Durability of high-performance concrete at high temperatures: effects of water-binder ratios and use of silica fume

ABSTRACT

This work examines the impact of altering the water-binder ratios (w/b) and cement/silica fume (SF) replacements on the strength at the compression of High-Performance Concrete (HPC), both before and during prolonged contact with extreme temperature. After preparation and testing, eighteen mixtures were produced. Based on the variation in weight/bulk density, the compressive strength test results at room temperature varied from 58 to 102 MPa. In addition, a novel technique known as “heat endurance” has been implemented to compare HPC responses at high temperatures. The findings demonstrate that pozzolanic interaction with the fillers component of SF improves HPC’s residual compressive strength following exposure to high temperatures. Comparative measurements of retained strength of compression were greatest for blends containing 6%, 12%, and 15% of SF at w/b ratios of 0.30, 0.35, and 0.40. As a consequence, altering the w/b ratio had a substantial impact on the outcomes. Lastly, a variety of measuring methods were offered to assist with the study, such as CT, SEM, and thermogravimetric (TG) analysis to evaluate the microstructure modification, porosity, and mass loss of HPC.

Keywords:
High performance concrete; Water-binder ratio; Silica fume; Compressive strength; Heat endurance

1. INTRODUCTION

High-strength concrete (HSC) has been manufactured extensively as a suitable replacement material for regular concrete since the 1950s. High cementitious material concentrations and low water/cement ratios (w/b) are crucial for the formation of HSC. When SCM (supplementary cementitious materials) such as fly ash (FA) and silica fume (SF) are used in part place of ordinary Portland cement (OPC), it has been demonstrated that several economic, environmental, and performance criteria are met. A by-product of elemental ferrosilicon alloys containing around 30% mass-produced silicon is silica fume. SF is an extremely fine non-crystalline silica powder that emerges from the furnaces as a secondary material. The SF has small pores of dust, with distinctive diameters of 0.1 to 0.2 μm, accounting for around 90% of the SF’s chemical makeup [¹]. Apart from its physical characteristics, SF has a range of colors, mostly because of the iron and carbon oxides. Global production of SF is estimated to be approximately 900,000 metric tons (1,000,000 tons) per year, according to ACI Committee 234, 363, and TR 41 figures [²,³,⁴]. Consequently, in recent years, SF has become the most SCM and has greatly increased in popularity. When mixed with Portland cement, SF exhibits strong pozzolanic activity primarily due to its physicochemical properties. The median diameter of individual SF nanoparticles is 0.1 μm, which is roughly 100 times smaller compared to the average diameter of OPC particles [⁵]. It makes it logical that SF would have a discernible effect on concrete’s properties. In the 1950s, research on the use of SF in concrete initially began. A dense structure was produced and in turn, improved the micro hardness and fracture toughness when the silica fumes were added instead of cement in the range of 2 to 5%. The microstructure becomes denser and calcium silicate hydrates (C-S-H) are formed, which together contribute to the development of strength and boost the mechanical characteristics. If concrete is exposed to high temperatures, the qualities of the material alter, therefore SF’s benefits are not just restricted to room temperature [⁶].

