Performance evaluation of concrete containing fly ash, silica fume and m-sand under high temperatures: mechanical and microstructural approach

Abstract

Fire is one of the most serious threats that building structures may face during their lifespan. Experiencing a fire that spreads over the whole building might lead to unforeseen damage to the structural components. The structural element and concrete are affected by the mechanical and thermal characteristics of the concrete. The current study aims to investigate the thermal properties of concrete using silica fume (SF), flyash (FA), with natural sand (NS) and M-Sand (100%) in concrete at 27, 250, 500, and 750 °C at a steady state of 2.5 h. Six mixes were generally considered in two series: with NS as M1 mix and with M-Sand mixes (M2-M5). The SF (0%, 6%, 12%, 18% & 24%), FA (10%), and M-Sand (100%) at the substitution rates. Each mixtures are assessed for residual compressive strength, mass loss, density, and ultrasonic pulse velocity, and statistical and microstructural analysis, like using scanning electron microscopy (SEM), thermo gravimetric analysis (TGA), and energy dispersive X-ray spectroscopy (EDS), were performed at high temperatures. The results found that substituting SCMs for cement with FA, SF with M-Sand in concrete shows the improvement in the residual compressive strength (R-CMS) at individual high temperatures of the mixes compared with mixes (M1 and M2). The combination M4 (10% FA and 12% SF, substituting with cement and 100% M-Sand) shows good performance, having 47.65 MPa, 51.25 MPa, 44.5 MPa, and 38.63 MPa at high temperatures, respectively. Microstructural analysis revealed that a compact dense nano C-S–H structure at individual temperature levels of the mixes via pozzolanic reactions. Overall this study emphasizes the role of FA, SF & M-Sand performance in concrete subjected to fire, making a valuable contribution to the development of green and sustainable infrastructure to improve the life span of the structure under the spread of fire.

Introduction

Elements of concrete construction might have varying constituent compositions to meet their specific strength and performance needs. A few examples of concrete that are often utilized include high-performance concrete [1, 2], self-consolidating concrete [3, 4, 5, 6, 7–8], recycled concrete [9, 10], normal-strength concrete, and high-strength concrete [11, 12, 13–14]. The heating and cooling cycles, types of aggregates, water-cement ratio, chemical and physical changes to components, and changes in moisture content may all cause concrete's characteristics to vary at high temperatures [15]. Extreme weather, particularly heat and fire, may endanger designed structures in a number of ways during the course of their lifetime. At high temperatures, the material may experience irreversible physicochemical alterations that impact its thermal and mechanical characteristics. The Interfacial Transition Zone (ITZ) between cement paste and coarse aggregates exhibits weakening due to temperature fluctuations and changes in moisture content [16]. Rapid temperature escalation can lead to explosive spalling and structural failure in low-porosity concretes. Investigating the mechanical properties of blends exposed to high temperatures is essential [17, 18, 19–20]. Mixtures incorporating industrial by-products such as fly ash, silica fume, and M-Sand as supplementary cementitious materials and fine aggregates can decrease cement demand and enhance mechanical performance under extreme conditions, thereby promoting sustainability in construction for infrastructure development.

Fly ash, a by-product of coal combustion in thermal power plants, is a conventional mineral additive used in concrete to partly substitute portland cement. In the investigation, mortars including 0–50% fly ash (FA) were subjected to elevated temperatures of 200–800 °C for 20 min. The results indicated that the enhancement of FA content did not alter the mineral phase development of hardened cement paste (HCP) under sustained heating. The analysis of mass loss, fracture temperature, and microstructural development of HCP subjected to elevated temperatures indicated that the addition of FA content significantly enhanced their thermal resistance [22, 23]. In the study by Wei Wang et al. (2017) [25], it was found that following exposure to high temperatures, both the compressive strength and thermal conductivity of ordinary concrete and fly ash concrete significantly decreased; specifically, at 550 °C, the compressive strength decreased by approximately 26%.

