1. Introduction
Every year, India creates over three million tonnes of medical waste, which is predicted to grow at an annual rate of 8%. Bio-medical waste is defined as “any solid and/or liquid waste, including its container and any intermediate product, generated during the diagnosis, treatment, or immunisation of humans or animals, or related research activities, or in the manufacturing or testing of biological products in health camps”. Biomedical waste is dangerous for two reasons. The first is infectivity, and the second is toxicity.
Bio Human anatomical waste such as tissues, organs, and body parts, animal wastes generated during research from veterinary clinics, microbiology and biotechnology wastes, waste sharps such as hypodermic needles, syringes, scalpels, and broken glass are all examples of medical waste. discarded pharmaceuticals and cytotoxic drugs, soiled waste such as dressings, bandages, plaster casts, blood-contaminated material, tubes and catheters, and liquid waste from any diseased region Biomedical activities increase waste volume and have a substantial impact because it is mainly fatal. For the treatment and disposal of biological waste, an adequate waste management system is required. To handle hazardous waste, methods such as carbon adsorption, incineration, chemical precipitation, chemical disinfection, biological oxidation, membrane separation, and others have been used, although incineration is the best treatment for biomedical waste [1, 2]. Sabiha-Javied [3] noted that harmful acid gases such as (CO2, SO2, NO2 etc.) are generated during the burning of biomedical wastes combined with leftover ashes (bottom and fly ash). The qualities of biomedical waste are determined by the source of trash collection. For instance, research laboratories, blood testing laboratories, and so on.
Although incineration can reduce waste weight by more than 70%, considerable volumes of combustion leftovers, particularly bottom ash, remain after incineration. According to data from the Government of India’s website, the total amount of BMW produced in the country is 484 TPD (tonnes per day) from 1, 68,869 HCFs. Unfortunately, only 447 TPD are treated, leaving 37 TPD untreated. There are currently 198 CBMWTF in operation, with another 28 under development. CPCB [4] reported that the waste handling capacity of incineration plants is approximately 15.01 lakhs MTA, but waste produced from biomedical activities is 27.30 lakh metric tonnes per year, which is greater than the amount that can be treated at incineration plants.
There are various types of incinerators in use. Rotary kiln, fluidized bed, moving grate, liquid injection, multiple hearth, catalytic combustion, waste-gas flare, and fixed grate / direct-flame are some of the more widely employed. The combustion of collected biological waste in the presence of sufficient oxygen is associated with the incineration process. When garbage is incinerated at temperatures above 850°C, it is transformed into ash and harmful gases. The trash is broken down into CO2 and water. The majority of the ash produced is bottom ash, which is the remnants that remain inside the burner after incineration. Post-burner equipment, such as scrubbers, collects fly ash. When incinerated hospital waste ash is melted at 1,200°C, the ash becomes molten, and the molten ash is transformed to slag by cooling at normal temperature. Metals are not eliminated during incineration and are frequently discharged into the environment with the ash [5–7].
The improper disposal of ash in a landfill may result in groundwater contamination owing to leachate [8–11]. The ash is often sent to various dump sites following incineration. Pathogens are destroyed during the incineration process, which also reduces trash volume and weight by 90% and 75%, respectively. IBMW contains a high concentration of hazardous metals, inorganic and organic compounds. Even if the volume is reduced during the incineration process, harmful metals and inorganic compounds are not destroyed, and these toxic metals and inorganic compounds are frequently discharged into the environment when ash is placed in landfill sites. Pollution of groundwater and the environment can occur as a result of its seeping from the dumping of ashes in landfills without sufficient treatment.
A critical concern is the leaching of contaminants from IBMW into groundwater, which is relevant for considering the impact of biomedical waste on public health and the environment [12]. For example, lead has been detected in IBMW and has been linked to brain damage in children [13]. Lead is considered to be a fatal element because of its ability to produce cerebral impairment. As a viable waste treatment process, the use of cement stabilisation in building products has subsequently become a standard method of decreasing environmental pollution and repurposing leftover materials.
