The effect of ATP as a cement substitute and the amount of BF on the fresh properties of FC mixtures, including 100% PA, is shown in Fig. 3 in terms of flow diameter results. As the percentages of ATP and BF increase, the mixes' flow diameters, which range from 25 to 8.4 cm and are clearly shown to decrease in Fig. 3. The water/cement ratio was maintained at 0.25 throughout the sample preparation procedure to evaluate the impacts of BF and ATP in FC mixes. It was discovered that the high water absorption capacity of ATP, which may reach up to 180% of its mass, decreased the workability of FC mixes [43] was substituted for cement and BF was added, as depicted in Fig. 3. When ATP was substituted for PC in FC mixes, the flow diameter decreased [44], and with a rise in ATP content, this decline intensified [45]. The enormous specific surface area, water-absorption capacity, and nonporous structure of ATP, which reduces flow diameters, all contribute to its explanation [46]. Moreover, the fine-grained ATP-induced increase in the aggregate surface of FC mixes led to a decrease in the flow diameters. The lowest workability value of 8.4 cm which is lower than the reference mixture by about 66.4% was achieved when cement was replaced with 30%ATP and with 0.5 and 2%BF included, and the reference mixture with 0%ATP and 0%BF had the largest flow diameter, measuring 25 cm. BF addition also resulted in a decrease in the flow diameters of FC combinations, as can be seen clearly in Fig. 3. This is largely because of the BF's increased friction force [47] and improved cohesiveness with the matrix, which both reduce mixture fluidity [48]. FC mixes were shown to become less workable and more viscous when fibers were added. This observation suggests that the fibers may hinder the aggregates' free mobility. The flow diameters of 10%ATP-incorporated mixtures reduced to 17, 14.1, and 9.5 cm at 0.5, 1, and 2%BF content leading to flowability reduction of 47, 43.6, and 62%, respectively, compared to the reference mixture. Similar reduction in flowability was clearly seen as illustrated in Figs. 3 and 5 when 20 and 30%ATP were substituted for cement. For mixes containing 20% ATP, the flow diameters decreased to 11.1, 9.8, and 8.5 cm at 0.5, 1%, and 2% BF content, respectively, resulting in a reduction in flowability of 55.6, 60.8, and 66% as per the reference mixture. The flow diameters for mixes with 30% ATP dropped to 8.4, 8.5, and 8.4 cm at 0.5, 1%, and 2% BF concentration, respectively. This meant that the mixes' flowability was reduced by 55.6, 60.8, and 66% in comparison to the reference mixture. The reduced workability of the mixtures with 30% ATP and BF was attributed to ATP's high water absorption and the increased internal friction from the fibers, which restricted particle movement and significantly decreased the flow diameter compared to the reference mixture.
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Figure 4 displays the images obtained from the flow diameter tests that were conducted to evaluate the FC blends' flowability characteristics.
Figure 5 depicts the impact of BF content and ATP as a cement substitute on the 28- and 90-day compressive strengths of FC mixes containing 100% pumice aggregate (PA). Figure 5 illustrates the strength improve that was seen as the curing age increased. The specimens of FC mixtures with 10%ATP showed a compressive strength that varied from 6.95 to 7.73 MPa and from 8.36 to 8.85 MPa at 28 days and 90 days, respectively. As shown in Fig. 5, during both curing ages, the FC combinations' compressive strength was lower than the reference mixture when cement was replaced with 10% ATP. Strength reductions with the addition of 10%ATP as compared to the reference blend were in the range of 8.84 and 18% at 28 days, and 25.81 and 29.92% at 90 days, showing more strength loss at later ages. On the other hand, adding BF reduced the compressive strength loss of 10% ATP-incorporated mixes as BF content increased. Since the addition of BF enhanced the compressive strength, the mixes incorporating 10% ATP and 2% BF at both curing ages attained the highest strength. Numerous research have reported that the reason for these improvement in compressive strength is because BF occupy the pores of the concrete in different orientations, improving the strength of the material [49]. Because of their small sizes, BF can lower the stress concentrations that cause micro-cracks to propagate farther in concrete [50], improve the cracking resistance and ductility of the material, and generally enhance concrete performance [51]. Niaki et al. [52] reported that adding BF at a dosage of 0.5–3.5 wt% enhanced the compressive strength to an ideal fiber content of 2%. At a 2% fiber dosage, the compressive strength rose by almost 10% when compared to the control mix. Nevertheless, the compressive strength decreased by roughly 12% and 21%, respectively, upon the additional increase in BF dosage to 2.5% and 3%.
