Sustainable use of magnesite mine waste in self-compacting concrete and its study on strength, microstructure, cost and CO2 emission

Managing waste materials from mining is of universal interest owing to its massive volume, ecological impacts, health hazards, and disposal challenges despite high operational costs. Advancements advocate for recycling mine waste to sustainably support construction. As the construction sector heavily consumes resources, utilizing mine waste from magnesite mines (MMW) in concrete has gained attention. This experimental study assesses the viability of substituting MMW for natural fine and coarse aggregates in self-compacting concrete (SCC) at intervals of 10% up to 50% by weight. Evaluations were done on fresh (slump flow, T50 slump, V-funnel, J-ring, L-box) and hardened (compressive, splitting tensile, and flexural strength) properties, along with microstructural features, cost, and CO2 emissions. The findings unveil that nearly all mixtures exhibit commendable performance, where mine waste is replaced for fine and coarse aggregates showcasing superior fresh and hardened properties, respectively. Fresh property results reveal the SF1 flow category with VF1 and VF2 viscosity types for the SCC mixtures. Moreover, these SCC mixtures observed substantial strength enhancements of approximately 10% to 15% in compressive, splitting tensile and flexural test results at 28 and 90 days. Microstructural analysis corroborates the observed strength outcomes, indicating a denser concrete matrix. Significant environmental and economic benefits were observed, including a notable 20% reduction in CO2 emissions and 17% cost savings. These findings underscore the potential of integrating MMW into SCC mixtures as a sustainable approach towards construction materials, offering both performance and environmental advantages.