2. Review of Literature
IBMW contains a variety of hazardous heavy metals, including zinc (Zn), silver (Ag), arsenic (As), iron (Fe), lead (Pb), cadmium (Cd), and mercury (Hg). Anastasiadou [14] used energy dispersive spectroscopy (EDS) to examine the composition of IBMW and discovered that the primary constituents of the IBMW were Fe2O3 (4.53%), Al2O3 (5.16%), Na2O (9.13%), CaO (27.77 %), and SiO2 (39.74%), in that order. CaO, Al2O3, and SiO2 were also found in IBMW by Azni and Katayon [15]. The analysis of incineration IBMW found high levels of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) [16–19]. Gibbsite, albite, gehlenite, kaolinite, anhydrite, calcite, halite, and quartz were also discovered in several studies [20–22]. Varied scientists reported various attributes such as bulk density of 3,390.4 kg/m3 [23] specific gravity of 2.50–2.72 [24, 25], and used IBMW in a variety of civil engineering applications.
The mechanical properties of IBMW concrete/mortar at various binder ratios was examined in several studies (cement). The concrete created by adding IBMW was tested at high temperatures (25, 150, 250, 500, 600, and 800°C), and compressive strength improved up to concrete containing 5% IBMW [26]. Maximum strength has been measured at 200°C, and the reason given is dissipation of free moisture content from pores. IBMW was used by Genazzini et al. [27] to study mortars with a cement substitution of up to 50%. The findings of compression, flexural strength, setting time, water absorption, density, temperature development, and leaching were analyzed. The results suggest that the supplemental cementitious materials system can be employed to make use of this ash. Municipal incineration ash can be utilised as a fine aggregate and cement in mortars, according to Al-Rawas et al. [28]. After 28 days of curing, the compressive strength achieved was 36.4 N/mm2 when 20 % ash was utilised as sand replacement in the research work. At 14 and 28 days of curing, studies showed that up to 20% cement replacement level with ash provided better compressive strength than standard concrete, with a compressive strength of 27.4 N/mm2 observed at 28 days of curing.
Anastasiadou [14] examined four different ratios of cement with fly ash and cement with bottom ash mixtures (60:40, 50:50, 40:60, and 30:70 w/w), whilst Filipponi et al. [24] reported that IBMW to total solid substitutions were varied over the range 10–80 percent (10, 20, 30, 50, and 80 percent) with selected water/-solid (includes IBMW and cement) ratios of 0. At different curing times, the solidified matrix’s compressive strength was determined. If more than 50% IBMW is used, the results show that the matrix has a modest pozzolanic property after 28 days. Aubert et al. [29] investigated the effects of IBMW on the durability and compressive strength of hardened concrete and proposed that IBMW be used as an SCM in concrete. IBMW-optimized cement/sand replacement in mortar/concrete enhanced flexural, tensile, and compressive strength, as well as elastic modulus, while enhancing toughness and bond strength [14, 23, 24, 27, 28].
The mechanical qualities of IBMW concrete have been substantially investigated with cement replacement. However, research on the use of IBMW as a partial substitute for fine aggregates, as well as microstructure of IBMW concrete, is lacking in the published literature. Because heavy metals and deadly elements are present, IBMW is considered toxic, causing substantial harm to living beings even at low concentrations. In the literature, cement-based stabilisation was employed to bind the components of a hazardous waste. Cement-based stabilisation is a chemical treatment procedure that aims to stabilise insoluble forms of entanglement hazardous components in a solid matrix [30].
Lombardi et al. [31] investigated the strength qualities of cement-stabilized biomedical ash and the leachability of heavy metals, finding that cement binding with the IBMW reduced the amount of heavy metal immobilisation in the IBMW. Using this technique, it was able to dispose of it in landfills without further treatment. Toxicology must be evaluated for IBMW, and all necessary steps must be made to restrict the leakage of dangerous components into the environment [32]. Anastasiadou [14] investigated the IBMW-cement cemented stability matrix developed by utilizing IBMW to reduce leaching from it. The cement-stabilization method was shown to be the most appropriate for pollutant sequestration while also being the most cost-effective [33]. Akyildiz et al. [23] conducted TCLP leaching tests on IBMW and IBMW-cement matrix. According to the test, coarse material smaller than 9.5 mm in size was removed and dipped in NaOH and CH3-COOH solutions at 4.93 0.05 pH for 18 h. Heavy metal migration in IBMW concrete is decreased by adopting a cement-based stabilising procedure.
3. Experimental Procedure
3.1. Materials
The materials used in the production of concrete are cement, fine and coarse aggregates ater and IBMW as partial replacement of cement. In this section, the details of these ingredients and the casting procedure are discussed.
