3.1. Fresh State Properties and Unit Weights of Cement Based Composites
The amount of plasticizer of mixtures varies, since basalt fibers in different proportions and lengths are used. In addition, it was not possible to keep the amount of plasticizer constant since there are many factors affecting the workability. It was determined in preliminary tests that the main factor affecting the workability is basalt fiber. Figure 5 shows the factors affecting the plasticizer ratio.
As the fiber length increases, the plasticizer ratio increases. In the use of 12 mm long fiber, as the fiber ratio increases, the amount of plasticizer increases (Fig. 5a). In the case of increasing the fiber ratio from 1–3%, the plasticizer ratio increased approximately 55%. Similar situations were observed in 6 mm long fibers. However, the use of 2 and 3% fiber did not affect the amount of plasticizer much. If the fiber ratio was 2%, the plasticizer ratio increased about 115%. It is seen that less plasticizer ratio is used in short fibers. However, in short fibers, the increase in fiber ratio among themselves increases the plasticizer ratio more. In long fibers, this increase remains at lower levels. In the study conducted by Emdadi et al., workability losses occurred as PP fiber length increased (Emdadi et al. 2015). Workability decreases since the increase of fiber length increases the internal friction. Therefore, the plasticizer ratio should be increased.
Fig. 5 (b) shows the effect of copper wastes on the plasticizer ratio. The use of 25% copper aggregate in mixtures used 0% copper slag increased the plasticizer ratio, while the use of 50% copper aggregate decreased the plasticizer ratio relatively. Similar results were observed when using 15% copper slag too. The use of 25% copper aggregate increased the plasticizer rate while 50% copper aggregate reduced the plasticizer rate. In the use of 7.5% copper slag, the 25% copper aggregate content reduces the plasticizer rate. Generally, it has been observed that the use of copper slag increases the amount of plasticizer.
As seen in Fig. 6 (a), as the w/b ratio increases, the plasticizer ratio decreases. In addition, as a result of the increase in fiber ratio, the rate of plasticizer generally increases. In the case of decreasing of w/b ratio from 0.60 to 0.40 at 3% fiber ratio, the plasticizer ratio increased approximately 11 times. Increasing of the fiber ratio has a great effect on workability.
Increasing copper slag at low w/b ratio (0.40) increases the plasticizer ratio more. If the w/b ratio is 0.50 and 0.60, the copper slag ratio does not affect the plasticizer ratio very much. If the copper slag is 7.5% and over in mixtures with a ratio of 0.40 w/b, the amount of plasticizer is over 2% (Fig. 6b). It was observed that copper slag affects the amount of plasticizer more at low w/b ratios. This situation is thought to be due to the paste volume.
As can be seen in Fig. 7(a), if the fiber ratio is 2%, the flow diameters of the mixtures produced from 6 and 12 mm long fibers increase. The reason for this increase is the use of more plasticizers in mixtures with 2% fiber ratio. Using 3% fiber in mixtures of 6 and 12 mm long fibers reduced the flow diameters. In the case of using 3% fiber, more plasticizer was used, but the expected improvement in the flow diameters of the mortars could not be observed. This is because, with the increase in the number of fibers, the internal friction increased and the flow diameters decreased. Since more plasticizer was used in the mixtures produced from 12 mm long fibers, the flow diameters were higher. In the case of preferring of a fiber of 12 mm length in mixtures with 3% fiber, the flow diameter was increased by approximately 10%. However, this is related to the excess plasticizer used. It was observed that the optimum ratio for short and long fibers is 2%. As the PP fiber content increased at a constant w/b ratio, the slump values of concrete were decreased by Hasan et al. (Hasan et al. 2019). Qin et al. used different proportions of basalt fiber in the mixtures they produced from magnesium phosphate cement. As a result of the increase in basalt fiber content, the workability of the mixtures decreased (Qin et al. 2018). More cement paste is needed to wrap around filament fibers with large surface areas, and the water absorption capacity of these fibers also negatively affects the workability (Dias and Thaumaturgo 2005; Chen and Liu 2005). In their study, Jalasutram et al. increased the amount of plasticizer together with the basalt fiber content, but could not eliminate the loss of workability (Jalasutram et al. 2017). Similar results were observed in the study conducted by Kabay, with the content of basalt fiber, the amount of plasticizer was increased and the workability was arranged in this way (Kabay 2014).
When the copper slag is used at the rate of 7.5%, the flow diameter increases with the increase in copper aggregate (Fig. 7b). If 7.5% copper slag is used, the flow diameters of the mixtures vary between 151–185 mm. The flow diameters of mixtures containing 0 and 15% copper slag showed similar properties. Especially in the case of using 25% copper aggregate, the flow diameters increased, while the use of 50% copper aggregate decreased the flow diameters. Since more plasticizer was used, this situation was observed when 25% copper aggregate was used in mixtures with 0 and 15% copper slag. In the mixtures containing 50% copper aggregate, relatively less plasticizer was used and the flow diameters decreased. In addition, the angled structure of the copper aggregate like crushed stone aggregate increased the internal friction and affected the workability more negatively. Therefore, the amount of plasticizer was also increased. The flow diameters of the mixtures containing 15% copper slag vary between 133–142 mm. Khanzadi and Behnood increased the slump values of concretes by using copper aggregate in their study (Khanzadi and Behnood 2009). Al jabri et al. produced concretes using different proportions of copper aggregate. As the copper aggregate increased, the slump values of the mixtures increased (Al-Jabri et al. 2011). This positive effect of copper aggregate was stated by some researchers because of its low water absorption rate (Ishimaru et al. 2005; Sharma and Khan 2017a). The high specific gravity of copper aggregate generally increases the slump values (dos Anjos et al. 2017). Najimi et al used copper slag by replacing it with cement up to 15%. The use of 5% copper slag increased the slump value relatively. 10% and 15% copper slag did not cause any loss of workability. The absence of slump loss was explained by the high specific gravity of the copper slag (Najimi et al. 2011). Onuaguluchi and Eren reported that the flow diameters decreased with the increase of copper slag in their study (Onuaguluchi and Eren 2013).
Figure 8 shows the effect of w/b ratio on the flow diameter. If the W/B ratio is 0.40, the flow diameter varies between 122–182 mm. Due to the low W/B ratio, the variability in the amount of plasticizer used to improve the workability of the mixtures was reflected in the flow diameter. When the W/B ratio was 0.50, mixtures with a flow diameter of 129–169 mm were obtained. In mixtures with a ratio of 0.60 w/b, the change in flow diameter was less and values between 135–160 mm were observed. As the relative w/b ratio increases, the flow diameters decrease. The reason for this can be explained by the amount of plasticizer used. As the W/B ratio increased, the amount of plasticizer decreased and in this case the flow diameters partially decreased.