To assess how well HSC containing SF performs after being exposed to high temperatures, several investigations have been conducted. HSC subjected to temperatures as high as 600°C has mechanical characteristics. The testing settings included diversew/bproportions (0.22, 0.33, and 0.57) and SF substitutions with 0% and 10%. Based on the quantity of the w/b ratio, the results showed that the chance of spalling increased as the w/b ratio decreased, affecting the loss of residual strength. The results showed that the tendency of explosive spalling was not significantly affected by SF alone. Since spalling occurs suddenly in response to heat pressure and thick pore structure, the benefits of superficies filling are limited [⁷]. Concrete behaved at high temperatures when different SF substitutes for cement, such as 0%, 5%, 10%, and 15%, were used. For concrete to avoid spalling, the authors recommended replacing 10% of the total square footage [⁸]. Following high-temperature treatment with SF-produced HSC at a 0.3 w/b ratio [⁹].Poor performance was seen in the results, particularly when cement replaced more than 10% of the SF. After subjecting HSC to high temperatures, the effects of varying w/b ratios and SF to cement substitutions on the material’s residual compressive strength. At both 100^oC and 200^oC, the researchers found no significant difference in the relative residual compressive strength when SF was substituted. However, as the temperature rises over 300°C, the contribution of SF has been demonstrated to have a significant impact on the residual compressive strength. The optimal substitution ratio between SF and cement was determined to be 6%. The evaluation of HSC containing SF’s possible resistance to spalling was the primary goal of the aforementioned investigations. When temperatures rise, the strength of the concrete is less likely to spill than high-density concrete (HDC).The problem has to be evaluated and further research done [¹⁰]. Earlier studies have suggested that the primary mechanisms causing increased temperature concrete degradation include thermal gradients, microcracking within the frame, pore-structure coarsening, and the thermal deterioration of hardened cement paste. The grouting paste mix plays a major role in determining how well concrete bonds; hence, any impairment to the HPC decreases the bond and lowers the concrete’s capabilities. Therefore, it is very important to examine how high temperatures affect the micro and macrostructure levels of HPC to assess how HSC behaves overall. Studies on HPC with SCM exposed to high temperatures have been conducted in a limited number [¹¹, ¹²].

With relation to cement paste formulations, Researcher’s investigation into the HPC included several substitutions of SF to cement, including 0%, 5%, 10%, 15%, and 20%. They concluded that mixes comprising 10% and 15% SF had compressive strengths that were very resistant to high temperatures [¹³]. The temperature rises significantly speed up the pozzolanic process [¹⁴]. Lastly, a variety of parameters, including the w/b ratio, oxide compositions, particle grading, shape, and fineness of the cement particles assessing the microstructure of HPC is a difficult task. In the end, the results of this experimental investigation offer a resistant HPC against elevated temperatures to lessen the harm that elevated temperatures may do to structures. The test performed to look at the HPC’s mechanical properties is the compressive strength test. The additional measurements and their results were supported, including (i) mass loss assessed by thermogravimetric (TG) tests, (ii) computed tomography (CT) analysis of porosity and pore distribution, and (iii) to offer a trustworthy assessment of the HPC microstructure, scanning electron microscopy pictures were recorded.

2. EXPERIMENTAL WORK

2.1. Materials

With incremental replacements of 3%, 6%, 9%, 12%, and 15%, the cement weight replacements ranged from 0% to 15%. The cement paste combined with SF was created in up to six groups by mixing and manufacturing these substitutes. After that, the mixes were assessed at three different water-binder ratios: 0.30, 0.35, and 0.40. For each w/b ratio, figuring out the optimum replacement of SF is crucial to the HPC’s effectiveness. Otherwise, using SF instead of the optimum substitute might jeopardize the HPC’s longevity and mechanical properties. There were eighteen distinct temperature cubes in total 20, 50, 150, 300, 400, 500, 800, and 900°C in each of the eighteen mixtures. The detailed properties of eighteen various blends are shown in Table 1. The Indian firm, Sapphire Corporation Enterprise provided the OPC and SF materials utilized in this experimental inquiry.

Table 1
Design parameters and its experimental matrix details.

MIX PROPORTION
WATER/CEMENT RATIO – 0.30				WATER/CEMENT RATIO – 0.35				WATER/CEMENT RATIO – 0.40
MIX (%)	SILICA FUME	CEM I	H2O	MIX (%)	SILICA FUME	CEM I	H2O	MIX (%)	SILICA FUME	CEM I	H2O
0	0	480	144	0	0	480	168	0	0	480	192
3	14.4	465.6	144	3	14.4	465.6	168	3	14.4	465.6	192
6	28.8	451.2	144	6	28.8	451.2	168	6	28.8	451.2	192
9	43.2	436.8	144	9	43.2	436.8	168	9	43.2	436.8	192
12	57.6	422.4	144	12	57.6	422.4	168	12	57.6	422.4	192
15	72	408	144	15	72	408	168	15	72	408	192

Table 2 shows that SF and cement meet European standards for their physical and chemical characteristics. The European standard specification uses CEM I 42.5 N. This is mostly composed of CaO, SiO₂, and Al₂O₃. However, Table 3 shows that the crystalline phases can be distinguished as follows: Gypsum (CaSO₄•2H₂O), tricalcium silicate (3CaO•SiO₂), dicalcium silicate (2CaO•SiO₂), belite, larnite, and tetra calcium alumino ferrite (4CaO•Al₂O₃•Fe₂O₃) as the principal crystalline component. The cement particle grading sieve curve is depicted in Figure 1. The utilized SF is completely amorphous, with a 2% sieve rejection at 45 µm and <1 μm of the diameter of the particle. By EN 1008:2002 standard standards [¹⁵], tap water is utilized for mixing.