1. Introduction 1.1.Background Today's modern world runs at a pace where time is the most insufficient resource, reflected in the construction industry, as the rapidly increasing population requires infrastructural development at a similar rate [1].Every country in the world desires to strengthen its infrastructure to balance the demands of the increasing population and enhance the quality of living.Regardless of the country's economic status, the construction industry has been a significant area of concentration for infrastructural development [2].This development requires producing massive concrete quantities at high-quality standards with improved strength and durability [3].Every year, the globe consumes over 30 billion tonnes of concrete, which is increasing faster [4].In addition, many nations are experiencing rapid growth in the construction sector, which uses natural resources to develop infrastructure.Natural aggregate use for concrete manufacturing is currently over 48.3 billion tonnes per year, leading to an alarmingly rising land loss through the extraction of natural resources [5,6].Reducing the exploitation of these natural resources and reusing suitable non-renewable resources from industrial wastes can create sustainable materials for the construction industry as the world's annual sand and gravel consumption is increasing [7,8].Also, many countries globally, including India, have imposed massive restrictions and banned mines/quarries as natural resources are being depleted at a higher rate than expected [9].
Mass concreting with traditional practices (conventional concrete) engages high cement usage, higher workforce requirements, and a lack of adequate compaction that reduces the performance of concrete structures.Higher compaction produces tremendous vibrations, resulting in noise pollution, severely impacting the environment and leading to workers' health issues [10].The risk of over-vibration is also a key factor, as it reduces the strength and durability of concrete.Environmental impacts include noise pollution, higher energy consumption and increased cost.Vibrated concrete cannot be preferred for structural elements that exhibit congested reinforcements and height restrictions (pumping issues) [11].A severe health disorder called Hand-Arm Vibration Syndrome (HAVS) is caused by overexposure to vibrations during the casting of concrete structures [12].To overcome these problems, Professor Okamura from Japan developed the concept of SCC during the late 1980 s, which flows freely and gets compacted by its self-weight [13].SCC can resist segregation and deformation, which maintains the homogeneity of the concrete mix [14,15].It also exhibits the properties of fluidity (flowability), passing and filling ability without segregation or bleeding, which are the main characteristics of workability [16,17].Noise pollution and vibration issues can also be addressed using SCC, improving socio-environmental wealth [18].SCC enables concrete pumping to considerable heights and through tightly packed reinforcing bars without requiring further compaction methods beyond its self-weight.So, using SCC can decrease construction time, workforce costs, and noise levels on the job site [19].SCC also contributes to environmental sustainability and cost-effectiveness since major industrial by-products like fly ash, silica fume, etc, considered unwanted materials, were consumed as fillers or supplementary cementitious materials (SCM) in concrete [20].According to a 2021 survey, India produces fly ash of about 232 million metric tons annually, mainly used in cement production [21].Additionally, fly ash has been utilized as SCM in concrete because of its pozzolanic nature, especially in SCC, due to its ability to enhance workability [22].
Sustainability in construction can be explained as the ability to create infrastructural development for the present generation's requirements without sacrificing the future generations' needs.Today, mining is believed to be crucial to the socioeconomic advancement of nations with mineral resources as it contributes to the Indian GDP (Gross Domestic Product) of around 2.5% [23].In one way, mining significantly harms the environment, while in the other way, it creates jobs and supplies the raw materials needed for economic growth [24].As a result of mining, some financial and infrastructural Sustainable Development Goals (SDGs-1, 2, 8 and 9) are improved while other social and environmental SDGs are negatively impacted (SDGs-10, 13 and 15) [25].Mining, mineral processing and metallurgical extraction are the three fundamental mineral/metal mining steps.However, this sector faces criticism worldwide due to producing significant solid waste, which can approach 25 billion tonnes annually [26].Different kinds of waste have been produced by these processes in various amounts depending on the type of mine [2].Wastes generated at different stages are titled waste rocks (mining waste), tailings (mineral processing waste) and slag (metallurgical waste).Different wastes obtained from mining are pictorially represented in figure 1, inspired by the work of Lottermoser (2010) [27].The phrase 'mine waste' denotes that the substance is an undesired by-product of mining and has no current economic use.Most commonly, these wastes are dumped into lands or utilized as landfills.According to the UN Sustainable Development Goals, landfills are at the bottom of the waste hierarchy [28].Dumping mine waste in nearby lands creates serious environmental hazards like land and water pollution, making the surrounding lands unfit for agriculture [29,30].Mine waste stocking also risks human health because of its environmental impact [31].Numerous tonnes of waste from industries are dumped in landfills, which is to blame for the degradation of natural soil strata and groundwater contamination [32,33].At the same time, some researchers suggest reusing them in the production of bricks, concrete blocks, ceramic tiles, and aggregates in concrete [31,[34][35][36][37]. Several studies were conducted to convert the waste from mining sites into alternatives for construction materials.Bricks made of waste from gold and iron mines combined with cement and other SCMs showed satisfactory strength results and can be used for construction purposes [38].Mining wastes were also introduced in concrete as a fine aggregate replacement, which proved effective and efficient.Waste from Iron ore mines was used as an alternative for sand in standard and high-performance concretes, which showed good workability and mechanical properties [34,39,40].High volumes of fly ash for cement showed satisfactory strength results and were recommended as a filling material [41].
This study considers Magnesite mine waste (MMW) one of the construction industry's least-explored alternative materials.Magnesite, used mainly in making refractory materials, is produced worldwide with about 26 billion metric tonnes through mining.India currently ranks 10th, producing 150000 metric tonnes, while China tops with 18 million metric tonnes [42].The state of Tamil Nadu is the largest producer of Magnesite in India.It produces two-thirds of the total Magnesite manufactured in India, of which 47% is produced in the Salem district of Tamil Nadu state.As per the report of Indian Minerals Yearbook 2018-Part 3: Mineral Reviews, only 7% of the mined ores are converted into mineral extracts, while the rest are considered waste [43].It was observed that most of the waste produced in the mining process is just openly landfilled.These Magnesite-producing mines generate huge amounts of waste dumped along the roadside and become difficult to dispose.It also affects the aesthetic beauty of the surrounding regions [44].Storage of these wastes typically requires a huge area, which can result in expensive expenditures and environmental and ecological effects [45].Researchers reported that soil around the mine waste dumping area becomes unsuitable for plant growth due to nutrient deficiency, which hinders the agricultural sector [44].Recent studies reported that tailings from magnesite mines (MMT) can be used as a subgrade in road construction in combination/treatment with cement [46].Sibanda et al (2013) evaluated the physical characteristics of MMW and reported that it exhibited the properties of well-graded sand, which can be used for specific engineering applications like road construction and foundation filling [47].Shanmugasundaram and Shanmugam (2021) reported that MMT consists of stones and sand-like particles suitable as building materials.Additionally, the alkalinity, pH and specific gravity suit MMT's application in reinforced concrete (RC) and embankment-filling material [30].
Concerns over the environment and sustainable development are spreading around the world.The new 'sustainable construction' method emphasizes the importance of minimizing the ecological influence of buildings and infrastructure.At the same time, traditional design and assessment tactics are centred on maximizing economic efficiency and include quality, cost, and time [48,49].One such thing is CO 2 emission, which greatly impacts environmental sustainability.The cement and concrete industry is responsible for about 7% of the total carbon dioxide emissions recorded around the globe [50].We all know that aggregates engage in approximately two-thirds of the total concrete volume, but interestingly, they also account for about 20% of CO 2 emissions [5].Another way to look at sustainability is to say that since building costs have always been a big concern in the business, engineers should aim to design and complete projects as cheaply as possible, as long as the structural integrity of the construction is maintained [51].These parameters are included as evaluation factors in accomplishing the Sustainable Development Goal (17 SDGs), targeted for 2030 and initiated by the UN in 2015.SDG 9 encourages built resilient infrastructure and sustainable industrialization, while SDG 12 aims at sustainable consumption and production patterns connecting to the 3 R principle (reduce, reuse, recycle) [25].Hence, studies on proposing alternate materials for the construction industry, especially waste, are becoming a prime research area under SDG 12.