When 20% ATP-incorporated FC mixtures were considered, all mixtures exhibited higher strength enhancement as compared the mixtures with 10%ATP but only the mixture with 0.5%BF had a slight lower strength at 90 days than the reference blend. The other two 20% ATP-incorporated mixtures containing 1 and 2%BF attained higher strength than the reference blend. The compressive strength enhanced significantly and increased to 17.43 and 20.98 MPa at 28 and 90 days for the blend comprising 20%ATP and 2%BF(A20B20). The enhancements in the strength for this mixture were 105.54 and 75.85% as per the reference mixture. The American Concrete Institute standard (ACI 318) [53] states that concrete is adequate for structural purposes if, after 28 days of curing, its compressive strength values are equal to or greater than ∼ 17 MPa. The FC mix containing 20%ATP and 2%BF were suitable for structural purposes, because it had compressive strength greater than 17 MPa. The mixture with 20%ATP that performed the best in terms of compressive strength was the one that combined 20%ATP and 2%BF.
When FC mixes containing 30% ATP were taken into account, all of the mixtures showed greater strength enhancement when compared to the reference blend, the mixes comprising 10% ATP and the 20%ATP-incorporated mixtures with 0.5 and 1%BF. The A30B5 mixture, comprising 30% ATP and 0.5% BF, achieved the highest compressive strength, showing remarkable gains of 129.3% at 28 days and 85.33% at 90 days compared to the reference mix. This enhanced performance results from the synergistic effects of ATP’s pozzolanic reactivity combined with the optimal reinforcement level of BF, which together strengthen the FC matrix. The FC mixes containing 30%ATP and 1%BF were also suitable for structural purposes, because they had compressive strength greater than 17 MPa. At 30%ATP content, increase in BF content decreased the compressive strength slightly at 1%BF and more decrease was measured at 2% BF. The rise in compressive strength was verified by the thermos-gravimetric study's data, which also demonstrated that ATP shows increased the pozzolanic reactivity at later ages after 28 days [43]. ATP functioned as a pozzolanic material, reacting with calcium hydroxide generated during cement hydration to produce additional calcium silicate hydrate (C–S–H) gel. This secondary C–S–H formation led to a denser, more robust matrix by filling micro-pores and enhancing the overall cohesion and strength of the FC. The pozzolanic reaction of ATP was especially effective at later ages (beyond 28 days), accounting for the continued strength gains observed at 90 days. The addition of 0.5% BF offered effective crack-bridging and reinforcement while preserving the concrete matrix’s homogeneity. Basalt fibers enhanced the tensile and compressive properties of FC by inhibiting crack propagation and improving load distribution within the matrix [54]. At this optimal fiber content, the fibers were evenly dispersed, maintaining good workability and efficiently bridging cracks, which led to an increase in compressive strength [55]. Utilizing 30% ATP and 0.5% BF created a balance between the matrix densification provided by ATP and the crack resistance offered by BF. This equilibrium was crucial, as ATP strengthened the matrix through additional C–S–H formation, while BF reinforced it without the risk of excessive fiber content, which could lead to clustering, voids, or decreased workability. Increasing the fiber content would have upset this balance, while reducing the amount of ATP would have lessened pozzolanic reactivity and matrix densification.
Figure 6 depicts the impact of BF content and ATP as a cement substitute on the 28- and 90-day axial stress–strain behavior of FC mixes. Stress enhancement was noticed with an increase in curing age, as shown in Fig. 6, but strain was generally reduced with the increase in curing age. Peak stress values were the same as the compressive strength values illustrated in Fig. 5. Observations reveal that the reference combination containing 0%ATP and 0% BF reached the peak strength then it failed without no much strain, a brittle failure occurred. However, the mixtures with BF exhibited strength loss gradually with increasing strain after peak stress. When the content of ATP decrease in FC mixtures, the specimens demonstrated the ability to support load at increasing strains once the compressive peak strength was attained. As a result of the fiber reinforcement creating connection bridges on the concrete, it stopped cracks from growing, which was the behavior that was linked to the specimens' increased integrity. More strains were reported after peak strains with the increase in BF content at both curing age of 28 and 90 days. Particularly, for the specimens with BF fractions of 1% and 2%, the improved integrity of the specimens due to the fiber reinforcement is more noticeable.
Figure 7 depicts the impact of BF content and ATP as a cement substitute on the 28- and 90-day flexural strength of FC mixes containing 100% pumice aggregate (PA). As the curing age increased, strength augmentation was observed, as Fig. 7 illustrates. The addition of BF and the substitution of ATP for cement greatly increased the flexural strength; all ATP-incorporated mixes with BF showed greater flexural strength than the reference combination. More flexural strength was enhanced with increases in BF and ATP content; the maximum flexural strength was reached at 2% BF at each ATP content. The highest flexural strength was attained with 2% BF, where this fiber content offered optimal reinforcement. Furthermore, the pozzolanic reactivity of ATP helped form a denser and more robust matrix, enhancing resistance to flexural stresses. The reference mixture had the lowest flexural strength at both curing age of 28 and 90 days and the largest flexural strengths were found for the 30%ATP and 2%BF incorporated mixture which exhibited strength enhancement of 96.28 and 91.11% at 28 and 90 days, respectively, when compared to the reference mixture. According to Liu et al. [54], the compressive and flexural strengths of cement paste were improved by incorporating nano-ATP in the appropriate amount. This was partially explained by alterations to the mortar's pore structure, which reduced potentially hazardous pores and porosity. Alternatively, the pores can be filled in by the disorganized nano-attapulgite hydration activity of ATP, and its potential to boost cement paste's early stage bonding strength as well as its rod-like fibrous architecture, that could enhance fracture resistance, are the other reasons why it increases the flexural strength of the mortar [56]. The increase in flexural strength due to the BF can be attributed to the prevention of crack propagation by BF. Further emphasis was placed on the role that the fiber–matrix interfacial transition zone plays in the propagation of fractures [57]. Since BF had a high tensile strength, the enhancement in flexural strength can also be attributed to this property of BF. It was shown that the flexural strength of BF was positively correlated with its resistance to the mortar sample breaking [58]. Prior research has indicated that the flexural behavior of concrete and mortar can be markedly enhanced by the addition of fibers [59] ascribed to the excellent adherence of cementitious material to mixed BF, which helps to efficiently stop fracture propagation and improve flexural strength [33].
