WITHDRAWN: Performance of Plastic Aggregate along with Steel Slag in the Concrete through the Mechanical Tests and Microstructural Analysis

doi:10.21203/rs.3.rs-4354714/v1

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WITHDRAWN: Performance of Plastic Aggregate along with Steel Slag in the Concrete through the Mechanical Tests and Microstructural Analysis

https://doi.org/10.21203/rs.3.rs-4354714/v1

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The demand on the utilization of the industrial by-product increased in order to preserve the natural resources and create a sustainable environment for the future generation. Therefore, the present study is focused on the utilization of industrial by-product electric arc furnace slag (EAFS) as the replacement of cement and fine aggregates whereas the plastic aggregate (PA) were replaced with the coarse aggregate. The optimization of the mix design was performed by replacing the cementitious slag (SAC) upto 50%, whereas the slag as the replacement of fine aggregates (SAFA) upto 70%. Also, plastic aggregates (PACA) were replaced with coarse aggregates upto 50%. It is concluded that the 30% replacement of cementitious slag and fine slag aggregate found optimum whereas the same found to be 50% in case of coarse aggregate replacement with plastic aggregate. Further, the combined effect of mix with 30% cement, 50% fine aggregate along with 30% of PA was studied. The flexural strength of mix SCFPA found to be 5.5 MPa whereas the same found to be 5.17 MPa in the mix PACA3 and it shows that the cementitious slag and slag fine aggregate contribute marginally on the resistance under flexure. The tensile strength of combined mixes such as SCFA, SCPA, SFPA and SCFPA was found insignificant, whereas the compressive strength of SCPA mix was found as the worst performer. Further, SEM, XRD and EDS analysis were also conducted in order to study the microstructure of the different mixes. The CSH, ettringite and portlandite were majorly observed and the calcium silicate hydrate is an important compound for measuring the strength and rheological properties of concrete. The ettringite (3CaO•Al₂ O₃ •3CaSO₄ •32H₂ O) having a needle-like crystals structure plays an important role in the setting time, strength development and shrinkage of concrete, whereas the portlandite identified as a cubical smooth structure works as a bonding agent.

Steel Slag

Plastic Aggregate

Optimization

Mechanical Tests

Microstructural Analysis

The cement and plastic materials found to be a product of routine life which means the cement is required for the development of infrastructure (Li et al., 2023[1]), while the plastic is being used as a carry bag, wrapping product, food packaging etc. These materials are the prime factors which are responsible for the development of the nation and to make life very easy for each of us. On the other hand, several risks are also associated with the usage of these materials i.e. due to very fine suspended particles of cement dust which may affect the human health by entering in the body through air and the nanoparticles of plastic contaminated the drinkable water and ultimately affect the human body i.e. as the nanoparticles of plastic entered in the human body through several ways, Gigault et al. (2018) [2]. Beside this, manufacturing of cement and the plastic bags exerts several gases into the environment (19–20% of CO₂ produced during the cement manufacturing) which are the real threat to the environment. It is to note that different types of chemicals (i.e. around 8000) are supposed to be used in the manufacturing of the plastic which have more hazardous effects on the environment as well as on the human health in comparison to the CO_2, Wiesinger et al. (2021) [3]. Therefore, in order to preserve the natural resources and to create a sustainable environment, there is a need to propose some guidelines regarding the usage of these waste materials in different construction practices which may result in the sustainable environment, Gencel et al. (2021) [4]. Steel industry has become the one of the most important industries in the world which is responsible for producing waste material i.e. slag along with the production of raw materials, Santhosh et al., (2021) [5]. Due to the production of slag on a large scale, there are a lot of opportunities for the construction sector to utilize the slag in sustainable construction practices and help in reducing the environmental pollution, Lu et al., (2023) [6]. As per the report of Indian Bureau of mines (2015) [7], production of cement in India is booming by 10% annually. While, Mehraj et al (2013) [8] stated that the increment in the quantity of suspended cement dust particles in an open environment results in the increase in several diseases like skin related, heart disease and many more which may affect the quality of life. Boora and Dharma (2022) [9] stated that the several non-treatable diseases like chronic respiratory diseases, asthma, and chronic obstructive pulmonary which are responsible for 10.7% deaths out of 82% total deaths happening due to the entering of these suspended dust particles into the human body. Another important reason to focus on the application of industrial byproducts is because of scarcity of their dumping in an open landfill which contaminates the soil mass as well as reduces the land fertility, Vishwakarma and Uthaman (2019) [10]. Due to this alarming situation, there is a need to utilize different waste materials in different construction practices which may help in creating a sustainable environment and preserving the natural resources.

Concrete is a mixture of cement, fine aggregate, coarse aggregate and water. Therefore, steel slag could be utilized as the replacement of coarse or fine aggregate in the concrete. Earlier, an experimental study was conducted by focusing on the application of slag as the replacement of fine aggregate in order to check its impact on the mechanical properties of concrete mix, Teng et al. (2013) [11]. On the basis of the findings it was observed that the slag is a beneficial product as the replacement of sand as it speeds up the hydration process along with the increment in the early age strength. Earlier, Qasrawi et al., (2009) [12] reported reduction in the workability of the concrete mix due to the incorporation of steel slag as the replacement of fine and coarse aggregate. Though, the rate of decrease in the workability was found to be at the lower side. While in case of PA as the replacement of coarse aggregate, the workability was found to be increased due to the lower water absorption capability of PA. Choi et al. (2005) [13] reported an increase in the workability of PET bottles lightweight aggregate concrete (WPLAC) due to the inclusion of plastic aggregate as the replacement of coarse aggregates. The main reason was the availability of the extra water i.e. plastic aggregate water absorption capacity is very low. Azhdarpour et al. (2016) [14] observed that the inclusion of PET aggregates (i.e. up to 10%) having a size of 2–4.9 mm as coarse and 0.05–2 mm as fine aggregate increased compressive and flexural strength.

