Annual cement consumption has increased to approximately 4.6 billion tons worldwide [1
], and this number is expected to increase by about 6 billion by the year 2050 [2
]. Humans have been dealing with the problems caused by global warming for a long time. The rapidly increasing population and need for shelter caused the concrete industry to spread carbon dioxide into the environment, accounting for approximately 7–8% of total emissions [3
]. Clinker, which a crucial phase of cement production, is mainly responsible for this problem; further, the burning of raw materials around 1450 °C generates carbon dioxide [5
]. Efforts to reduce carbon emissions started with the use of high-volume fly ash in the 1950s; these efforts have continued with the evaluation of different industrial wastes today but have not yet reached the desired level [6
]. This does not allow existing standards to be used in high quantities of these wastes, or the fact that the quality of the resulting product cannot be controlled regularly. Huge demand for construction material contributes to environmental problems and poses a risk for living organisms. The term “sustainability,” first introduced in the Brundtland Report
in the 1980s [7
], offers the use of alternative supplementary cementitious materials in concrete construction. Cement can either partially replace various waste or be used during the cement’s manufacturing stage to attain sustainability goals. Nations have accepted to reduce carbon emissions using alternative binder materials in building construction by 2050 [2
]. During concrete production, natural resources, such as aggregates, are used; further, they are the main components of the concrete, accounting for 70–85% of its volume [8
]. The scarcity of natural resources has forced the building sector to find alternative solutions. To attain more sustainable construction, the consumption of natural resources should be minimized to a larger extent [9
]. Besides, fly ash [10
], marble powder [11
], bottom ash [6
], and rice husk ash [12
] are commonly used as cementitious materials to improve the fresh and hardened properties of the composites.
Bottom ashes are composed of larger particles and collected at the bottom of the furnace. Bottom ashes contain mainly silica (55%), alumina (20–25%), and iron (8–10%) oxides (5–10%). Lower amounts of calcium oxide are present [13
], mainly in the form of a glassy and amorphous structure composed of irregular and sphere particles [14
]. Many studies have focused on concrete [19
]. Recently, some researchers have evaluated its properties in cement pastes. In these studies, the addition of bottom ash reduced the composite’s strength and its durability against the harsh environment. The researchers searched for alternative solutions to improve the hardened properties to satisfy the general concrete requirements. However, up to date, none of them have been fully satisfied. Abdulmatin et al. [22
] examined the possibility of using bottom ash as a pozzolanic material. They reported that mortar containing up to 20% bottom ash could be satisfactorily used as a pozzolanic material in concrete construction. The majority of bottom ash particles fall within 325 micrometer sieve size, which was found to be an ideal sieve size for bottom ash particles to produce excellent pozzolanic activity. Singh et al. [23
] compiled the published literature on the use of bottom ash as a fine aggregate replacement. The authors mentioned that the incorporation of a larger amount of bottom ash reduced the concrete’s compressive strength. Additionally, bottom ash improved the microstructure of concrete, and a 25–30% replacement level was found optimum for bottom ash. Le et al. [24
] examined the triaxial behavior of the composites composed of bottom ash. They found that the bottom ash performed the same as dense sand and satisfied the requirements for designing road base structures.