Silica fume is an ultrafine powder obtained as a by-product during the manufacturing of silicon and ferrosilicon alloys. It improves the microscopic structure of concrete by pozzolanic processes that generate supplementary C-S–H gel and occupy micro-voids, resulting in a denser matrix with less permeability. This densification enhances fire resistance by mitigating explosive spalling, decreasing thermal conductivity, and preserving structural integrity at high temperatures via improved interfacial transition zones [17, 18]. In the study, silica fume (SF) utilized at 0 to 10% can greatly enhance the bond strength and compressive strength of mortar as a partial cement substitute. Silica fume positively influences the fire resistance of insulating mortar, with mortar containing a 10% silica fume admixture exhibiting approximately 28.30% lower mass loss rate and about 14.73% higher relative compressive strength after exposure to high temperatures, following 28 days of curing [21]. However, H.M. Khater et al.[24] examined how nano-silica fume affects the mechanical properties and thermal stability of geopolymers when subjected to high temperaturesexposure to 500, 700, 800, and 1000 °C.Strength and microstructure were both improved by adding 5% NSF to the MK geopolymer binder. Mixtures of MK and geopolymers with NSF have an improved mechanical strength of up to 700 °C and an even greater increase with NSF concentrations of up to 5%. Both the microstructure and physico-mechanical characteristics were improved by adding 5% NSF, both before and after high-temperature exposure, but Davood Mostofinejad et.al [27]. observed that the thermal conductivity was found to decrease by 45% and 43%, respectively, in concrete that included 50% fine and 50% coarse rubber particles together with 15% silica fume. Mohammed Abd El-Salam Arab et.al [29] investigate the effects of calcination at temperatures of 600, 800, and 1000 ᵒC for eight hours on nano-silica fume and nano-alumina blends in their physically mixed state, as presented in the experimental work. The findings show that the two types of nano-silica fume, when combined, have a synergistic impact that significantly increases the mechanical strength of the mix. For these performance increases to be maximized, calcination at 800 °C was shown to be the best temperature. With the help of nanomaterial additives, especially nano-silica fume, the microstructure was modified, which led to a significant decrease in water sorptivity and an increase in acid resistance, both of which were indicators of better durability. In the study, the combination of FA, SF is carried out by Mohammed Abd El-Salam Arab et.al [29], showing that without fly ash and silica fume, strength is lost at high temperatures. Specimens doped with 15% and 10% silica fume, on the other hand, showed the greatest improvement in strength. Structures resembling ceramics may be created by sintering phases and amorphous particles produced by K-struvite breakdown with the help of silica fume and fly ash. An efficient method for improving MKPC's resistance to high temperatures is to mix fly ash with silica fume [28, 30].

In the current context of sustainable development strategies, M-Sand is an eco-friendly alternative to natural sand that has less of an influence on the environment during building. Based on that, some studies discuss the M-sand exposed to fire, Xinjie Wang et.al 2024 [19], findings demonstrate that RCM specimens have marginally superior compressive strength after 7 days of water curing in comparison to the 30-day conventional curing. Moreover, after conducting high-temperature tests between 200 °C and 800 °C, the decline in strength of the specimens is not markedly evident when compared to typical curing at elevated temperatures.

The existing literature reveals that only a limited number of studies have been conducted on the combined effects of fly ash or silica fume on conventional concrete at elevated temperatures. (2) No comprehensive systematic research has been undertaken that simultaneously varies fly ash (FA) and multiple dosages of silica fume (SF) using 100% manufactured sand (M-sand) as fine aggregate under elevated temperature conditions. (3) There is inadequate correlation between microstructural characteristics and mechanical properties at elevated temperatures. This research study addresses that gap by formulating mixtures and providing innovative insights into the thermal properties of sustainable concretes.

Research significance

Fire and elevated temperatures are essential considerations in the design of concrete structures, relevant to both conventional and nuclear facilities. A fire that consumes the whole building may cause unexpected damage to the structural elements. This study investigates the mechanical and microstructural characteristics of materials and their prospective uses in improving fire safety and environmental sustainability in buildings. This research is particularly important for towering structures, infrastructure in fire-prone areas, and nuclear reactors, where fire resistance is paramount. Utilizing fly ash and silica fume, industrial by-products, in an eco-friendly manner minimizes the necessary quantity of cement and the volume of carbon emissions.

An attempt is made to substitute cement with fly ash at 10% across all mixtures, while cement was also replaced with silica fume at concentrations of 0%, 6%, 12%, 18%, and 24%. Additionally, natural sand was entirely replaced with M-Sand subjected to 27, 250, 500, and 750 °C at a steady state of 2.5 h, respectively. This study investigates Residual compressive strength (R-CMS), R-Mass loss (R-ML), R-Residual density(R-D), and R-Ultrasonic pulse velocity (R-UPV) after 28 days of curing samples. Apart from the statistical analysis and micro microstructural analyses like TGA, DTG, SEM, and EDS were performed. The experimental framework is represented in a schematic form as shown in Fig. 1.