When 10% ATP-incorporated FC mixtures were considered, the mixture A10B20, which was incorporated with 2% BF, exhibited the maximum strength of 4.88 MPa and 5.02 MPa after 7 and 28 days of curing. This represents a 64.86% and 59.36% enhancement above the reference mixture. Similarly, when 20% ATP-included FC mixes were taken into account, the mixture A20B20—which was combined with 2% BF—showed the highest strength at 7 and 28 days of curing. Above the reference combination, this indicates an improvement of 81.08% and 75.23%. Combined use of ATP and BF improved the flexural strength notably due to the increase in the strong interface areas between the BF and binder matrix.
Figure 7b depicts the findings of measurements taken over 7 and 28 days for compressive strength and flexural strength. For each of the correlation factors, R2 was determined to be 1.00 for all FC mixtures with 0.5,1 and 2% BF content at 28 and 90 days. There was a strong link between the FC mixes' compressive and flexural strengths.
Figure 8 illustrates the impact of BF content and ATP as a cement substitute after 90 days on the oven dry density of FC mixtures containing 100% PA. The FC mixes displayed oven dry density values ranging from to kg/m3, as illustrated in Fig. 8. According to the findings, the FC combinations with 20% ATP and 2% BF had the highest dry density (dry density enhancement of about 20%) in contrast to the reference combination that is devoid of BF and ATP, while the mixtures with 10% ATP and 0.5% BF had the lowest dry density with the dry density reduction of about 17% as compared to the reference blend. The dry density variation in this investigation was quite similar to the compressive strength change, since the w/b ratio and quantity of the foam agent were fixed. Put another way, the dry density increased as the compressive strength increased. Three mixtures A20B20, A30B5, and A30B10 which exhibited dry density values above kg/m3 also indicated the largest compressive strength above 20 MPa at 90 days. The other two mixes containing 1 and 2% BF likewise showed a density drop of 3.5 and 7.4%, respectively, at 10% ATP content in comparison to the reference mixture. At 20% ATP content, the mixtures with 0.5 and 1% BF also revealed the density decline of 3.5 and 7.4%, respectively, but 2% BF incorporated mixture exhibited the largest dry density of kg/m3 with a density enhancement of 20%. This improvement can be attributed the more compact, dense structure, and less porosity of the mixture.
At 30% ATP content, all mixtures exhibited larger dry density than the reference blend, the blends with 10% ATP, and 20%ATP-incorporated mixtures with 0.5 and 1%BF. Two mixes with 0.5 and 1% BF (A30B5 and A30B10) showed dry densities above kg/m3 very slightly lower than the mixture with the largest density. These two mixtures depicted density enhancement of 19.59 and 19.66% as per the reference mixture. The combination containing 2% BF showed the dry density of kg/m3 which also higher than the reference mixture with an enhancement of 8.8%. Combined use of 20%ATP in place of cement and 2%BF and 30%ATP with 0.5 and 1% BF was found to have the highest compressive strength that satisfies the criteria for structural bearing load strength, which ASTM C 330-17a [60] states must be greater than 17 MPa with densities above kg/m3. It may be possible to employ the expressed FC mixtures in building applications. The FC blend containing 20% ATP and 2% BF achieved the highest dry density. This was due to ATP’s pozzolanic reaction, which densified the matrix by filling voids with additional calcium silicate hydrate (C–S–H) gel, and the 2% BF content, which reinforced the structure by minimizing larger air gaps. In contrast, the blend with 10% ATP and 0.5% BF exhibited the lowest dry density, as the lower ATP content resulted in less densification, and the limited fiber reinforcement led to a more porous structure with increased air-filled spaces.
One of the main elements influencing compressive strength of foam concretes is density. Age, porosity, and dry density were found to affect the compressive strength of foamed concrete [61]. Figure 8b reveals that there was a 0.91 correlation coefficient between the dry density and compressive strength. The test findings reveal an excellent relationship between compressive strength and dry density.
