Effect of Fly Ash on Some Properties of Reactive Powder Concrete

To achieve sustainability in the field of civil engineering, there has become a great interest in developing reactive powder concrete RPC through the use of environmentally friendly materials to reduce the release of CO2 gas produced from cement factories as well as contribute to the recycling of industrial wastes that have a great impact on environmental pollution. In this study, reactive powder concrete was prepared using total binder content of 800 kg/m, water to binder ratio (0.275), and micro steel fibers 1% by volume of concrete. The experimental program included replacing fly ash with (8, 12, 16) % by cement weight to find the optimal ratio, which achieved the best mechanical properties of (RPC) at 7, 28, and 90 days with standard curing. Some mechanical properties of reactive powder concrete (flow, compressive strength, tensile strength, and density) were verified. The results at 28 days showed that the compressive strength (96.5) Mpa, tensile strength (9.38) Mpa, and density (2395 kg/m). The results showed that the percentage of replacement of 8% of fly ash with cement is the optimal percentage, which achieved the highest resistance compared to the others. The results also indicated that it is possible to develop RPC using fly ash with a high withstand stress, tensile strength, and density.


INTRODUCTION
The need for high strength requirements with the high performance of concrete was the main reason for the production of reactive powder concrete (RPC). The first person who began to search for this type of concrete was Richard Cheyrezy from 1990 to 1995. It became possible to produce RPC concrete by establishing and presenting basic structure, preparation, and mixing (Richard, 1995). Reactive powder concrete (RPC) is a modern cement composite material with high performance and good durability properties and has been applied in a wide field of protective engineering. The addition of fiber to RPC improved its compressive strength properties compared to conventional concrete. (Xu, 2016). Fly ash is one of the wastes generated from electric power plants, which relies on coal as fuel. When coal is burned, it will pass into the furnace through the high-temperature region, in which the volatile material is burned with carbon. fly ash was used to produce a type of cement called blended cement, where it was added to clinker and gypsum and crushed. ( Zongjin Li, 2011). The main benefits of using fly ash in concrete are summarized as follows:  Due to the spherical nature of fly ash particles and the packing effect property, which helps to improve workability and reduce the amount of water required.  The high surface area increases pozzolanic activity and thus improves the gain strength.  Improve the consistency of the concrete mixture and reduce its variability through the harmonic distribution of particles within the mixture (Zongjin Li, 2011; Yu J and Leung CKY,2017). ( Kadhum, 2015) assessed the mechanical properties of the compressive strength and flexural strength of the RPC by using steel fibers. The results show that the addition of steel fibers led to a significant improvement in compressive strength and bending compared to concrete without steel fibers. (Algburi et al., 2019) also recorded the ratio of the compressive strength of RPC reinforced with fiber at the age of seven days to the compressive strength of RPC at the age of 28 days which was higher than that of non-fiber reinforced RPC by 33%, which made it suitable for use in structural elements that need high early compressive strength such as bridges, buildings rising and structural columns. (Elsayed et al., 2019) succeeded in obtaining a compressive strength of 121 MPa by producing PRC by using silica and fly ash at a life of 28 days and under normal conditions where the percentage of silica that achieved the highest resistance was 25% replaced by cement weight with a w/c ratio of 0. 25 and the proportion of steel fibers is 2%. It was also noted that the preparation of reactive powder concrete using fine iron fibers at a ratio of (1-1.5%) and through water treatment, the amount of compressive strength achieved at the age of 28 days was 70 MPa, where the results showed that the sample containing (1.5%) of fibers is the best for protection because it is very dense (Fawzi et al., 2020). In another study (Yu et al., 2018) concluded that a new type of environmentally friendly green concrete could be developed by including a percentage of no less than 80% of fly ash in concrete production. Also, he proved that the use of fly ash in the production of concrete will lead to obtaining suitable workability and good strength in structural construction. ( Hüseyin et al., 2012) concluded that it is possible to produce RPC by replacing 60% of the cement weight with fly ash. After conducting the tests to verify the mechanical properties, the results indicated a significant improvement in compressive strength, tensile and flexural strength, as the compressive strength under standard water treatment was 200MPa. (Sarika and John, 2015) checked the mechanical and fresh concrete properties of reactive powder concrete. They show the ability to produce RPC at a compressive strength of 130 MPa under standard processing conditions at 28 days of life. The strength obtained at 28 days of curing is equivalent to approximately 70% of the strength achieved at 7 days. This research aims to use sustainable fly ash as a partial substitute for cement weight and to ascertain its effect in improving the mechanical properties of reactive powder concrete. Thus, reducing the amount of cement that makes up the bulk of the components of reactive powder concrete, as the cement manufacturing process is one of the main sources responsible for the release of carbon dioxide and the cause of global warming, in addition to contributing to the protection of the environment from pollution through the recycling of fly ash waste.

Cement
The Ordinary Portland cement (OPC) used in this study is within the Iraqi Specification IQS No. 5/2019 type / (42.5 N). Table.1 shows the chemical and physical analysis of the cement used in this work. Table.2 shows physical analysis of cement.  Table 2. Physical analysis of cement.

Fine Aggregate
AL-Ukhaider Natural sand as a fine aggregate was used in concrete mixes of this work. The chemical and physical properties of the sand are within the limit specified by Iraqi standard, zone /4. IQS No.45/1984. As shown in Table 3. The water used in this research within Iraqi to specification No.1703/1992 .