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Fig. 1

Schematic diagram for the experimental process

Experimental program

Materials

Ordinary Portland Cement of 53 Grade, according to Is 12269–2015 [31], supplied from the local market, was utilized for the research. Class C fly ash collected from Vijayawada Thermal Power Station (VTPS) located in Ibrahimpatnam, Andhra Pradesh, and silica fume is collected from Astra Chemicals, Tamil Nadu. The aggregates were categorized as fine aggregate and coarse aggregate, adhering to the specifications of IS 383–1970. This study utilized locally sourced coarse aggregate with a maximum size of 20 mm and a minimum size of 12.5 mm. The aggregates were cleansed to eliminate dust and grime and subsequently dried to a surface-dry state. The coarse aggregate possesses a specific gravity of 2.7, a bulk density of 1.6 kg per cubic meter, and a fineness modulus of 7.17 as per IS383-2016 [33]. Locally sourced M-Sand as fine aggregates is utilized in the project. The specific gravity is 2.38, the fineness modulus is 2.5, and the bulk density is 1.45 kg/cu.m, and PCE-based superplasticizer (SP) is used as per Is 9103: 1999 [32].

Mix proportions and preparation

The concrete mixes are carried out as per Is 10262 − 2019 [30] guidelines, and the designation of the mix is represented in Table 1. Six mixtures are proposed for the study, in all the mixes, the cement is replaced with 10% Flyash (FA) as a constant, including the control mixture. The blend with a combination of 100% natural sand (NS) and cement (C) replaced with 10% FA, along with coarse aggregates (20 mm, 12 mm), water, and SP is labeled as "M1" (control mixture with NS). The blend combination with 100% M-Sand and cement replaced with 10% fly ash, 0%SF, along with coarse aggregates, water and SP is labeled as "M2" (Control mixture with M-Sand), while the mixture with 100% M-Sand and cement replaced with 10% fly ash, 6%SF, along with coarse aggregates, water and SP is labeled as "M3", while the mixture with 100% M-Sand and cement replaced with 10% fly ash, 12%SF, along with coarse aggregates, water and SP is labeled as "M4", similarly, other mixes are designated as follows upto "M6".

Table 1. Mix proportions, Kg/cu.m

Sl.No	Mix Id	C	FA	SF	NS	M-Sand	20 mm	12 mm	W	SP
1	M1	364.5	40.5	0	624.29		766.85	511.23	180	4.1
2	M2	364.5	40.5	0	-	624.29	766.85	511.23	180	4.1
3	M3	340.2	40.5	24.3	-	624.29	766.85	511.23	180	4.1
4	M4	315.9	40.5	48.6	-	624.29	766.85	511.23	180	4.1
5	M5	291.6	40.5	72.9	-	624.29	766.85	511.23	180	4.1
6	M6	267.3	40.5	97.2	-	624.29	766.85	511.23	180	4.1

The preparation of the mixtures was carried out in the laboratory using pan mixing. The dry components (aggregates, fly ash, and silica fume) were mixed for three minutes, followed by the gradual addition of the water + sp, while mixing continued for 5 to 10 min until a homogeneous mixture was achieved. All mixing operations were conducted at a controlled temperature of 27 ± 2 °C with relative humidity maintained at 65 ± 5%. Thereafter, concrete, poured into the cube moulds for the corresponding mixes as per the study. Figure 2a represents pan mix preparation, and Fig. 2b shows the casting of moulds. The moulds are kept open for a maximum of 24 h, after which samples are immersed in the curing tank for the proposed study.

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Fig. 2

Experimental Process. a Mix prepration in pan, b Casting of cube samples

Experimental test procedures

To assess the effect of the elevated temperature on the thermal properties of 28-day samples are subjected to 250 ᵒC, 500 ᵒC, and 750 ᵒC at 150 min standard steady state as per IS 834–1:1999 [36]. The samples of 100 × 100 × 100 mm cubes based on IS: 516–2015 [34] were kept in an electrical oven, as shown in the muffle furnace used in the study, as shown in Fig. 3. Figure 3a represents the view of furnace temperature, and Fig. 3b represents the internal view of the furnace. The samples were subjected to a temperature that was gradually increased at a rate of 5 degrees centigrade per minute until the desired temperature was achieved. Subsequently, the samples were kept at the desired temperature to reach the ambient temperature. After reaching the ambient temperature, the residual compression strength test, Residual Rebound hammer as per Is 13311 (Part 2):1992 [35], Residual-ultrasonic pulse velocity as per 13,311 (Part 1):1992 [37], mass loss, residual density, microstructural analysis, TGA SEM, and EDS are assessed at high temperature.

