Geopolymer hybrid fly ash concrete for construction and conservation in peat environment: A review

Fly ash is industrial residue from the coal combustion in electric and steam generating plants and recognized as an environmental pollutant. Fly ash contains a high concentration of silica and alumina and can be used as geopolymer concrete. Studies the use of fly ash as a geopolymer concrete mixture has been developing in recent decades. This review focuses on the characteristics of fly ash as hybrid geopolymer concrete, and its resistance is peat environment. The hybrid geopolymer is made by activating fly ash using an alkaline activator and Portland cement. The addition of fly ash geopolymer concrete with Portland cement forms N-A-S-H bond structure from fly ash, and C-S-H hydration bond from Portland cement forms C-A-S-H, which is more solid and stronger against aggressive environment such as peat. In addition, the geopolymer concrete is proven to be able to mobilize heavy metals in fly ash so the concrete has low leaching rate and not harmful to the peat ecosystem. This review proves that hybrid geopolymer concrete using fly ash is a potentially innovative and promising material in the future. The geopolymer hybrid not only has its resistance to peat and acts as conservative material in an environmentally friendly peat environment.


Introduction
Indonesia's coal production is continuously increasing over time. In 2017, the realization of coal production reached 456 million tons and increased to 548 million tons in 2018; according to Dewi [1], fly ash waste resulting from coal combustion is classified as a toxic and hazardous substance to the environment and living creatures. Fly ash contains heavy metals such as Pb, Cu, Cd, Cr and Zn. These five elements are hazardous if dissolved in water and potential to disrupt the peat ecosystem. Therefore, fly ash waste needs to be transformed using stabilization and solidification methods such as geopolymer material. Geopolymer is an effective method to mobilize heavy metals in fly ash. The heavy metals in fly ash used in geopolymer concrete remain physically bonded solidly in concrete and will not cause long-term environmental pollution [2]. Therefore, geopolymer can be a construction material and can be conservative material that will minimize pollution potential from heavy metals in fly ash waste.
The peatland map compiled by BB Litbang SDLP [3] shows that the total area of peatland in three major islands is around 14.9 million ha. The details of the peatland area are in Sumatra 6.40 million ha, Kalimantan is 4.70 million ha and Papua 3.60 million ha. The largest peatland area in Sumatra is located in Riau Province, with an area of 3.861.401 ha or around 24% of the total peatland area in Indonesia [4]. The majority of construction in Riau Province is carried out on peatland, and around 2 57% of the total peatland area in Riau has been converted to infrastructure, agriculture, industry and even human settlement.
Peat environment generally has poor physical properties to concrete construction because it has high organic contents, high acidity (pH 3-5) and low bearing capacity [5]. Zivica & Bajza [6] explained that acid ions in peat would attack the cement paste bond in concrete and cause interference to the concrete bonding system. As a result, the concrete is easily porous, with reduced serviceability and decreased concrete performance. The study result [7] showed that OPC (Ordinary Portland Cement) experience a significant compressive strength decrease starting from 28 days to 150 days in peat water soaking.
Geopolymer is a polymerization material synthesized from silica and alumina, resistant to high temperatures [8]. The silica and alumina can be obtained from industrial residue such as fly ash. Geopolymer concrete is made from the aggregate mixture, water and binder material resulted from polymerization reaction between activator alkaline with silica and alumina in fly ash material [9]. Fly ash geopolymer almost has the same and even better physical and mechanical properties than OPC concrete, especially in an aggressive environment such as peat [10][11][12][13]. Bakharev [47] showed that fly ash geopolymer concrete compressive strength did not experience a significant decrease in the sulfuric acid environment.
The study by Mejía et al. [14], Suwan & Fan [15], Nath & Sarker [16] and Wijaya et al. [17] showed that the addition of OPC could increase setting time and increase the fly ash geopolymer strength without high temperature. Yanuari et al. [18] conducted a study on concrete geopolymer using palm ash, and it also showed the same trend. Therefore, the addition of PPC to geopolymer concrete can increase geopolymer strength without high temperature. Although the effect of PCC and OPC addition to hybrid geopolymer concrete has been studied, the relationship between the durability and the environmental performance of fly ash geopolymer concrete with Portland cement such as PCC and OPC has not been studied. Therefore, a literature review is conducted, which is purposed to discover the relationship between the durability and the environmental performance of hybrid geopolymer concrete in a peat environment