Figure 9 shows the flow diameter of cement based composites in which fiber and copper waste are used. The flow diameters of the mixtures vary between 117–199 mm. The lowest flow diameter was obtained in mixture number 1, and the highest flow diameter was obtained in mixture number 13. It was also observed that there was some segregation in the mixture numbered 13. Some segregation occurred due to the plasticizer ratio being 2.75%.
Fig. 10 shows the effects of fiber and copper wastes on unit weight. As the fiber ratio in the miztures increases, the unit weight values of the composites decrease. This applies to both 6 and 12 mm long fibers. Unit weight values decreased as the fibers increased the internal friction during insertion into the mold (Fig. 10a). If the fiber ratio is 2% and over, the unit weight values of the mixtures fall below 2400 kg/m3. When 12 mm long fibers were used at 3% instead of 1%, unit weight decreased by about 5%. The change in fiber ratio affected the unit weight values, even a little. Using 1 and 3% of 12 mm long fibers further reduced the unit weight. As the basalt fiber content increased in the study conducted by Qin et al. the unit weight of concrete decreased (Qin et al. 2018). Similar results were observed with mixtures made with glass fiber (Tassew and Lubell 2014; Hong and Lubell 2015). Hanafi et al. observed that unit weight values did not change much with the increase in fiber ratio in cement pastes that they obtained by using different proportions of basalt fiber (Hanafi et al. 2020). In the study conducted by Borhan, as the basalt fiber content increased, the unit weight values of the concrete decreased (Borhan 2012).
When 7.5% and 15% copper slag is used, the unit weight values increase as the copper aggregate content increases (Fig. 10b). However, this situation was not observed in mixtures without copper slag. The unit weight value of the mixtures without copper slag did not changed much, approximately 2350 kg/m3 value was obtained. In the case of using 7.5% and 15% copper slag, the unit weight values were between 2400–2500 kg/m3 as a result of the increase in copper aggregate. Especially when 50% copper aggregate and 15% copper slag were used, the unit weight value of the mixtures was approximately 2500 kg/m3. No significant relationship was observed between the flow diameters and the unit weight values of mixtures produced from copper waste. This is because copper slag and copper aggregate have high specific gravity. Although the workability of the mixtures is bad, the unit weight values increased due to their high specific gravity. Anjos et al. stated that unit weight values increased as a result of the increase in the aggregate content in the concrete they produced using copper aggregate (dos Anjos et al. 2017). Khanzadi and Behnood used copper slag instead of aggregate and the unit weight values of concretes increased with the increase in slag ratio (Khanzadi and Behnood 2009). In the study conducted by Al jabri et al. copper slag was evaluated as aggregate. As the copper slag ratio increased, the unit weight values of the concrete increased. The unit weight values of concretes in which 100% copper slag was used were determined to be approximately 2700 kg/m3 (Al-Jabri et al. 2009). Najimi et al. used copper slag up to 15% instead of cement and achieved increases in the unit weight of the mixtures (Najimi et al. 2011). Similar results were observed in the study conducted by Gupta et al. (Gupta et al. 2017). Studies indicated that when copper slag is used instead of aggregate and cement, the unit weight increases. This is explained by the fact that copper wastes have a higher specific gravity than both aggregate and cement.
As seen in Fig. 11, as the w/b ratio increases, the unit weight values of the mixtures decrease. The unit weight values of mixtures with a w/b ratio of 0.40 vary between 2300–2570 kg/m3. In mixtures with a W/B ratio of 0.50, the unit weight values vary between 2250–2390 kg/m3. The unit weight values of mixtures with a ratio of 0.60 w/b are generally below 2400 kg/m3. Since increase of the w/b ratio in the mixtures increases water content the unit weight value of the mixtures decreases. The unit weight values over 2500 kg / m3 were obtained in mixtures with a ratio of 0.40 w/b.
3.2. Compressive and Flexural Strength of Cement Based Composites
Figure 12 shows the effect of fiber and copper wastes on the flexural strength of 7, 28 and 91-day mixtures. The flexural strength of the basalt fiber doped mixtures varies between 4.5–6.8 MPa (Fig. 12a). If 1% and 3% of 6 mm long fibers are used, the flexural strength is 6 MPa and over. However, when the fiber ratio was 2%, the flexural strength decreased. The increase in fiber ratio in the use of 12 mm long fibers increased the flexural strength. However, when 12 mm long fiber was used, the flexural strength did not exceed 6 MPa. The use of 6 mm long fiber increased the 7-day flexural strength relatively more than the 12 mm fiber.
When the 28-day flexural strength is examined, the increase in the fiber ratio in the use of 6 mm long fiber increases the flexural strength. However, when the fiber length is 12 mm, the increase in the fiber ratio decreased the flexural strength (Fig. 12a). The 28-day flexural strength of the mixtures generally exceeded 8 MPa with the effect of hydration. When 3% of 6 mm long fibers were used, the 28-day flexural strength was found to be approximately 8.8 MPa. As a result of the increase in fiber ratio, negligible decreases occur in flexural strength. In the case of using 2 mm long fiber at 3% instead of 1%, approximately 4% strenght loss was occured.
As seen in Fig. 12a, 91-day flexural strength generally improves as the fiber ratio increases. Strength loss occurred when only 2% of 12 mm long fibers were used. However, this strength loss value was approximately 13%. Similar properties were observed when 1% and 2% of 6 mm long fibers were used. It is seen that 91-day flexural strength is generally over 9 MPa. In terms of flexural strength of 28 days, it is seen that using 3% of 6 mm long fibers is more positive.
Borhan et al. stated in their study that the ratio of basalt fiber decreases the tensile splitting strength in some intermediate values, but increases it at some rates (Borhan 2012). However, losses in tensile splitting strength remained at negligible levels. Kabay used 2% and 4% basalt fiber in the concrete in his study. It was observed that the higher the rate of basalt fiber, the higher the flexural strength (Kabay 2014). Qin et al found similar results in their study and stated that as the rate of basalt fiber increases, the tensile splitting strength increases (Qin et al. 2018). In the study of Hanafi et al., the use of basalt fiber up to 0.75% generally increased the flexural strength, while the use of 1.5% basalt fiber decreased the flexural strength (Hanafi et al. 2020). These different behaviors in flexural strength may be due to the tendency of the fibers to coagulation.
Since basalt fiber is composed of SiO2 and CaO, it is expected to show better mechanical performance. C-S-H gel formation provides a higher compressive and flexural strength. In addition, basalt fibers have an amorphous structure. This helps to form a densified matrix with homogeneous basalt fiber distribution in the cement paste. Saloni et al. (Saloni et al. 2020) reported that basalt fiber acts as an aggregate and increases strength more when strong matrix properties are formed. Similar results were observed in other studies (Pehlivanlı et al. 2016; Sadrmomtazi et al. 2018; Ralegaonkar et al. 2018; Chidighikaobi 2019).