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Fig. 3

Muffle furnace for the study. a view of furnace temperature, b internal view of furnace

TGA analysis

Utilizing a thermogravimetric analyzer (TGA) type Hitachi STA-7200 equipment, we examined the samples' weight loss. Under nitrogen purging of 20 ml/min and a heating rate of 20 °C/min, samples ranging from 10 to 20 mg were heated from 27 to 800 °C for the investigation. Derived thermo-gravimetric analysis (DTG) was used to examine the sample composites' maximum weight loss temperature.

SEM/EDS analysis

The crystalline structure of the mixture samples is explored using microstructural examination. In the residual compressive strength test, the sample was taken from the sample's inner core and subjected to scanning electron microscopy (SEM). By directing a stream of electrons onto a sample's surface, SEM can produce images of the material. To conduct the research, the SEM, VEG3, and SBHTESCAN were equipped with backscattered electron imaging capabilities for the mixes at elevated temperatures.

Results and discussion

To investigate the mixes, cube samples were cast and cured, and then subjected to an elevated temperature at 250 °C, 500 °C, and 750 °C in a Muffle furnace by increasing the temperature to 5 °C until it reached 250 °C, 500 °C, and 750 °C at 2.5 h steady state. The samples were taken out of the muffle furnace and allowed to reach the ambient temperature, and the tests like carried out. The test results of Residual Compressive strength (R-CMS), Residual Ultrasonic Pulse velocity (R-UPV), Residual Rebound hammer number (R-RbNo), Residual-Mass loss (R-ML%), and Residual density (R-D), along with microstructural properties, are discussed below as follows:

Residual compressive strength results

The residual compressive strength (R-CMS) at elevated temperatures of 27 °C, 250 °C, 500 °C, and 750 °C at 2.5 h, as shown in Fig. 4. At the room temperature, 27 °C, the R-CMS ranges from 39.62–47.65 MPa, at 250 °C, the R-CMS ranges from 40.72–51.25 MPa, at 500 °C, the R-CMS ranges from 34.25–44.50 MPa, at 750 °C, the R-CMS ranges from 28.65- 38.63 MPa for the mixes M1-M6, respectively. Figure 5 represents the Increment/Decrement (%) of R-CMS correlation with control mix (M2) to mixes at elevated temperature. At 250 °C, the % rise of compressive strength ranges from 1.85% to 18.53% it values of about 2.46%,6.66%,18.53%,10.69%, and 1.85%. At 500 °C, the % rise of compressive strength ranges from 6.7% to 17.48% it values of about 6.72%,7.38%,17.48%,8.5%, and a drop of 3.3%. At 750 °C, the % rise of compressive strength ranges from 5.7% to 15.89% which is about 11.81%, 5.79%, 15.89%, and8.53%, and a drop of 6.4% for the cement replacement of SF0% % % to 24% with M-Sand Fig. 6. Increment/Decrement (%) R-CMS correlation with control mix (M2) to the mixes at ambient vs elevated temperature. At 250 °C, the % rise of compressive strength ranges from 2.44% to 20.52% it values of about 2.44%, 8.94%, 20.52%, 12.87%, and 4.25%. At 500 °C, the % drop of compressive strength ranges from 2.65% to 11.93% but at SF12% slight rise of compressive strength, about 4.34% for the M-4 mix, and at 750 °C, the %drop of compressive strength ranges from 5 to 20% for the cement replacement of SF0% % to 24% with M-Sand.The linear regression equations for the mixes at the elevated temperatures of 250 °C, 500 °C, 750°Cwhich show a strong relationship between the samples. The coefficients of determination (R²) observed for CMS vs R-CMS mixes as 0.96 (96.0%), 0.918 (91.8%), and 0.89 (89.9%) for the elevated temperature, as shown in Fig. 7. When compared with ambient temperature, the elevated temperature shows better CMSupto250°C.A slight decrement is observed at 500 and 750 °C, but it is nominal at 500 °C at SF12% % % for M4, showing an increment in the strength of about 4.34% and the M4 mix has a compressive strength is 42.5 MPa.At 250 °C, the strength is increased, and at 500 °C, at M4 slight rise is due to the improvement of CSH gel due to binder materials of SF, and flyash with M-Sand respectively [38, 39, 40–41].