Figure 9a illustrates the impact of BF content and ATP as a cement substitute after 90 days on the porosity and water absorption of FC mixtures containing 100% PA. Figure 9a illustrates the porosity range of the FC mixes, which was 9.7–16.6%. There was a range of 6.1–15.6% found in the FC mixes' water absorption. The mixture containing 30%ATP and 0.5%BF had the lowest porosity and water absorption with reductions of 40.0 and 56.73%, respectively, with respect to the reference mixture. This decline makes sense by the mixture's more compact and dense structure, which also had the highest compressive strength. However, the blend with 10%ATP and 1%BF had the largest porosity with an increase of 4.4%, and the mixture with 10%ATP and 0.5%BF had the largest water absorption with an increase of 10.6%. All 10%ATP-incorporated mixtures exhibited larger porosity and water absorption that the reference mixture. Conversely, all 20 and 30%ATP –incorporated mixtures revealed lower porosity and water-absorption results with respect to the reference mixture. Addition of BF affected the porosity and water absorption positively or negatively depending on the ATP and BF content. At 10%ATP content, inclusion of 2%BF resulted in lowest porosity and inclusion of 1%BF led to the lowest water absorption. At 20%ATP content, increase in BF content reduced the both values notably and the lowest results were achieved at 2%BF incorporated mixtures. On the other hand, at 30% ATP content, increase in BF content enhanced the both values and the largest results were achieved at 2%BF incorporated mixtures. Fiber insertion has two distinct effects on porosity. First, a larger pore size results from the infiltration of fiber into the aggregate dispersion. The enlargement is usually influenced by the elasticity of the fiber modulus. Nonetheless, by filling the empty space with fiber, the porosity might be reduced [62]. As expressed, using 10% ATP in place of cement enhanced the porosity and absorption slightly but replacing cement with 20 and 30%ATP decreased the water absorption and porosity. The hydration reaction of the cement mortar may be accelerated by the nano-attapulgite, producing more calcium C–S–H gels, according to a study by Lindgreen et al. [63]. This would result in a decrease in the size and quantity of pores in the mortar while also increasing its density. Through the promotion of cement secondary hydration, the pozzolanic activity of calcined nano-ATP can significantly increase the compressive strength of concrete while reducing its porosity and water absorption. The mixture containing 30% ATP and 0.5% BF had the lowest porosity and water absorption, attributed to ATP’s high pozzolanic reactivity at this level, which densified the matrix by filling voids with C–S–H gel. In contrast, mixtures with 10% ATP showed higher porosity and water absorption, because the lower ATP content resulted in less matrix densification. Furthermore, the blend with 10% ATP and 1% BF likely exhibited increased porosity due to fiber clustering, while the 10% ATP and 0.5% BF mix had the highest water absorption due to limited matrix compaction.
One of the main elements influencing compressive strength of foam concretes is density. Age, porosity, and dry density were found to affect the compressive strength of foamed concrete [61]. Figure 9b reveals that there were 0.93 and 0.83 correlation coefficients between the porosity, compressive strength, and dry density, respectively. The test findings reveal that the porosity, compressive strength, and dry density are clearly correlated. The porosity increased as the compressive strength and dry density decreased.
One of the most important aspects of building design and construction is making sure there is enough thermal insulation. Foam concrete's cellular structure gives it excellent thermal insulation qualities. The degree to which thermally conductive foam concrete is affected by pore size, density, aggregate type, fiber content, and mineral admixtures [2].
Figure 10a illustrates the impact of ATP as a cement substitute and the amount of BF on the thermal conductivity of FC mixtures containing 100% PA after 90 days. The results displayed in Fig. 10a demonstrate that the FC mixes' thermal conductivity ranged from 0.606 to 0.741 (W/mK). Because the porosity and dry density of the specimens have a significant impact on thermal conductivity, lower dry density corresponds to higher porosity and lower thermal conductivity. The blend with 10%ATP and 0.5%BF which had the lowest dry density exhibited the least thermal conductivity 4.2% lower than the reference blend. With the lowest porosity, the blend containing 30% ATP and 0.5% BF showed the maximum thermal conductivity, 17.06% more than the reference mixture. The differences in thermal conductivity between the FC blends containing 10% and 30% ATP and 0.5% BF were due to variations in density and porosity. The blend with 10% ATP and 0.5% BF had a lower density and higher porosity, which created additional air pockets that served as insulators, leading to a 4.2% reduction in thermal conductivity compared to the reference mixture. Conversely, the blend with 30% ATP and 0.5% BF featured a denser, less porous matrix as a result of ATP’s pozzolanic reaction, which filled voids and enhanced heat transfer through the solid material, causing a 17.06% increase in thermal conductivity. All mixtures containing 10%ATP as a cement replacement revealed lower thermal conductivity values varying from 0.606 to 622 (W/mK) compared to the reference mixtures. Having higher porosity, lower dry density, and compressive strength of these mixtures supported these results. The mixtures containing 0.5 and 1% BF similarly showed very slightly greater and lower thermal conductivity values than the control mixture at 20% ATP concentration, respectively, but the thermal conductivity of the mixture with 2%BF increased by 15.6% parallel to the incline in the dry density and compressive strength.