Research significance
The use of MMW in SCC represents a significant and novel advancement in sustainable construction materials research.While previous studies have extensively investigated the incorporation of industrial by-products into concrete, the specific application of mining waste, particularly from magnesite mines, remains largely unexplored.Though previous research suggests using magnesite mine tailings as foundation/embankment filling and coarse aggregate in road construction, the potential of MMW as fine and coarse aggregates in SCC remains unexplored.Hence, the current study addresses this gap by comprehensively analyzing SCC mixtures containing varying levels of MMW replacements for fine and coarse aggregates.Through rigorous experimentation and analysis of fresh and hardened properties, microstructural characteristics, cost implications, and CO 2 emissions, this research elucidates the feasibility and effectiveness of utilizing MMW in SCC.Moreover, the exploration of MMW offers innovative solutions to environmental and economic challenges in the mining industry, aligning with circular economy principles and sustainable development.By repurposing mining waste as a valuable resource in concrete production, this study contributes to resource conservation and waste reduction efforts in the construction sector while advancing knowledge on eco-friendly materials.In summary, the investigation of MMW as aggregates in SCC fills a critical research gap and underscores the importance of exploring unconventional sources for sustainable construction practices.

Materials and method
2.1.Materials Ordinary Portland cement (OPC) of 53 grade with specific gravity 3.15 meeting the requirements of [52] and fly ash with specific gravity 2.1 conforming to [53] were used as binders.The cement showed a specific surface area of 419.75 kg/m 2 and initial and final setting times of 75 and 420 min, respectively.This study used fly ash from a thermal power plant in Ennore, Tamil Nadu, India.Based on the chemical composition shown in table 1 and IS 3812 Part 1 (2013) guidelines, the SiO 2 , Al 2 O 3 and Fe 2 O 3 compositions propose that the Class F fly ash was procured [53].Fly ash was preferred in the place of viscosity modifying agent (VMA) as it involves additional chemicals in the concrete and increases the cost of production, which can be prevented [54].The use of fly ash or SCM instead of VMA is referred to as powder-type SCC, which exhibits a self-compacting nature through increased powder content [55].Fly ash also increases the concrete's strength and reliability by enhancing the C-S-H gel formation [18].
As fine and coarse aggregates, locally available M-sand and 12.5 mm crushed stones with specific gravities of 2.56 and 2.71 were used.Fine and coarse aggregates exhibited a water absorption of 2.56% and 0.75% with bulk densities of 1697 and 1400 kg per cubic metre, respectively, while m-sand was considered medium sand (fineness modulus 2.83).The fine aggregate exhibited a powder content of 10% (particle size < 0.125 mm) through sieve analysis, which accounted for the SCC mix design.Waste rocks from Magnesite mines in Salem, Tamil Nadu, India, were used to replace the natural aggregates in SCC.Mine waste was pulverized into smaller sizes and was replaced for fine (size less than 4.75 mm) and coarse aggregates (greater than 4.75 mm and up to 12.5 mm), as shown in figure 2. MMW powder and stones exhibited 1650 and 1450 kg bulk densities per cubic metre, respectively, with specific gravities of 2.70 and 2.72, respectively.They also had a water absorption rate of 6% and a fineness modulus 2.36 (fine sand).The microstructure of MMW under the scanning electron microscope, shown in figure 3, reveals the shape of MMW to be irregular with rough surface texture.The chemical composition of cement, fly ash, MMW and m-sand utilized are shown in table 1 and portrays that magnesite mine waste majorly contains oxides of magnesium and silicon with little contributions from oxides of calcium, iron and aluminium.From the x-ray diffraction results, it can be observed that Magnesite (MgCO 3 ) and quartz (SiO 2 ) were the predominant phases present in the MMW, while lime (CaO) and calcite (CaCO 3 ) being the rest, which is shown in figure 3. Polycarboxylic ether (PCE) based superplasticizer as per IS 9103 (1999-Reaffirmed 2018) was the chemical admixture utilized in concrete with different dosages on the mass of cement as per the workability requirements of SCC mixtures [56].The superplasticizer exhibited a specific gravity of 1.08, and the optimum dosage was identified through the marsh cone test.Concrete was mixed and cured using potable tap water.