Table 2
Physical and chemical characteristics of silica fume and cement.

TYPE OF PROPERTY	CEMENT	SILICA FUME
Density (g/cm3)	3.09	2.18
Surface area (cm2/g)	3928	19958
SiO2	19.59	95.96
Al2O3	5.19	0.00
Fe2O3	3.18	0.08
CaO	63.89	0.81
MgO	1.41	0.74
SO3	2.88	0.03
K2O	0.74	1.19
K2SO4	0.02	0.00
Loss in ignition (%)	2.97	2.81

Table 3
Cement used and its phase composition.

COMPOSITION OF CEMENT	MASS IN %
Tri-calcium Silicate (C3S)	68.65
Dicalcium Silicate (C2S)	14.82
Tri-calcium Aluminate (C3A)	4.77
Tetra-calcium alumino ferrite (C4AF)	9.51

Figure 1
Cement - Grading plot.

2.2. Methods of mixing

In compliance with EN 196-1 and EN 197-1 [¹⁶, ¹⁷], the mixing procedure was conducted at room temperature (20 ± 2°C). To uniformize the binding property, SF and cement were mixed dry, and combined for 30 seconds at the start. The cement and water were then mixed for two minutes. 75% of the cement was combined with the entire amount of water to prevent the agglomeration of dehydrated particles; the remainder was added while the mixture was being mixed. The cement paste was shaped into 30-by-30-millimeter cubes. To get rid of air bubbles, a vibrating table was used to stir the new mixture. The cast cubes were kept in a lab setting (20 ± 2°C) for a full day until they began to demould. The cubes were kept in storage in the lab following a seven-day curing period in a water tank.

2.3. Methods of testing and heating

HPC mixes were analyzed both while they were fresh and when they had solidified. A Haegermann-Flow Table, with a Ø 300 mm diameter aluminum/glass plate and a Ø 70/100 mm funnel cone, was utilized for the fresh property. The funnel needs to be positioned in the middle of the table throughout the exam. There were two levels of fill in the funnel and a tamper was used ten times to flatten each layer. We took out the funnel just fifteen seconds later. Once the procedure is complete, the measurement of the diameter will be determined in two directions and the table gets smashed fifteen times, or one shock every second. DIN EN 459-2 [¹⁸] compliance is met by the test. HPC specimens with toughened characteristics underwent a 1.40 kN rate of load strength of compression test. The ALPHA 3/3000S test equipment, which has a compressive stress capability of 200 kN, is used to conduct the test. The following is how the heating program and test technique have been used: Tests were done when the animals were 90 days old. Every set of three cubes had the heating program implemented; the temperature was raised logarithmically to reach the desired level and then fixed for two hours by ISO 834 [¹⁹]. The specimens cooled naturally after being subjected to different temperatures (20 to 900°C). The study employed a temperature range that is comparable to what may be seen in a real fire event. The HPC’s physical changes and chemical response served as the basis for choosing the temperature ranges. Between 50°C and 150°C, which are quite high temperatures, there was a brief decrease in strength; for this reason, these temperatures were selected to measure the rate of degradation. At 400 to 500 degrees Celsius, Portlandite begins to decompose. Given the measuring inaccuracy of the furnaces, CSH breakdown occurs after an 800°C heat load. The propensity of the strength decline changes as a result of the extra heat load was investigated using a temperature load of 900°C.

