Mechanical Properties and Microstructure of Geopolymer Binder Based on Umeanyar Slatestone Powder

This research focuses on the mechanical and microstructure properties of geopolymer binder with slatestone waste as the base material. This geopolymer binder comes from industrial waste crushing slate in the Umeanyar area. This waste is processed into stone powder (USSP) which contains SiO 2 (49%), Al 2 O3 (11%), CaO (11.2%). This USSP uses a sodium hydroxide (SS) activator with a concentration of 14 M. The proportion of the mixture of precursor and activator (P/A) is 70%: 30%; 75%: 25%; 80%: 20% and alkaline activator Na 2 SiO 3 : NaOH (SS/SH) of 1:1; 1.5:1; 2:1, by weight. Samples of specimens were made in the form of a cube with a side of 50 mm and tested at the age of 7 and 28 days. Mechanical properties tested include density and compressive strength based on ASTM-C39. Meanwhile, the microstructural analysis used X-Ray Diffraction (XRD) and Scanning Electronic Microscope-Energy Dispersive X-Ray (SEM-EDX) analysis. The results of the density test were 1.90g/cm 3 and 1.85g/cm 3 respectively and the compressive strength test results were 7.40 MPa and 12.73 MPa at the age of 7 and 28 days, respectively.


Introduction
Concrete is one of the materials needed in the construction sector. The constituent materials of concrete generally consist of cement, aggregate, and water, if necessary there are materials added to change certain properties of the concrete [1]. The use of Portland cement as a concrete-forming material, in its production, requires a very large amount of energy due to heating to a temperature of 1500°C [2][3][4][5][6][7][8][9]. The cement-forming materials consist of limestone (CaCO 3 ), silica sand/clay (SiO 2 and Al 2 O 3 ), and iron oxide (Fe 2 O 3 ). These raw materials are processed through heating and calcination to release carbon dioxide gas (CO 2 ) which is the main contributor to greenhouse gas emissions in the atmosphere [10]. The product in the form of granules in the form of gray marbles with a diameter of about 2 cm is called Portland cement clinker The production of each tonne of cement clinker including its fuel is almost equivalent to one ton of CO 2 gas released into the atmosphere [2,5]. In recent years, the world's cement production has been recorded at 4 billion tons per year, which means about 4 billion tons of CO 2 gas has been released into the atmosphere every year [9,10]. With the results of CO 2 gas emissions that are quite large and have harmed the environment, the mitigation action that can be taken is to reduce the use of cement in the manufacture of concrete.
Geopolymer binder is a geosynthetic binder that uses materials that are not derived from cement. The term geopolymer was first introduced by Davidovits in 1978 who found a polymerization bond between alkaline activators and the main ingredients in the form of fly ash and rice husk ash [13][14][15]. Geopolymer is the synthesis of natural materials through a polymerization process with the main ingredients in the manufacture of geopolymer materials are materials containing silica and alumina elements [12,[15][16]. The raw material in this geopolymer binder is called a precursor.
The precursors in the geopolymer binder are the main ingredients that are rich in alumina and silica. The raw materials or precursors of geopolymers usually use fly ash, rice husk ash, metakaolin, white clay, or other materials that contain a lot of silica and alumina. The effect of using this precursor can affect the physical and mechanical properties of the geopolymer binder. Research on the use of fly ash as a precursor containing more than 50% silica and alumina activated with an activator solution can to produce compressive strength exceeding conventional cement [18][19][20][21][22].
In this study, slatestone waste was used as a geopolymer binder precursor. Slatestone is generally referred to as slatestone and belongs to the metamorphic or metamorphic rock group [23]. Slatestone has physical characteristics that are slate with regular thickness, shiny and hard, often found in mountainous areas, one of which is in the Umeanyar area. Until now, slatestone is not only used as a building material (coarse aggregate) or wall and floor decoration material. The use of slatestone for the roofs of buildings is in many areas of Galicia, Spain [24].
The stone crusher industry in the area processes slatestones of various sizes (1-4 cm) depending on consumer demand. This stone-cutting business results in useless parts becoming waste that accumulates and even builds up so that it can disrupt the local environment. A preliminary research study on the mineral content in slatestones has been carried out by Karyasa [25]. The results of the study showed that slatestone contains 37% silica, 11.13% aluminum, 12.3% calcium, 5.47% iron and, 34.1% other metals. With the high content of silica and alumina minerals, these materials have the potential to be used as an alternative as a geopolymer precursor. The slatestone waste will be processed into powder which is used as a precursor by using NaOH and Na 2 SiO 3 as alkaline activators in the manufacture of geopolymer binder.
In the manufacture of geopolymers, many factors affect the characteristics of the resulting geopolymer both mechanically, physically, and microstructured, including raw materials or precursors, calcination temperature/thermal treatment, particle size, activating solution/activator, and the treatment process [12,[24][25][26][27][28]. The results of the compressive strength test as one of the mechanical properties of the binder are very important as a parameter of a test object. Several studies have been carried out using several precursors activated with alkaline activators [31][32][33][34][35][36]. In addition to compressive strength as a mechanical property of binders, physical properties also need to be considered in the manufacture of geopolymer binders. One of them is the volume weight or theoretical weight of the test object can be determined by dividing the weight by the volume of the test object [37].
Activation of the precursor by providing an alkaline activator to a mixture of geopolymer binders with a certain ratio of sodium silicate (SS) and sodium hydroxide (SH) can affect the results of the compressive strength test. Research using fly ash, crushed stone dust, lunar rock sand as precursors have been carried out and give an increase in compressive strength when the SS/SH ratio increases [7,[29][30]. The increase in compressive strength also occurs when the age of the test object increases when tested. This is evidenced by the test specimens aged 7 and 28 days [31][32], the compressive strength produced by each specimen at the age of 28 days is higher than at the age of 7 days. The effect of the ratio of precursors and activators (P/A) also affects the compressive strength of geopolymer binders. The greater the P/A ratio, the higher the compressive strength value {Formatting Citation}. As for the mechanical property in the form of volume weight, the value increases when the P/A and SS/SH ratios increase [35][36][37]. In addition to mechanical properties in the form of compressive strength and volume weight tests, this study will also discuss the microstructure of geopolymer binders based on Umeanyar slatestone powder. Microstructural tests were carried out using X-RD (X-Ray Diffraction) and SEM (Scanning Electronic Microscope).