3.1.1. Cement
3.1.2. Aggregates
Locally available basalt, confirming to IS requirements: 2386-1963 (Revision 2016) [35] and 20 mm down particle size was used. These were thoroughly washed with tap water and dried in the air for 24 h. This ensured the elimination of deleterious materials such as silt, dust and, unsound particles detrimental to concrete. The properties of coarse aggregates are mentioned in Table 3. All the tests were done confirming IS 2386-1963(Revision 2016) [35]. It is seen that the aggregates have excellent impact (10.48%) and crushing value (20.62%) and very low water absorption of 0.45%. River sand confirming to IS: 383-2016 [36] was used in this work, with particle size distribution as shown in Fig. 1. The fineness modulus was found to be 2.48 while specific gravity was found to be 2.62. From the particle size distribution.
3.1.4. IBMW
IBMW is a greyish colored ash, obtained by incinerating the biomedical waste at 900°C in the combustion chamber of a biomedical incinerator present in Gwalior, Madhya Pradesh. The fineness modulus according to IS: 2386 Part III-1963 [39] was found to be as 3.14. The other relevant physio-chemical properties of IBMW are presented in Table 4. From the Table it is seen that the IBMW has water absorption of 3.01%, showing that when mixed with ingredients of concrete it will absorb water and thus make the cement paste sticky. The bulk density of IBMW is less than 1 g/cc (i.e. 0.68 g/cc).Thus the resulting concrete is expected to be lighter in density compared to the control mix.
4. Testing Methods
4.1. Compressive Strength
150 mm cubes were made for the five different replacements and tested at 7, 28, and 56 days of curing in accordance with BIS 516–1959 [40] (compressive strength test). The incremental force of 4.5 kN/s was applied consistently until the cube disintegrated and no larger load could be sustained during compression testing. After 7, 28, 56 and 90 days, the broken samples from each replacement were subjected to SEM and EDS analyses.
4.2. Split Tensile Test
BIS:5816–1999 [41] was used to conduct the split tensile test, which consisted of preparing cylinders of 150 mm diameter and 300 mm height and testing them at 7, 28, and 56 days, according to BIS:5816–1999 [41]. Testing was carried out on the specimens using a universal testing machine, capable of supporting a force of 2,000 kN. The gradual load was imposed without the use of a shock and was gradually increased at a rate of 1.2 to 2.4 N/(mm2/min) over a period of time.
4.3. Flexural Strength
The beams were evaluated at 28 days using a flexural testing machine under four point loading till failure. A constant loading setup with a 300 mm shear span and a 3.0 shear span depth ratio was used. When the specimens were moulded, they were put on the supporting bearing blocks with one side in relation to the other. At one quarter distance from the end of the supports, the upper surface of the test specimen was brought into contact with the load-applying block. The load applying block is then brought into complete contact with the beam surface as a result of this operation. The beam was examined to confirm that it was in uniform contact with the bearing and load-bearing blocks. The specimen was loaded constantly until it failed and the dial stopped moving. The testing machine indicated the maximum applied load, which was recorded. The following equation is used to compute the flexural strength:
where
4.4. Sorptivity
Three 30 mm slices were taken from three concrete cubes measuring 100 mm × 100 mm × 70 mm to conduct the sorptivity test. The age of these cubes was 90-d. The specimens were dried in an oven at 55°C for three days and then chilled in desiccators. Epoxy resin was covered all along the sides to prevent water sorption from the sides and only allow sorption from the bottom. The specimens were immersed in tap water in pans that had a water level of 5 mm above the pan’s bottom. The experimental set-up is depicted schematically in Fig. 2. After draining the surplus water with an absorbent towel, the mass of these specimens was properly measured at regular intervals. The slope of a line fitted to the plot of cumulative absorbed volume of water per unit area of inflow surface vs square root of time was obtained using data on absorbed volume of water. The sorptivity co-efficient is calculated as follows:
Here,
4.5. Casting and Curing of Concrete
Table S1 represents the nomenclature of the concrete mix and the mix proportion according to IS 10262-2019 [42]. Concrete mixing was done using a Pan mixer (AIMIL-India made the capacity of 100 L). All the aggregates used in concrete mixture were in the saturated surface dry condition. All the mixes were prepared by replacing fine aggregates in various percentages of replacements: 0, 2.5, 5.0, 7.5, and 10.0%. For each composition and test, 3 members were casted and cured for 7, 28, 56 and 90 days, respectively.