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Fig. 4

R-CMS of mixes at elevated temperature

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Fig. 5

Increment/Decrement (%) of R-CMS correlation with control mix (M2) to mixes at elevated temperature

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Fig. 6

Increment/Decrement (%) R-CMS correlation with control mix (M2) to the mixes at ambient vs elevated temperature

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Fig. 7

Regression analysis of CMS vs R-CMS mixes at elevated temperature

Residual -ultrasonic pulse velocity results

The residual ultrasonic pulse velocity (R-UPV) at elevated temperatures of 250 ^°C, 500 °C, and 750 ^°C at 2.5 hrs represented in Fig. 8. At 27 °C, the residual ultrasonic pulse velocity (R-UPV) is 4.75, 4.85, 4.89, 5.32, 5.21, and 4.9 km/s.At 250 °C, the R-UPV are 4.425, 4.29, 4.35, and 4.58 km/s.At 500 °C, the R-UPV are 3.6, 3.7, 3.9, 4.1, 3.8, and 3.5 km/s.At 750 °C the R-UPV are 2.65, 2.78,2.95, 3.3, 2.91and 2.7 km/s for M-1, M-2, M-3,M-4, M-5 and M-6 respectively. The R-UPV of the control mix (M2) to the remaining mixes at individual elevated temperatures of 250 ^°C, 500 °C, and 750 ^°C at 2.5 hrs shows the rise in the UPV values. The results showed that the UPV values showed a slight drop at the elevated temperature when compared with ambient temperature, but when compared with individual temperature, they showed better UPV. At 27°, all the mixes show that the quality of concrete is excellent. At 250 °C, the quality of concrete is excellent for the M4 mix (FA 0% and SF12%) and M-5 mix (FA 0% and SF18%) with MS100% and the remaining mixes exhibit a quality of concrete that is good, respectively. At 500 °C, all mixes exhibit the quality of concrete. At 750 °C, all the mixes exhibit the quality of concrete is moderate and the M4 mix (flyash 10% and silica fume 12%) exhibits the quality of concrete is good. In all elevated temperatures, with M-sand of 100% and SF% mixes, exhibit a good quality of concrete compared to the mixes M1 and M2 as per the recommended code, as per Is 13311-1: 1992 [37]. Figure 9 shows the regression analysis of R-UPV vs R-CMS of mixes at elevated temperatures.The regression analysis at the elevated temperatures of 250 °C,500°C, and 750 °C mixes which shows a strong relationship. At the elevated temperatures, a strong correlation with mixes having the (R²) observed as 0.93 (93.6%), 0.895 (89.5%), and 0.908 (90.8%), respectively

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Fig. 8

R-UPV mixes at elevated temperature

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Fig. 9

Regression analysis of R-UPV vs R-CMS of mixes at elevated temperature

Residual-Rebound hammer number(R-RbNo) results

The residual rebound hammer number (R-RbNo) at elevated temperatures of 250 °C, 500 °C, and 750 °C, at 2.5 h, as shown in Fig. 10. At room temperature, 27 °C, the R-RbNos are 40, 42, 43, 47, 45, and 40.At 250 °C, the 42,44, 45, 50, 46, and 42. At 500 °C, the R-RbNo are 33, 34, 38, 42, 41, and 33. At 750 °C, the R-RbNo are 27, 31, 35, 36, 33, and 30 for M1-M6, respectively. When compared with ambient temperature, the elevated temperature shows that the mix is distributed uniformly so that there is no predominant change in the rebound number value subjected to elevated temperatures for the mixes [35].

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Fig. 10

Residual-Rebound Number of the mixes at elevated temperature

Residual density (R-D) results

Figure 11 represents the residual density (R-D) mixes at elevated temperatures of 250 °C, 500 °C, and 750 °C at 2.5 h. At room temperature, the residual density (R-D) [38] Is 2617, 2672, 2564, 2632, 2540, and 2500 kg/cu.m.At 250 °C, the 2568, 2625,2515, 2585, 2520, and 2485 kg/cu.m.At 500 °C, it Is 2460, 2565, 2472, 2520, 2421, and 2325 kg/cu.m. At 750 °C, the R-D are 2360, 2420, 2340, 2359, 2219, and 2138 kg/cu.m from M1-M6, respectively.

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Fig. 11

R-Density of the mixes at elevated temperature

Residual Mass Loss (R-ML%) mix results

Figure 12 represents the residual mass loss (R-ML%) at elevated temperatures of 250 °C, 500 °C, and 750 °C at 2.5 h. At 250 °C the residual mass loss(R-ML%)are 2.48,1.83,1.64, 3.57, 2.57 and 2.5%. At 500 °C the (R-ML%) are 2.42,3.52,5.17,1.36,1.34, and 1.25%. At 750 °C the(R-ML%) are 2.2,4.58,3.82, 3.68,3.5 and 3% from M1-M6, respectively.