SCC mix design and mixing procedure
Mix proportion for SCC was designed based on IS 10262 (2019) and the European Guidelines for Self-Compacting Concrete [57,58].The main objective was to develop an eco-friendly and economical SCC mix by reducing the quantity of cement and natural aggregates in SCC.Hence, the total quantity of cement in the conventional mix (SCC1 mix) was kept at 389 kg/m 3 (below 400 kg/m 3 ), meeting the minimum requirement of 320 kg/m 3 as per Indian Standards.Further reduction of cement content is possible only by adding SCM like fly ash, slag, etc, which is quite common in SCC mix design.Accordingly, the total cement content was replaced with 20% fly ash (SCC2 mix) to the weight of cement, which also improved the SCC mixtures' workability and maintained the strength requirement of M40 grade (40 N/mm 2 ) at 28 days as achieved in the previous work [59].Conventional fine and coarse aggregates were replaced MMW at 10% intervals from 0 to 50% to the weight of aggregates.Potable tap water was used with a w/c ratio of 0.45.Superplasticizer (SP) was used at 0.6% dosage for conventional concrete (with and without 20% fly ash) and 0.5 to 1.1% for SCC containing MMW. Mixing of concrete was performed as per ASTM C192/C192M; aggregates and binders were mixed dry for one minute, then wet for 120 s after adding 70% of the total water, and lastly, 2 min for mixing the remaining water and superplasticizer [60].Mixture proportioning details adopted in the study are listed in table 2.   unconfined environment without any physical barriers.The T 50 slump test visually inspects the SCC mix for its uniformity in flow, viscosity and resistance against segregation.Viscosity in SCC is an important parameter that shows the fluidity of the mix.T 50 slump test and V-funnel test indirectly measure the viscosity by determining the flow rate of SCC.Passing ability determines the potential of SCC to surpass and flow through highly packed and very narrow reinforcement bar spacing without segregation and external compaction (compacting on its own weight).L-box and J-ring tests are performed to evaluate the passing ability of physical barriers that block the flow of concrete when subjected to confined conditions.This passing ability is important as the fresh concrete should pass through all the congested reinforcements without any segregation and maintain the uniformity of the mixtures.

Experimental methods
Compressive strength was tested at 7, 28 and 90 days, while flexural and splitting tensile strength tests were performed at 28 and 90 days as per IS 516 (Part 1/section 1) 2021 [61].A minimum of 3 samples were tested for every hardened property, and the average values were reported as mentioned in IS 516 (Part 1/section 1) 2021.Samples were studied under scanning electron microscopy (SEM) for microstructural studies.8% of all carbon emissions come from the concrete sector, primarily from the cement industry, which is of two types: embodied CO 2 emission and operational CO 2 emission.Hence, material selection for concrete plays a huge part in carbon footprint [62].The cost of concrete mixtures is a vital subject that must be considered while producing concrete since it helps to identify the areas where savings can be made, leading to improved efficiency and profitability.Hence, cost and CO 2 emission (embodied emission) analyses were performed considering only the basic parameters (material cost and emission factor-EF m ) to foresee the significance of incorporating mine waste towards sustainability and economic viability in construction.Equations (1) and (2) represent the formula for calculating the cost and CO 2 emission incurred in producing 1m 3 of SCC.
Where, M c -Material cost, INR Q m -Materials quantity, kg per cubic meter EF m -Emission factor for concrete ingredients, kgCO 2 per kg of material The CO 2 emission factor (EF m ) is a conversion factor used to measure the amount of CO 2 emitted per unit quantity of the material produced.

Experimental results and discussions
The experimental testing was performed in the laboratory, and the results are discussed below,