Materials
The precursor used in this study was Umeanyar slatestone powder (USSP). The material sample is sieve at 200µm (Figure 1). The results of the XRF analysis of Umeanyar slatestone powder are shown in Figure 2 and Table 1. From the results of XRF analysis, the mineral content contained in USSP consists of SiO 2 (49%), Al 2 O 3 (11%), CaO (11.2%), Fe 2 O 3 (22.35%), and 6.45 other oxide minerals. With silica and alumina content of more than 50%, it is expected to produce good geopolymer precursors. There are two types of alkaline activators used, namely sodium silicate (Na 2 SiO 3 ) and sodium hydroxide (NaOH). The concentration of sodium hydroxide (SH) solution was 14 M. The criteria for selecting the proportion of geopolymer binder are based on several studies that have been carried out previously which resulted in high compressive strength. Geopolymer binder was made with 3 variations of precursor and activator (P/A), namely 70%:30%; 75%:25%; 80%:20%. Meanwhile, the ratio of sodium silicate and sodium hydroxide (SS/SH) activator is 1:1; 1.5:1; 2:1.

Density
The weight of the volume or the theoretical weight of the test object can be determined by dividing the weight by the volume of the test object [37]. The volume weight of the geopolymer binder test object can be formulated by equation (1) below:

Compressive Strength
Compressive strength is the amount of load per unit area that causes the test object to crumble when it is loaded with the compressive force generated by the press machine. The testing process begins by placing the test object into the compressive strength testing machine, but the slate surface of the test object is placed on the top base. The compressive strength testing machine is turned on, wait a few seconds for the test object to crack. The compressive strength test is intended to determine the actual compressive strength value of the binder sample under hard conditions using a compressive strength testing machine until the sample is completely crushed [44]. The value of the binder compressive strength can be calculated by equation (2) as follows: (2) by: σ = Compressive strength (N/mm 2 or MPa) P = Ultimate Load (N) A = Cross-sectional area (mm 2 )

Microstructure Analysis
After the volume weight is measured and a compressive test is carried out on the geopolymer binder, the flakes of the specimen will be tested for XRD and SEM-EDX microstructures. XRD is a tool used to characterize the crystal structure, crystal size of a solid material. All materials containing certain crystals when analyzed using XRD will show specific peaks. Diffraction methods are generally used to identify unknown compounds contained in a solid by comparing the diffraction data with the database released by the International Center for Diffraction Data in the Powder Diffraction File (PDF).
While the SEM-EDX test is a type of microstructural testing using an electron microscope that uses an electron beam to describe the surface shape of the material being analyzed. The working principle of this SEM is to describe the surface of an object or material with a reflected electron beam with high energy. The surface of the material that is irradiated or exposed to the electron beam will reflect the electron beam or called a secondary electron beam in all directions. But of all the reflected electron beams there is one electron beam that is reflected with the highest intensity. The detector contained in the SEM will detect a high-intensity electron beam reflected by the object or material being analyzed. Besides that, it can also take advantage of the reflected beam from the object so that information can be known by using an image processing program contained in the computer.