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Fig. 12

R-Mass loss (%) mixes at elevated temperature

Microstructural analysis

The analysis of microstructure was performed on mixed samples at ambient and elevated temperatures of 27, 250, 500, and 750 ◦C, respectively, to examine the morphology and crystalline structure.

TGA analysis

The TGA analysis of the mixtures at the elevated temperature is shown in Fig. 13. Flyash (10%) and Silica Fume (0%, 6%, 12%, 18%, and 24%) in the mixtures as measured by thermogravimetric analysis (TGA). The expulsion of adsorbed water molecules from the mixtures was the initial cause of the weight loss (< 2.5%) that occurred between 100 and 200 ◦C. The weight loss is less than 3.5% at temperatures ranging from 200 to 400◦C. The rate of weight loss is less than 7% at temperatures between 400 and 600^◦C and less than 10% at temperatures between 600 and 800℃and the overall mass reduction is around 10% from M1 to M4. Following M5 and M6, the weight loss is about than 16.5%. At increased temperatures, the DTG analysis of the mixtures is shown in Fig. 14. The dihydroxylation/hydroxyl (OH) groups in flyash and silica fume cause a mass loss of around 10% at peaks of 300^◦C, 500◦C, and 700◦C when the largest peaks are seen [44]. Calcite, calcium silicate hydrate, Ca(OH)₂, and C-A-S–H are hydrates that decompose to leads to largest peak.

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Fig. 13

TGA analysis of the mixes at the elevated temperature

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Fig. 14

DTG analysis of mixes at elevated temperature

SEM observation of the samples

Figure 15 represents the SEM image at elevated temperatures of cement replacement of 10% Flyash and 100% M-Sand (M2). The Fig. 15(a) SEM image at 27 degrees centigrade shows the dense CSH matrix, cracks, and holes along with the Portlandite structure. Figure 15(b) shows the SEM image of the sample subjected to 250 degrees centigrade at a 2.5-h study state. The SEM image shows several chemical changes are observed, and no cracks were observed at the surface of the sample up to 250 degrees centigrade. Figure 15c shows the SEM image of the sample at 500 degrees centigrade. It was observed that Portlandite structure and CSH deterioration lead to the formation of dense C-S–H gel in the concrete matrix. Figure 15d shows the SEM image of a 750ᵒC. In the SEM image observed CSH gel and a series of voids are observed. Due to the temperature rise, the calcium hydrates were lowered upto500 degrees centigrade beyond to rise in temperature of about 700 −800ᵒC leads to dehydration of calcium hydrates and breakdown of C-S–H nano structure.

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Fig. 15

SEM analysis of M2 sample. a 27ᵒC, b 250 ᵒC, c 500 ᵒC and (d) 750 ᵒC

Figure 16, represents the SEM image of the cement replacement of Flyash at 10% and Silica Fume (6%) with 100% M-Sand (M3)at elevated temperature. Figure 16a represents the SEM image of the ambient temperature. The SEM image shows the dense C-S–H gel, and the formation of Portlandite, along with C-S–H nanostructures, is abundant in the SEM image. The growth of nanostructures over time in the hydration process covers the small holes and cracks. Figure 16b represents the SEM image at 250 °C. The SEM image shows the dense C-S–H structure observed. On the surface of the sample, no changes are seen in the cement phases and C-S–H structure up to 250 °C. Figure 16c represents the SEM image at 500 °C. The SEM image observed the portlandite structure, and the nano C-S–H structure is strongly observed along with cavities. It is due to the effect of elevated temperature that the water escapes from the gaps, and the portlandite phases are converted into calcium oxides. Figure 16d represents the SEM image at 750 °C. The SEM image shows the complete decomposition of the portlandite structure, and due to the high temperature changed to a glassy phase. It leads to damage of cement paste and decomposition of C–S–H nanostructure, and conversion of CaCo₃ to CaO and CO₂ leads to increased cracking, which possibly leads to a reduction in the concrete strength [47, 48–49]