Fresh properties
The consistency of SCC and the ability to fill and compact on its own weight can be expressed by four main attributes: flowability, passing ability, viscosity and segregation resistance [63].The fresh property results are given in table 3, along with the standard references specified by EFNARC and IS 10262 (2019) for determining the classes of SCC.The test results of slump flow indicate that all 12 mixtures of SCC fall under the flow category of SF1 (slump flow diameter ranging between 550 to 650 mm), with 650 mm being the highest and 600 mm being the lowest flow diameter.All the mixtures exhibited flow in the 610 to 630 mm range, showing the mix design's uniformity, making it eligible for application in lightly reinforced concrete members (EFNARC 2005 and IS 10262-2019).The incorporation of MMW gradually reduced the flowability of SCC as the rise in dosage of MMW decreased the fresh concrete flow.The same trend also extended to T 50 flow as the replacement levels increased; the time to flow through 500 mm diameter also increased.T 50 flow test results reveal that all the SCC mixtures can be categorized under the VS2 type, which takes more than 2 secs to flow through a diameter of 500 mm.All mixtures exhibited a time range of 5 to 6 s to spread 500 mm diameter, which shows the uniformity of the mix design [63].
The trend was different in the V-funnel test results as both conventional concrete (SCC1) and control concrete (SCC2) exhibited V1/VF1 (flow time less than 8 s).In contrast, concrete mix incorporated with MMW exhibited both types of flow, i.e., V1/VF1 and V2/VF2.The combination with 50% replacement of MMW as fine aggregates (SCC50MWFA) only showed V2/VF2 flow type, while other mixtures exhibited V1/VF1 flow type.In contrast, only the mix with 10% of MMW as coarse aggregates (SCC10MWCA) falls under the VF1 category, while the other SCCMWCA mixtures come under the VF2 flow type.L-box test results showed good performance of SCC with MMW as the h 2 /h 1 ratio of all mixtures was recorded in the range between 0.81 and 0.93 with three bar testing (recommended for highly congested reinforcement), progressing as per the requirements of the EFNARC and Indian Standards.All mixtures of coarse aggregate blended SCC (SCCMWCA) displayed a lesser range (0.81 to 0.83) of the h 2 /h 1 ratio, while a few mixtures of MMW replaced for fine aggregates (SCCMWFA) exhibited higher range values (above 0.90), showing the better passing ability of SCCMWFA mixtures than SCCMWCA mixtures.The higher powder content and fineness modulus of MW fine aggregate (MWFA) improved the SCC mixtures' overall cohesiveness, enhancing the fresh properties.
All the concrete mixtures performed well in the J-ring test as the concrete did not fail to pass through the obstruction provided by the reinforcement bars of 10 mm diameter attached to the ring.By comparing the results of the J-ring test, it can be observed that there was an average drop in flow diameter of around 40 mm in all the SCC mixtures, which falls under the minor blockage zone of ASTM C1621 (2009) except for the mixture containing 50% MMW as fine aggregates, which exhibited a flow drop of about 80 mm [64].Analyzing the height difference (blockage step) when passing through the J-ring setup, it is observed that all the 12 mixtures of SCC performed as per the requirements of EFNARC guidelines, i.e., h 2 -h 1 of all the mixtures were lesser than or equal to 2 mm, which falls well below the maximum value of 10 mm.This clearly shows that all SCC mixtures have good passing abilities, which might be due to the non-bleeding nature of SCC.
The workability of mixtures containing MMW as fine aggregates was retained/slightly reduced due to the presence of finer particles in mine waste powder (fineness modulus lesser than that of m-sand) that fills the voids created in concrete up to a certain level, resulting in the formation of denser matrix.Beyond the filling level, it pushes away the aggregates that disturb the concrete matrix, reducing the workability.A slight reduction in workability was observed, which may be caused by the friction that the waste from the magnesite mine's rough surface created, as shown in figure 3.An increase in superplasticizer dosage can also be a reason for the observed workability of SCCMWFA mixtures since the PCE-based admixtures cause dispersion between the cement's unhydrated granules, resulting in improved flowability.A slight loss in workability of SCCMWCA mixtures can be interpreted given that the irregular shape of MMW stones creates interparticle friction between the aggregates and internally resists the flow, i.e., fluidity.Overall, the relationship between these fresh properties (flowability, passing and filling ability) in SCC is centred around achieving a consistent, fluid mixture that can efficiently fill formwork, pass through congested reinforcement, resist segregation, and minimize bleeding.Proper mixture design, suitable admixtures, and careful quality control during mixing are crucial in maintaining the desired properties and performance of SCC.The workability assessment of SCC mixtures is shown in figure 4.

Hardened properties
All the SCC mixtures were subjected to compression, splitting tensile and flexural strength evaluations.Compressive strength reflects overall load-bearing capacity, flexural strength indicates its behaviour under bending loads, and splitting tensile strength highlights its resistance to localized tensile stresses.A compressive strength test was conducted at 7, 28, and 90 days after curing, while at 28 and 90 days, the splitting tensile strength and flexural strength tests were conducted.Figure 5 shows the hardened property tests conducted, while table 4 summarises the test results of different SCC mixtures.The result comparison of mine waste replaced SCC mixtures with SCC1 and SCC2 are discussed below; 3.2.1.Conventional and control SCC versus SCCMWFA series SCC2 and SCC containing MMW (MWFA and MWCA series) exhibited a lower seven-day compressive strength than conventional SCC (SCC1 without fly ash) due to the presence of fly ash that exhibits long-winded (slow) hydration or pozzolanic reactions [21,65].Similar results were also noticed for 28 and 90 days tests, as SCCMWFA mixtures showed a lesser compressive strength of up to 30% than the SCC1 mix.In addition, 40MWFA and 50MWFA mixtures only achieved the minimum compressive strength of 40 MPa at 90 days.When comparing the same with the SCC2 mix (i.e., SCC with 20% fly ash), mixtures with MMW fine aggregates did not attain better strength than SCC2 at 7 and 28 days.It is observed that even at 90 days, the SCCMWFA mix did not perform better than SCC2 except for the 20MWFA mixture, as it achieved equal strength as that of SCC2.The fineness modulus of fine aggregates (discussed in section 2) indicates that the m-sand and mine waste can be categorized as medium and fine sand, respectively.As medium sand is replaced by finer sand, water required for  workability increases, decreasing the strength of concrete, which was also reported in a similar investigation [66].The additional water requirement was adjusted by increasing the superplasticizer dosage to improve/maintain the workability, as shown in table 2. Silica (SiO 2 ) in the sand also contributes to concrete strength development, which can also be a reason for the reduced compressive strength of SCCMWFA mixtures, as mine waste powder exhibits lesser silica compositions than m-sand, as mentioned in table 1.
The splitting tensile strength results revealed that the strength of mixtures with MMW fine aggregates (SCCMWFA) improved by around 10% and 14% at 28 and 90 days than the SCC1 mix, respectively.In addition, mine waste dosage of 30% and above as fine aggregate slightly reduces the strength at 28 days, which is negligible.Comparing the same with the SCC2 mix, a similar trend was observed in splitting tensile strength as all the mixtures containing MMW attained higher strength than the SCC2 mix.Maximum strength improvement was around 20% more than the SCC2 mix for the SCCMWFA mixtures.The capability of mine waste to act as a filler in concrete, along with its surface roughness that restricts the propagation of cracks during load transfer might be one of the reasons for the strength achieved by the concrete mixtures.Comparing the flexural strength test results, almost all mixtures of MWFA attained higher strength than SCC1 except for the 50% replacement level (SCC50MWFA), which exhibited a slight reduction in strength at 28 days.Improvement in strength was observed at about 8%, while a negligible reduction was observed in MWFA mixtures compared to SCC1.Comparing the flexural strength results of MWFA mixtures with SCC2, all MWFA mixtures exhibited better flexural strength with a maximum improvement of around 15%.The enhancement in flexural strength might be due to the asymmetrical shape and roughness of the mine waste that helped to transform the stresses by creating resistance to shear stress through its interlocking mechanism.
An interesting fact to be noticed is the rate of strength gain (7 to 28 days and 28 to 90 days) in different combinations of SCC.SCC1 (without fly ash) has a rate of gaining around 17 to 25% for different ages, while SCC2 (with fly ash) and mine waste blended SCC mixtures (MWFA and MWCA series) showed around 25 to 65% strength-gaining rate.The main reason behind this is the presence of fly ash that enhances the pozzolanic reactions, forming more C-S-H gel and resulting in the improved strength-gaining ability of the concrete mix upon ageing.The same can be explained by the strength-gaining rate of mixtures from ages 7 to 90 days.Also, the rate of losing strength between the mixtures of SCCMWFA is around 15%, which can be due to the minimal loss of silica content in the composition of fine aggregates, as stated by Kannur and Chore (2023) [67].