At 30%ATP content, thermal conductivity values of the blends with 0.5 and 1%BF were the highest with the enhancement of about 17% parallel to the increase in the dry density and compressive strength with respect to the control blend and the thermal conductivity of the blend with 2%BF decreased slightly as compared to the other two blends with 30%ATP. The blend with 10% ATP and 0.5% BF exhibited the lowest thermal conductivity, attributed to its higher porosity and lower density, which formed insulating air pockets that limited heat transfer. In contrast, the blend with 30% ATP and 0.5% BF displayed the highest thermal conductivity due to its denser, less porous structure, enabling easier heat flow through solid material pathways.
Figure 10b reveals that there were 0.95 and 0.93 correlation coefficients between the thermal conductivity, dry density, and porosity, respectively. The results of the tests indicate that the thermal conductivity, dry density, and porosity are clearly correlated. The thermal conductivity increased as the dry density increased and porosity decreased.
Sorptivity is the capacity of a porous material to extract water from a free water source and transfer it through capillarity. All three methods are highly dependent on the pore volume and the pore network's connection [64]. The water sorptivity/absorption rate is a suitable measure of the fluid transport properties of building materials and, hence, a trustworthy indicator of material durability [65]. A decline in this rate consistently indicates increased durability. Sorptivity seems to be a more helpful criterion than permeability for categorizing the performance of concrete [66].
The effects of BF content and ATP as a cement substitute on the sorptivity of FC mixes containing 100% PA after 90 days are shown in Fig. 11a. As illustrated in Fig. 10a, the sorptivity of FC mixtures ranges from 3.22 to 6.88 kg/m2. The statistics on porosity, oven dry density, and compressive strength that were previously discussed in the sections above were perfectly consistent with the sorptivity results. The blend with the greatest compressive strength, 30%ATP and 0.5%BF, also had the lowest sorptivity, 50.53% less than the reference blend. With the largest porosity, the mixture containing 10% ATP and 1% BF showed the maximum sorptivity, 5.7% higher than the reference mixture. At 10%ATP content, the sorptivity increased by 3.2 and 5.7% at 0.5 and 1%BF content, respectively, and decreased by 0.3% at 2%BF content compared to the reference mixture. When 20%ATP was added instead of cement, all mixtures had lower sorptivity and 2%BF incorporated mixture achieved the sorptivity of 3.89 kg/m2 showing the 40.2% reduction with respect to the reference mixture. All mixes exhibited also reduced sorptivity when 30% ATP was added in place of cement; the mixture incorporating 0.5% BF which had also the largest compressive strength attained the lowest sorptivity of 3.22 kg/m2 due to the formation of more compact and dense structure resulting from the combined use of 30%ATP and 0.5%BF [62], indicating a 50.53% drop in comparison to the reference mixture. Through the production of additional calcium silicate hydrate (C–S–H), the nano-attapulgite can accelerate the hydration reaction of cement mortar, reducing the amount and size of pores while increasing the density of the mortar. The pozzolanic activity of calcined nano-attapulgite can promote the secondary hydration of cement, increasing the unit weight and compressive strength of regular concrete [63].
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Figure 11b illustrates the relation between the compressive strength and the sorptivity values and a very strong correlation coefficient R2 of 0.99 was found between them indicating a clear correlation between sorptivity and compressive strength as demonstrated by the test results.
Figure 12 reveals the microstructure of the reference blend A0B0 at various magnifications. SEM images show that the cement matrix is dense. In addition, pumice aggregates embedded in the matrix are visible. It was determined that the cellular structure was formed thanks to the foam used in the mixtures. Because the presence of independent spherical air bubbles was observed. The diameter of these voids decreases up to 20 µm. The thermal conductivity coefficient of the A0BO mixture was 0.633 W/m K, and the 28-day compressive strength was measured as 8.48 MPa.
Figure 13 shows the microstructure of the A10B10 mixture. SEM image with 250 × magnification shows the surface of pumice aggregate. It is observed in these images that the pumice aggregate has a porous structure. In addition, the presence of BF was frequently observed in SEM analyses. The presence of hydration products formed due to the reaction of cement with water was detected on the BF surface. It was observed that these products adhered to the fiber surface during fiber debonding. This proves that the adherence between BF and matrix is relatively good. Comparing the A10B10 mixture to the reference blend, the 28-day flexural strength rose 1.85 times.
Figure 14 depicts the SEM of the A30B20 blend at different magnifications. SEM images at 250 × magnification show that BFs are locally densified. This appearance is due to the agglomeration tendency of BF. In addition, the hollow surface of the pumice aggregate was also observed in SEM images. These voids are usually independent of each other. This situation affects the thermal performance of foam concretes. For instance, the A30B20 blend's thermal conductivity coefficient rose by around 1.1 times when compared to the reference combination. This is because the attapulgite used in the blends enhances the cake volume and decreases the pumice content. As a result, the thermal conductivity coefficient increases relatively.