Marble dust is another waste that causes environmental problems. During production, a huge amount of it goes to the dumpsite near the manufacturing units. Disposal of this waste affects not only the ecology but also the groundwater quality. Bostanci [25
] evaluated the marble dust for sustainable concrete construction. He concluded that marble dust causes a significant reduction in compressive strength at day 1. However, this can be compensated for at later ages. Ashish [26
] evaluated the use of marble waste as a sand replacement together with two different supplementary cementitious materials in concrete. Based on the experimental tests, no adverse effect of using marble wastes was observed. Additionally, marble waste helps the hydration process and improves the concrete microstructure. Ma et al. [27
] used marble powder in cement-based materials to improve mechanical properties. They reported that 10% waste marble and 3% of nano-silica was optimum for better strength properties. They also suggested that the negative effect of marble powder at higher rates can be offset by using nano-silica. Nežerka et al. [28
] investigated the microstructure of cement pastes composed of marble powder. They mentioned that marble powder increases the porosity of cement pastes. Additionally, they concluded that marble powder incorporation had minor effects on the stiffness of the cement paste and the interfacial transition zone. Kabeer and Vyas [29
] produced mortar mixes composed of marble powder as a fine aggregate replacement. Their results revealed that 20% is an optimum replacement level for better sustainable building construction, as it helps reduce the demand for river sand. Khodabakhshian et al. [30
] investigated the durability performance of the structural grade concrete composed of marble powder. They reported that 10% is the optimum value, and, beyond this replacement, level strength tends to decrease. Sutcu et al. [31
] studied the effect of the addition of 35% marble powder to laboratory-produced bricks. They reported a decrease in bulk density and compressive strength. However, bricks containing up to 30% marble powder addition satisfied the building requirements. Corinaldesi et al. [32
] studied the characterization of marble powder in concrete and mortar. They concluded that a 10% replacement of sand by marble powder was stronger. Furthermore, Vardhan et al. [33
] evaluated the performance of concrete by incorporating marble powder as a fine aggregate replacement. They reported that strength and drying shrinkage improved by 20% and 30%, respectively, with marble powder incorporation.
The addition of fibers can improve the weakness in tension and other strength properties. The cracks and drying shrinkage can be minimized by using various fibers [34
]. Notably, cement-based materials are vulnerable to temperature changes and loading. Recently, carbon fiber was introduced into concrete to improve the flexural strength and reduce the crack width. Carbon fibers contain mainly carbon atoms and are commonly used in civil engineering works. The optimum carbon fiber amount should be 0.3% by volume of concrete. The best performance was reported in carbon fiber and basalt fiber. Polypropylene fiber also shows superior performance [34
]. Pirmohammad et al. [36
] investigated the fracture toughness of asphalt concrete containing carbon fiber, indicating that fracture toughness improved by 42% compared with plain concrete. Additionally, the authors noted that 4 mm in length showed superior performance compared with those at 8 and 12 mm. Ostrowski et al. [38
] investigated the effect of carbon fiber on the strengthening of concrete columns, stating that carbon fiber improved the resistance of concrete columns to buckling and enhanced the strength against confining pressure. Liu et al. [39
] examined the flexural behavior of coral concrete composed of carbon fibers. They concluded that using carbon fibers extended the deformation and that the fibers absorbed more energy compared with reference concrete. Additionally, flexural toughness increased by 367% to 586% with different concrete grades. The optimum carbon fiber was found to be 1.5% for coral concrete. Safiuddin et al. [40
] investigated the mechanical and microstructure of carbon-fiber self-compacting concrete. The concrete was prepared with various carbon fiber content (0–1% by volume) and two different water-to-binder ratios (0.35 and 0.40). The authors concluded that due to the high length to diameter ratio of carbon fiber, they are not able to resist the compression. The reduction in compression strength was recorded in the range between 35–60%. Song et al. [41
] studied the flexural strength, drying shrinkage, and creep behavior of concrete composed of slag and carbon fiber. They concluded that carbon fiber decreased the compressive strength but increased the flexural strength of composites. The authors also stated that drying shrinkage decreased by 29% when incorporated with 0.3% carbon fiber.