The cube, beam, and cylinder specimens were cast to test the workability, strength, and durability of various prepared concrete mixes. All moulds were compacted on the table vibrator and demoulded after 24 h of casting. The test specimens were put underwater at 27 ± 2°C till the testing age for curing. Table S2 gave descriptions of all the experiments carried out in terms of the age of specimen while testing, specimen dimensions, the apparatus used for testing and the standard code referred to, during the testing.
The flowchart for the entire process of production to testing is presented in Fig. 3.
4.6. Toxicity Characteristic Leaching Procedure
The IBMW was subjected to a toxicological characteristic leaching technique (TCLP) in order to study the leaching of harmful heavy metals from it. In this experiment, two different extraction solutions (solution #1, HOAc, pH 4.900.05; solution #2, HOAc, pH 2.880.05) were employed. The liquid to solid ratio was 20:1 in a rotary tumbler, which was employed for the experiment. The sample was spun at a rate of 30 revolutions per minute. After agitating the sample for 18 hours, the leachates were filtered through whatman filter paper with a 0.45 m pore size. The leachates were acidified with 1M HNO3 to remove the toxins. With the help of the Ultima 2 Inductively Coupled Plasma-optical Emission Spectrometer, the elemental characterisation of IBMW was completed (ICP-OES).
4.7. Characterization Using SEM and EDS
The microstructure and morphology of IBMW concrete has been studied using an energy dispersive spectrometer and scanning electron microscopy. Strength tests were performed 28 days later, and small broken fragments were removed from the core of the tested sample. The samples were placed in a desiccator and dried overnight at 80°C to remove moisture before SEM examination. Broken pieces from tested concrete specimens were mounted on brass stabs using carbon ribbons, gold coated, and studied for microstructure using Hitachi S-3400N. The analysis of samples was carried out with a 2 μm probe diameter, 15 kV accelerating voltage and 50 nA probe current. The error of the SEM measurements is estimated to be about ±2 at.%.
5. Results and Discussions
5.1. Slump Test
The fresh properties obtained from all replacements (both with and without IBMW) are depicted in Fig. 4. All of the concrete mixes were made with the same amount of water applied at the same time. Addition of IBMW to concrete resulted in stiff, yet cohesive and sticky fresh concrete. The workability of the mixes thus reduced in direct proportion to the amount of sand replaced by IBMW. Lowest slump value was found in IBMW12.5, nearly 36 mm. Because IBMW has high water absorption rate and low water content, it has a low lubricating effect and consequently has a low workability. The permeable particles of IBMW ash absorbed more water internally during the mixing than the natural fine aggregate employed in the mix, which was a result of the mixing procedure. Similarly, in another study on bottom ash, Jaturapitakkuland Cheerarot [43] demonstrated that the presence of larger pores in the particle results in a greater water absorption capacity. The rough texture and irregular shape of the material, which enhances the interlocking and hardness of the material and hence diminishes the ball bearing effect. All of these factors result in reduced slump and increased water demand of concrete containing IBMW. When the results of the slump test were compared to those of earlier research, it was found that there was a high degree of agreement that increasing IBMW led in a decrease in slump value [26, 32].