Devi and Gnanavel (2014) [15] examined the effect of steel slag on the workability of concrete mix. Based on the results, reduction of 20% in the workability was reported after the use of 30% steel slag as the replacement of coarse aggregates. Although, Monosi et al. (2016) [16] reported that the reduction in workability was overcome with the use of plasticizer and improved mechanical properties for concrete mix. It was recommended to utilize the EAFS after proper crushing to ensure the proper grading. It was also concluded that the use of EAFS as coarse aggregate could achieve higher strength in comparison to mixes containing natural coarse aggregate alone. Several other studies also recommended the use of steel slag as a replacement for aggregate due to the improvement in tensile strength and shrinkage properties of concrete mix, Rondi et al. (2016) [17]. The use of EAFS was recommended due to its ability to reduce the shrinkage problem in concrete mix as well as volume expansion characteristics of EAFS, Santamaría et al. (2018) [18]. Besides, it was also observed that the use of slag helps in reducing the cement content in mix for the required strength. Fronek et al. (2012) [19] concluded that the slag becomes no expansive after processing it. An observation was made that incorporating treated slag in concrete mixture resulted in the attainment of the highest strength after a 28-day curing period, surpassing the strength achieved by using untreated or as-received slag. This improvement is attributed to the enhanced cementitious behavior of the slag, which occurs only after undergoing reprocessing, Muhmood et al. (2009) [20]. The use of copper slag upto 40% was recommended as a replacement of fine aggregate, beyond which increased number of voids may occur which led to the failure, Wu et al. (2010) [21]. Pellegrino and Faleschini (2013) [22] concluded that the EAFS may be used as 100% replacement of coarse aggregate which will be beneficial from economical as well as environmental point of view. However, Pellegrino et al. (2013) [23] stated that the coarse aggregate was replaced with steel slag up to a maximum proportion of 50% without affecting the mechanical properties of the mix.

Based on the detailed literature review, it was observed that most of the studies reported the use of slag as a partial replacement of fine and coarse aggregate. A limited number of studies were found heretofore to use as partial replacement of slag with cement and FA. Also, the performance of plastic aggregate along with the steel slag found to be limited. Therefore, the present study is focused on a sustainable environment by minimizing the depletion of natural resources such as river sand and coarse aggregate and reducing CO₂ emissions by reducing the cement usage in the concrete production. Hence, the optimization of the mix design was performed by replacing the cementitious slag up to 50% (0, 10, 20, 30, 40 and 50%) of cement whereas fine slag aggregate replaced upto 70% 0, 10, 20, 30, 40, 50, 60 and 70%) of river sand, see Section 2. Further, the optimization of the design was performed by replacing the plastic aggregate up to 50% (0, 10, 20, 30, 40 and 50%) of coarse aggregate, see Section 2. The optimized mix is proposed based on the tests on fresh concrete and hardened concrete, see Section 3. Based on the optimization, the cement and fine aggregate both in combination were replaced by the steel slag whereas the coarse aggregate is replaced by the plastic aggregate and studied against compression, flexure and tension tests, see Section 4. Further, SEM, energy dispersive X-ray (EDS) was performed to identify the types and the quantity of elements present in a concrete whereas the X-ray diffraction analysis was performed in order to examine the crystallographic structure of a material, see Section 5. The major findings are elaborated and presented at Section 6.

2.1 Materials

EAFS was procured from Jindal Stainless (Hisar) Limited, Haryana, India. Other materials such as 43-grade of Ordinary Portland Cement (OPC), coarse aggregate and fine aggregate were collected from local vendors. The physical tests on cement with or without slag were conducted. The EAFS particles were sieved using a 75-micron sieve to be used as a replacement of cement. As per the guidelines of IS 4031-Part 5 [24], the initial and final setting time of the cement with or without EAFS was found to be varying from 50 to 500 minute and 20 to 309 minutes (i.e. 30% EAFS), respectively. While following the guidelines of IS 4031- Part 11 [25], the specific gravity of 3.18 for the cement having EAFS (30%) was reported in comparison to the specific gravity of OPC i.e. 3.15. The soundness of the modified mortar by steel slag was studied as per IS 4031-Part 3 [26]. On the basis of the results, the soundness for cement with EAFS was found to be 4 mm whereas the same in the case of conventional cement is 3 mm, see Table 1.

The physical properties of fine aggregate and coarse aggregate (i.e., alone or in combination with EAFS) were also tested based on IS codes and shown in Tables 2 and 3, respectively. Chemical composition of the cement and EAFS was analyzed using X-Ray fluorescence (XRF), see Table 4. Coarse aggregates used in the present study were found to be varying in sizes i.e. 20 mm to 10 mm. While, fine aggregates having a size of 150 µm passing sieve no. 4 were used in the present study. The guidelines of IS 383 [29] were followed to obtain the proportion of coarse and fine aggregates. Figure 1 shows the particle size distribution of coarse and fine aggregates used in the present study. All the materials including EAFS (except PA) were heated for 24 hours in a temperature-controlled oven at 100°C to remove the moisture.