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Fig. 16

SEM analysis of M3 sample. a 27ᵒC, b 250 ᵒC, c 500 ᵒC and (d) 750 ᵒC

Figure 17 shows the cement replacement FA10% and SF 12% and M-Sand 100% (M4) at the elevated temperature. Figure 17a represents 27 °C SEM images of the M4 mix. The SEM image shows needle-like structures and dense CSH nanostructures, along with no visible cracks observed, and a large number of accumulated CSH abundantly found in the image. Figure 17b, represents the SEM image at 250 °C. The SEM image shows the accumulation of CSH nanostructure, and Portlandite crystals are visible. Figure 17c, represents the SEM image at a 500-degree centigrade. The SEM image shows an accumulated and dense CSH gel. The Portlandite is observed in the same image, along with voids. Figure 17d, represents the SEM image at 750 °C. The SEM image shows the dense gel apart from the voids and cracks due to high temperatures. The changes in the volume increase in decrease in the commercial strength of concrete compared with the ambient temperature [42, 43]. Figure 18(a) represents the SEM image at 27 degrees centigrade. The SEM image shows the accumulation of CSH nanostructure, and Portlandite crystals are visible. Due to the growth of the Nanostructure being observed apart due to the secondary Portland diet secondary CS gel leads to improvement in the strength compared to the control mix at elevated temperature. Figure 18b represents the image at 250 °C, the SEM image shows accumulated and dense CSH gel. The Portlandite is observed in the SEM image, along with voids. Figure 18c represents the SEM image at 500 °C.The SEM image shows the dense and accumulated CSH gel along with Portlandite, and cavities are observed. The SEM image of 750 ᵒC the SEM image shows accumulated and dense gel along with voids and gaps formed. This is due to high temperatures, the damage to cement paste and composition of structure leads to the formation of cavities and voids in the matrix, which leads to a decrease in the compressive strength.

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Fig. 17

SEM analysis of M4 sample. a 27ᵒC, b 250 ᵒC, c 500 ᵒC and (d) 750 ᵒC

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Fig. 18

SEM analysis of M5 sample. a 27ᵒC, b 250 ᵒC, c 500 ᵒC and (d) 750 ᵒC

In EDS analysis the EDS analysis shows the Si, Ca, C, and O under EDS analysis. Table 2 represents the atomic weights and shows the EDS analysis of M2 samples. The range Ca/Si varies from 0.57 to 3.36. The Table 2 represents M2 mix at 27ᵒC,the Ca/Si = 3.36, at 250ᵒC,Ca/Si = 2.84,at 500 ᵒC, Ca/Si = 3.0,and at 750ᵒC, Ca/Si = 0.576. Table 2 shows the EDS analysis of M3 samples. The range of Ca/Si varies from 2.64 to 7.04.In Table 2 represents the M3 mix 27℃, Ca/Si = 7.04,at 250ᵒC,Ca/Si = 3.22,at 500ᵒC, Ca/Si = 2.64 and at 750ᵒC, Ca/Si = 6.08.The EDS analysis of M4 samples is represented in Table 2. Table 2 represents M4 mixat27ᵒC, Ca/Si = 1.99, at 250ᵒC,Ca/Si = 2.12, at 500ᵒC, Ca/Si = 1.717, and at 750ᵒC, Ca/Si = 2.The EDS analysis of M5 samples is shown in Table 2. Table 2 represents the M5 mix at 27ᵒC, Ca/Si = 2.97,at 250ᵒC,Ca/Si = 1.61, at 500ᵒC,Ca/Si = 0.69, and at 750ᵒC, Ca/Si = 0.36.

Table 2. EDS analysis of mixes at elevated temperatures

Specimen Code_Mix @ temperature	Atomic (wt%)
	O	Al	Si	Ca	Fe	Ca/Si
M2 @ 27 ᵒC	66.84	1.59	7.58	25.49	0.5	3.36
M2 @ 250ᵒC	64.76	1.84	8.48	24.13	0.78	2.84
M2 @ 500 ᵒC	61.97	1.2	8.9	26.71	1.21	3
M2 @ 750ᵒC	69.86	2.06	17.8	10.27	-	0.57
M3 @ 27 ᵒC	58.85	1.39	4.46	31.42	3.89	7.04
M3 @ 250ᵒC	68.32	0.91	7.29	23.48	-	3.22
M3 @ 500 ᵒC	67.31	1.01	8.61	22.76	0.31	2.64
M3 @ 750ᵒC	66.71	0.97	4.45	27.09	0..78	6.08
M4 @ 27 ᵒC	70.22	2.04	10.96	16.8	0.7	1.53
M4 @ 250ᵒC	65.67	1.64	10.53	20.96	1.19	1.99
M4@ 500 ᵒC	68.57	2.22	9.09	19.36	0.76	2.12
M4 @ 750ᵒC	64.61	3.43	11.22	19.27	1.47	1.717
M5 @ 27 ᵒC	53.14	1.39	11.03	32.76	1.67	2.97
M5 @ 250ᵒC	69.12	1.6	11.21	18.08	-	1.61
M5@ 500 ᵒC	66.5	4.67	15.96	11.07	0.82	0.69
M5 @ 750ᵒC	45.69	45.94	6.14	2.23	-	0.36