5.2. Mechanical Properties
5.2.1. Compressive strength
Compression strength results are provided as an average of three specimens at 7, 28, 56 and 90 d for each mix, as illustrated in Fig. S1. The compressive strength of IBMW0 was determined at the age of 7 d at 22.32 MPa. The compression strength of IBMW2.5, IBMW5.0 and IBMW7.5 showed a nominal increase in strength of 2.15, 5.15 and 10.67% compared to the control mix (i.e. IBMW0), the strength of IBMW10 and IBMW12.5 was found to be 6.67% and −3.34%, respectively. Thus, the strength of IBMW12.5 was found to be lesser than the control mix at 7 d. Maximum increase in compressive strength was found at 7.5% replacement level. Similar trend was observed in 28, 56 and 90 days and thus the maximum compressive strength was reported at 7.5% replacement (Fig. 6). For example, at 7.5% replacement level on 28th day, 40.3% increase in compressive strength was obaserved and on 56th and 90th day this value was 51.61% and 56.14% respectively. These results are consistent with that of Akyildiz et.al. [23] and Lombardi et al. [31]. Some of the possible explanations for the improvement in strength up to 7.5% of IBMW could be as follows:
Effect of pozzolanic activity
Calcium and silica occurred in IBMW and increased compressive strength due to pozzolanic activity between IBMW and water. Both the hydration of Portland cement and the pozzolanic reaction from the IBMW to the calcium hydroxide part of Portland cement is attributed to the compressive quality of these blends. To assess the reactivity of IBMW as a pozzolanic material for use as potential substitute of cement, lime reactivity test was performed in accordance to IS: 1727–1967 [47]. Six cubes of size 50 mm × 50 mm × 50 mm were cast by mixing the ingredients in an electrically driven epicyclic mixer. Dry materials for the test were calculated as per the standard norms as 100 N: 400:1500 (IBMW: cement: sand), where N is the ratio of specific gravity of IBMW to that of cement. The IBMW used for mixing was washed and dried in order to remove contaminants. The quantity of mixing water was ascertained by flow table test and found to be 220 mL. After 48 h of casting, the cubes were de-moulded and carefully placed in humidizing chamber at 27°C and a relative humidity of 90 for 8 d. The cubes were tested in compression testing machine by applying a uniform compressive force at the rate of 35 Kg/cm2/min. The average compressive strength for all cubes was found to be 3.09 MPa. But, for a material to be categorized as pozzolanic, the average compressive strength value must be greater than 3 MPa. This implies that IBMW used in the current work shows marginal pozzolanic activity and hence would be more effective as a substitute for cement and not as a constituent for partial replacement of fine aggregate. This is also supported by the results of EDS spot analysis which shows maximum amount of Calcium in IBMW7.5. This is consistent with the results of Tzanakos et al. [48], Kaur et al [49] and studies carried out by various other researchers [50, 51].
1) Effect of microfilling
The maximum particle size of IBMW was found to be less than 1 mm and more than 95% particles were passing 600 micron sieve. Incorporation of these particles could have resulted in the micro-filling effect by filling the intergranular voids and thus enhancing the compactness of concrete.
2) Effect of chemical reaction
The effect of chemical reaction can be possible in two ways. At the surface level, IBMW particles can act as nucleation sites and enhance hydration, thereby becoming an intergral part of cement paste and contributing to a rich matrix. At the chemical level, the IBMW particles could interact with the ingredients of cement such as Ca(OH)2, forming CSH gels.
3) The two-wall effect
Kronolov initially described this phenomenon, which occurs when the amount of water/solution necessary to fill the space between the finer and coarser particles is more than the amount of water/solution contained in the interior of the cement paste. Concentration of anhydrous clinker grains decreases around bigger aggregates. The larger aggregates form a barrier, increasing the concentration of smaller clinker grains near the aggregates. The less dense packing results in a greater water/cement ratio and consequently a more porous paste, reducing the strength in certain places and hence affecting the overall strength of the concrete. By adding ultrafine filler to concrete, the packing will be improved by filling the fine space between the cement particles and the aggregate wall. Additionally, the tiny particles aid in the diffusion of the hydration product.
5.2.2. Split tensile strength
Split tensile strength was measured using IBMW replacement in various percentages as shown in Fig. S2. It was observed that the split tensile strength of IBMW2.5, IBMW5.0 and IBMW7.5 concrete mixes increased compared to the control mix on all days. Compared to the 7th day, the 28th day split tensile strength showed significant improvement. It was found that incorporation of IBMW into concrete increased the split tensile strength when 7.5 percent of the sand was replaced with IBMW, as illustrated in Fig. S3. The concrete strength obtained in IBMW10 and IBMW12.5 was lesser than that obtained in control mix.
The properties of IBMW also have an impact on the nature of the resulting cement paste and the interfacial transition zone, both of which have an impact on the tensile strength. It was found that the split tensile strength of IBMW0 was 2.2 MPa at the age of 7 d, while the split tensile strength of other mixes IBMW2.5, IBMW5.0, IBMW7.5, IBMW10 and IBMW12.5 were 2.3, 2.5, 2.8, 2.69 and 2.48 MPa at the age of 7 d. Augustine et al. [46] also reported similar trends.