Table 1

Physical properties of cement
Sr. No.	Experiment		Experimental Values		IS code
Sr. No.	Experiment		Cement	Cementitious slag
1	Settling Time Test	Initial	50 min	20 min	IS 4031-Part 5 [24]
1	Settling Time Test	Final	500 min	309 min	IS 4031-Part 5 [24]
2	Specific Gravity Test		3.15	3.18	IS 4031- Part 11 [25]
3	Soundness by Le-Chatelier Method		3mm	4mm	IS 4031-Part 3 [26]
4	Consistency Test		32%	35%	IS 4031-Part 4 [27]
5	Fineness Test		6.14	4.5	IS 4031-Part 1 [28]

Table 2

Physical properties of fine aggregate (River Sand)
Sr. No.	Experiment	Experimental Values		IS code
Sr. No.	Experiment	Fine Aggregate	Slag	IS code
1	Specific gravity Test	2.80	3.17	IS 2386-Part-III) [30]
2	Fineness Test	4.53	4.53	IS 4031-Part-1 [28]

Table 3

Physical properties of coarse aggregates
Description	Natural Aggregate	Plastic Aggregate	IS code
Specific Gravity (g/cm³)	2.74	0.99	IS:2386-Part-III [30]
Water Absorption (%)	1.0	0.02	IS:2386-Part-III [30]
Impact Value (%)	22	0	IS: 2386-Part-IV[31]
Crushing Value (%)	21	0	IS: 2386-Part-IV [31]
Los Angeles Abrasion Value (%)	18	0	IS: 2386-Part-IV [31]
Flakiness (%)	13	14	IS: 2386-Part-I [32]
Elongation (%)	12	11	IS: 2386-Part-I [32]

Table 4

Chemical composition of cementitious materials
Material /compound	SiO₂	Al₂O₃	Fe₂O₃	CaO	MgO	K₂O	Na₂O	SO₃	MnO	LOI*
OPC-43	20.27	5.32	3.56	60.41	2.46	0.41	0.33	2.48	-	4.76
EAFS	24.43	5.47	20.53	37.53	6.21	0.01	0.02	1.0	4.79	0.01
*Loss on ignition

Table 5

Mix design in terms of cementitious slag replacement with cement
Sample Name	Water (kg/m³)	Admixture (%)	Cement (kg/m³)	Coarse Aggregate (kg/m³)	Fine Aggregate (kg/m³)	Cement (%)	Cementitious slag (%)
SAC0(NC)	165	1	414	1282	666	100	0
SAC1						90	10
SAC2						80	20
SAC3						70	30
SAC4						60	40
SAC5						50	50
Particle Size						< 90 micron

Table 6

Mix design in terms of slag fine aggregate replacement with river sand
Sample Name	Water (kg/m³)	Chemical admixture (%)	Cement (kg/m³)	Coarse aggregate (kg/m³)	River sand (kg/m³)	River sand (%)	Slag fine aggregate (%)
SAF0 (NC)	165	1	414	1282	666	100	0
SAF1						90	10
SAF2						80	20
SAF3						70	30
SAF4						60	40
SAF5						50	50
SAF6						40	60
SAF7						30	70
Particle Size						< 4.75 mm	4.75 mm -63µm

Table 7

Mix design of plastic coarse aggregate slag replacement with coarse aggregate
Sample Name	Water (kg/m³)	Admixture (%)	Cement (kg/m³)	Coarse Aggregate (kg/m³)	Fine Aggregate (kg/m³)	Coarse Aggregate (%)	Plastic aggregate (%)
PACA0(NC)	165	1	414	1282	666	100	0
PACA1						90	10
PACA2						80	20
PACA3						70	30
PACA4						60	40
PACA5						50	50
Particle Size						< 20mm

2.2 Methodology

The optimization of the mix design was performed by replacing the cementitious slag (SAC) up to 50% (0, 10, 20, 30, 40 and 50%) of cement and six mixes (SAC0-SAC5) were prepared. Also, the as slag fine aggregate (SFA) replaced up to 70% (0, 10, 20, 30, 40, 50, 60 and 70%) of fine aggregate (river sand) and eight mixes (SAF0 -SAF7) were studied against compression tests as shown in Tables 5 and 6. The guidelines of IRC 44:2008 [33] were followed to finalize the mix design of M-40 grade of concrete for use in pavement construction. Besides this, IS 456:2000 [34], was also followed for further reference. The samples for the different concrete mixes were prepared for a curing period of 7, 14 and 28 days by employing Naphthalene based superplasticizer conforming IS 9103:1999 [35] to achieve the desired workability. Consequently, the samples were prepared using slag as the replacement of cement and fine aggregate in varying proportion by weight having a water-cement ratio equal to 0.40 as per the specification of IRC 15:2011 [36]. Further, the plastic aggregate as the replacement with coarse aggregate (PACA) was considered varying from 0 to 50%. Also, the optimization of the mix design was performed by replacing the coarse aggregate up to 50% (0, 10, 20, 30, 40 and 50%) with plastic aggregate and six mixes (PACA0-PACA5) were prepared, see Table 7.