Additionally, creep development was reported to decrease with carbon fiber. Yakhlaf et al. [42
] investigated the new properties of self-compacting concrete containing carbon fiber (0–1% by volume). They stated that carbon fiber had no significant effect on segregation, but it decreased the workability of the mixes by reducing the passing ability of the fresh concrete. The authors found that 0.75% of carbon fiber was optimum for fresh concrete workability. Rangleov et al. [43
] investigated the possibility of using carbon fiber in previous concrete applications. They mentioned that using carbon fiber had a beneficial effect on pore structure and helped to improve the strength of the composites. Tanyıldız [44
] evaluated the effect of carbon fiber on the mechanical properties of lightweight concrete. He prepared the samples composed of various carbon fiber percentages from 0% to 2% by a mass fraction. He concluded that the optimum carbon fiber should not exceed 0.5% for the best compressive strength. Giner et al. [45
] examined the dynamic properties of concrete containing carbon fiber and silica fume. They stated that the addition of carbon fiber is more effective for reducing vibrations compared with silica fume additions. Furthermore, the addition of carbon fiber decreased the compressive strength but increased the flexural strength. Moreover, Díaz et al. [46
] investigated the phase changes of cement paste with carbon fibers. They concluded that carbon fiber increased the pores significantly. The poor microstructure of cement paste affected the permeability of the paste. This can be more effective beyond higher carbon fiber volume fractions. Hambach et al. [47
] produced pastes containing carbon fiber (3% by volume) with flexural strength greater than 100 MPa. They reported that if the carbon fiber aligned in the stress direction, this could help the tremendous improvement in flexural strength. Kim et al. [48
] investigated the effect of carbon fibers on autogenous shrinkage and electrical properties of cement pastes and mortars. They reported that flow values declined significantly with the addition of carbon fiber. However, the electrical resistivity of pastes improved with the addition of carbon fiber. The authors also mentioned that the negative effect of fine aggregate was mitigated with the addition of carbon fiber. Fu and Chung [49
] studied the effect of carbon fiber on the thermal resistance of cement paste. They reported that the addition of short carbon fibers (0.5–1% by weight of cement) decreased the thermal conductivity. Belli et al. [50
] examined the effect of recycled carbon fibers up to 1.6% by volume fraction on the thermal conductivity of mortars. They stated that the carbon fibers decreased water absorption by 39%. Additionally, their analyses revealed a decrease in thermal conductivity. Wei et al. [51
] evaluated the performance of carbon fiber lightweight concrete. The toughness of the composites increased by 26–37% when carbon fiber was added. Additionally, the tensile strength of the composites increased significantly with the addition of carbon fiber.
The use of bottom ash and marble dust waste has increased recently, thus reducing the amount of cement. Although it is recommended to use fiber to improve the strength and durability of composites produced using these materials, studies on this subject are limited. Most studies focused on concrete. In this study, composites from cement paste were prepared by mixing 20% of bottom ash and marble powder with cement. Four different carbon fiber ratios (0.3%, 0.75%, 1.5%, and 2.5%) were added to improve the performance of the composites, and their physical, mechanical, and durability properties were investigated. The material has been designed as a cement paste, and an alternative binder material has been attempted to be used in the concrete component by using waste materials. No previous study, to our best knowledge, on strengthening the cement paste of carbon fiber has been encountered. In addition, this study contributes to the concrete industry by using two different wastes and investigating the physical, mechanical, and durability of these composites produced in a laboratory environment. Using bottom ashes in structural grade applications could be the best solution for future sustainability trends.
Though a significant amount of industrial waste, e.g., marble dust and bottom ash, is readily for use in civil-engineering-related works, its use still remains low in the building sector. However, the use of such waste in nonstructural-grade applications (i.e., masonry works, controlled low-strength applications, subways, paving application, etc.) has great potential toward achieving a sustainable construction industry and reducing CO2 emissions. We believe that using them in structural concrete applications provides a better sustainable approach in the concrete sector. However, more research should be conducted, especially for those applications that check safety regulations. The International Union of Laboratories and Experts in Construction Materials, Systems and Structures, RILEM, have carried out research to offer suitable standards and design methods for quality assessment of pastes, mortars, and building materials used in civil engineering works.
Additionally, as we mentioned above, alternative binders must be developed and used for building sector; further, existing tests mentioned in standards are not appropriate for practice in their assessment. Therefore, modified or totally alternative testing procedure must be required. Because of limited understanding of their site performance, novel provisions for those materials also required to address the limits on physical performance and strength development found in existing standards. Thus, we checked the performance of our marble dust cement paste and bottom ash cement paste composites through various tests for physical and mechanical durability. In these evaluations, cement paste strength was not considered to be equivalent to concrete strength. We evaluated the laboratory-produced composites according to the current international standards and tried to optimize the performance of our composites based on those mentioned tests. The performance-based requirement could possibly apply to produced cementitious composites and allow for direct evaluations. However, the lack of agreement in sector concerning the engineering properties that need to be tested, along with the absence of suitable testing techniques that would fit well with material properties to performance when these alternative materials are used in various forms in concrete applications. Moreover, based on the safety considerations with concrete structures, there is reluctance in the sector to use novel materials because there is less information regarding performance and durability-related issues in long term. In this study, bottom ash and marble dust were used as a cement replacement, and the composites were enriched with various carbon fiber volume fractions. Physical, mechanical, and durability properties were evaluated at 7, 28, and 56 days. According to the issues mentioned above, this study covers the wide-range use of waste, and the laboratory-produced composites can be considered to be novel composites. The study designed a pure paste that does not contain any natural raw-materials (i.e., fine and coarse aggregate). Thus, the composites are said to be sustainable; further, ecological building composites deserve full consideration in the building sector. Based on the experimental results, the following conclusions can be drawn.