Figure 12b shows the effect of copper wastes on the flexural strength properties. It is seen that the 7-day flexural strength varies between 4.2-7.0 MPa. On the 7th day, the highest bending strength (~ 7.0 MPa) was observed in mixtures using 7.5% copper slag but not using copper aggregate, and mixtures using 15% copper slag and 25% copper aggregate. The increase in copper aggregate in 7-day mixtures generally led to a decrease in flexural strength.
In the 28-day flexural strength, the increase in the copper aggregate content of the mixtures using 15% copper slag provided an increase in strength. In the case of using 7.5% copper slag, the increase in copper aggregate decreased the flexural strength. The flexural strength of mixtures without using copper slag increased only at 25% copper aggregate content. In the case of using 50% copper aggregate, the 28-day flexural strength decreased by approximately 15%. The 28-day flexural strength of the mixtures varies between 7-8.55 MPa. In terms of 28-day flexural strength, the most suitable mixture ratios were determined as 7.5% copper slag 0% copper aggregate or 15% copper slag 25% copper aggregate.
When the 91-day flexural strength is examined, in mixtures containing 0 and 7.5% copper slag, as the copper aggregate increases, the flexural strength decreased. In the case of being 15% of copper slag, the 25% copper aggregate increased the flexural strength by approximately 29%. The mixture with 50% copper aggregate and the mixture without copper aggregate showed similar properties. It is observed that the 91-day flexural strength varies between 8-11.5 MPa. The highest flexural strength was obtained in mixtures without 7.5% copper slag and copper aggregate. The flexural strength of the mixtures without copper aggregate and slag was approximately 11.5 MPa.
In the study conducted by Anjos et al., copper aggregates used instead of aggregate reduced the tensile strength (dos Anjos et al. 2017). Some studies reported that by using copper slag in place of fine aggregate, the compressive and tensile strengths are significantly higher than control mixes, almost in line with those of normal concrete (Hwang and Laiw 1989; Shoya et al. 1997; Khanzadi and Behnood 2009). In the study conducted by Al Jabri et al., when copper slag is used instead of aggregate, flexural strength decreases at some rates and increases at some rates (Al-Jabri et al. 2011). Wu et al. stated in their study that with the increase of copper slag ratio, the flexural strength decreases and the strength loss is due to excessive trapped water (Wu et al. 2010a).
Moura et al. used 10% copper slag instead of cement and observed a decrease in flexural strength (Moura et al. 1999). Antonio and Mobasher determined the flexural toughness by using 10% copper slag in concretes with 0.50 w/b ratio. It was observed that the flexural toughness decreases when copper slag is used instead of cement (Ariño and Mobasher 1999). In the study conducted by Onuaguluchi and Eren, copper slag used instead of cement by 10 and 15% decreased the flexural strength (Onuaguluchi and Eren 2013). The reason for the loss of strength is due to copper slag, which has less reactivity compared to cement (Onuaguluchi and Eren 2012). In the study conducted by Gupta et al., flexural strength increased when copper slag was used instead of cement (Gupta et al. 2017).
As seen in Fig. 13, as the w/b ratio increases, the flexural strength of the mixtures decreases. The increase in the w/b ratio at all test times affects adversely the flexural strength. As a result of the increase in hydration period, the flexural strength increases. Especially on the 91st day, the flexural strength of the mixtures is 8 MPa and over. When the w/b ratio is increased from 0.40 to 0.60 in 28-day mixtures, the flexural strength decreases by approximately 25%. In the case of increasing the w/b ratio from 0.50 to 0.60 in 28-day samples, some strength development was observed.
Figure 14 shows the compressive strengths of mixtures obtained from basalt fiber and copper waste at different times. The use of 6 mm long fiber in 7-day compressive strength provided more positive results (Fig. 14a). In the use of 6 mm long fiber, choosing 1% and 3% fiber increases the compressive strength. The use of 2% fiber reduced compressive strength by 11%. The use of 12 mm long fiber also decreased the compressive strength relatively compared to the use of 6 mm long fiber. If 12 mm long fiber is used, the increase in fiber ratio decreases the compressive strength. It was observed that 7-day compressive strength varied between 39.8–45.8 MPa. It is more appropriate to use 1% or 3% of 6 mm long fibers in 7-day compressive strength.
It is seen that the fiber properties do not cause a significant increase or decrease in 28-day compressive strength. It is seen that thecompressive strength is between 49.2–54.1 MPa (Fig. 14a). It is observed that when 6 mm long fiber is used, 2% of it causes strength loss. However, it is seen that this strength loss is a small value as 6%. As the fiber ratio increases in the mixtures produced from 12 mm long fibers, the compressive strength decreases. Therefore, it has been determined that the optimal ratio is 1%, which provides the value of 52.6 MPa.
In 91-day mixtures, the increase in hydration process contributed to fiber-matrix adherence. In particular, using 1% of 12 mm long fibers has increased the compressive strength to approximately 80 MPa. However, the use of 2% and 3% fiber reduced the compressive strength by approximately 30%. In the case of using 6 mm long fiber, different results were obtained from other experiment days. Using 2% of the 6 mm long fiber increased the compressive strength relatively slightly. Compressive strength varies between 57.8–59.7 MPa in the use of 6 mm long fiber. The optimum mixture ratio in terms of 91-day compressive strength was the use of 1% of 12 mm long fiber. Similar results were observed in the study conducted by Borhan et al. and as a result of the increase in the ratio of basalt fiber, the compressive strength of some mixtures decreased (Borhan 2012). Dias and Thaumaturgo stated that the addition of 1.0% basalt fiber by volume reduced the compressive and tensile splitting strength of concrete by 26.4% and 12%, respectively. They also stated that concretes with lower fiber content (0.5% by volume) showed negligible changes in compressive and tensile splitting strength compared to non-fibrous concrete (Dias and Thaumaturgo 2005). In the study conducted by Kabay, the increase in basalt fiber ratio at low w/b ratio (0.45) decreased the compressive strength. However, the increase in fiber ratio at high w/b ratio increased the compressive strength (Kabay 2014). In the study conducted by Qin et al., the increase in basalt fiber ratio increased the compressive strength. Concretes with a compressive strength of about 80 MPa were obtained on the 28th day (Qin et al. 2018). In general, the addition of basalt fibers to the mixture increases the load-bearing capacity of composites. This increase in performance can be explained by the adherence between fiber-matrix (Simões et al. 2017). Sun et al. stated that the addition of basalt fiber up to 2% by volume increases the compressive strength, but adding more fiber affects the strength negatively (Sun et al. 2019). In the study conducted by Jalasutram et al., as the basalt fiber content increased, the compressive strength of concrete decreased (Jalasutram et al. 2017). The addition of basalt fibers did not provide a significant improvement in the dynamic compressive strength of the geopolymer concrete, but improved deformation and energy absorption capacities significantly (Li and Xu 2009). Jiang et al showed that the addition of basalt fibers to the mixture increased the compressive and flexural strengths in the early hydration period significantly (Jiang et al. 2010).