Figure 15a illustrates the impact of ATP as a cement substitute and basalt BF concentration on the average compressive strength of FC mixtures containing 100% PA following exposure to high temperatures. Figure 15b shows the strength fluctuations when comparing the compressive strength values obtained on exposure to high temperatures with those cured after 90 days. When the compressive strength on the exposure to 200 °C was compared to the strength at 90 days, the reference mixtures and the blended mixtures of ATP and BF both shown an increase in strength. Results demonstrate that applying 200 °C to FC specimens increased their compressive strength. This improvement was mostly ascribed, in the literature, to the hydration of anhydrous cement and the accelerated pozzolanic process [67]. The strength of all mixtures, however, decreased when they were subjected to the high temperature of 400 and 600 °C. Regular exposure to extremely high temperatures weakens concrete for a number of causes, all of which have been extensively studied in the literature. It is thought that several physicochemical alterations inside the concrete matrix are what lead to degradation in the case of a fire. The main causes of strength loss are aggregate disintegration, dehydration with concurrent matrix cracking, and the breakdown of calcium hydroxide (CH) and calcium silicate hydrate gel (CSH) [3]. The thermal performance of FC mixtures was reduced by substituting 10%ATP for cement when compared to the reference combination at 400 and 600 °C. The blend containing 10%ATP and 1%BF demonstrated the highest thermal performance across all high temperature exposures, demonstrating a 9.5% rise in strength at 200 °C and strength loss of 16.7 and 35.5% at 400 °C and 600 °C. 10%ATP-incorporated mixture with 0.5%BF which had also the lowest compressive strength exhibited the worst thermal performance at 600 °C with a strength loss of 47.7%.
When 20% ATP was substituted for cement, the thermal performance of FC mixtures with 0.5 and 1%BF was much lower than that of the reference combination at 400 and 600 °C. However, the mixture containing 2% BF demonstrated the best thermal performance across all high temperature exposures, with a 6.4% strength increase at 200 °C and a 10.7% and 26.9% strength loss at 400 °C and 600 °C, respectively. FC mixes with 0.5%BF provided the highest thermal performance with the least amount of strength loss at 600 °C when 30% ATP was used in place of cement. However, the mixture containing 2% BF demonstrated the worst thermal performance, with a 46.9% strength loss at 600 °C.
When ATP was used in place of cement, the strength loss upon exposure to high temperatures was frequently reduced, especially at 10% ATP content, as Fig. 15 illustrates. This is due to the fact that high temperatures, between 400 and 600 °C, alter the mineral composition of ATP and lead to the calcination and dissolution of SiO2, which alters the hydration activity of materials that contain cement [68].
Figure 16 depicts the impact of basalt fibers (BF) content and attapulgite powder (ATP) as a cement substitute on the axial stress–strain behavior of FC mixes at 200, 400 and 600 °C. As expected, Stress decrease was noticed with an increase in high temperature, as shown in Fig. 16 but strain was enhanced with the increase in high temperature. Peak stress values were the same as the compressive strength values illustrated in Fig. 15. At 200 °C, the blend containing 30%ATP and 0.5%BF (A30B5) exhibited a sharp increase in stress values with a very small strains. After reaching peak stress, a large strains happened with a slight increase in stress then failed at strain value of above 0.02. On the other hand, other mixtures generally exhibited large strains with a low stress increment during the initial loading period and then stress increase up to the peak stress. More strains can be recorded for the mixtures comprising 10%ATP with relatively low stress values. With an increase in BF content, more strains were also observed following peak strains. At 400 °C, all mixtures had lower peak stress than the mixtures exposed to 200 °C, but as expected, more peak and ultimate strains were gained for the mixtures containing BF due the high-temperature resistance of BF showing bridging behavior and prevented high-temperature-induced cracks [69]. The reference mixture exhibited the lowest ultimate strain, whereas the blend with 10%ATP and 2%BF indicated the largest ultimate strain on exposure to 400 °C. The mixture with 0.5%BF and 30%ATP which had one of the largest peak stress indicated an approximately linear stress–strain behavior until the peak stress then failed without strain after peak stress. On the other hand, the mixture with 2%BF and 20%ATP which had the largest peak stress exhibited more rigid stress–strain behavior until the peak stress then the strain increased with a slight decrease in stress and reached the strain of nearly 0.04 before failure. At 600 °C, all mixtures exhibited more peak stress loss than the mixtures exposed to 200 and 400 °C but as expected, because BF exhibits bridging behavior at high temperatures and inhibits cracks caused by high temperatures, mixtures containing BF achieved greater peak and ultimate stresses. Strains increased more with the slight increase in stress. When exposed to 600 °C, the blend comprising 10% ATP and 2% BF showed the biggest ultimate strain, whereas the reference mixture showed the lowest ultimate strain. The combination containing 0.5%BF and 30%ATP had the highest peak stress values, suggesting a somewhat linear stress–strain behavior up until the peak stress, after which the mixture broke without strain.