3.1 Tests on Fresh Concrete

There are seventeen mixes along with conventional concrete mix resulting in eighteen mixes were prepared by utilizing the slag and plastic aggregate as the replacement of cement, fine and coarse aggregate individually. The workability in terms of slump of concrete containing different replacements are compared with the control mix, Fig. 2(a)-(b). It was observed that the significant reduction in the workability of the concrete mix with cementitious slag as well as fine aggregate slag. The slump value of the concrete with cementitious slag of 10, 20, 30, 40 and 50% are 48, 41, 38, 33 and 30, respectively, whereas the same was found to be 53 in case of normal concrete. The workability of concrete mix with 50% cementitious slag found to be reduced 43% as compared to the normal concrete. Also, the performance of fine slag aggregate was found to be similar to that of cementitious slag. However, the workability of concrete with plastic aggregate found to be increasing with increase of volume of plastic aggregate. It was observed that the concrete with plastic aggregate of 10, 20, 30, 40 and 50% found to be 55, 58, 59, 61 and 65, respectively. The workability of concrete mix with 50% plastic aggregate (PACA5) was found to have increased 22.6% as compared to the normal concrete and the similar results were reported by Qasrawi et al., (2009) [12].

3.2 Tests on Hardened Concrete

3.2.1 Density of Concrete mixes

The density of the concrete containing different replacements is compared with the control mix at 28 days, Fig. 3(a)-(d). The density of the concrete with cementitious slag of 10, 20, 30, 40 and 50% are 2379, 2385, 2379, 2311 and 2293, respectively, whereas the same was found to be 2355 in case of normal concrete. It was observed that the density of the concrete mix with 20% cementitious slag was found higher among the chosen mixes, however, the density of the mix was reduced with increase of slag, see Fig. 3(a). The reason may be due to the poor packing by the slag. Also, the density of the concrete with fine slag aggregate of 10, 20, 30, 40, 50, 60 and 70% are 2353, 2375, 2444, 2410, 2400, 2320 and 2242 kg/m³, respectively, whereas the same was found to be 2355 kg/m³ in case of normal concrete, Fig. 3(b). Further, the density of the concrete with plastic aggregate was studied, see Fig. 3(c). The reduction in the density of concrete with 50% plastic aggregate found to be 17% as compared to the normal concrete. It was observed that the density found to be reduced marginally by the plastic aggregate upto 40%, however significant reduction found when the plastic aggregate exceeds 40% of coarse aggregate. The maximum reduction in density of modified concrete mixes was due to the incorporation of plastic aggregate whereas, the incorporation of slag as replacement of cement and Slag fine aggregate was increasing the overall density of concrete.

3.2.2 Influence of EAFS as Replacement of Cement

A total of 54 samples with varying proportions of EAFS i.e., 0 to 50% as the replacement of cement were prepared and tested at 7, 14 and 28 days curing time. The compressive strength test was performed as per the guidelines of IS: 516 (Part 1/Sec 1) [37]. The compressive strength in normal concrete (NC) was found to be 41.3 MPa, Fig. 4. However, the mix incorporating EAFS (SAC1-SAC3) as the replacement of cement with 10, 20 and 30% revealed the compressive strength found to be almost the same as 30 MPa. In case of the mix SAC4 and SAC5, the compressive strength was found reduced significantly, i.e., 15 and 9.7 MPa, respectively. It was observed that the strength in the mix SAC5 was found to be 75% higher as compared to normal concrete. Whereas, the same in the mix SAC1-SAC3 found to be 25% higher as compared to normal concrete SAC0. It was also observed that the strength in the normal concrete is 16, 29 and 41 MPa at 7, 14 and 29 days, respectively. The development of the strength in the normal concrete SAC0 mix at 7 and 14 days was found to be 40 and 70%, respectively as compared to 28 days. However, the insignificant strength gaining was observed after 14 days in case of mix SAC4 and SAC5. It is concluded that the compressive strength on the mix with 30% replacement of slag with cement found 25% reduced as compared to the normal concrete however, the addition of slag (> 30% of cement) led to reduction of the same by 75% as compared to the normal concrete.

3.2.3 Influence of EAFS as Replacement of Fine Aggregate

A total of 72 samples with varying proportions of EAFS i.e., 0 to 70% as the replacement of fine aggregate (SAF) with river sand were prepared and tested at 7, 14 and 28 days curing time. The compressive strength in normal concrete (SAF0) was found to be 41.3 MPa which is the same as the NC mix, Fig. 5. Initially, compressive strength was found to decrease 19 and 15% with an incorporation of 10 and 20% slag fine aggregate in comparison to the conventional mix SAF0. The reason may be due to the improper or error during the mixing and preparation of the samples. However, the mix incorporates EAFS (SAF3-SAF5) as the replacement of slag fine aggregate with 30, 40 and 50%, compressive strength found to be almost same as 41 MPa which is equal to the conventional mix SAF0. The reason may be due to the SiO₂ and CaO contain higher content on EAFS which can provide higher strength to the mix due to the formation of CSH gel. In case of mix SAF6 and SAF7, the compressive strength was found reduced significantly, i.e., 31 and 28 MPa, respectively. Thus, it was observed that the compressive strength in the mix SAF6 and SAF7 was reduced 25 and 30%, respectively as compared to SAF0. It is concluded that the steel slag up to 50% replacement with river sand exhibited a compressive strength same as compared to the conventional concrete mix. Further, the addition of steel slag (> 50% - <70% of slag fine aggregate) led to reduced compressive strength of about 30%.

3.2.4 Influence of Plastic Aggregate as Replacement of Coarse Aggregate

A total of 45 samples with varying proportions of plastic aggregate i.e., 0 to 50% as the replacement of coarse aggregate were prepared and tested at 7, 14 and 28 days curing time. The compressive strength of concrete with 10, 20, 30, 40 and 50% plastic aggregate was found to be 26, 28, 29, 23 and 15.5 MPa, respectively, whereas the same was found to be 41.34 MPa at normal concrete, at 28 days, see Fig. 6. It was observed that the concrete with 30% plastic aggregate found to be reduced 30% as compared to normal concrete. The influence of plastic aggregate upto 30% on the compressive strength found to be almost the same. Further, the concrete with 40% plastic aggregate found to be reduced 43% as compared to normal concrete. It is concluded that the plastic aggregate above 40% found to be unsuitable for the application of reinforced concrete structures. It is also concluded that the plastic aggregates up to 30% replacement with coarse aggregate exhibited a better compressive strength as compared to the normal concrete.