Mixing Geopolymer Binder
The USSP-based geopolymer binder was prepared first by making an alkaline solution by mixing the 14 M sodium hydroxide activator alkaline which had been left for 24 hours previously with a sodium silicate solution according to the ratios listed in Table 2. The USSP precursor was put into a 3-liter bowl mixer. Turn on low speed for 15 seconds, then add an alkaline activator and turn the mixer on medium speed again for 30 seconds and mix well. After thoroughly mixed, prepare a cube mold with a side of 50 mm (Figure 3). After being printed into the mold, tightly wrapped in airtight plastic for 24 hours. After being allowed to stand for 24 hours, the cube test object was put into an oven at a temperature of 70°C for 24 hours. After being removed from the oven, the specimens were removed from the mold and tested at the age of 7 days and 28 days. For each variation, 3 specimens were made, and the tests were carried out according to the standards of SNI 2008 [37] and ASTM C39 [44].

Density Test
The results of the density test are presented in Figures  4-10 and Tables 3-4. Each according to variation with the ratio of P/A and SS/SH with an increase in age of 7 and 28 days.  The results of the density test showed that with an increase in the P/A ratio, the volume weight value increased at the age of 7 and 28 days. In the 1:1 SS/SH group, the lowest volume weight was 1.75 g/cm 3 at the age of 7 days and decreased to 1.71 g/cm 3 at the age of 28 days. Meanwhile, the highest volume weight was in the 2:1 SS/SH group with a P/A ratio of 80%:20% with a value of 1.90 g/cm 3 and 1.85 g/cm 3 at the age of 7 and 28 days, respectively. This shows that the increasing percentage of precursors causes an increase in the volume weight of the geopolymer binder [45]. The precursor with the highest amount (80%) contributed to the high volume weight of the geopolymer binder mixture because the amount of solid content was more than the amount of liquid contributed by the alkaline activator in the amount of 20%. So along with the increase in age from 7 to 28 days, the polymerization bond reaction continues so that the volume weight when the specimen is 28 days old decreases [46][47][48].

Sodium silicate to sodium hydroxide ratio
Increasing the SS/SH ratio can affect the density value of geopolymer binders with USSP base materials. This can be seen in Tables 4 and Figures 8-10. The more the value of the SS/SH ratio increases, the density value of the test object also increases. The average increase occurred in each group with the highest density value at a 2:1 SS/SH ratio of 1.87 g/cm 3 , 1.89 g/cm 3 , and 1.9 g/cm 3 at the age of 7 days. Meanwhile, at the age of 28 days, the average density of the specimens decreased. During the geopolymerization reaction process until the age of 28 days, the decrease in density occurred slowly. Evaporation that occurs during the binding process is also the cause of the decrease in density. This is due to water loss on the binder surface, and the initial bonding process is faster [26,34,[41][42].
In group 1 (Table 4), the lowest density value was 1.71 g/cm 3 and the highest was in group 3, which was 1.85 g/cm 3 . The increase in density values in each group with variations in SS/SH was caused by the increase in the alkaline activator content, especially the increasing SS/SH ratio. However, the results of the density of each test object along with the increase in the value of the SS/SH ratio, are not too large, namely an average of 2%. So the SS/SH ratio factor in geopolymer binders contributes less to the density increase [42].    The increase in the P/A ratio affects the compressive strength of geopolymer binders. The value of the compressive strength of each variation of the test object is shown in Tables 5 and Figures 11-14. The increase in the compressive strength value occurs with every increase in the P/A ratio. The percentage increase in compressive strength values is between 10-15% for each increase in the SS/SH ratio at the age of 7 days. While at the age of 28 days, the percentage increase almost reached 40%. The highest compressive strength of 12.73 MPa is in the test object coded D33 with the highest P/A ratio of 80%:20%. The greater the P/A ratio, the higher the compressive strength of the geopolymer binder as it ages. The use of a precursor of slatestone powder at 80%:20% P/A composition has the highest compressive strength. This is because, at this ratio, the binder has the highest silica and alumina content compared to other P/A ratios. In addition to producing a denser binder structure, the high P/A content can contribute to increasing the compressive strength of geopolymer binders [43][44].

