Effect of polypropylene fiber on workability and strength of fly ash-based geopolymer mortar

. Geopolymers do not require Portland cement as a binder; hence, the binder is replaced by a material containing high SiO2 and Al2O3 reacted with an alkaline activator (NaOH and Na2SiO3). In addition, mortar or concrete is highly susceptible to cracking, requiring the addition of polypropylene fiber (PPF). This research belongs to an experimental study examining the addition of PPF with percentages from 0%, 0.25%, 0.5%, 0.75%, and 1% by utilizing coal fly ash (FA) as a precursor. This study also added a superplasticizer (SP) and extra water (EW) to maintain the flowability. The tests were carried out on the workability, strengths, and microstructure of geopolymer mortar. Workability obtained values of 140 - 220 mm. Furthermore, the highest compressive strength of 73.3 MPa and flexural strength of 9.92 MPa were identified in the geopolymer mortar with the addition of 0.5% PPF. In addition, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) tests were applied to investigate the geopolymer microstructure.


Introduction
Development in the construction sector can cause such as damage impacting climate change.Cement manufacture, for example, is one of the major contributors to greenhouse gas emissions released during its manufacture.Boden et al. [1] stated that the cement industry can produce up to 8% of global CO2 emissions annually, with more than 4 billion tons of cement production.Every 1 tonne of cement production generates 0.87 tons of CO2 emissions [2].Therefore, engineers and scientists intend to obtain alternatives in producing environmentally friendly construction materials without compromising the structural performance of buildings.One of the potential alternatives to achieve green concrete is a geopolymer material, with a high-content precursor of silicon dioxide (SiO2) and aluminium oxide (Al2O3).This material can be used as a binder with alkaline activator solutions to form a polymerization reaction.The high silica content in either geopolymer or normal concrete can strengthen the hardened properties [3].In addition, using geopolymer in concrete also reduces the dependence on conventional cement and CO2 emissions by 80% [4].
Fly ash is the most widely utilized aluminosilicate material in the research and development (R&D) of geopolymers [5][6].It is correlated with the abundance of production each year, approximately 2.8 billion metric tonnes and about 58% of its application [7].Ahmed et al. [8] asserted that the final oven temperature for FA-based geopolymer is 70 o C, accelerating polymerization.Moreover, Alhaji et al. [9] conducted an experimental study with varying temperatures for curing *Corresponding author: rahmad.a.ft17@mail.umy.ac.id geopolymer between 50 o C and 110 o C, discovering an optimum temperature of 70 o C. In addition to materials, the ratio of alkaline activator solution affects flowability, setting time, and hardness properties [10][11].Nevertheless, a superplasticizer (SP) and extra water (EW) are required to achieve better flowability in fresh properties [12][13].
On the other hand, geopolymer manufacture has a high risk of shrinkage, which lead to micro-cracks that reduce strength and create higher creep.Consequently, researchers began to innovate to minimize cracks by adding fibers to the material [14].Subsequently, reinforcement such as fibers is beneficial to enhance the mechanical properties of mortar.Fibers are divided into two types, synthetic and natural.Synthetic fibers are man-made, such as steel, polypropylene, glass and polyvinyl alcohol, while natural fibers, for example, are sisal, coir, and jute [15].
In addition, several researchers have conducted research on geopolymer incorporation fiber.For example, Abdullah et al. [16] conducted research on geopolymer concrete reinforced steel fiber with a proportion of 1% -7% which resulted in an increase in compressive strength of 13.92%.However, over time steel has negative impacts on construction because steel is susceptible to corrosion, so this will adversely affect its strength and not create a sustainable material.Therefore, this study investigated geopolymers reinforced with PPF.In this case, PPF has many advantages over steel fiber, such as not being easily corroded, good chemical resistance, has a low coefficient of friction, and very resistant to moisture.Moreover, After polyvinyl alcohol fiber, polypropylene fiber (PPF) is the most commonly used fiber [17].PPF possesses good elongation and resistance to acidic and alkaline environments [18].In addition, Nguyen et al. [19] investigated the effect of composites reinforced with PPF, uncovering that it could increase deflection, hardening behaviour, multiple cracks, and bridging capacity of fiber composites.
Following this potential, this study aims to study geopolymer mortar using a mixture of coal FA and PPF with different percentages of 0%, 0.25%, 0.5%, and 0.75% by testing compressive and flexural strengths.Geopolymer using 100% FA without the addition of PPF was utilized as a reference to determine the optimum proportion of PPF incorporating geopolymer.In addition, this study also tested microstructures such as X-ray diffraction (XRD) to identify the crystalline phase in coal FA, X-ray fluorescence (XRF) to discover chemical compounds in FA, as well as scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) to analyze morphology, composition, elements and geopolymer phase properties.