5.2.3. Flexural Strength
The results of 7, 28, 56 and 90 days for flexural strength of beam are shown in Fig. S3. The strength characteristics follow similar trend as that of split tensile strength and show relatively low strength at 7 d and a significant improvement is seen in 28 days. Since the hydration process continues, a marginal increase in strength is observed at 56 and 90 d also. Similar the case of compressive and split tensile strength, the values of flexural strength also achieve a maximum value at 7.5% replacement.
5.2.4. Sorptivity
At 28 days after curing, sorptivity test was used to determine capillary suction of all samples. Sorptivity is defined as the slope of the straight line that shows the relationship between absorption and the square root of time. The absorption is plotted as a function of time and the square root of the absorption coefficient for the IBMW concrete mixes in Table S2. Concrete mixes containing IBMW0 has the lowest sorptivity, followed by IBMW2.5 and IBMW5.0, IBMW7.5, IBMW10 and IBMW12.5, respectively. The variation in sorptivity levels can be linked to the pores’ properties. The low sorptivity was caused by the smaller particle size of IBMW, and increasing production of calcium silicate hydrate (CSH) gel results in a reduction in the size of the pores, and hence the sorptivity. Increased sorptivity was seen in samples containing higher percentages of IBMW due to the presence of calcium and silica, which caused the concrete to expand. As a result, pores became more connected, resulting in increased absorption. These findings are consistent with those obtained for water absorption. Similar results were also reported by Kaur et al. [49] and Singh et.al. [50]
5.2.5. Leachate Analysis
Leaching is the term used in environmental applications to describe the release of possibly lethal chemicals. When assessing the impact of solid wastes on human health and the environment, one of the most important processes to consider is the leaching of contaminants into groundwater. Leachate is a liquid that flows from a landfill after garbage has been leached. Its composition varies greatly depending on the age of the landfill and the waste it contains.
Landfill sites that are correctly built and engineered can reduce the risk of leachate generation. Aubert et al. [29] investigated the leachate of concrete-containing municipal solid waste incinerator fly ash and found that the technique of utilizing the fly ash allows for the production of products without posing significant concerns to the environment. Furthermore, these findings as a whole show that using garbage in concrete could be a way to increase value. Lombardi et al. [31] investigated the leaching behavior and mechanical qualities of cement solidified hospital solid waste incinerator fly ash. An acetic acid standard leaching test and a dynamic leaching test were used to conduct leaching tests on both fly ash and solidified/stabilized goods.
Sukandar et al. [50] used sequential extraction and toxicity characteristics leaching procedure (TCLP) study to investigate metal leachability from medical waste incinerator fly ash in each particle size category. According to the findings, Ba, Cd, Ni, Pb, and Zn in medical waste incinerator fly ash had a high mobility. With the exception of a range of particle sizes 150–106 mm, they tended to bind to carbonate and exchangeable fractions. In particle size fractions of 150e106 mm, arsenic (As), cadmium (Cd), chromium (Cr), nickel (Ni), lead (Pb), sulfide (Sn), and zinc (Zn) tended to attach to the Fe-Mn oxide matrix. Sequential extraction also revealed that in the larger particle size sample, both exchangeable and carbonate related Cr concentrations were higher, but Ba was discovered in the smaller particle fraction. Cd, Cr, Cu, Hg, Ni, Sn, and Zn. The particle sizes were not significantly different when using the TCLP approach. Arsenic leachability was found to be higher in the 38 mm particle size fraction than in the other particle size fractions. In the particle size fractions of 150–106 mm and 75–38 mm, respectively, Ba and Pb had the highest leachability. Lyckova et al. [33] conducted a Toxicity Characteristics Leaching Procedure (TCLP) test of medical waste incinerator bottom ash, which revealed that the leached levels of heavy metals were considerably below the limits. The total quantities of carcinogenic PAHs ranged from 4.09 to 16.95 mg kg 1, surpassing the legal limits, while the sum of 16 US EPA priority PAHs (SPAHs) ranged from 10.30 to 38.14 mg kg 1. Thike et al [51] and Tan et al. [52] investigated the effects of leaching period, pH, and particle size on heavy metal leaching properties, as well as the impact of melting on the stability of medical waste fly ash. These findings revealed that as the leaching duration increased, heavy metal leaching concentrations and lixiviate toxicity of heavy metals increased in fly ash. The leaching concentrations of heavy metals were lowest at pH 7, but the leaching concentrations of heavy metals were lower when the grain size was larger or smaller, but greater when the grain size was between 250 and 900 mm. The lixiviate toxicity of heavy metals was considerably reduced when the fly ash melted at a high temperature, which explains why melting has such a good effect on heavy metal stability.
