ash of 10%, 15%, and 20% using a concentration of 16M NaOH solution produced compressive strengths of 25.42 MPa, 22.41 MPa, and 19.91 MPa.The difference between 10% and 15% substitution is 3.01 MPa and the difference between 15% and 20% is 2.5 MPa.It can be seen that there is a decrease in strength with the increasing amount of rice husk ash used.The results of the compressive strength of concrete from the results of research and this journal with the amount of fly ash substitution with 10% rice husk ash have values that are close to 23.4 MPa and 25.42 MPa with a difference of 2.02 MPa.From the results of this study, an experimental study was carried out to determine the compressive strength of geopolymer concrete in which fly ash was substituted with rice husk ash.From the observations made by comparing with other studies, it can be concluded that the substitution of 7.5% to 10% fly ash with rice husk ash is suitable to produce geopolymer concrete.This study only conducted research by substituting 10%, from other studies which conducted research by substituting 15% fly ash and 20% rice husk ash experienced a decrease in compressive strength.
The decrease in the compressive strength of geopolymer concrete at 15% and 20% can be attributed to the increase in unreactive silica content in rice husk ash, which causes the SiO2/AI2O3 ratio to be incorrect, creating a repulsive effect on the strength value.The correct and optimum percentage in the use of rice husk ash substitution is 10% from the results of tests that have been carried out in this study and other studies.
The different results in each previous study and this study are due to differences in the specific gravity of the concrete used depending on the aggregate test, differences in the use of the ratio '(fly ash + rice husk ash): fine aggregate: coarse aggregate', and differences in the molarity of the alkaline solution used in the concrete mix because the molarity of NaOH used is directly proportional to the compressive strength of the concrete obtained.From the compressive strength values generated in this study, it can be concluded that substituting fly ash with rice husk ash can increase the compressive strength of existing concrete and concrete can be used for structural parts because it has a compressive strength value that is accordance with the standard.

Conclusion
An experimental study was performed to test the material of fine and coarse aggregate and measure the compressive strength of the FA and RHA geopolymer concrete.It showed that a 10% replacement of fly ash with rice husk ash gives the best result of compressive strength.The result of using RHA at a lower burning temperature, e.g. at 500 o C should be studied whether it might show a similar outcome as this study.It could become the priority preference as it required less energy in the production of RHA.

Fig. 5 .
Fig. 5. Compressive strength results in 28 days compare to other studies [18] substitute fly ash with rice husk ash of 10%, 15%, and 20% using a concentration of 12M NaOH solution to produce concrete compressive strength of 28.1 MPa, 25 MPa, and 15.8 MPa.The difference between 10% and 15% substitution is 3.1 MPa and the difference between 15% and 20% is 9.2%.There is a decrease in strength as the amount of rice husk ash used increases.The results of the compressive strength of concrete from the results of research and this journal with the amount of 5% fly ash substitution have values that are close to 19.7 MPa and 23.4 MPa with a difference of 3.7 MPa and the compressive strength results of concrete at 10% fly ash substitution also have a value that is close to 28.1 MPa and 23.4 MPa with a difference of 4.7 MPa.Compared to the study by 26 [25], which substituted fly ash with rice husk ash of 5%, 7%, and 10% produced concrete compressive strengths of 25 MPa, 27 MPa, and 26.3 MPa.The difference between 5% and 7% substitution is 1.3 MPa and the difference between 7% and 10% substitution is 1.3 MPa.There is an increase in strength as the amount of rice husk ash used increases, but if the amount of husk ash added is too much, the compressive strength can decrease.The results of concrete compressive strength from the results of research and this journal with the amount of fly ash substitution with 5% rice husk ash have a close value difference of 19.7 MPa and 25 MPa with a difference of 5.3 MPa, at 7% it has a value of 25.2 MPa and 27 MPa with a difference of 1.8 MPa, and at 10% has a value of 23.4 MPa and 26.3 MPa with a difference of 2.9 MPa.From the results of this study, an experimental study was carried out to determine the compressive strength of geopolymer concrete in which fly ash was substituted with rice husk ash.From the observations made by comparing with other studies, it can be concluded that the substitution of 7.5% to 10% fly ash with rice husk ash is suitable to produce geopolymer concrete.This study only conducted research by substituting 10%, from other studies which conducted research by substituting 15% fly ash and 20% rice husk ash experienced a decrease in compressive strength.

Table 4 .
Mix design of the study

Table 5 .
Physical properties of fine and coarse aggregate