Figure 17a shows the impact of ATP as a replacement for cement and BF amount on the mean loss of weight of FC blends, comprising 100% PA, upon exposure to high temperatures. The reduction in weight is a result of decomposition, spalling, and moisture loss through hydration products. The weight loss increases as temperature rises and accelerates beyond 200 °C, as evident in Fig. 17a. The weight loss of FC mixes was mainly dependent on the amount of BF and ATP. The weight loss of FC mixtures after high temperatures is influenced by density. Greater mass loss in foamed concrete is also a function of increased porosity and moisture content [70]. This is the reason that the mixtures that showed the biggest porosity and lowest dry density also showed the largest weight loss. It seems that replacing cement with ATP tends to decrease the weight loss of the mixtures and 30%ATP-incorporated mixtures exhibited the lowest weight loss. The ATP concentration was the primary determinant of the effect of BF on the weight loss of the combinations following high temperature. When cement was replaced with 10%ATP, all mixtures revealed larger weight loss than the reference mixture and the mixture with 0.5%BF which had the lowest compressive strength demonstrated the maximum weight loss on temperature exposure of 200, 400 and 600 °C. Increasing content of BF tended to decrease the weight loss for the mixtures with 10%ATP. The mixes with 0.5 and 1%BF showed greater weight loss than the reference mixture when cement was substituted with 20%ATP, and the blend with 2%BF, which had the compressive strength above 20 MPa at 90 days, had lower weight loss at 200, 400, and 600 °C than the reference mixture. For the mixes with 30%ATP, all mixtures showed lower weight loss than the reference blend and the blends with 10%ATP. Adding more BF tended to increase the weight loss and maximum weight loss was determined for the 2%BF incorporated blend. Figure 17b indicates the compressive strength and weight loss relation on exposure to high temperature. Extremely high correlation values R2 of 0.98, 0.94, and 0.97 were ascertained between the compressive strength and weight loss of the samples at 200, 400, and 600 °C, respectively, showing a parabolic relationship with the increase in strength corresponding to the weight loss decrease.
The A0B0 mixture's microstructure alterations after being subjected to 600 °C are depicted in Fig. 18. It became apparent that the effect of the high temperature led to the formation of micro-cracks in the matrix. Nonetheless, it was noted that the matrix remained dense. Even its adherence to pumice was observed to be good. In SEM images, it was observed that spherical air bubbles were formed, and these bubbles were independent of each other. In this way, the heat propagation caused by high temperatures is reduced. This reduced the damage to the microstructure. High-temperature exposure led to the formation of micro-cracks in the cement matrix; however, the matrix remained relatively dense, and the bond between the cement and pumice particles was well preserved. Spherical air voids were also observed, potentially enhancing the material's thermal insulation properties. Micro-cracks are a common response of cementitious materials to heat, as thermal expansion induces internal stresses that can lead to cracking. The extent of cracking depends on factors, such as material composition, heating rate, and maximum temperature. The maintenance of matrix density indicates that the structural integrity was not significantly compromised, contributing to the material's mechanical strength and durability. Strong adherence between the cement matrix and pumice is crucial for stress transfer and preventing particle detachment. The spherical air voids further improve thermal insulation by reducing thermal conductivity and minimizing stress concentrations, which lowers the risk of additional cracking. The A0B0 mixture exhibits reasonable thermal stability and resistance to high temperatures. Although micro-crack formation is not ideal, it did not significantly degrade the material's properties. The effective bond between the matrix and pumice, along with the presence of air voids, positively influences the material's overall performance.
The microstructure of the A10B10 blend exposed to high temperature is given in Fig. 19. As in other blends, it was observed that BF agglomerated in some regions. In addition, pumice aggregate with a hollow structure was observed in 50 × magnification SEM images. Micro-cracks were not observed in the matrix. The reason for this is that BF prevents crack propagation. The A10B10 blend reached a compressive strength of roughly 5.5 MPa at 600 °C after that.
The SEM image of the A30B20 mixture is shown in Fig. 20. Fiber agglomeration was observed in this blend using 2% BF. The distance between two BFs was measured as 35 µm in the SEM image. Spherical air bubbles formed in other blends were also observed in this blend. BF and these air bubbles reduced the damage caused by high temperatures. As a matter of fact, map cracks that may occur due to high temperature were not observed in the matrix.
A prevalent issue with concrete's durability is sulfate attack, which can lead to various problems such spalling, increased permeability, cracking, and loss of strength. Once sulfate ions and hydration products penetrate interior pores, a chemical reaction takes place that produces ettringite and gypsum as the final products [71]. Following 90 of exposure to 5% MgSO4, the compressive strength and weight loss of FC mixtures were evaluated to determine the effect of the attack.