The TCLP values for IBMW are presented in Table S3. The corresponding U.S. Environmental Protection Agency (US EPA) series referred for the tests are also mentioned in Table 9. The leached values of heavy metals was compared to the limits provided by USEPA guidelines and was found to be lower than the permissible limits set by the guidelines. Thus, from the TCLP test, IBMW can safely be considered as non-toxic, with negligible risks of reuse. Similar results were also reported by Woolley et al. [32].
5.2.6. Microstructure analysis
Scanning electron microscopy facilitates the characterization of the microstructure of concrete and aids in determining the components that influence its mechanical qualities and durability [48, 49]. CSH gel, calcium hydroxide, calcium sulfoaluminate hydrate (ettringite and monosulfate), coarse and fine aggregate, and an interfacial transition zone (ITZ) between aggregate and cement hydration products make up the concrete microstructure. Calcium, silica, and alumina content in IBMW were found in percentages of 35.24, 29.3, and 12.87 in EDS spot analysis findings, respectively. Fig. S4(a) and (b) reveal the microstructure of concrete IBMW7.5. This composition was selected for microstructural analysis due to its optimal values of compressive strength, split tensile strength and flexural strength.
The microstructure of the specimens changed after 28 days of curing in all of the replacements. Table S4 shows EDS spot analysis of concrete containing IBMW. It reveals the presence of Alumina, Silica, Iron oxide and Calcium oxide in significant amount. The main hydration product CSH gel, which was responsible for improved mechanical properties, is present in significant quantity. Hence a sharp rise in value of strength parameters was observed at 28 days. A pozzolanic reaction and the production of a CSH gel were observed after 28 days of curing.
Fig. S4 shows the microstructure of NAC, IBMW2.5, IBMW7.5 and IBMW12.5 at 50 μm magnification. Fig. S4(a) shows the microstructure of the control mix. Micropores upto 5 μm are seen scattered along the matrix. Thus, the control mix showed presence of pores and voids in it. IBMW2.5 (Fig. S4(b)) and IBMW5 (Fig. S4(c)) show a progressively improving microstructure with smaller voids and less in number. The ITZ is intact as seen in IBMW5 (Fig. S4(c)). In case of, IBMW7.5 (Fig. S4 d), EDS spot analysis revealed dense microstructure and minimal voids. This is consistent with the findings of compressive strength and further establishes the two wall effect. The aggregates of IBMW7.5 are well covered with the matrix and exhibit an excellent bond with the matrix. Micro-cracks and micropores are nearly absent in this sample. The presence of IBMW thus reduces the voids in the concrete. As the percentage of IBMW increases to 10 and 12.5, the strength parameters are found to decrease while water absorption and sorptivity increase. This is because of the following reasons:
Formation of cracks and due to expansive nature of IBMW reduces the strength parameters.
Due to porous and soft nature of IBMW, larger amount of water is absorbed, which increases the water absorption and sorptivity of concrete.
6. Conclusions
The current work discusses the mechanical properties of concrete containing IBMW and durability aspects such as water absorption and sorptivity. Studies on leaching characteristics are also done in this work. Based on this, the following conclusions can be drawn:
IBMW shows marginal pozzolanic activity through lime reactivity test.
The addition of IBMW to concrete reduces the slump value, and increases the water absorption and sorptivity. This is due to the expansive nature of IBMW and porosity. Hence, super-plasticizer is essential to improve workability.
The experimental study results suggest that including 7.5% IBMW as a partial cement replacement improves the compressive strength by 20%; improves the split tensile strength by 17% and flexural strength by 14% compared to control mix in 28 days
The possible reasons for increase in compressive strength such as pozzolanic action, densification, presence of Calcium and Silica in the ash, two wall effect and improved packing by micro-filling of IBMW in the concrete matrix are discussed.
All metal values are within the TCLP test’s allowed limits, as suggested by the US EPA.
The addition of IBMW upto 7.5% was found to decrease the voids and provide a dense matrix. Thus, it showed an improved microstructure.
The use of IBMW as a partial replacement for sand in concrete solves the problem of limited area and high land disposal costs, resulting in a pollution-free environment.
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