3.3 Discussion on Mix Optimization

On the basis of the findings, it was revealed that the slag up to 30% could be used as the replacement of cement (SAC) and fine aggregate (SAFA) up to 50% and similar observations made by Devi and Gnanavel (2014) [15]. The compressive strength was found lower than the normal concrete mix, Choi et al. (2005) [13], however, the proposed mixes may be utilized for low traffic volume rural roads as per the guidelines of IRC: SP 62-2014 [38]. Hekal et al (2013) [39] also stated the inclusion of EAFS as the replacement of cement more than 20%, reduces the compressive strength significantly in comparison to normal concrete. However, the compressive strength of SAFA mix was found to be 1% lesser than the normal concrete. The reason may be due to the higher packing density offered by the slag fine aggregate which is lesser in size as compared to the natural river sand. Therefore, it is recommended that the SAFA up to 50% could be used in the construction of National and State highways. Also, it was observed that the plastic aggregate as the replacement of coarse aggregates (i.e. up to 30%) found better. Based on the findings from the comprehensive experiments, there are four mixes in combination of slag fine aggregate-cementitious slag (SCFA), cementitious slag-plastic aggregate (SCPA), slag fine aggregate- plastic aggregate (SFPA) and cementitious slag-slag fine aggregate-plastic aggregate (SCFPA) were proposed. Table 8 depicts the mix design of a combined approach with varying proportions of cementitious slag, fine aggregate and plastic aggregate. A total of 36 cubes having a standard size of 150×150×150 mm were prepared and tested at 7, 14 and 28 days.

Table 8

Mix design optimized replacement with cementitious slag, fine aggregate slag and plastic coarse aggregate
Sample ID	Water (kg/m³)	Admixture (%)	Cement (kg/m³)	Coarse aggregate (kg/m³)	River sand (kg/m³)	Slag fine aggregate (%)	Cementitious slag (%)	Plastic aggregate (%)
SCFA	165	1	414	1282	666	50	30	0
SCPA						0	30	30
SFPA						50	0	30
SCFPA						30	50	30
Particle Size						< 4.75 mm	4.75 mm -63µm	< 20 mm
Note: Slag as the replacement of cement and fine aggregate – SCFA, Slag as the replacement of cement along with plastic as the replacement of coarse aggregate – SCPA; Slag as the replacement of fine aggregate along with plastic as the replacement of coarse aggregate – SFPA and slag as the replacement of cement and fine aggregate along with the plastic aggregate as the replacement of coarse aggregate – SCFPA.

4.1 Workability of Concrete

There are four mixes along with conventional concrete mix resulting to five mixes were prepared by utilizing the slag and plastic aggregate as the replacement of cement, fine and coarse aggregate such as SCFA (30% + 30%), SFPA (50% + 30%), SCPA (30% + 30%) and SCFPA (30% + 50% + 30%). The workability in terms of slump of concrete containing different replacements are compared with the control mix, Fig. 7(a). The slump values of the SCFA, SFPA, SCPA and SCFPA are 33, 43, 38 and 29, respectively, whereas the same was found to be 53 in case of normal concrete. It was observed that the workability of the concrete mix SCFA found to be 38% lesser than the normal concrete. On the other hand, mixes having PA were showing significant improvement in the workability due to the presence of plastic aggregate as the mix SCPA found to be 19% lesser than the normal concrete. Further, the mixes having cementitious slag, fine slag aggregate and plastic aggregate were showing significant decrement in the workability as the mix SCFPA found to be 45% lesser than the normal concrete.

4.2 Density of Concrete

The density of the concrete containing different replacements in combinations are compared with the control mix at 28 days, Fig. 7(b). The density of the SCFA, SFPA, SCPA and SCFPA are 2372, 2385, 2324 and 2295, respectively, whereas the same was found to be 2355 in case of normal concrete. It was observed that the maximum density reduction on the concrete mix SCFPA found to be 2.5% than the normal concrete. However, the maximum increase in density was observed on the concrete mix SCFA found to be 0.75% than the normal concrete. Therefore, it is concluded that the maximum reduction in density of 2.5% on mix SCFPA among the chosen combination.

4.3 Compressive Strength

The compressive strength of the concrete with 30% cementitious steel slag replacement of cement and 50% of steel slag as fine aggregate replacement with river sand were studied through the compression tests, i.e. SAC3 and SAFA5, respectively. Also, the influence of 30% plastic aggregate replacement of coarse aggregate was studied, i.e., PACA3. The compressive strength of mix SAC3, SAFA5 and PACA3 are compared with the NC. The 28-days compressive strength of NC, SAC3, SAFA5 and PACA3 is found to be 41.3, 30, 41 and 29 MPa, respectively, see Fig. 8(a). It is observed that the compressive strength of the SAFA5 mix achieved the target strength of the NC mix, whereas the SAC3 mix found to be reduced 25% as compared to the strength of NC mix. However, the compressive strength of PACA3 mix was found to be reduced 30% as compared to the strength of NC mix. The early compressive strength, i.e., 7days, found to be highest for the mix PACA3 is 63% as compared to the 28 days strength. However, the compressive strength in the PACA3 mixes at 7 and 14 days found to be the same.