Figure 14 (b) shows the effect of copper wastes on compressive strength. In 7-day mixes, copper slag and aggregate generally reduce the compressive strength. In particular, using 50% copper aggregate in mixtures without copper slag reduced the compressive strength by approximately 48%. The use of 50% copper aggregate in mixtures using 7.5% copper slag reduced the strength loss. In mixtures using 15% copper slag, the use of 25% copper aggregate increased the compressive strength and a compressive strength of approximately 45 MPa was obtained. It has been observed that the optimum ratio for 7-day compressive strength is 7.5% copper slag and 0% copper aggregate.
It is seen that the 28-day compressive strength of the mixtures varies between 34.7–69.1 MPa (Fig. 14b). Generally, the increase in copper slag and aggregate reduces 28-day compressive strength. An increase in strength was observed in mixtures using only 15% copper slag and 25% copper aggregate. It is seen that 7-day and 28-day compressive strength are parallel to each other. Compressive strength of approximately 70 MPa was obtained in mixtures where copper waste is not used.
91-day Compressive strength was observed to be similar to other experiment days. Compressive strength of 90 MPa and over was obtained, especially in mixtures without copper waste. If the copper aggregate is 50% in mixtures without copper slag, the compressive strength is reduced by approximately 53%. With the increase in copper slag, the rate of strength loss caused by copper aggregate is reduced. The compressive strength of the mixtures in which 15% copper slag and 25% copper aggregate were used was over 60 MPa value. It was observed that it was possible to produce high-strength cement based composites with these rates.
In the study conducted by Anjos, copper slag was used instead of aggregate and the increase in slag ratio decreased the compressive strength of concrete (dos Anjos et al. 2017). Khanzadi and Behnood used copper slag as aggregate in their work. They observed that copper slag used as aggregate increased the compressive strength. The reason for the strength increase was explained by the strength of the copper slag aggregate. They also argued that copper slag aggregates had better adherence to the matrix than chalk (Khanzadi and Behnood 2009). In their study, Al Jabri et al. obtained strength increase by using copper slag instead of aggregate (Al-Jabri et al. 2011). Wu et al claimed that copper slag grains improve the cohesion of the concrete matrix thanks to their angular sharp edges. Due to its rough surface, the adherence between cement paste and aggregate increases. The angular sharp edges of the copper slag grains reduce the negative effects of fine aggregate to some extent and thus have the ability to further improve the cohesion of the concrete. In addition, the glassy surface texture of copper slag grains has a negative effect on cohesion. The low water absorption properties of copper slag cause excess water in the concrete. In high copper slag content, excessive bleeding may occur. This situation may cause micro voids or capillary voids in the concrete and lead to a decrease in concrete quality. Therefore, the strength of concrete with low copper slag content can be improved by the positive effect of copper slag, but if the copper slag content exceeds 40%, the strength of the concrete can be significantly reduced by a reduction in the effective cohesion (Wu et al. 2010a, b).
Mobasher et al. and Tixier et al. examined the effects of copper slag on cement hydration. Copper slag up to 15% by weight was used as a pozzolanic reaction activator with up to 1.5% hydrated lime in Portland cement replacement. The results showed a significant increase in compressive strength for up to 90 days of hydration. In addition, a decrease in capillary porosity and an increase in gel porosity were observed (Mobasher et al. 1996; Tixier et al. 1997). Moura et al. suggested that copper slag can be a potential alternative to additives used in concrete and mortars (Moura et al. 1999). Al Jabri et al. increased the compressive strength by using 5% copper slag in concretes with 0.50 w/b ratio (Al-Jabri et al. 2006). Zain et al. searched the use of copper slag in different proportions instead of cement. They found that copper slag extends the setting time of mortars and reduces the compressive strength. They stated that the optimum rate of replacement should be between 5-7.5% (Zain et al. 2004). In the Onuaguluchi and Eren’s studies, while the strength increase up to 5% was achieved in the copper slag used instead of cement, strength loss was observed in a higher rate. Ranganath et al. (Ranganath et al. 1998) demonstrated that large particles are less reactive in cement mixtures and the delay of cement hydration due to the presence of Cu (II) ions was stated by Hashem et al. (Hashem et al. 2011).
Figure 15 shows that the compressive strength of mixtures decreases as the W/B ratio increases. Increase in w/b ratio on all test days decreases compressive strength. On the 91st day, the compressive strength of mixtures with different w/b ratio varies between 43–75 MPa. On the 28th day, the compressive strength of mixtures with a rate of 0.40 w/b exceeded 50 MPa, on the 91st day, mixtures with a ratio of 0.40 and 0.50 w/b exceeded 50 MPa. It was determined that mixtures with 0.60 w/b ratio do not exceed 50 MPa.
As seen in Fig. 16, the flexural strength of the mixtures increases as the compressive strength increases. Although the R2 coefficient between compressive and flexural strength is 0.65, the increase in compressive strength generally increases the flexural strength.
In Fig. 17, the cross-sections for aggregate distribution of copper aggregate doped and undoped composites for which flexural strength tests have been applied are given. In Fig. 17 (a), it is seen that the aggregates with black color are the aggregates obtained from copper slag. It is also seen that the copper aggregates are homogeneously distributed.
3.3. Sorptivity Properties of Cement-Based Composites
Figure 18 shows the time-dependent water penetration depth of mixtures produced from 6 mm long fibers.
In Fig. 18, the highest water penetration depth was observed in the mixture numbered 3 in the early days of the sorptivity test. It is observed that the water penetration depth of the mixture number 3 increased in the following process. The reason for the high water penetration depth of the mixture numbered 3 is that the w/b ratio is 0.60. In addition, 50% copper aggregate is also effective in this process. The fact that the fiber content in the mixture number 3 is 3% is another factor that increases the water penetration depth. Since the w/b ratio of mixtures numbered 9 and 6 is 0.60, their water penetration depth is higher than other mixtures. The water penetration depth of the mixtures numbered 3,6,8 and 9 at the end of the 28th day varies between approximately 1.5–1.8 mm. Although the w/b ratio of the 8th mixture was 0.50, the fact that the fiber content was 3% led to an increase in the water penetration depth. The water penetration depth of the mixture number 1 is less than 0.6 mm. The w/b ratio of the mixture number 1 is 0.40 and the fiber content is 1%. With the decrease in the W/B ratio, water treatment depth generally decreases. The water penetration depth of the mixtures numbered 4 and 7 with a w/b ratio of 0.40 was determined as approximately 0.8 mm. The water penetration depth of the mixture number 6 increases continuously over time. It is estimated that this occurs because the 15% copper slag used in mixture number 6 reduces the content of the paste. On the other hand, in mixtures numbered 5 and 7, using 7.5% copper slag, the water penetration depth has not increased much since the 8th day. Since the copper aggregate content of the mixtures numbered 3 and 9 is 50%, it is observed that the depth of water penetration of the mixtures is higher. The use of copper aggregate, which has a higher specific gravity than crushed stone aggregate, increases the porosity of the mixtures. In this case, the water penetration depth of the mixtures can increase. In Fig. 19, the water processing depths of the blends produced with 12 mm long fibers are given.