Material properties and mix proportion
This research utilized a coal fly ash (FA) derived from PT. Tjiwi Kimia as a precursor containing aluminosilicate compounds of 32% SiO2 and 13% Al2O3 that can create a polymerization reaction when mixed with alkaline activators.This FA contained CaO with 19.5%, categorized as high calcium FA based on BS EN 450-1:2012 [20].Other chemical ingredients are described in Table 1.Fig. 1 presents a dark grey FA was derived from local waste in Indonesia and XRD pattern that FA had an amorphous phase, and the main mineral in the peak ranging 20o2Ө -28o2Ө were quartz and sillimanite.Moreover, possessing specific gravity of 2.53.In addition, This study employed short PPF, with a length of about 12 mm.This size could make PFF equally distributed in the geopolymer mixture.Furthermore, the proportion of PPF from 0% to 1% was taken by the volume of FA.Table 2 depicts the specifications of PPF.Table 1.Chemical properties of FA.The mortar manufacture requires fine aggregate.This study utilized local sand sourced from the Progo River, Yogyakarta, as fine aggregate.It had 2.4 specific gravity, 2.27% fineness modulus, and 3.31% moisture content.In addition, the most widely used alkaline activator solutions (AAS) in geopolymer research are NaOH and Na2SiO3.The ratio of alkaline activator (NaOH/Na2SiO3) used in this study was 1:2.5, and the molarity of NaOH solution was 12 M produced by mixing 480 grams of pellets of NaOH dissolved with distilled water to reach one liter of solution.NaOH was prepared at least one day before the mixture.This experiment applied an SP and EW to maintain flowability and workability in geopolymer.The mixture proportion of geopolymer mortar with the addition of PPF is summarized in Table 3, with the composition of FA, PPF, alkaline activator, fine aggregate, SP, and EW.Furthermore, Sukontasukul et al. [21], conducted research on PPF-reinforced geopolymer in which the PPF content was taken by volume fraction, while Yoosuk at al. [22], PPF content was the by-weight of FA.However, in this research, the proportion of PPF from 0% to 1% was taken by the volume of FA.

Mixing, Casting and Curing
The mortar mixing was carried out in several stages for 6 minutes using a planetary mixer.Initially, FA and alkaline activator were mixed for 2 minutes, then fine aggregate in a saturated surface dry (SSD) condition was added and mixed for 3 minutes.SP and EW were also added for one minute.Finally, polypropylene fiber was spread gradually and evenly on the mixture.Furthermore, the fresh mortar mixture was tested for flowability.The fresh mortar was placed in 5 x 5 x 5 cm cube molds for the compressive strength test and 4 x 4 x 16 cm beam molds for the flexural strength test.After casting, each variation of the test sample was removed from the mold following the final hardening time.After that, the specimens were cured in an oven at 70 o C for 24 hours.In the following steps, all samples were wrapped in plastic and cured at ambient temperature until the age of 28 days for testing in mechanical properties.

Flowability, Strengths and Microstructure Tests
The flowability test using the flow table test under ASTM-C1432-70 [23] aimed to identify the distribution of the fresh mortar to achieve optimal conditions, and it is categorized as Table 4.The geopolymer workability was measured by a flow test due to the high viscosity of the geopolymer paste.The flowability test was carried out by mixing the geopolymer paste, pouring it, and compacting it into three layers into a flow mold with a base diameter of 80 mm.Furthermore, each layer was compacted 25 times using a tamper.The flow mold was lifted, and the machine was driven up to 25 beats.Moreover, the diameter of the mortar spread was measured using a meter to determine the mortar flowability in millimeters.Furthermore, all completely mixed mortar specimens were cast into 5x5x5 cm and 4x4x16 cm molds.Each mixture variation was tested after a curing period of mortar at 28 days.Both compressive and flexural strengths of mortar were tested using a universal testing machine (UTM).
In addition, microstructure studies on geopolymer mortar employed SEM and EDX to analyze morphology, composition, elements and geopolymer phase properties.SEM and EDX testing have different functions and purposes.SEM is a type of electron microscope that produces images of test samples by scanning the surface using a focused electron beam with magnification up to a certain scale.In contrast, EDX is a tool used to analyze chemical elements.The characterization ability is largely due to the basic principle that each element has a unique atomic structure, with the possibility of having a varying series of peaks in its electromagnetic emission spectrum.

Flowability
Based on Table 4, PPF variations with percentages of 0%, 0.25% and 0.5% are classified as high, with values of 220 mm, 200 mm, and 180 mm.Meanwhile, PPF variation of 0.75% belongs to the moderate category, with a value of 155 mm.Subsequently, the addition of 1% PPF is included in the stiff category, with a value of 140 mm.The results are presented in Fig. 2. Al-majidi et al. [25] discovered that adding SP and EW in geopolymers could increase the flowability with a longer hardening time.However, adding PPF to mortar could cause workability problems where a higher PPF percentage could reduce the flowability [26].Therefore, the composition in this study used an SP and EW to maintain the flowability in fresh mortar.

Compressive Strength
Fig. 3 illustrates the results of the flexural strength of the geopolymer mortar.The compressive strength test yielded an increase in values from 32.98 MPa (0% PPF) to 54.13 MPa (0.25% PPF), an increase of 64.13%.The results continued to increase until they peaked at the addition of 0.5% PPF variation, with a compressive strength value of 73.3 MPa, an increase of 122.26%.It is similar to the research of Chindaprasirt et al. [17], discovering that the optimum PPF fiber content could be observed at a PPF variation of 0.5%, while an increase in PPF content beyond the optimum limit led to an increase in porosity and a reduction in compressive strength.On the other hand, the results on the compressive strength test decreased after adding a PPF proportion of 0.75%, obtaining a compressive strength value of 68.89 MPa, a decrease of 6.40%.Meanwhile, adding 1% PPF acquired a compressive strength value of 65.92 MPa, decreasing by 10.06% from the 0.5% PPF.