Conventional and control SCC versus SCCMWCA series
Table 4 shows that after 28 days, every mixture of mine waste coarse aggregates (SCCMWCA) met the minimum strength of 40 MPa, and it was also noted that it attained a strength of 50 MPa at 90 days.Notably, out of the 5 mixtures of SCCMWCA, 3 replacement levels (20%, 30% and 40% MWCA) performed better than SCC1 at 28 days and 90 days of compressive strength testing.Compared with the SCC2 test results, all the mixtures containing mine waste coarse aggregates started showing better strength than the SCC2 mix right from the age of 7 days.This indicates that mine waste in the form of coarse aggregates can improve the compressive strength of concrete by up to 15% at an early age.Analyzing the compressive strength of MWCA mixtures at 28 and 90 days, results reveal that all the mine waste SCC mixtures achieved higher strength of up to 20% except for SCC50MWCA, where it performed slightly less than SCC2 at the age of 28 days.All these results prove that MMW in their natural form can improve the compressive strength of concrete because of its better physical properties than natural aggregates.The mine waste aggregates' rough surface and irregular shape create friction that enhances the bonding of aggregates and mortar, resulting in a denser microstructure.It also improves the adhesion involving binders and aggregates, which enhances strength.Also, the crushing value of MMW aggregates (7%) is almost 50% less than the conventional aggregate (13%), which shows better crushing resistance of MMW stones, enhancing the overall strength of SCC mixtures.Splitting tensile strength test results reveal that SCC with mine waste coarse aggregates (MWCA series) exhibits strength enhancement up to 20% and 11% at 28 and 90 days compared to SCC1, respectively.Comparing the SCC2 mix, MWCA mixtures showed higher strength of up to 29% and 18% at 28 days and 90 days, respectively.Comparing the flexure test results, SCC with MWCA did not perform better for lesser replacement levels (10% and 20% replacement).In comparison, it improved/maintained the strength for the replacement levels higher than 20%.Improvement in strength was observed at about 8%, while a reduction of up to 12% was observed in MWCA compared to SCC1.Aggregates' surface roughness and shape play vital roles in compressive strength development, extending to flexural strength.The same reason was reported while introducing ceramic waste aggregates in concrete by Gautam et al (2022) and Zareei et al (2019) [68,69].Comparing the same with that of the SCC2 mix, almost all mixtures of SCC with MMW performed well at 28 and 90 days, while lesser dosages of mine waste (20% and less) exhibited lesser strength, which was well below 5%.Strength improvement was observed to be around 13% for MWCA mixtures.
Similar to the strength-gaining ability of mine waste fine aggregate concrete, coarse aggregate blended SCC also showed around 25 to 70% strength gain upon ageing.In addition, it can be noticed from the results in table 4 that SCCMWCA mixtures showed 100% strength gain from 7 days to 90 days, which shows their ability to gain strength upon ageing against SCC1 and SCC2.Between the mixtures of SCCMWCA, the maximum strength gain was around 11%.