The use of 12 mm long fibers generally reduced the depth of water penetration (Fig. 19). However, the increase in the w/b ratio increases the water penetration depth of the mixtures. Especially, the water penetration depth of the 18 numbered mixture with a w/b ratio of 0.60 and fiber content of 3% was found to be approximately 1.6 mm. Similar results were observed in the mixture number 12. The water penetration depth of the mixture numbered 12 with a W/B ratio of 0.60 and a copper aggregate ratio of 25% was found as 1.46 mm. Although the w/b ratio of the mixture number 17 was 0.50, the fact that the fiber content was 3% and the copper aggregate content was 50%, increased the depth of water penetration greatly. In addition, although the w/b ratio of the mixture numbered 10 is 0.40, since the copper aggregate content is 50%, the water penetration depth exceeded 1.20 mm. The water penetration depth of the mixture number 11 is less than 0.60 mm. Because no copper slag and aggregate are used in the mixture number 6. Capillar voids are thought to be less due to the fact that the paste volume and aggregate volume do not decrease. At the beginning of the experiment, it is seen that the mixture numbered 10 has a greater water penetration depth. The lowest water penetration depth was also observed in the 11th mixture. On the 28th day, the water penetration depth of the mixture numbered 18 is 63% more than the mixture number 11.
As a result, the minimum water penetration depth was obtained as 0.54 mm in the mixture number 1. The w/b content of the mixture number 1 is 0.40 and the fiber content is 1%. In addition, copper wastes were not used in its structure. The maximum water penetration depth value was obtained as 1.77 mm in the mixture numbered 6. The fiber content of the 6th mixture with a W/B value of 0.50 is 2%. The 28-day compressive strength of the mixture numbered 6, in which no copper aggregate is used but 15% copper slag is used, is 37.91 MPa. Generally, mixtures with low compressive strength have more water penetration depths. It is thought that this situation is caused by the copper slag and aggregate used in the mixtures.
Nagarajan et al. used basalt fiber up to 0.25% in lightweight concretes. Sorptivity coefficients of concretes increased with the increase in basalt fiber ratio (Nagarajan et al. 2020). Basalt fiber was used up to 0.20% by volume in the study conducted by Niu et al. As the basalt fiber ratio increased, the water penetration depth of the concretes increased (Niu et al. 2020). When the fiber content is too high, the fibers are not distributed homogeneously, they overlap in the matrix and coagulation occurs, thus causing an increase in the coarse spaces between the fibers. As a result, higher water absorption occurs (Niu et al. 2020). In the study by Karthikeyan et al., basalt fiber was used up to 1% by volume. While the increase in basalt fiber ratio in some mixtures decreased the sorptivity coefficient, in some mixtures sorptivity coefficient increased (Karthikeyan et al.). In the study conducted by Adesina et al., 4% and 8% basalt fiber was used in concrete production. As a result of the increase in basalt fiber ratio, the water penetration depth of the mixtures also increased (Adesina et al. 2020).
In high performance concrete, sorptivity was reduced by using 40% copper slag as fine aggregates and 2% nanosilis as cement replacement (Chithra et al. 2016). Sharma and Khan used copper slag instead of aggregate and showed that sorptivity was better than control concrete (Sharma and Khan 2017a, b, 2018). In the study by Geetha et al., as the copper slag content increased, sorptivity values decreased (Geetha and Madhavan 2017). In the study by Gupta and Siddique, copper slag used instead of fine aggregate generally reduced sorptivity values (Gupta and Siddique 2020). Rajasekar et al. used copper slag instead of aggregate up to 100% and determined that capillarity values decreased (Rajasekar et al. 2019).
Sorptivity properties of mixtures obtained by using copper slag instead of cement were not studied much. However, as copper slag is effective on the paste volume, its effect on sorptivitycan be explained in this way. Sorptivity values generally decrease as a result of the increase in paste volume. There are many studies on this in the literature (Kolias and Georgiou 2005; Chen et al. 2014; Zhong and Wille 2015; Chu 2019).
3.4. Drying Shrinkage Properties of Cement Based Composites
Figure 20 shows the drying shrinkage time-dependent behavior of the mixtures produced from 6 mm long fibers. Drying shrinkage measurements were performed up to the 91st day after 7-day of water treatment.
Among the mixtures, the lowest shrinkage value (for 91 days) was obtained with the mixture numbered 1 as 1568x10− 6. No copper slag and aggregate were used in the 1 mixture with a W/B ratio of 0.40. The shrinkage value of the other mixture (number 4) with a w/b ratio of 0.40 was also under 2000x10− 6. Although the w/b value of the mixture numbered 7 was 0.40, the shrinkage value increased relatively and exceeded the value of 2000x10− 6. Although the fiber content of the mixture numbered 7 was 3%, it did not show a very distinctive behavior in reducing drying shrinkage. The highest shrinkage value was obtained in the mixture numbered 9 with a w/b ratio of 0.60 and a copper aggregate ratio of 50%. The shrinkage value of mixture number 9 on the 91st day was measured as approximately 3500x10− 6. The shrinkage values of the mixtures with 3% fiber content were over 2000x10− 6. The shrinkage values of the mixtures with 2% fiber content were generally under 2000x10− 6. The increase in fiber ratio generally increased the shrinkage value. The shrinkage values of the mixtures numbered 3, 6 and 8 with 15% copper slag were determined to be close to each other. The use of copper aggregate generally increases the drying shrinkage values. Because the aggregate volume decreases and therefore the drying shrinkage increases.
Figure 21 shows the drying shrinkage time-dependent behavior of the mixtures produced from 12 mm long fibers. The lowest shrinkage values were observed in mixtures numbered 10, 11 and 12. The common point of these mixtures is that they have 1% fiber ratio. Drying shrinkage values increase in fiber ratios after 1%. The drying shrinkage values of the mixes numbered 14 and 15 with 2% fiber ratio approached approximately 4000x10− 6. The w/b ratio of the mixture numbered 14 is 0.5 and the copper slag is 15%. The w/b ratio of the mixture numbered 15 is 0.60 and the copper aggregate content is 25%. In addition to the W/B ratio, the increase in copper slag and aggregate ratio increased the drying shrinkage behavior. Although the w/b ratio of the mixture numbered 18 was 0.60, the shrinkage value was measured as 3344x10− 6. It is thought that the shrinkage value is reduced by 3% fiber used.