Further, the compressive strength of the mix SCFA, SFPA, SCPA and SCFPA are measured and compared with the normal concrete. At 28 days curing time period, compressive strength of SCPA mix is the worst performer and was found to be 36% less than the normal concrete. It was observed that the strength of mix SCPA at 7 and 14 days achieved only 9.4 and 13.5 MPa, respectively whereas the same was found to be 26.5 MPa at 28 days, Fig. 8(b). The reason may be due to the inactive cementitious slag particles in the mix which may get activated after 14 days and found similar results by Barnett (2006) [40]. It is concluded that the mix SCPA is not achieving an early strength (7 and 14 days) which is found to be below 30% of the target strength. The strength on mix SCFA at 7 and 14 days achieved 24.9 and 29.5 MPa, respectively whereas the same was found to be 36.1 MPa at 28 days. It is observed that the mix SCFA is achieving significant early strength (7 and 14 days) which is found to be above 70% of the target strength. In the case of mix SFPA, the pattern of strength performance at 7, 14 and 28 days was found to be almost the same as that of NC. In the case of SCFPA mix, the strength at 7 days was found to be 22.1 MPa whereas the same was found to be 10.5 MPa in the mix PACA3. It was observed that the reduction in the early strength (7 days) on mix PACA3 is (10.5 MPa) significantly less as compared to the SCFPA (22.1 MPa) mix. The reason may be due to the cementitious slag and slag fine aggregate that led to the early hydration and reaction with the cement matrix. On the basis of the experimental results, it was observed that the PA and SAC are the main parameters which are responsible for the reduction in compressive strength of the modified mix as compared to the normal mix. It is to note that the PA led to build weak bonding with the mortar (i.e. due to its smooth surface) and some amount of the slag as the replacement of cement are remain not activated (i.e. due to the availability of less water which may get activated after 14 days curing time period). The reason for higher strength by the modified mixes (having slag as fine aggregates) in comparison to other modified mixes (at 28 days curing time period) is due to the good packing density as lesser in size in comparison to the fine aggregate. Therefore, it is concluded that the plastic aggregate along with the cementitious slag and slag fine aggregate led to performance similar to that of normal concrete with maximum reduction of 15%.

4.4 Flexural Strength

The flexural strength of mix SAC3, SAFA5 and PACA3 compared with the NC. The 28-days flexural strength of NC, SAC3, SAFA5 and PACA3 are found to be 4.6, 5.25, 9.08 and 5.17 MPa, respectively, see Fig. 9(a). At 28 days, it is observed that the flexural strength of SAFA5 mix achieved the highest i.e., 98% than the NC mix, whereas the SAC3 mix also found to be higher 15% as compared to NC mix. Also, the increase in compressive strength of PACA3 mix found to be 12% higher as compared to the NC mix. At 14 days, it is observed that the flexural strength of SAC3, SAFA5 and PACA3 mix found to be increased 8, 93 and 24% as compared to the strength of NC mix. Similarly, the flexural strength of SAC3, SAF5 and PACA3 mix was found to be 25, 70 and 68% as compared to the NC mix at 7 days. In addition to that, the flexural strength of SACF3050 mix was found to increase, 26, 35 and 58% as compared to the strength of NC mix at 7, 14 and 28 days. Overall, it was observed that the flexural strength of the concrete increases significantly by the use of EAFS as the replacement of cement and FA as well as coarse aggregate with plastic aggregate at all curing ages. Therefore, it is concluded that the EAFS as the replacement of cement and FA as well as coarse aggregate with plastic aggregate lead to improve the flexural strength significantly which is very important to the concrete pavement.

Further, the flexural strength of the mix SCFA, SFPA, SCPA and SCFPA are measured and compared with the normal concrete. At 28 days curing time period, flexural strength of NC, SCPA and SFPA mix was found to be almost same to that of the normal concrete, see Fig. 9(b). The reason for the lower flexural strength was due to the incorporation of PA as the replacement of coarse aggregate. It was observed that the strength of the SCFA mix achieved the highest flexural strength among the chosen mixes and a similar result was reported by Qasrawi et al. 2009 [12]. In case of SCFPA mix, the strength at 28 days found to be 5.5 MPa whereas the same found to be 5.17 MPa in the mix PACA3 and it shows that the cementitious slag and slag fine aggregate doesn’t contribute to the resistance under flexural load. While in case of SCFPA and SCFA found increased flexural strength by 20 and 60%, respectively as compared to the normal concrete.

4.5 Split Tensile Strength

The tensile strength of mix SAC3, SAFA5 and PACA3 compared with the NC. The 28-days tensile strength of NC, SAC3, SAFA5 and PACA3 are found to be 5.17, 5.31, 5.57 and 5.23 MPa, respectively, see Fig. 10(a). At 28 days, it is observed that the flexural strength of SAFA5 mix achieved the highest i.e., 7.7% than the NC mix, whereas the SAC3 and PACA5 mix found to be almost same to that of NC mix. The split tensile strength was found to exhibit no significant change by using slag as the replacement of cement (i.e. 3%) and fine aggregates (i.e. 7.7%) as compared to the normal concrete. It was observed that the inclusion of 40% of slag as the replacement of river sand proves beneficial in terms of increase in strength values due to the formation of dense concrete. While further increase in the proportion of slag, strength was found to decrease due to the high water absorption characteristic of steel slag and its porous structure in the mix SAFA and the same was witnessed by Fronek (2012) [19].