2.4. TG (Thermogravimetric) analysis

Thermogravimetric analysis is used to define various thermal decays of unique High-performance concrete and stages, and mass loss under static circumstances is assessed concurrently. In addition, phase transitions and alterations in the material characteristics were tracked using Thermogravimetry/Derivative Thermogravimetry/Differential Thermal Analysis (TG/DTG/DTA) apparatus. Al₂O₃ serves as the reference material for the measurements, with a 300 mg sample mass. At 1000°C air environment the specimens were heated at 10°C/min. The thermogravimetric samples were measured in TG/DTA after being pulverized into a fine powder to ensure that there was no carbonation present. The powders were taken from the center of the specimens. Winder (Version 4.4.) program was used to determine the thermo-analytical test results. Samples of ambient temperature (20°C) were used to evaluate the powders. The reference combination (SF0%) and the mixture having the ideal SF replacement obtained at increased temperature for the mixture made with a 0.30 w/b ratio were the two mixtures for which the powders under investigation were intended. After the samples were 90 days old, the TG/DTA investigations were conducted.

2.5. Computed tomography

It is possible to examine the pore size, pores distribution, the permeability of HPC with the CT approach. The CT examinations were performed using a Siemens Somatom 16. The slices measured 1.5 mm in thickness and 0.225 mm in pixel spacing. Before the CT analyses began, the samples were unloaded. Without requiring human input, automatic algorithms in the Matlab environment processed the CT slices using predetermined parameters. For reference and SF mixes, CT measurements have been performed on cylindrical specimens measuring 100 mm in diameter and 50 mm in height. The CT test findings were meant to show pore distributions and porosity through HPC both with and without SF. The total values of the slices collected along the vertical axis of the sample were used to calculate the pores content.

2.6 Microstructure analysis

Using a Phenom XL scanning electron microscopy (SEM), we examined the microstructure of high-performance concrete (HPC). At the microstructural level of the modified HPC, SEM provides excellent magnification and resolution. SEM photos were captured of the samples that contained SF as well as the reference sample (SF 0%). Samples from the center of the specimen’s cross-sectional regions were chosen following the compressive strength test. Afterward, a 30-second golden spray coating was applied to the sample faces, and SEM analysis was performed on the samples. Under various experimental conditions, the surface properties of SF concrete mixes (SF 0% and SF 6%) were investigated using energy-dispersive X-ray spectroscopy (EDX). To improve the mechanical strength of concrete mixes, the efficiency of the HPC and the SF interactions with concrete particles were investigated through the analyses. The best mixtures, which were identified based on each concrete mix’s mechanical strength findings in this study, underwent microstructural investigation.

3. RESULTS AND DISCUSSION

3.1 Fresh concrete – characterization

By lessening the bleeding, SF enhances the matrix’s physical characteristics. Nucleation sites are thus created, enabling the hydration products to easily precipitate. The findings of Table 4’s slump test demonstrate that, for a certain cement matrix composition, the amount of slump is not considerably impacted by the presence of SF material. However, as the SF percentage was raised, the paste combinations with SF demonstrated more cohesiveness than regular cement paste and were a little harsher, especially for blends with low w/b ratios.

Table 4
Slump value of concrete mixes in (mm).

WATER/CEMENT RATIO	0%	15%
0.3	230	200
0.35	270	260
0.4	300	300

3.2. At-room-temperature measurements

3.2.1. Compressive strength

Every compressive strength test result was performed on specimens that were 90 days old. Figure 2 shows the compressive strength values of eighteen mixes before heating. Compiling the standard deviation for each number and displaying it as an error bar showed how consistently the compressive strength data were collected. The w/b ratios (0.3, 0.35 & 0.4) of high-performance concrete with SF replacement (9%, 9%, and 9%) provide the maximum level of compressive strength for the concrete specimens. For compressive strength, these produced comparable findings of 87.73, 98.59, and 99.29 MPa respectively. The compressive strength of the concrete mix and its rise was mostly due to the filling role of SF particles and how they combine with the cement hydration product to create C–S–H [²⁰]. As a result of the w/b ratio fluctuating, significant variations in compressive strengths were observed, as seen in Figure 2. Additionally, the compressive strength values of the specimens with the ideal SF replacements increased more than those of the reference specimens (SF 0%). Up to 9% of SF replacement the rise in compressive strength was observed based on their w/b ratios.