Increasing fiber length generally increased the drying shrinkage values of the mixtures. Mixtures using 12 mm long fibers generally have lower unit weight values. This situation indicates that the mixtures are more porous. It was determined that drying shrinkage values increased due to the porous structure. It is also known that mixtures with high compressive strength cause less shrinkage. Generally, high shrinkage values were observed in some mixtures with high fiber ratio, as the increase in fiber ratio reduces the compressive strength. The shrinkage value in mixtures numbered 13 and 17 with a copper aggregate content of 50% was measured as approximately 3500x10-6. Since the copper aggregate content decreases the total aggregate content, it generally increased the drying shrinkage values. In addition, the increase in w/b ratio also increased the drying shrinkage values. However, in some cases the high fiber content reduced this disadvantage.
Valeria and Nardinocchi stated that drying shrinkage can be reduced by fiber reinforcement in cement-based composites (Corinaldesi and Nardinocchi 2016). Jiang et al. observed in their study that the drying shrinkage at an early age is reduced by the addition of basalt fiber (Jiang et al. 2010). Punurai et al. stated in their study that as the basalt fiber content increases, the drying shrinkage decreases. They stated that basalt fiber acts as a micro-aggregate due to its reactivity and therefore its shrinkage values decrease (Punurai et al. 2018). Jiang et al. obtained lower shrinkage values than the control mixture using the basalt fiber (Jiang et al. 2016). Ruijie et al used 0.1% and 0.3% basalt fiber in their study. The increase in basalt fiber ratio increased the shrinkage value of some mixtures, but generally the shrinkage values decreased (Ruijie et al. 2017). In the study conducted by Li et al., it was observed that drying shrinkage increased in some mixtures containing basalt fiber (Li et al. 2020). In addition, studies indicated that drying shrinkage is mainly affected by transition pores with diameters less than 50 nm (Zhong and Zhang 2020). This situation also shows the effect of w/b ratio on drying shrinkage. The researchers stated that as the curing time increases, the decrease in the shrinkage rate is related to the rate of water loss. The drying shrinkage rate of basalt fiber doped mixtures was lower than that of control concrete during all curing times. This result shows that basalt fiber has a positive effect in reducing the drying shrinkage rate. There are three reasons for this result: (1) Adding fiber to the mixture increases the tensile strength of the concrete matrix, which physically contributes to restrictive shrinkage. (2) With the fiber addition, crack development is prevented by the bridging effect and (3) the stresses resulting from shrinkage are transferred by the interconnections between the fibers (Li et al. 2006). The increase in drying shrinkage values as a result of the increase in basalt fiber can be explained by the fact that the fibers increase the internal friction and make it difficult to settle in the mold. It is estimated that the drying shrinkage values increase as the porosity of the mixtures increases.
Drying shrinkage in mixtures produced from copper slag used instead of fine aggregate showed similar properties to the control mixture. It was even observed that some mixtures containing copper slag show less shrinkage than the control mixture (Hwang and Laiw 1989; Shoya et al. 1997). In the study conducted by Gupta et al., copper slag used instead of aggregate increased the shrinkage values of concretes (Gupta et al. 2017). In the study by You et al., using 60% copper slag instead of aggregate increased the drying shrinkage values (You et al. 2020). In addition, in some studies, if copper slag is used at a high rate, segregation and bleeding occur and drying shrinkage increase (Zhang et al. 2015; Han and Wang 2016; Mastali et al. 2018). In the study conducted by Sharifi et al., copper slag used instead of coarse aggregate up to 100% reduced the drying shrinkage. This positive effect can be explained by the high adherence of copper aggregate to cement paste (Sharifi et al. 2020). There is no study in the literature that deals with the drying shrinkage behavior of copper slag by using it instead of cement. Therefore, a comment was made on the effect of paste volume on drying shrinkage. Because there are studies in the literature showing that drying shrinkage is directly related to the paste volume. Drying shrinkage may increase as a result of the increase in paste volume (Bissonnette et al. 1999; Rozière et al. 2007).
3.5. Sulphate Resistance Properties of Cement Based Composites
The behavior of the mixtures produced from 6 mm long fibers under the effect of 5% sodium sulphate is given in Fig. 22. After the mixtures numbered 1,4,5,6 and 7 were put into sodium sulphate solution, there was a slight decrease in their size. These mixtures have shown an expansion feature mostly from the 14th day. It is seen that the mixture number 1 with a W/B ratio of 0.40 and without copper slag and copper aggregate shows the least expansion. However, the occurrence of shrinkage for the first 14 days in this effect was an important situation. The expansion values of the other mixtures (4 and 7) with a W/B ratio of 0.40 were also relatively low. The expansion value of the 6th mixture with a W/B ratio of 0.60 and fiber ratio of 2% was found to be 759x10− 6. The reason for the low expansion despite the high w/b ratio is the 15% copper slag used in its content. It was stated in the studies that the C3A content in the cement structure affects the sulphate resistance. As the C3A content in cements decreases, sulphate resistance increases (Kaplan and Öztürk 2019). The highest expansion value was obtained in the mixture numbered 9 as approximately 2500x10− 6. Mixture numbered 9 has a w/b ratio of 0.60 and is produced with 50% copper aggregate. Since copper aggregate decreases the total aggregate volume, the porosity of the mixture increased, as a result, also the expansion values increased. The expansion values of mixtures numbered 6 and 8 using 15% copper slag were determined as approximately 1000x10− 6. The use of copper slag made an important contribution to sulphate resistance. A significant effect of the fiber ratio on sulphate resistance was not determined. In Fig. 23, time-dependent sulphate resistance of the mixtures produced from 12 mm long fibers is given
As seen in Fig. 23, mixtures numbered 10, 11 and 12 showed similar properties. It is seen that the mixture numbered 12 with a ratio of 0.60 w/b has an expansion value of approximately 1500x10− 6. It was observed that the 7.5% copper slag used in the mixture numbered 12 contributed positively to this process. It is seen that the mixtures numbered 13, 15, 16,17 and 18 exceed the expansion value of 2000x10− 6. The w/b value of the mixture numbered 15 is 0.60 and no copper slag is used instead of cement. The fiber ratio of 2% in the mixture is not very effective in preventing sulphate expansion. Although the w/b ratio of the mixture number 16 is 0.40 and the copper slag is 15%, it is seen that a relatively high expansion occurs in its structure. This is thought to occur with increased porosity as a result of reduction in aggregate and paste volume with 25% copper aggregate and 15% copper slag. It is seen that the mixture numbered 18 shows a high expansion due to the w/b ratio of 0.60.
As a result of the increase in fiber length, the sulphate-based expansion values of the mixtures also increased. In addition, it was determined that the mixtures numbered 1, 4, 7 and 11 with high compressive strength (> 60 MPa) had less expansion than other mixtures. More expansions occurred in mixtures numbered 9, 17 and 18 with low compressive strength (< 40 MPa). The reason for the high sulfate expansion value of the mixture numbered 9 was explained by the high depth of water penetration (~ 1.5 mm). It was observed that the mixture numbered 6 with a high water penetration depth (~ 1.8 mm) did not cause excess sulphate expansion. This is due to the 15% copper slag used instead of cement. This situation is also reflected in sulphate expansion, since the water penetration depth of the mixture numbered 18 is approximately 1.60 mm.