Further, the tensile strength of the mix SCFA, SFPA, SCPA and SCFPA are measured and compared with the normal concrete. At 28 days curing time period, tensile strength of NC, SCPA and SFPA mix was found to be almost same to that of the normal concrete, see Fig. 10(b). It was observed that the split tensile strength was not showing any significant changes and the reason may be due to the more porous structure of slag and its higher water absorption ability and similar results were reported by Yu et al., (2023) [41]. It was also observed that the SAC and PACA mixes showed lower early age strength due to lower hydration activity of EAFS in comparison to the cement and poor bonding of PA with mortar. Therefore, it is concluded that EAFS and plastic aggregate could be utilized as the replacement of cement, fine aggregate and coarse aggregate without compromising the tensile strength of concrete mix. In addition to that, EAFS as the replacement of cement could be found useful in the construction of rural roads as the tensile strength was found to be the same as the normal mix which is minimum requirements of rural roads.

5.1 XRD Analysis

The X-ray diffraction (XRD) technique based on Bragg’s law was used in order to study the phase identification of a crystalline material present in the modified concrete due to the cementitious materials and different additives used in the present study at 28 days curing period. The step size was maintained at 0.0170 for all the test specimens. Initially, all the specimens were crushed and sieved through 900 µm. Consequently, the powder materials of all the specimens were tested and different peak positions along with their relative pattern intensities were analysed to gather the required information as shown in Fig. 11.

In the present study, the test was performed at room temperature with CuKα and the scanning was carried out for 2θ angle from 5 to 80°, in order to examine the crystallographic structure of a material. It was observed that the presence of quartz, CSH, ettringite and portlandite help in increasing the strength of concrete mix. It is to note that, during the hydration of cement two different silicate compounds are produced namely calcium silicate hydrate and calcium hydroxide. The calcium silicate hydrate was found similar to a natural mineral named as tobermorite (tobermorite gel), an important compound for measuring the strength and rheological properties of cement - concrete. It was also observed that the ettringite (3CaO•Al₂ O₃ •3CaSO₄ •32H₂ O) having a needle-like crystals structure as the by-product of hydrated Portland cement. Ettringite plays an important role in the setting time, strength development and shrinkage of chosen concrete mix. While portlandite having a cubical smooth structure identified as a major bonding agent in mix. The portlandite was identified as the performance indicator of concrete in terms of durability of concrete and its resistivity against carbonation as it helps in maintaining the pH of 12.6. Another important compound of concrete is calcium silicate and calcium aluminate which play an important role in the binding property and early strength, respectively. It was also observed that the xonotlite was found beneficial for enhancing the mechanical properties of concrete mix because of its compact structure and needle-like structures confirmed the presence of xonotlite in concrete mix. Also, the presence of gyrolite in the mix helps in the hydration of OPC at early stages.

5.2 SEM and Energy Dispersive X-ray Spectroscopy (EDS) Analysis

The scanning electron microscopy (SEM) technique was used to study the microstructure and the interfacial transition zone (ITZ) of the modified concrete. The samples were prepared after crushing the modified samples i.e. whose compressive strength was calculated after 28 days curing time period. Initially, samples were polished and covered with a gold palladium coating. All the SEM images were captured using the Carl Zeiss machine at Dr. B R Ambedkar NIT Jalandhar laboratory as shown in Fig. 12. It was observed that the presence of CSH, ettringite and portlandite help in increasing the strength of concrete mix. The ettringite (3CaO•Al₂ O₃ •3CaSO₄ •32H₂ O) having a needle-like crystals structure plays an important role in the setting time, strength development and shrinkage of concrete. It was also observed that the portlandite having a cubical smooth structure identified as a major bonding agent in mix.

Table 9

EDX analysis of combined concrete mixes
Element	Weight (%) on different mixes
Element	SFPA	SCFPA	SCFA	SAFA	SCPA	SAC
C K	23.71	0.00	78.52	19.18		20.57
O K	44.86	48.26	17.56	51.97	51.76	43.16
Na K	1.23	0.00	0.24	0.00	0.00	0.00
Mg K	0.91	4.57	0.00	0.45	0.00	2.56
Al K	3.39	1.43	0.33	3.54	2.34	0.79
Si K	19.59	7.28	0.72	19.44	28.26	3.10
K K	1.44	0.00	0.00	1.54	0.00	0.00
Ca K	1.54	30.90	2.63	1.83	13.42	23.99
Fe K	3.33	2.49	0.00	1.49	3.61	0.00
Cr K	0.00	3.81	0.00	0.00	0.00	0.00
F K	0.00	0.00	0.00	0.00	0.61	1.45
Mn K	0.00	1.26	0.00	0.00	0.00	0.00
S K	0.00	0.00	0.00	0.00	0.00	4.38
Rh L	0.00	0.00	0.00	0.12	0.00	0.00
Pd L	0.00	0.00	0.00	0.44	0.00	0.00

An X-Ray technique named as energy dispersive X-ray (EDS) was used to identify the types and the quantity of elements present in a specific material type. In this technique, a specific sample comes in contact with an electron beam having energy in the range of 10–20 keV and produces X-rays which depend on the characteristics of the material. The acceleration voltage has a significant role in the quantitative analysis of a specific element i.e. it should be 2 to 3 times of the absorption edge of voltage, Wang et al. (2008) [42]. Figure 13 showing the EDS analysis for SAC and SAFA mix type. Table 9 is showing the weight percentage of different elements observed from the different mixes with the help of EDS analysis. Based on the findings of EDS spectra, C, O, Si, Al, Ca and Fe were found dispersed on SFPA, SCFPA, SCFA, SAFA, SCPA and SAC mixes. Among all the mixes, SFPA, SAFA and SCPA were showing higher dispersion of Si in comparison to other mixes. On the other hand, Ca dispersion was found higher in case of SCFPA, SAC and SCFA. It is to note that higher Si percentage led to form a quartz which is responsible for the denser microstructure. Besides this, very low percentage or absence of other elements like Al, Mg, Na etc. was reported among all the mixes.