Microstructural study through SEM analysis
To validate the strength achieved by concrete, it is essential to comprehend its microstructure.The production of hydration products such as calcium silicate hydrates (C-S-H), calcium hydroxide (Ca(OH) 2 ), and ettringite should be examined using SEM analysis to understand the concrete materials' microstructure.SEM analysis links the strength parameters obtained through the incorporation of various materials in concrete by exploring its microstructure [66].The microstructure of concrete is mainly composed of aggregates, interfacial transition zone (ITZ), and hydrated cement paste.Concrete microstructure highly depends on the hydration reaction duration, water-binder ratio, admixtures, and cement type [21].Fly ash and superplasticizers are critical in microstructure because they accelerate cement hydration reactions and optimize pore distribution, creating a denser matrix [70].SEM analysis was performed on all mixtures with the samples collected from cube specimens used for hardened property investigation at 28 days, and the images of the samples that performed better in compressive strength are shown in figure 6.
Based on the analysis of the SEM images, the formation of C-S-H gel, portlandite and ettringite has been identified after 28 days of curing.The C-S-H gel and portlandite production, which enhanced the strength of the concrete, were the most commonly found entities.It is evident that mixes containing fly ash have higher concentrations of C-S-H gel.Additionally, denser C-S-H (DC-S-H) gel formation appears predominantly more in the SEM images of mine waste coarse aggregates (SCCMWCA) mixtures, indicating the better strengthgaining capability of SCC.Another interesting fact to be noticed is that the DC-S-H formation is more likely higher in mixtures of SCC2 and other mine waste SCC mixtures than SCC1 due to fly ash's presence that enhances pozzolanic reactions and reduces the voids in the concrete microstructure.A better adhesion between aggregates and surrounding cement paste was also observed, which allows the equal distribution of loads without any segregation.The same principle of adhesion between aggregates and cement paste was reported earlier by Sosa et al (2021) while analyzing the durability aspects of SCC [71].
Early or primary ettringite formation (thin needle-like structure) was observed only in SCC1 (figure 6(a)).In contrast, secondary ettringite formation (thick and long needle-like structure) was only observed in the 50MWFA mix (figure 6(d)).Primary ettringite formation involves the development of strength, while secondary ettringite formation (delayed ettringite formation-DEF) lessens the strength of SCC [72].Batic et al (2000) stated that C-S-H gel's ability to absorb sulphate rapidly in early ages and release the same in later stages is responsible for secondary ettringite formation [73].Ettringite in SCC1 was surrounded by DC-S-H gel, which acts like a bridge between the C-S-H layers, making the concrete more compact and improving its strength.The secondary ettringite formation in the 50MWFA mix is considered the primary factor in the drastic reduction in compressive strength.Another reason for strength loss in most of the mixtures of SCCMWFA is the presence of voids in the concrete.It is evident that the dense microstructure of C-S-H gel and its development are majorly responsible for the higher strength achievement of mixtures with MWCA.Figures 6(e) and (f) show that the entire microstructure is occupied by DC-S-H gel formation, which is responsible for the higher strength properties of SCC with mine waste coarse aggregates.Additionally, there were no evident cracks in the SEM images, which credits the improved strength properties of SCC blends, including mine waste as coarse aggregates.

Cost study and CO 2 emission analysis
The importance of cost and CO 2 emission analysis of concrete is significant, as both aspects are essential for achieving sustainable and economically viable concrete production.Understanding the carbon footprint in concrete production is critical to assess its environmental impact.In contrast, evaluating the production cost helps to identify the areas where costs can be optimized for economically efficient applications.The cost involved in producing 1m 3 of SCC with different proportions of mine waste aggregates calculated using equation (1) with data obtained from tables 2 and 5 is represented in figure 7(a).The price of the raw materials was determined using the Schedule of Rates database, established by the Tamil Nadu State Government in India and listed in table 5 [74].CO 2 emission engaged in the making of one cubic meter of SCC was calculated using Since industrial waste such as fly ash is widely available, the Government of India have imposed policies to all construction organizations, including Government and private agencies, that fly ash-based products (bricks, concrete, pavements) should be used for construction which is located within 300 km radius of a thermal power plant [76].This initiative encourages using fly ash to replace cement in the SCC mixtures, producing more reasonably priced concrete than the conventional SCC (SCC1).Similarly, mine waste replaced in the place of aggregates also reduces the production cost of SCC but not in the higher ranges due to their lower price range than cement.SCC with mine waste as fine and coarse aggregates achieve a cost reduction of 13% and 17%, respectively (figure 5(b)), compared with conventional SCC (SCC1).Compared with SCC2, SCC with mine waste reduces 2% and 6% for fine and coarse aggregate replacements.The main reason is that the aggregate cost is not very high compared to cement and admixture.The superplasticizer's role was crucial as it was necessary to enhance/maintain the flowability and workability without sacrificing the durability and strength of SCC mixtures made with MMW.Hence, the cost of the superplasticizer increased in the production of mine waste incorporated SCC, which compensated for the cost savings achieved by mine waste replacements.A similar trend can be observed in CO 2 emission analysis as only cement replaced with fly ash reduces almost 20% of the emission (SCC1 versus SCC2).Compared with SCC1 and SCC2 mixtures, CO 2 emission is almost similar in all mine waste substituted SCC as the aggregates emit very low CO 2 compared to cement, which is nearly 200 times higher.It can be observed from table 6 that one cubic meter of mine waste incorporated SCC emits around 400 kg of CO 2 compared to the 499 kilograms emitted by conventional SCC (SCC1).Like cost analysis, increased admixture dosage compensates for the CO 2 emission reduced by MMW.This reduction in cost and CO 2 emission highly contributes to the UN's Sustainable Development Goals (SDG), which targets 2030.Specifically, incorporating MMW in concrete adds to achieving SDGs 9 and 12, promoting sustainability in the construction sector's consumption, production and industrialization.