Based on these results, the moderate improvement in compressive strength should be closely correlated with the reduction in the Ca/Si ratio compare with individual elevated temperatures is observed in M4 mix it is due to the formation of thick C-S–H gels from silica fume and flyash is anticipated to correlate with enhanced strength at temperatures, apart the fine aggregate as M-Sand helps to make better bond matrix in concrete. However, at high temperatures, the decrement in the compressive strength is due to decomposition of C-S–H gel, which leads to improvement in the porosity it adverse the formation of series of voids, cracks are observed in the SEM image. Furthermore, in practical applications, it is well-known that blended binders with silica fume (amorphous SiO₂) result in improved mechanical performance, but this is commonly improved particle packing and microstructural refinement [21]

The increase in the atomic Ca/Si ratio results in a decrease in the compressive strength of the concrete and the existence of Si, Ca generates a dense amount of CSH gel at the IT zone of the concrete matrix phase, resulting in closely packed homogenous integrity, which in turn leads to a reduction in the synthesis of calcium silicate hydrate (C-S–H) gel due to decomposition of hydration products exposed too fire [26, 45, 46].

Conclusion

The findings of the study investigating the thermal parameters of concrete mixtures. Based on the experimental investigations, mixes incorporating FA (10%), SF (0%, 6%, 12%, 18% & 24%), and 100% M-sand subjected to elevated temperatures at 27,250, 500, and 750 °C, of a steady state of 2.5 h, the following conclusions can be drawn:

1. In the study, M4 mix demonstrated better residual compressive strength, having 44.45 MPa at the elevated temperatures up to 500◦C, this is due to the blending of silica fume (amorphous SiO2) and flyash, resulting in the improvement in the mechanical performance, apart due to particle packing density of ternary blends facilitated the development of a more compact C-S–H matrix at microstructural refinement. After 500 to 750◦C, a decrement in strength is observed, which is due to the decomposition of the C-S–H matrix, leading to the formation of a series of voids, cracks observed in SEM, EDS analysis.

2. The quality of concrete is observed for the mixes containing 100% M sand and cement replacement with fly ash and silica fume show the quality of concrete is excellent at 250 °C, good at 500 °C and moderate at 750 °C, while the mix M4 shows the quality of matrix is good, it is due to the cohesiveness of the mixes, pozzolanic reactivity, and refined microstructure.

3. In the TGA analysis, after 250 degrees centigrade, the weight loss < 2.5%. From 400–600◦C °C, the weight loss is about 7%. From 600 to 800 °C, it is about 10% only. The highest weight loss was observed after 600 °C, which is due to the carbonation loss, decomposition of portlandite, and breakage of bonds in the C-S–H structure. It leads to a drop in the compressive strength and the quality of the concrete due to the formation of voids, cavities in the matrix.

4. SEM/EDS analysis observed that with an increase in temperature, there is a breakage of bonds and decomposition of the concrete matrix.M4 mix shows better accumulation of dense matrix at elevated temperature in the SEM compared to the other mixes. In the EDS analysis, low Ca/Si is observed, which resembles improvement in the hydration of dense C-S–H structure, and it is about 1.9,1.46 1.7 at 250, 500, and 750 °C. Based on the finding, M4 mix shows good performance at individual temperature levels when compared in the same temperature, like 250, 500, 750 °C, about 51.25, 44.5, and 35 MPa at 250, 500, 750 °C, but in overall compressive strength declines when the specimens are subjected to high temperatures. The drop in the compressive strength is due to the breakdown of C-S–H nanostructures.

Overall, it is observed that M4 (10% flyash,12% silica fume, and 100% M-Sand) shows good performance and advantages compared with the control mix at the elevated temperature. The findings possess significant practical relevance in the design and construction of buildings exposed to extreme environmental conditions such as elevated temperatures and fire.

Authors' contributions

Bypaneni Krishna Chaitanya: Writing original draft, Experimentation, Y.Madhavi : Visulavalization, Chava Venkatesh: Editing,Chereddy Sonali Sri Durga: Proof reading and Visualization, A.Srinivasa Prasad : Supervision and Editing, Meseret Getnet Meharie: Writing original draft, Experimentation, Draft correction and Visulavalization, George Uwadiegwu Alaneme :Visulavalization, N.Satya Vijay Kumar : Visulavalization.

Funding

This research received no external funding.

Data availability

Data are available upon request.

Declarations

Competing interests

The authors declare no competing interests.

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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