Flexural strength
Fig. 4 demonstrates the results of the flexural strength test of the geopolymer mortar.The highest flexural strength was identified in the geopolymer mortar sample with a PPF variation of 0.5%.Adding 0.5% PPF could increase the flexural strength because the PPF effect could bind and minimize cracks.However, adding PPF exceeding the optimum level could gradually reduce the flexural strength as it causes the non-uniform distribution of PPF, making it difficult to mix.Furthermore, it could also decrease workability and increase porosity.In this study, the flexural strength test obtained an increase in values from 4.95 MPa (PPF of 0%) to 6.78 MPa (PPF of 0.25%), an increase of 36.97%.The results continued to increase until they peaked at the addition of 0.5% PPF, acquiring a flexural strength of 9.92 MPa, an increase of 100.40%.In addition, the flexural strength decreased after adding a PPF proportion of 0.75%, obtaining a value of 9.53 MPa, a decrease of 4.09%.Meanwhile, adding PPF of 1% achieved a flexural strength value of 8.92 MPa, experiencing a decrease of 11.21% from the PPF of 0.5%.The results of this study have similar conclusions to Yoosuk et al. [22], revealing that geopolymer mortar experienced an increase in flexural strength with the addition of PPF up to 0.5% and a decrease when PPF substitution exceeded 0.5%.

Microstructure
This study observed the morphology of the sample surface at high magnification using the SEM test, as depicted in Fig. 5.All materials in the SEM image were homogeneous and perfectly integrated, as in Fig. 5(a).Zhang et al. [27] stated that FA particles have a smooth shell and porous interior, as demonstrated in Fig. 5(b).Fig. 5(c) portrays some PPF from the fracture surface, indicating that PPF has functioned as a puller between materials when tested for strength.The more PPF, the stronger the attraction between the materials.It proves that PPF has effective toughness and can prevent mortar fracture.Fig. 5(d) illustrates the cracks bend and stop crossing the PPF on the mortar surface tested for strength.In addition, Fig. 5(e) and 5(f) depict the mortar surface and some micro-cracks and micro-pores caused beyond the optimal PFF level.The microstructure on the surface of the geopolymer mortar incorporating PPF was analyzed using the SEM and EDX tests.EDX data were obtained from the area where the SEM image was taken.The EDX test was run to analyze the elements of chemical compounds, with the results displayed in Fig. 6.Following the EDX spectrum, the main components of the FA geopolymer are Si and Al, with a small addition of Na and K [28].This statement is proven in this study, in which the EDX analysis generated atomic mass with silica (Si) of 8.08%, alumina (Al) of 3.35%, calcium (Ca) of 1.84%, iron (Fe) of 1.60%, natrium (Na) of 3.66%, magnesium (Mg) of 0.75% and kalium (K) of 0.32%.

Conclusion
Geopolymers have greatly interested researchers because they use innovative and eco-friendly materials, not using Portland cement as a binder.Therefore, geopolymer concrete is the most revolutionary development of building construction materials in new concrete technology.Indeed, adding PPF in geopolymer can be a solution in mortar or concrete, which is highly susceptible to cracking.
The results of this geopolymer mortar have been tested under the specified reference standard.Furthermore, it has become a sustainable potential in concrete research with the addition of coarse aggregate.Geopolymers can be used in precast structural elements because they harden quickly.In addition, using FA as a material makes it environmentally friendly because it utilizes industrial waste.The following conclusions were drawn based on the test results.
1.This study utilized the basic ingredients of industrial waste in coal FA and PPF with percentages of 0%, 0.25%, 0.5%, 0.75%, and 1% as the geopolymer mortar.The ratio of NaOH/Na2SiO3 was 1:2.5, with a molarity of 12 M NaOH to obtain the optimum strength based on some literature recommendations.The addition of 2% SP and 5% EW from the binder aimed to improve the flowability and workability of geopolymers.
2. Increasing the PPF content resulted in higher flowability and workability values.Thus, this study applied five variations of PPF to obtain optimum results.The results disclosed the flowability with a diameter of 220 mm, and the lowest was at 140 mm.
3. The highest compressive strength of 73.3 MPa and flexural strength of 9.92 MPa were identified in the geopolymer mortar with the addition of 0.5% PPF.
4. The microstructure on the surface of the PPF geopolymer incorporating mortar was analyzed using EDX and SEM tests.The EDX test obtained the atomic mass of silica (Si) of 8.08%, alumina (Al) of 3.35%, calcium (Ca) of 1.84%, and iron (Fe) of 1.60%.Meanwhile, the SEM test has proven that PPF has effective toughness and can prevent mortar fracture.

Fig. 5 .
Fig. 5. Morphology of the sample surface at high magnification.