Sodium silicate to sodium hydroxide ratio
The results of the compressive strength produced in each SS/SH variation group are presented in Table 6 and Figure 15-17. There is an increase in the compressive strength value along with the increase in the SS/SH ratio and the age of the test. The highest compressive strength was in the 80%:20% P/A group with a 2:1 variation of SS/SH. The alkaline activator with high silicate content can accelerate the geopolymerization process so that it also increases the compressive strength of geopolymer binders [31,36,[52][53][54]. The average percentage increase was 18% in each group. The highest increase in compressive strength occurred in group 3 with a SS/SH ratio of 2:1. There was an increase in the value of compressive strength up to 30%, which was 12.73 MPa. This shows that the presence of sufficient silica in the activator can increase the binder strength so that the geopolymerization process can run more quickly and perfectly to produce a higher compressive strength.

XRD (X-Ray Diffraction)
The XRD test process with analysis was carried out after the compressive strength test of the geopolymer binder test object. Figure 18 shows the sample refining process for material characterization according to the XRD test requirements. The results of the XRD analysis can be shown in Figure  19-23 and Table 7-11. Umeanyar slatestone powder has an XRD graph as shown in Figure 19. The results of XRD analysis on USSP precursors with primary mineral content are shown in Table 7. The highest peak of 2θ between 20°-30° indicates the highest mineral content in USSP powder. Each sample was tested at the age of 7 and 28 days by taking samples with the lowest and highest compressive strength values.  In sample Y11 at 7 days old, the mineral content of cristobalite was 7% and albite 93%, this mineral content increased at 28 days by 11% and 89%, respectively. While the sample Y33 at the age of 7 days has a mineral content of 9% cristobalite and 91% albite, and an increase of 14% and 86% respectively at the age of 28 days. Test the sample with an angle of 2θ from 0°-90°. The diffraction peaks showing the binder structure of the USSP geopolymer can be seen in the range of 20°-30°. X-ray diffraction results in the formation of different reaction products in geopolymer binders depending on the ratio of the activator used. Characteristically sharp diffraction peaks indicate the development of the crystalline phase from the partially amorphous phase of the geopolymer binder [41,55]. The presence of new compounds in the form of cristobalite and albite indicated that the crystalline phase was more dominant than the amorphous phase. The content of cristobalite compounds which increased along with the increase in the P/A ratio and the SS/SH ratio showed that at the age of 28 days the geopolymer binder had a higher amorphous phase than when the specimen was 7 days old. [56,57].

SEM (Scanning Electronic Microscope)
The SEM-EDX test was carried out on specimens with variations in age of 7 and 28 days with the lowest and highest ratios of P/A and SS/SH. The results of the SEM-EDX test for USSP precursors are shown in Figures  24-25 and Table 12. The results of the analysis show that USSP has a microstructure with the K electron shell having the highest OK content followed by SiK, CK, AlK and several other compounds with a small percentage.    When sample Y11 was 7 days old, a new compound appeared, namely MgK, but the value was small, namely 0.58%. There was an increase in SiK content by 23.68% at the age of 28 days. This shows that increasing the age and the SS/SH ratio can increase the silica content of the geopolymer binder. This also happened in studies using fly ash precursors [29,49,[58][59][60][61][62]. Samples with the highest compressive strength values were also tested by SEM-EDX, the results are shown in Figures 30-33 and Tables 15-16. The content of MgK compounds also appeared in sample Y33 (28 days) which was 1.62%, this value was greater than the variation of Y11. SiK content is also higher up to 23.92%. With the increase in silica content that has been activated with alkaline activator, it can help the polymerization reaction that occurs in geopolymer binder. Binder with high silica and alumina content are factors that can increase compressive strength. Samples with a high SS/SH activator ratio also affected the presence of MgK and SiK, so that the resulting compressive strength was higher.

Conclusions
This research resulted in several things related to the mechanical properties and microstructure of geopolymer binders based on Umeanyar slatestone powder (USSP), namely:  The density test which affects the physical properties of the geopolymer binder is influenced by the ratio of P/A and SS/SH as well as age at the time of testing. The higher the ratio of P/A, SS/SH, and age, the higher the density value  The compressive strength test was carried out at the age of 7 and 28 days, giving the highest results at the age of 28 days with the highest ratio of P/A (80%:20%) and SS/SH (2:1)  Microstructural testing was carried out using XRD and SEM-EDX analysis. XRD analysis was carried out to determine the diffraction peaks of the crystalline and amorphous phases of the compounds formed. Meanwhile, the SEM-EDX test was conducted to determine the surface shape of the USSP-based binder sample. The result is that a new compound is formed, namely Mg in the binder, in addition to the Si content which is formed more and more after 28 days of age with the highest compressive strength.