Silica fume (SF)
Micro silica from Company (CONMIX) was used in this work. The percentages used were 10%, replacement with cement weight, and it is within the requirements of ASTM C1240-15. Chemical and physical analysis of silica as shown in Table 4. Fly ash was used in this research according to the standard to American Standard ASTM C 618-15. In this research Fly ash Class (F) with (8) % proportion was used as replacements of cement weight. Chemical and physical analysis of Fly ash is given in Tables 5 and 6.  The (Msf) used in this test program was straight steel fibers manufactured in China. The diameter of fiber used 0.2 mm and the length 13mm so (L/d = 65). Fibers were added at 1% of the concrete volume.

Superplasticizer (SP)
Master Glenium 51(SP), according to ASTM C494 -17 was used in this research. The manufacturer's recommended dosage was in the range (0.5-1.6) liters /100kg of the cement. The cement dosage used in this study was 1 liter /100 kg.

Concrete Mixes
The mixtures were prepared in this work by experimental mixtures based on specifications and guidelines presented in the previous research (Osama, 2018) (El-Louh,2014). The batches were prepared as a trial to obtain a compressive design strength of 90 MPa, as the reference mixture for comparison with other mixtures. A total number of four Reactive powder concrete (RPC) mixtures were prepared by using a water to binder ratio of about ( 0.275). The content of the binder used in this research is 800 kg/m 3 , HRWR (Glenium 51) used 8 liters by weight of cement. The experiment program included made the control mix with only Portland cement in addition to silica fume (PC+SF) as the binder, while other mixtures included replacing the cement with Fly ash (FA) (PC+ +SF+FA) in which a percentage of PC was replaced with Fly ash (FA) at 8%, 12%, and 16 %. Mix proportions details for RPC are shown in Table 7.  Table 7. Details of mix proportion of RPC.

2.3.1Pozzolanic Activity Index
Pozzolanic Activity Index found in this study for both fly ash with silica and according to  [96] and (ASTM1240-15) [92] respectively.

Flow test
The RPC flow was calculated in this study according to the ASTM C-1437-15 specification requirements. Flow is an increase in the original diameter of the concrete source as a percentage to Flow Table Cone (100 mm). Equation 1. Illustrate the flow of PRC.

Compressive strength test
Compressive strength test was done according to ASTM C109 by using standard cube specimens of 50x50x50mm. The Compressive strength (fm) is calculated with the following equation. The tensile strength was determined in this study according to ASTM C 496. This test was performed using a standard cylinder model of 100 ×200 mm. (T) calculated according to the following equation:

fm = P/A
Where: Τ: Splitting tensile strength (MPa) P: Max. Applied load (N) l: is the length of specimen (mm) d: Diameter (mm )

Density Test
The density test in this paper was performed according to the American standard ASTM C642-13 .The density was calculated by dividing the weight of a cube by its volume. Samples were tested at 7, 28, and 90 days old.

Strength Activity Index
The results of the pozzolanic activity test at the age of 7 days of the materials that were used in this research were illustrated as shown in Table 8.

Workability
The results of the flow test in Table 9 and Fig. 1 show that the flow increased with increasing the replacement rates of fly ash by about 35% and that the water demand has decreased compared to the reference mixture while maintaining stable workability. This is due to the small size of the fly ash particles in addition to the glassy texture and the spherical shape that makes it work as spherical bearings lubricate the mix. Thus it acts as a superior plasticizer that increases workability and reduces the amount of water required that increases workability. Also, the spherical shape and smooth surface of the fly ash particles helped reduce the friction that occurs between the particles and make them have greater fluidity. The fly ash particles present on the cement dough surface have an opposite charge, which helps to create a greater dispersion and prevent the Prevent clumps that occur in it (Mehta, 2006).

Compressive Strength
The results of the average compressive strength of three cubic samples that were prepared in this study are summarized as shown in Fig. 2. and Table 10. The results showed that the increase in the replacement ratios of cement with fly ash led to an improvement in the compressive strength of concrete during different ages. The sample M1 achieved a compressive strength of (75.7, 96.5, 115.5) and the percentage of increase in strength was (2.6, 4.9, 5. 5)% compared to the reference mixture during ages 7, 28, and 90 days, respectively. This increase is a result of the pozzolanic activity of fly ash resulting from the interaction of SiO2 and Al2O3 with CH and the formation of C-S-H, and these reactions result in the formation of a matrix with high density and low porosity and thus an improvement in concrete strength (Tahir, 2005( and(Malvar, 2006), as well as particle capacity. Fly ash to fill the voids in the cement paste through its contribution to filling led to an increase in density and thus increased strength.

Splitting Tensile Strength Test:
After determining the optimum ratio of fly ash (FA) in the previous stage through the compressive strength test, the tensile strength of the samples that included 8% FA were tested. The results are shown in Table 11, and Fig. 3 summarized the tensile strength of the samples at ages (7, 28, and 90).
Table11. Splitting tensile strength of RPC mixtures. The apparent improvement in tensile strength is due to the effect of fly ash. The increased hydration leads to the continuation of pozzolanic reactions. This increase may be related to the presence of pozzolanic materials such as fly ash in the composition, whose fineness is less than cement by about five times; this led to improving and strengthening the structure of the concrete material, thus reducing the porosity and increasing the strength of RPC, (Aljalawi, 2009

Dry bulk density
Dry density results illustrated in Table 12 and Fig. 4 matched what (Shaheen and Shrive, 2006) recorded that RPC density ranges from 1760 to 2410 kg / m 3 . The density of FA samples was higher than the reference samples during ages 7, 28, and 90 days. The reason for this was attributed to the pozzolanic interactions of fly ash with cement and form a dense gel of amorphous C-S-H with the reduction of the porosity and the number and length of the microscopic cracks, thus increasing the bond strength within the concrete network, which results in a high-density matrix compared to the reference samples.