Conclusions
This study investigated one of the unexplored sustainable building materials by addressing magnesite mine waste (MMW) as a potential replacement material in self-compacting concrete.Extensive laboratory investigations were conducted, and its findings primarily demonstrated the potential for MMW to be used as building materials in self-compacting concrete.Analyzing the results of SCC mixtures obtained through various experimental evaluations, the conclusions are summarised below; • Utilizing waste rocks (MMW) as aggregates favoured the characteristics of SCC as it exhibited sufficient fresh properties based on the requirements of EFNARC (2005) and IS 10262 (2019).Results of filling and passing abilities showed that SCC mixtures with mine waste as fine aggregates exhibited an upper hand over SCC with mine waste as coarse aggregates in fresh properties.
• Compressive strength results involving MMW as fine aggregate (SCCMWFA) displayed satisfactory results, while SCC with mine waste as coarse aggregate (SCCMWCA) exhibited better strength than SCC1 and SCC2.SCCMWFA mixtures showed a maximum strength reduction of 30%, while SCCMWCA mixtures showed a maximum strength enhancement of 15%.
• Notably, except for the mixtures with 40% and 50% mine waste for fine aggregate, all the other mixtures achieved a minimum of 40 MPa at 28 days, which is very promising for structural applications.
• Splitting tensile results revealed a maximum improvement of 10% and 21% for MWFA and MWCA mixtures, respectively.It is evident that almost all mixtures with MMW achieved equal/improved results than SCC1 and SCC2.
• Similar results were also obtained in flexural strength, as most of the mine waste replaced SCC mixtures attained better flexural strength than SCC1 and SCC2.Maximum improvement in strength was observed to be around 7% for mine waste replaced SCC mixtures.
• Comparing the hardened property results of SCCMWFA and SCCMWCA mixtures, it is clear that mine waste as coarse aggregate (natural form) attained way higher strength than fine aggregate (crushed powder) replacement.
• SEM analysis revealed the microstructural characteristics of SCC containing MMW. Introducing SCM and well-gradated mine waste aggregates enhanced the strength of SCC mixtures, resulting in the development of denser C-S-H and microstructure.SEM images of mine waste incorporated SCC also revealed the absence of cracks or voids (except SCCMWFA mixtures) in concrete.
• A maximum cost of 17% and CO 2 emission of 20% can be saved by featuring MMW as partial aggregate replacements in SCC.
• Sustainable Development Goals (SDGs) 9 and 12 can be promoted by incorporating mine waste into concrete.
Summarising all the results, it is evident that the magnesite mine waste has a promising future as an alternative to conventional construction material.This proposed method contributes to sustainable construction by efficiently reusing MMW, currently disposed in landfills, and consuming less energy in concrete production.Using mined waste will protect the environment by minimizing the effects of global warming and produce green and affordable concrete.

Figure 2 .
Figure 2. Magnesite mine waste as fine and coarse aggregates.

Figure 3 .
Figure 3. Microstructure of MMW using SEM and XRD.

Figure 7 .
Figure 7.Comparison of Cost and CO 2 emission analysis.

Table 1 .
Chemical composition of different materials in percentage (%).

Table 2 .
Mix proportioning of SCC in kg per cubic metre.Conventional SCC without fly ash, SCC2-Control SCC with 20% fly ash, SCCxxMWFA-SCC with mine waste fine aggregate, SCCxxMWCA-SCC with mine waste coarse aggregate, xx-Percentage replacement (10% to 50%) of mine waste in SCC.

Table 3 .
Results of fresh properties.

Table 4 .
Results of hardened properties.

Table 6 .
CO 2 emission involved per m 3 of SCC.