Figure 2
Results of concrete compressive strength.

3.3. Exposure to high temperatures

This section presents the results on residual compressive strength following exposure to high temperatures. By the RILEM (Réunion Internationale des Laboratoireset Experts des Matériaux) [²¹] guidelines, the specimens were assessed at 3 months of age. The compressive strength ratio remaining at every Celsius threshold to the compressing strength of the same composition at the ambient temperature was used to compute the relationship between the residual compressive strength findings as a function of temperature. Figure 2 shows the compressive strength of each combination at room temperature. The heat endurance technique may be applied in this study by computing the area under every bend in addition to the particular curve data.

3.3.1. Impact of SF quantities

The formation of high-performance concrete with SF replacements and w/b ratio modifications was analyzed and shown in Figure 3 (a), (b), and (c) respectively. Furthermore, Figure 3displays the results of the computation of residual strength of concrete. At temperatures as high as 150°C, the dehydration of cement products particularly the breakdown of ettringite often results in a drop in the relative residual strength. After that, the passage of steam hydrates the unhydrated cement grains in the microstructure, causing the residual strength to increase to 300°C [²²]. The residual strength then drops once more at temperatures over 300°C. ZHANG et al. [²³], also reported on this outcome. It demonstrates that when temperatures rise, particularly over 150°C, the strength of SF combinations improves. This increase may be explained by the fact that SF mixes include more fine particles than regular mixtures do and that this increases the hydration of the unhydrated particles [²³]. Following this, at temperatures as high as 500°C, there were notable decreases in the reference mixes’ relative residual compressive strength (SF 0%), mostly as a result of portlandite’s (Ca(OH)₂) breakdown.The corresponding residual strength for 0.30, 0.35, and 0.40 w/b ratios at 500 °C is 24%, 34%, and 40%, as seen in Figure 3.

Figure 3
High-performance concrete and relative compressive strength for (a) 0.3 w/b, (b) 0.35 w/b, and (c) 0.4 w/b at different temperature levels.

Alternatively, the relative residual strength of concrete is improved when silica fume (SF) is present. It was shown that the residual compressive strength values were much greater in specimens with a certain water-to-cement (w/b) ratio that had been exposed to a temperature 500°C below room temperature. Upon heating, the specimen with 6% SF preserved 97% of its compressive strength, but the specimen with 9% SF retained 92% of its compressive strength, as demonstrated by the comparison between the two specimens. This highlights how silica fume helps the concrete withstand higher temperatures, indicating that it may be useful for strengthening thermal or fire-resistant qualities [²⁴]. The test findings, as shown in Figure 3, demonstrate that specimens with 6% and 9% replacement of silica fume (SF) saw a considerable decline in compressive strength values to 63.58% and 66.49%, respectively, following exposure to a temperature of 800°C. Furthermore, at higher temperatures, the relative strength drops when SF is replaced by more than 9%, which may be the result of a dense microstructure. The improvement in each paste mixture’s resistance to heat for 0.30 w/b after adding SF is shown in Figure 4. As such, the maximum heat endurance value that might be considered a suitable substitute is 6% of SF. Also, figure 4 displays the results of mixing HPC at 0.35 water-binder ratio. The specimens containing 12% silica fume (SF) were exposed to the temperature ranges of 500°C and 800°C and they showed compressive strength values of 83.59% and 52.83%, respectively. Notably, out of all the studied permutations, the combination containing 12% SF showed the highest heat tolerance. As Figure 4 illustrates, this suggests that adding SF greatly improves the concrete’s resistance to high temperatures. Also, the relative residual strength of the 0.4 w/b concrete mix was shown in that figure 4.

Figure 4
Heat endurance of High-Performance Concrete with different w/b ratios.