There is little information in the literature about the contribution of basalt fiber to chemical durability. In the study conducted by Hanafi, the use of basalt fiber reduced the expansions caused by sodium sulfate (Hanafi et al. 2020). Myadaraboina et al. stated in their study that chloride and sulphates do not directly damage basalt fiber (Myadaraboina et al. 2014). There are many studies in the literature indicating that the durability properties of cement-based composites are improved by using fibers (Brandt 2008; Karahan and Atiş 2011; Yehia et al. 2016).
Toshiki et al evaluated copper slag as fine aggregate. As a result of the study, it was observed that copper slag improved the durability properties of concrete (Ayano et al. 2000). When copper slag is used instead of aggregate, the carbonation rate decreased as well as the increase in sulphate resistance (Hwang and Laiw 1989).
Partial replacement of industrial wastes such as slag, fly ash and silica fume instead of cement is known to be a useful technique to increase the durability of concrete to sulphate attack (Freeman and Carrasquillo 1991; Al-Dulaijan et al. 2003). In the study conducted by Onuaguluchi and Eren, copper slag used instead of cement increased the expansion values due to sulphate. This negative property of copper slag was explained by the permeability. This was observed since the permeability of the mixtures increases as the copper slag ratio increases (Onuaguluchi and Eren 2012).
3.6. Freeze-Thaw Resistance of Cement Based Composites
Fig. 24 shows the effect of basalt fiber properties and copper wastes on the freeze-thaw resistance. It was determined that 50 F-T cycles do not decrease the flexural strength much (Fig. 24a). Especially in mixtures with a w/b ratio of 0.40, a slight increase in flexural strength was observed. This situation may be caused by the incomplete hydration due to impermeability at low w/b ratios. The water leaking from the cracks formed in the mortar with the F-T effect enables the non-hydrated cement grains to participate in hydration. As a result, strength increase was observed in low F-T cycles. When using 2% of 6 mm long fibers, the flexural strength is generally reduced. However, if the fiber ratio is 3%, the loss of flexural strength after the F-T cycle may decrease. Increasing the fiber length decreases the flexural strength after F-T cycles. As the fiber ratio increases in the mixtures produced from 12 mm long fibers, the flexural strength decreases. The flexural strength after 50 F-T cycles varies between 7.1-8.8 MPa, while the flexural strength after 200 F-T cycles varies between 7.0-6.1 MPa. For the F-T effect, it is more appropriate to use 1% or 3% of 6 mm long fibers.
As seen in Fig. 24b, as the number of F-T cycles increases, the flexural strength of the mixtures decreases. After 50 F-T cycles, the flexural strength decreased as the copper aggregate increased in mixtures without copper slag. However, with the increase of copper slag, the use of copper aggregate relatively increased the flexural strength. This situation was also observed in mixtures 100 and 200 F-T cycles applied. After 50 F-T cycles, the greatest flexural strength was observed in mixtures using 7.5% copper slag and 50% copper aggregate. After 100 F-T cycles, the use of 15% copper slag and 25% copper aggregate reduced the losses in flexural strength. A similar situation is observed with the 200 F-T effect. Using copper slag in combination with copper aggregate reduced the flexural strength losses resulting from the F-T cycle.
In Fig. 25, it is observed that after all F-T cycles, the flexural strength decreases as the w/b ratio increases. As a result of the 100 and 200 F-T cycles, the flexural strength of the mixtures showed similar properties and fell below 7 MPa. It is observed that mixtures with a ratio of 0.40 W/B are not affected much by F-T cycles. This is related to the fact that the mixtures become more impermeable as a result of the decrease in the w/b ratio.
In Fig. 26, the effect of fiber properties and copper wastes on compressive strength after F-T cycles is given. After 50 F-T cycles, as the ratio of both 6 mm and 12 mm long fibers increased, the compressive strength of the mixtures increased (Fig. 26a). However, the increase in fiber length caused a slight decrease in compressive strength. Especially, the compressive strength of the mixtures obtained from 6 mm long fibers after 50 F-T cycles was over 50 MPa. As the fiber ratio increases in the mixtures obtained from 12 mm long fibers after 100 F-T cycles, the compressive strength increases. For 6 mm long fibers, it was observed that the optimum ratio was 1% or 3%. The compressive strength of all mixtures fell below 50 MPa after 200 F-T. However, the strength loss of the mixtures produced from 6 mm long fibers was less.
As with the flexural strength, the combination of copper aggregate and copper slag has a more positive effect on compressive strength after F-T cycles (Fig. 26b). Using 7.5% of copper slag after 50 F-T cycles increased the compressive strength. However, the compressive strength of the mixtures using copper aggregate was relatively lower. In the case of using 15% copper slag after 100 F-T cycles, the increase in copper aggregate affected positively the compressive strength. A similar situation was observed after 200 F-T cycles. If the copper slag is 7.5%, 50% copper aggregate generally reduces the strength losses after the F-T cycle.
Figure 27 shows the effect of w/b ratio on compressive strength after F-T cycles. As in the flexural strength, the freeze-thaw resistance of the mixtures decreases as the w/b ratio increases. It is seen that mixtures with a ratio of 0.40 w/b have a compressive strength over 50 MPa even after 200 F-T cycles. A slight increase in compressive strength was observed after 50 F-T cycles of mixtures with a W/B ratio of 0.40.
Using 2% of 6 mm long fibers generally reduces the compressive and flexural strengths after the F-T cycle. One of the reasons for this situation is that the 28-day compressive strengths of these mixtures are generally low (< 40 MPa). It is more appropriate to use 6 mm long fibers in obtaining the mixtures. The combination of copper slag and copper aggregate provided positive results for the F-T effect. The case, that if the copper slag is 7.5%, the copper aggregate is 50%, and if the copper slag is 15%, the copper aggregate is 25%, can improve the mechanical properties after the F-T cycle. It was observed that mixtures with low water penetration depth are usually cases where copper slag and aggregate are used in combination. It is more appropriate to have a w/b ratio of 0.40 for mixtures with this feature.
Researches showed that freezing-thaw resistance can be increased as a result of adding fiber to concrete. In addition, fibers can increase Matrix strength by preventing crack formation and development. The addition of fibers can increase the number of harmless pores, which can reduce the expansion pressure caused by the freezing event, and therefore reduce the degree of damage caused by freezing-thaw (Tiberti et al. 2014; Nam et al. 2016; Zhang et al. 2016). In some studies, the freezing-thaw resistance of concrete was increased by using basalt fiber (Jin et al. 2014; Zhao et al. 2018).