Methods
The study method used was a literature review using a tracing of secondary data, which is indirectly obtained from laboratory tests (primary data) or the primary study data. The flowchart of this review is presented in Figure 1.
The stages of data collection, data analysis and conclusion drawing are explained as follows: 1. A literature review is searching for information and data related and relevant to the keyword of fly ash, geopolymer concrete, hybrid, heavy metals and peat, through a searching database such as Google Scholar, ResearchGate and Scopus of 2010-2020. Two thousand articles related to the keywords were found, which will be selected and reviewed to be 50 full-text articles most relevant to the research topic and article review. 2. We are reducing and summarizing all research data obtained during data collection and focusing on the reviewed aspect. This study examines the strength performance (compressive strength and the change of compressive strength) and environmental performance (heavy metals leaching). 3. The data was analysed by tabulating, mapping the quantitative data, deriving them into other equations and comparing them to various comparable data and related regulations. 4. We present the analyzed data into narrative text, graphs, or tables for ease of understanding. 5. Conclusion drawing in the secondary data-based study is obtained from synthesizing preceding data, normalizing data, and making data from various sources as equal as possible to conclude the best recommendations.  Figure 1. Flowchart of research method

Result of Coal Fly Ash Chemical Composition Test
Fly ash is an industrial by-product of coal combustion. Coal combustion produces approximately 5% solid pollutants, 80-90% fly ash, and 10-20% is bottom ash. Fly ash is formed from inorganic mineral material and fine particles passing No. 200 sieves [19]. Fly ash is pozzolanic because it contains silica (Si) and alumina (Al) but little calcium (Ca). Fly ash handling is currently limited as an ameliorant material and mostly hoarded on empty spaces around steam power plants ( Figure 2). Fly ash can be used both as addition or partial replacement material of cement in concrete mixture. The reaction between fly ash and calcium will form adhesive properties similar to cement and has a better density compared to conventional concrete. Fly ash can be used as filler to reduce porosity in concrete thereby increasing the strength of the concrete. Based on the test result by Research and Development Center of Coal and Mineral Technology [20] it was found the heavy metal elements in fly ash as presented in Table 1. The heavy metal elements are not hazardous in open-air. However, they will be poisonous and deadly in water. Likewise in peat water, the presence of these elements indirectly causes living creatures around the peat water poisoned.

Literature Review
Keywords: Concrete, Geopolymer, Fly Ash, Peat, Heavy Metals Ombilin fly ash 87 15 153 120 tt ASTM C 618 [21] classifies fly ash into three classes: C, N and F. The C class has a minimum of 50% of SiO 2 + Al 2 O 3 +Fe 2 O 3 and more than 10% CaO (low-calcium fly ash) content requirement. Meanwhile, the F class has a minimum of 70% of SiO 2 + Al 2 O 3 +Fe 2 O 3 and a maximum 10% CaO (low-calcium fly ash) content requirement. Therefore, F class fly ash is more suitable for a concrete mixture in an acid environment such as peat water because it contains low calcium that is not easily dissolved by the peat water acidity. From Table 2, it can be seen the majority of fly ash from steam power plants in Indonesia contains approximately 70%-90% of SiO 2 +Al 2 O 3 +Fe 2 O 3 and is included in the F class pozzolan has a low content of calcium. Therefore, according to Hardjito & Rangan [29], this fly ash is suitable for being applied as a geopolymer material because it has a high silica and alumina content and low calcium. Thereby, the geopolymer can react to alkaline to form a polymerization bond, having good performance and not quickly disintegrate in an acid environment.

Characteristics of Peat Water
Based on Wetland International calculation, the peatland area worldwide is 400 million m 2 where it covers approximately 2% of the whole land area on earth. Meanwhile, Indonesia is the world's fourthlargest peatland area after Canada, Russia, and United States [30]. Indonesia is estimated to have 20.6 million hectares of peatland or 10.8% of the land area. Peat has a significant role in hydrology and the environment for the life of human beings and other living creatures. However, peat is a prone ecosystem to damage due to toxic substances. The damage occurring in peatland will result in disaster around the environment and disrupt biota living in it. Therefore, peatland must be protected and conserved.
Peat is formed from the accumulation of organic material from weathered plants decomposition found mostly in waterlogged conditions, swamps, and river basins [31]. Continuous stagnant water on the soil causes a change of water characteristics such as turbid, brownish red in colour, high content of mineral salt and low pH number between 3-5 resulting in acid peat water [32]. For example, peat water in Riau Province is highly acid, pH 3-5, contains high organic substances and low hardness levels. Generally, the characteristics of peat water in several locations in Indonesia can be seen in Table 3.

Performance of Conventional Concrete in Peat Water Acid Environment
Peat environment is acid that will impair the mechanism and performance of concrete. (Neville & Brooks [33] explained that Portland cement is not resistant to acidity. The acid ion in peat water will unravel calcium (Ca) in cement paste so that the concrete is easily porous. The higher the degree of acidity, the easier the concrete to suffer from structural damage and serviceability decrease and remain residual sediment in peat environment [6]. The compressive strength of concrete in peat water environments tends to decrease until the day of 180, by 40% [12]. This is because of the unwell cement hydration process and cement matrix decomposition with acid ion, thereby decreasing the density and compressive strength of the concrete. The formation of gypsum and ettringite in concrete reacting with acid will damage concrete durability. Excessive calcium in cement paste will unravel, resulting in the concrete experiencing cracks and pores, decreasing the compressive strength of the concrete. In addition, gypsum and ettringite are hazardous elements if released and decomposed in peat water. The acid endemic animals and even plants are no longer able to survive due to peat environment damage.