The combination with 15% SF had the greatest comparative strength residual values. After the temperature exposure (500°C and 800°C), the relative strength values were measured at 77% and 51%, respectively. Figure 4 shows graphically that performance is improved with mixtures containing 15% silica fume (SF) according to heat endurance data. Test findings show that SF significantly improves blend performance, particularly when the blends contain the ideal SF replacements identified in the three w/b ratios. Throughout the process, the very thick microstructure is produced by the pozzolanic reactions of the ultra-fine SF particles, which serve as effective fillers and provide the HPC with a high degree of thermal resistance. The HPC’s SF efficiency showed a significant improvement as the temperature surpassed 400°C. Reduced effects of the pozzolanic reaction on the degradation of (Ca(OH)₂) might account for this [²⁵].

3.3.2. Water–Cement ratio and its effect

The compressive strength of high-performance concrete after exposure to the high temperature was examined by altering the w/b ratio and represented in Figure 5. Based on how the experiments conducted changed when exposed to high temperatures, the results may be split into two groups. Relative residual strength data between 20 and 400°C are grouped into the first group, while those beyond 400°C are grouped into the second group. When compared to specimens generated with a w/b ratio of 0.35, specimens created with a 0.30 w/b ratio demonstrated greater values up to 400°C, as shown in Figure 5. Then, there was a discernible behavioral shift: specimens produced with a w/b ratio of 0.35 performed better than those prepared with a w/b ratio of 0.30. The emergence of severe fractures at increased temperatures may be explained by the very thick microstructure of high-performance concrete (HPC) at low w/b ratios (0.30). Both the HPC’s permeability and moisture content rise with the w/b ratio. At higher temperatures, the high permeability of the HPC would cause the hardened paste to behave positively, while the high moisture content would have the opposite effect [²⁶]. The amount of physically confined water rises with moisture content, leading to an increase in temperature gradient and pore pressure at higher temperatures. Furthermore, reducing the quantity of moisture has been shown by several researchers to lessen spalling [²⁷]. The study’s findings suggest that in the HPC subjected to high temperatures, mixes with a significant w/b ratio and improved permeability fared only slightly better than mixes with a larger moisture content [²⁸].

Figure 5
Reference mix (SF 0%) its w/b ratio effect in various temperature.

The findings of the research’s tests indicate that at higher temperatures, the benefits of adding SF to HPC on strength in compression outcomes are larger. The ideal silica fume (SF) mixtures for each of the three water-to-cement (w/b) ratios are shown in Figure 6. The optimal amount of SF needed was calculated regardless of the selected w/b ratio, with an emphasis on combinations that showed the least amount of activity in terms of deterioration of residual compressive strength. The high-density microstructure of silica fume (SF) replacement is the main factor responsible for fractures, hence restricting its overall activity. Furthermore, this process is influenced by the amount of free water that develops porosity in the high-performance concrete (HPC) structure. Consequently, the restricted use of the given square footage may be altered by adjusting the w/b ratio.

Figure 6
High-performance concrete and its heat endurance with SF optimum replacements for different w/b ratios.

3.3.3. Thermogravimetricanalysis

Figure 7 displays the results of thermogravimetric tests conducted on samples of hardened paste for both the combination containing SF (SF 6%) and the reference mixture (SF 0%). In thermo-analytical studies, high-performance concrete (HPC) experiences many heat reactions between 20 and 1000°C. The test findings indicate that the majority of these reactions are endothermic, with three main stages. Ettringite dehydration (C₃A•3CaSO₄•H₃₂), mono-sulfate dehydration (C₃A•CaSO₄•H₁₂), and the evaporation of physically coupled water are essentially related at 20–200°C temperature levels. The breakdown of Ca(OH)₂ is the cause of the ­second temperature range, which is 430–540°C.The degradation of calcium carbonate (CaCO₃) and calcium silicate hydrate (C-S-H) has been linked to the third temperature range, which is normally between 600°C and 900°C. The amounts of calcium hydroxide (Ca(OH)₂) and calcium carbonate (CaCO₃) in the high-performance concrete (HPC) have a major impact on the strength deterioration. The breakdown of Ca(OH)₂ and CaCO₃ resulted in reduced mass loss in the mixes including SF, as seen in Figure 8. Thus, these results were the primary causes of the combinations including SF’s superior performance.

Figure 7
Loss of mass for reference mix (SF 0%) at 0.3 w/b.

Figure 8
Pore distribution through HPC.