The optimization of the mix design was performed by replacing the cement up to 50% whereas the river sand upto 70% by steel slag and the coarse aggregate replaced with plastic aggregate upto 50%. Further, the influence of combined effect of cementitious steel slag, steel slag fine aggregate and plastic aggregate was studied under compression, flexure and tensile tests. In addition to that, microstructural analysis was also done using XRD and SEM techniques in order to study the phase identification of a crystalline material present in the modified concrete. Based on the comprehensive experimental investigation, following conclusions were drawn;

The reduction in the density of concrete with 50% plastic aggregate found to be 17% as compared to the normal concrete. It was observed that the density found to be reduced marginally by the plastic aggregate upto 40%, however significant reduction found when the plastic aggregate exceeds 40% of coarse aggregate. The mixes having cementitious slag, fine slag aggregate and plastic aggregate were showing significant decrement in the workability as the mix SCFPA found to be 45% lesser than the NC. It was observed that the maximum density reduction on the concrete mix SCFPA found to be 2.5% than NC, however, the maximum increase in density was observed on the concrete mix SCFA about 0.75% than NC. Therefore, it is concluded that the maximum reduction in density of 2.5% on mix SCFPA among the chosen combination.
Based on the optimization study, it was revealed that the slag up to 30% replacement of cement (SAC) and fine aggregate (SAFA) up to 50% could be used. It was also observed that the plastic aggregate as the replacement of coarse aggregates (i.e. up to 30%) found better and the proposed mixes may be utilized for low traffic volume rural roads as per the guidelines of IRC: SP 62-2014 [38].
It was observed that the strength of SCFPA mix at 7 days found to be 22.1 MPa whereas the same was found to be 10.5 MPa in the mix PACA3. It was observed that the reduction in the early strength (7 days) on mix PACA3 is (10.5 MPa) significantly less as compared to the SCFPA (22.1 MPa) mix. The reason may be due to the cementitious slag and slag fine aggregate that led to the early hydration and reaction with the cement matrix.
In case of SCFPA mix, the strength at 28 days found to be 5.5 MPa whereas the same found to be 5.17 MPa in the mix PACA3 and it shows that the cementitious slag and slag fine aggregate contribute marginally on the resistance under flexural load.
It is concluded that EAFS and plastic aggregate could be utilized as the replacement of cement, fine aggregate and coarse aggregate without compromising the tensile strength of concrete mix. In addition to that, EAFS as the replacement of cement could be found useful in the construction of rural roads as the tensile strength was found to be same to the normal mix which is the minimum requirements of rural roads.
Based on the microstructural analysis, the presence of CSH, ettringite and portlandite were observed. The calcium silicate hydrate was found similar to a natural mineral named as tobermorite (tobermorite gel), an important compound for measuring the strength and rheological properties of cement - concrete. The ettringite (3CaO•Al₂ O₃ •3CaSO₄ •32H₂ O) having a needle-like crystals structure plays an important role in the setting time, strength development and shrinkage of concrete. It was also observed that the portlandite having a cubical smooth structure identified as a major bonding agent in mix.

It is concluded that usage of SAC and PACA can be feasible in the construction of sustainable low traffic volume rural roads as the strength was found to be 30 MPa. SAFA could be used up to 50% in the construction of NHs and SHs as it exhibits a similar type of compressive strength as that of normal concrete. In case of combined approach, further curing (i.e. 90 or 120 days) of SCFA, SFPA, SCFPA may be required to achieve desired strength. SCPA could be used in the construction of rural roads after being subjected for further curing only as the compressive strength was found around 26 MPa.

Author Contribution

Mrs. Kavita Rani has performed the experiment, analyzed the data's and preparation of the manuscript.Dr. K. Senthil arranged the resources, Supervising of the experiment, Editing of the manuscript and Review of the manuscript.

Data Availability

The authors confirm that the data supporting the findings of this study are available within the article.

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WITHDRAWN: Performance of Plastic Aggregate along with Steel Slag in the Concrete through the Mechanical Tests and Microstructural Analysis

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Editorial Note

Abstract

1. Introduction

2. Material and Methodology

2.1 Materials

2.2 Methodology

3. Optimization of the Mixes

3.1 Tests on Fresh Concrete

3.2 Tests on Hardened Concrete

3.2.1 Density of Concrete mixes

3.2.2 Influence of EAFS as Replacement of Cement

3.2.3 Influence of EAFS as Replacement of Fine Aggregate

3.2.4 Influence of Plastic Aggregate as Replacement of Coarse Aggregate

3.3 Discussion on Mix Optimization

4. Mechanical Strength of Concrete Mix with Combined Replacement

4.1 Workability of Concrete

4.2 Density of Concrete

4.3 Compressive Strength

4.4 Flexural Strength

4.5 Split Tensile Strength

5. Microstructure Analysis

5.1 XRD Analysis

5.2 SEM and Energy Dispersive X-ray Spectroscopy (EDS) Analysis

6. Conclusion

Declarations

Author Contribution

Data Availability

References

Additional Declarations

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