Carbon fiber was effective up to 0.75% for bottom ash, which is 0.30% for bottom ash mixture groups basically when considering the physical properties at the early period of hydration. However, the authors’ advice is to not use beyond a 0.3% carbon fiber dosage. Beyond 28 days, higher dosages cannot be effective, and improper compaction will lead to the formation of voids and reduce the ability of carbon fiber to hold the particles.
Decrease in flow was more notable in marble dust mixture groups compared with bottom ash mixture groups. An approximately 54% reduction was observed in marble dust mixtures when compared with the reference mixture. The reduction is mainly due to the surface characteristics of the carbon fiber with a high-length-to-diameter ratio. The surface of the carbon filaments absorbs more water during mixing and causes a decrease in workability at higher rates. The addition of carbon fiber above 0.75% was associated with significantly reduced flowability of marble dust mixture groups.
Porosity was higher in marble dust mixture groups. The addition of carbon fiber increased porosity values for marble dust and bottom ash mixture groups beyond the 0.75% addition.
Water absorption values decreased with the addition of carbon fiber. However, the addition of carbon fiber beyond 0.75% increased water absorption values for both mixture groups. However, the increase was greater in bottom ash mixture groups. The authors believe that the higher absorption values can possibly create additional water for later ages and the pores acting as a reservoir for providing extra water available for curing. This can be compatible with the mechanical and durability test results. No adverse effect of higher absorption capacity at lower dosages was reported by considering the compressive and flexural strength. Additionally, no loss of resistance against sulfate and seawater environments was observed during this study.
Unconfined compressive and flexural strength values increased with the addition of carbon fiber. However, beyond a 0.75% carbon fiber addition level, both strength values decreased. The decrease in intensity was greater for bottom ash mixture groups. The compressive strength reached 48.4 and 47.2 MPa for marble dust and bottom ash mixture groups, respectively, at 56 days of curing. The strength gain was greater after 28 days for both mixture groups. The laboratory produced carbon-enriched bottom ash; further, marble cement paste composites showed excellent performance at later ages. The composites can be satisfactorily used in various civil engineering works and applications (e.g., manufacturing of bricks, tiles, paving stone). The composites’ weakness against mechanical loading can be reduced with the addition of small carbon fibers. For structural-grade applications, compressive strength values are also appropriate at lower carbon dosages. The mechanical performance for both mixture groups is appropriate for using composites in the building sector as an alternative sustainable material. The use of industrial waste such as bottom ash and marble dust have a positive effect on the economy and sustainability. The authors believed that using 20% waste and reducing the amount of cement in pastes will have a major impact on concrete, especially in highway or other bulk applications. This study considers the cement paste application with lower-scale applications. The impact would be more when used in concrete. The reduction in carbon dioxide amount decreases the negative effect of environmental pollution. Additionally, in this study, 6mm carbon fiber was used. Thus, the effectiveness of long carbon fiber on mechanical performance should be checked before use.
Both composites were resistant to sulfate and seawater attacks. Carbon fiber seemed to be useful for protecting the composites against harsh environments at low volume fraction. The addition of carbon fiber beyond 0.75% adversely affected the composite’s resistance.
It is highly recommended to investigate the effect of carbon fibers on the microstructure of these novel-based composites. Additionally, different water binder ratios might be helpful for better understanding the behavior of such composites. In this study, the length of carbon fiber is 6 mm; further, the authors strongly recommend using different lengths of carbon fiber to investigate the performance of the composites on a large scale.