3.3.4. Outcomes of computed tomography

Using the CT technique, the pores bigger than 0.01 mm were measured to assess the amount and placement of pores in the sample. Elevated accurateness may be achieved with this procedure, which can serve as a substitute for other common laboratory tests. The distribution of pores in high-performance concrete (HPC) at room temperature (20°C) is shown in Figure 8 for both specimens that include silica fume (SF 6%) and reference specimens (SF 0%), in that order. The data pattern indicated that the sample with SF had fewer pores overall. It is possible to examine Figure 8 to illustrate the protective impact that SF adds to the HPC microstructure. The entire volume of the pores was also computed, revealing that the HPC with 6% SF had 3.5% of its complete volume, whereas the HPC without SF had 5.93% of its whole volume.

3.3.5. Results of microstructural analysis

SEM allows for the visual identification of HPC microstructures, as seen in the pictures shown in Figure 9. The pictures show how the HPC microstructure changed for specimens that were at room temperature (20 °C) and after adding SF. In contrast, there was no detection of Ca(OH)₂ in the HPC containing SF, and the sample’s microstructure exhibits a significant concentration of C–S–H. These outcomes were consistent with SF’s pozzolanic response with Ca(OH)₂.The breakdown of Ca(OH)₂, which accounts for around 28% of the mass of pure HPC, is the primary factor contributing to the degradation of HPC at high temperatures, as previously mentioned. That is why at higher temperatures, mixes containing SF performed better [²⁹, ³⁰].

Figure 9
SEM analysis through HPC at (a) SF 0% and (b) SF 6% respectively.

The results of Energy Dispersive X-ray Spectroscopy (EDX) shown in Figure 10, which show the amount of calcium hydrate in the SF 0% and SF 6% specimens, corroborate the findings. Higher silica fume (SF) ratios were associated with lower calcium-silicate-hydrate (C-S-H) contents, which in turn led to higher calcium hydroxide (CH) and lower silicate hydrate (S_i) contents. The specimens’ increased porosity was made possible by this decrease in C-S-H. Prior studies have looked at the decrease of calcium-silicate-hydrate (C-S-H) bonds brought on by the usage of silica fume (SF) [³¹]. The compressive strength and porosity test results from the methanol exchange method are consistent with the results of the scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX) investigations.

Figure 10
EDX analysis through HPC at (a) SF 0% and (b) SF 6% respectively.

4. CONCLUSION

The high-performance concrete subjected to high temperatures combined with silica fumes was investigated in this research study. Temperature variations, water-to-cement (w/b) ratios, and SF substitutions are all investigated in this study. The authors emphasize the originality of their study strategy by pointing out a dearth of previous work addressing the testing of SF additions at higher temperatures with brief incremental replacements. Also, to better comprehend the mechanism underlying the SF effect, the study includes other measuring techniques, such as CT, SEM, and TG/DTG/DTA. These were the principal conclusions reached:

• After being exposed to increased temperatures, the optimal replacements of SF achieved differed from those obtained at room temperature.

• When compared to combinations with a low water-to-cement (w/b) ratio, silica fume (SF) was more effective at room temperature at increasing compressive strength. The highest compressive strength was achieved at a w/b ratio of 0.30 and a silica fume replacement percentage of 9% at room temperature.

• Mixtures with 6% silica fume (SF) showed the greatest absolute residue compressive strength and the most heat endurance when heated to high temperatures, especially at 0.30 w/b ratio. The maximum compressive strength of 96.84% and 63.23% was observed at the maximum temperature exposure of 500°C and 800°C respectively.

• At a w/b ratio of 0.35, the SF 12% replaced concrete showed the best heat endurance even at high temperatures. The compressive strength values of 83.28% and 52.23% were measured at the temperatures of 500°C and 800°C respectively.

• The maximum heat endurance was observed at a w/b ratio of 0.40 for SF 15% concrete mix subjected to high temperatures. Values of 76.69% and 50.24%, respectively, for relative residual strength were obtained at 500°C and 800°C temperature exposure.

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Publication Dates

• Publication in this collection
  03 Feb 2025
• Date of issue
  2025

History

• Received
  17 Oct 2024
• Accepted
  22 Nov 2024