In the study cınducted by Shoya et al., it was reported that the freezing-thaw resistance of concrete is lower than control mixtures if copper slag is used instead of aggregate (Shoya et al. 1997). Ayano and Sakata stated that the freezing-thaw resistance of concrete increases if copper slag is used instead of aggregate (Ayano and Sakata 2000). However, there are a limited number of studies in the literature showing the effects of copper slag on freeze-thaw resistance.
Among the samples that 200 F-T cycles are applied, the samples with the highest loss of strength and visual damage are given in Fig. 28.
3.7. Optimization and Verification Tests of Cement Based Composites
For the mixtures produced according to the Taguchi L18 test matrix, optimization was carried out under three main headings. Optimization groups are presented below.
Group 1: Mechanical Properties
Group 2: Freeze-thaw resistance
Group 3: Dimensional stability and impermeability
For the mechanical properties in the first group, optimization was performed to maximize the compressive and flexural strengths of 7, 28 and 91 days. For the freeze-thaw resistance in group 2, optimization was performed to maximize the mechanical properties (compressive and flexural strength) after 50, 100 and 200 F-T cycles. In the third group, the optimum mixture was found to minimize the sodium sulphate-induced expansion, shrinkage after drying and water penetration depth. Experimental verification was carried out after the estimated properties of the optimum mixes were found. In groups 1 and 2, the target function was chosen as maximize, while in group 3, target function was determined as minimize. Estimated and experimental results of the 1st group are given in Table 7.
Table 7 Optimum mixture ratios for mechanical properties and results of verification tests
Optimum mixing ratios
|
Fiber length (mm)
|
Fiber ratio (%)
|
Copper slag (%)
|
Copper aggregate (%)
|
W/B
|
6
|
1
|
0
|
0
|
0.40
|
Predicted and experimental results
|
7 Day-Flexural strength (MPa)-Pre.
|
7 Day-Flexural strength (MPa)-Exp.
|
Diff. (%)
|
6.71
|
7.21
|
7.45
|
28 Day-Flexural strength (MPa)-Pre.
|
28 Day-Flexural strength (MPa)- Exp.
|
Diff. (%)
|
9.93
|
9.05
|
8.86
|
91 Day-Flexural strength (MPa)-Pre.
|
91 Day-Flexural strength (MPa)- Exp.
|
Diff. (%)
|
12.12
|
10.97
|
9.49
|
7 Day-Compressive strength (MPa)-Pre.
|
7 Day-Compressive strength (MPa)- Exp.
|
Diff. (%)
|
67.19
|
63.46
|
5.55
|
28 Day-Compressive strength (MPa)-Pre.
|
28 Day-Compressive strength (MPa)- Exp.
|
Diff. (%)
|
72.39
|
69.62
|
3.83
|
91 Day-Compressive strength (MPa)-Pre.
|
91 Day-Compressive strength (MPa)- Exp.
|
Diff. (%)
|
95.53
|
89.04
|
6.79
|
As seen in Table 7, it may be more appropriate not to use copper wastes when the mechanical properties are desired to be maximum. It was observed that the difference between the estimated and experimental results is less than 10% and there is less deviation percentage in the estimation of compressive strength. Using 1% of the 6 mm long fiber affected the mechanical properties positively.
Table 8 shows the results of optimization for the maximum compressive strength of mixtures as a result of 50, 100 and 200 F-T cycles.
Table 8
Optimum mixture ratios for Freeze-Thaw resistance and results of verification tests
Optimum mixing ratios
|
Fiber length (mm)
|
Fiber ratio (%)
|
Copper slag(%)
|
Copper aggregate (%)
|
W/B
|
6
|
3
|
7.5
|
0
|
0.40
|
Predicted and experimental results
|
After 50 F-T Compressive Strength (MPa)-Pre.
|
After 50 F-T Compressive Strength (MPa)-Exp.
|
Diff. (%)
|
69.39
|
71.87
|
3.57
|
After 100 F-T Compressive Strength (MPa)-Pre.
|
After 100 F-T Compressive Strength (MPa)-Exp.
|
Diff. (%)
|
61.75
|
64.04
|
3.71
|
After 200 F-T Compressive Strength (MPa)-Pre.
|
After 200 F-T Compressive Strength (MPa)-Exp.
|
Diff. (%)
|
56.11
|
52.82
|
5.86
|
In optimization for the freeze-thaw effect, it is more appropriate to use 3% of 6 mm long fiber. The degree of damage was reduced by controlling the stresses caused by the freezing effect with increasing fiber content. In addition, the use of copper slag at the rate of 7.5% also made a positive contribution to the freeze-thaw resistance. After 200 F-T cycles, it was observed that it is possible to obtain mixtures with compressive strength over 50 MPa by using a low percentage of copper slag. It was also observed that the w/b ratio of the mixture of 0.40 has a significant effect in reducing the damage after 200 F-T cycles. Low w/b ratio and high fiber content were the factors that decreased freeze-thaw damage.
In Table 9, the required mixture ratios are given to minimize the depth of water penetration, drying shrinkage and sodium sulphate-induced expansion.
Table 9
Optimum mixture ratios for impermeability and dimensional stability and results of verification tests
Optimum mixing ratios
|
Fiber length (mm)
|
Fiber ratio (%)
|
Copper slag(%)
|
Copper aggregate (%)
|
W/B
|
6
|
1
|
15
|
0
|
0.40
|
Predicted and experimental results
|
Water penetration depth (mm)-Pre.
|
Water penetration depth (mm)-Exp.
|
Diff. (%)
|
0.93
|
1.01
|
8.60
|
Drying shrinkage (10− 6)-Pre.
|
Drying shrinkage (10− 6)-Exp.
|
Diff. (%)
|
1195
|
1307
|
9.37
|
Sulphate expansion (10− 6)-Pre.
|
Sulphate expansion (10− 6)-Exp.
|
Diff. (%)
|
365
|
401
|
9.86
|
As seen in Table 9, the fact that copper slag is 15% contributes to the dimensional stability of the mixtures. It was particularly effective in reducing the sodium sulphate-induced expansions. Copper slag used instead of cement contributed to sulphate resistance as it reduced C3A and CH content. As a result of the increase in copper slag, it can be said that the depth of water penetration increases and accordingly the freeze-thaw resistance increases. The increase in copper aggregate content did not contribute to dimensional stability. Because it has a higher specific weight compared to crushed stone aggregate, increasing the amount of aggregate decreases the aggregate volume. As a result, the mixture becomes more porous and the shrinkage values may increase. The aggregate volume has an important role in reducing drying shrinkage.
In some studies, it was stated that basalt fiber causes coagulation in concrete and mortar (Meng et al. 2016; Alnahhal and Aljidda 2018). The coagulation of basalt fiber was determined by SEM images by Yan et al. and it was determined that this effect also affects the mechanical properties (Yan et al. 2017). Similar results was observed for shotcrete (Yan et al. 2020). In this study, it is thought that coagulation formed in basalt fiber causes variation in mechanical properties. The variation that occurs with the increase in fiber content may be due to the tendency to coagulation.