The effects of ATP as a cement substitute and BF concentration on the average compressive strength of FC blends comprising 100% PA after being subjected to the sulfate assault over a 90-day period are depicted in Fig. 21a. Figure 21b illustrates the differences in compressive strength between the 90-day FC combination results and the results following sulfate attack. All FC mixtures exhibited strength loss ranging from 6.2 to 40.1% on the exposure to sulfate attack. The blend comprising 10% ATP and 1% BF indicated the least strength loss, whereas the blend containing 20% ATP and 1% BF produced the greatest strength loss. The blend containing 10% ATP and 1% BF exhibited the least strength loss due to an optimal combination of matrix densification and crack resistance, which effectively limited sulfate ingress and expansion. In contrast, the blend with 20% ATP and 1% BF experienced the greatest strength loss, likely because the higher ATP content led to a less cohesive matrix, rendering it more susceptible to sulfate-induced damage, even with the reinforcement provided by BF. Strength loss mainly depended on the BF content. When cement was replaced with 10%ATP, the mixture with 0.5%BF revealed the worst sulfate resistance showing strength loss of 35%. On the other hand, the greatest sulfate resistance was found in the mixture containing 1%BF, which showed a 6.2% strength decrease. When 20%ATP was used in place of cement, the combination containing 1%BF showed the weakest sulfate resistance, with a 40.1% reduction in strength. However, the mixture with 2%BF, which had a 12.2% strength loss, had the highest sulfate resistance. The mixture containing 2%BF exhibited the worst sulfate resistance when 30%ATP was substituted for cement, with a 26.9% drop in strength. However, sulfate resistance was best in the mixture containing 0.5%BF, with an 8.3% strength loss. The principal reason for this low strength loss is because the pozzolanic and filling properties of 30%ATP and 0.5%BF restrict the permeability of concrete, impeding the ingress of harmful ions and postponing the degradation of concrete subjected to harsh conditions like acid and sulfate attacks [72].
Figure 22 depicts the impact of BF content and ATP as a cement substitute on the axial stress–strain behavior of FC mixes after sulfate attack for 90 days. Stress loss was noticed for all mixtures when compared the strength values at 90 days. The reference mixture exhibited a linear stress–strain behavior until the peak stress then failed without strain. The reference mixture also exhibited the lowest ultimate strain, whereas the blend with 10%ATP and 2%BF indicated the largest ultimate strain on exposure to sulfate attack. The blend with 0.5%BF and 30%ATP which had the largest peak stress indicated an approximately linear stress–strain behavior until the peak stress. The mixture with 0.5%BF and 30%ATP also had the second lowest ultimate strain after the reference mixture. However, the blend with 2%BF and 10%ATP revealed the largest ultimate strain with a modest rise in strain after peak stress before failure. The worst stress–strain behavior was observed for the blend comprising10%ATP and 0.5%BF showing an increase in stress and strain up to the peak stress and then declined in stress with increasing strain. The mixture with 2%BF and 20%ATP exhibited the second largest peak stress with an approximate stress–strain behavior with a strain value of about 0.035.
The influence of BF concentration and ATP as a cement substitute on the mean weight loss of FC blends comprising 100% PA following sulfate exposure ranging from 8.79 to 12.76% is displayed in Fig. 23a. Very similar response in weight loss to the compressive strength was observed. Replacing cement with 10%ATP exhibited higher weight loss than the reference blend and the largest weight decline of 12.76% was assessed for the blend comprising 10%ATP and 0.5%BF. Cement replacement with 20%ATP showed also greater weight reduction than the reference mixture at 0.5 and 1%BF content; but the mixture 2%BF showed lower weight loss indicating that the two substances can increase the durability of FC blends to sulfate assault and effectively lower the rate of sulfate attack while producing less erosion products. Cement replacement with 30%ATP showed the lowest weight reduction at 1%BF content and other two mixtures with 0.5 and 2%BF showed also lower weight loss than most of the mixtures including the reference blend and the blends with 10, 20% ATP.
After the samples were exposed to the sulfate assault, their weight loss and compressive strength showed very significant correlation coefficient (R2 of 0.94), indicating a parabolic connection with the increase in strength corresponding to the decrease in weight loss as illustrated in Fig. 23b.
According to Fig. 24, the A0B0 mixture's microstructure after being exposed to the MgSO4 action. Ettringite crystals were seen in densely packed areas inside the matrix in SEM images. Because they expanded, these sulfate-formed crystals caused tiny fissures to appear in the matrix. Nonetheless, the impact of MgSO4 caused the compressive strength to drop 1.1 times when compared to the compressive strength after 90 days.
Figure 25 shows the microstructural changes of the A10B10 mixture. It was noted that BF was present in the matrix. Thanks to BF, micro-cracks caused by sulfa-induced expansions were significantly reduced. However, MSH structures were observed in the matrix. MSH was formed by replacing Ca in CSH. Due to this cation exchange, The A10B10 mixture's compressive strength dropped 1.5 times.
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