Fly Ash Geopolymer Concrete
Davidovits [8] first proposed the geopolymer in the 1990s. Initially, geopolymer was used as an antifire building material. The material then was developed as an alternative substitution of cement using the silica and alumina activator from industrial waste such as fly ash, palm oil fuel ash, and rice husk ash. The activation process of silica and alumina uses an activator solution of sodium hyroxide and sodium silicate [8].
Hybrid fly ash geopolymer concrete is the mixture of sand, gravel, water, and binder material from polymerization reaction between alkaline with silica and alumina from fly ash and Portland cement as a mixture. However, the pozzolanic reaction from fly ash reacts adequately slowly that the initial strength of fly ash geopolymer concrete is lower. Therefore, it is necessary to use the combination of PCC and alkaline activated fly ash geopolymer [34]. Figure 3 illustrates the composition of Portland cement and fly ash in hybrid geopolymer mortar.   Table 4. Based on the study results, generally, Portland cement can function as an agent to accelerate the reaction, curing time, increase mechanical properties and reduce porosity due to the formation of a stronger hydration product than the pure geopolymer bond. [17] Hybrid fly ash geopolymer mortar using OPC has higher compressive strength than PCC from the age of 7 to 28 days under room temperature curing.
[35] Replacement of fly ash with partial Portland cement can increase the compressive strength under room temperature curing. [34] The addition of fly ash geopolymer concrete with OPC forms C-S-H hydration structure from OPC and N-A-S-H from fly ash resulting in stronger C-A-S-H. [14] The addition of OPC or GBFS increases compressive strength (at 28 days of drying) up to 127% compared to fly ash-based geopolymer system. [16] The addition of OPC helps heat process from fly ash geopolymer concrete; thereby, the concrete no longer requires high temperature (oven) curing but adequate room temperature. [36] The replacement of fly ash with cement in geopolymer will decrease bonding time, porosity and increase compressive strength and modulus of elasticity.

Resilience Performance of Hybrid Fly Ash Geopolymer Concrete in Peat Environment
Based on Table 5, several tests present the relationship between the type and composition of concrete with the compressive strength of concrete. The study result by [37,38] showed decreased compressive strength of OPC and PCC concrete with peat water immersion kept increasing along with the age of the concrete. The study by Olivia et al. [37], Nath et al. [39] showed a similar decrease trend with OPC concrete by pure fly ash addition. Meanwhile, the fly ash geopolymer concrete with the addition of OPC showed a different result. The compressive strength in peat water immersion increased along with the concrete age.  Furthermore, [40] conducted study on the change of compressive strength of hybrid geopolymer concrete in peat environment. The formula used to calculate the compressive strength change is based on ASTM C-267 [41].
Compressive Strength Change = S2-S1 S1 × 100 (1) Where S1 = the average compressive strength of concrete after 28 days curing (initial strength) period, S2 = the average compressive strength of concrete after a test period of peat water exposure. A change of compressive force with a positive number shows that the strength (durability) increases after peat water exposure from its initial strength. Otherwise, the change of compressive strength with a negative number indicates that the strength decreases after peat water exposure.  (Geopolymer with the addition of PCC cement) increases at the age of 14 days in peat water immersion. GP PCC has the most significant change of compressive strength compared to the other concretes because PCC contains high pozzolanic material and less Ca (calcium) than OPC. After 28 days of immersion, GP OPC and GP PCC concrete decrease slightly in strength.

Environmental Performance of Hybrid Fly Ash Geopolymer Concrete in Peat
In this study of environmental performance in peat environment, earlier research data of heavy metals leaching of fly ash hybrid geopolymer concrete in peat environment is conducted and compared to other concretes. Fly ash has leachate potential with a high concentration of heavy metals, resulting in terrible and severe environmental impacts. Heavy metals are metals having high toxicity if released into the environment. The toxic substances in heavy metals pollute soil, air and water but will also cause damage to human health through the food chain [42]. One of the tests to determine heavy metals leaching level is TCLP (Toxicity Characteristic Leaching Procedure) test. This test is used to identify the toxic concentration of waste. Table 6 presents the leaching concentration of heavy metals in various types of concrete mixtures. Based on the study of [40], fly ash geopolymer concrete with the substitution of OPC 15% and PCC 15% by fly ash did not experience a significant change of heavy metals leaching after 28 days of immersion. However, if compared to the study of Prasanda et al. [43] and Aziz & Azhari [44], the concrete with the addition of fly ash has a higher leaching concentration.