Concrete has been used as one of the most popular construction materials for decades, while the concerns regarding the use of limited resources for concrete production have accumulated. Concrete technology developers and industries have discussed solutions to meet the global consensus of sustainable development and promote sustainability for concrete construction. Potential solutions include utilizing waste materials (e.g., fly ash and slag), saving materials in design, recycling concrete, and maximizing concrete performance under the given resources. Hence, the use of waste materials as an additive or a substitute for concrete components to enhance concrete performance has recently received considerable attention, owing to waste management. Kim et al. [1
] studied the effect of recycled aggregates on the mechanical properties and durability of concrete. Blending recycled coarse aggregates generally promotes the flowability of concrete while satisfying the design compressive strength. Batayneh et al. [2
] reported that crushed glass aggregates as a partial substitute for fine aggregates improved the strength of concrete. Binici et al. [3
] blended marble and limestone dust as additives with the concrete mixture. They found that those waste materials increased the compressive strength of normal concrete and promoted the resistance to sodium sulfate.
In recent years, activated carbon has also received favorable attention as a reinforcement of cement-based composites. Chowdhury et al. [4
] found that activated carbon could promote moisture resistance in cement and mitigate the risk of moisture attacks. Mahoutian et al. [5
] evaluated the effect of activated carbon on the air void characteristics of concrete. They reported that activated carbon reduced the air content in concrete, leading to an increase in strength. Activated carbon can be also used to improve indoor air quality. Krou et al. [6
] researched the adsorption capacities of hardened cement pastes that contained activated carbon as an additive. They exposed the cement specimen to volatile organic compounds (VOCs) and measured the number of hazardous compounds, such as acetaldehyde and toluene. The results revealed that activated carbon powder was able to absorb the greater part of toluene, and thus, hardened cement paste containing activated carbon could be a potential construction material that helps to improve the depolluting effect. Zhang et al. [7
] also observed that activated carbon could reduce the radon exhalation rate in concrete.
Activated carbon and biochar share many properties, and it is difficult to differentiate one from the other. While biochar is primarily used for soil amendment, activated carbon is widely used to adsorb dissolved contaminants in water and wastewater treatment [8
]. The large surface area per unit mass of activated carbon makes it a great absorbent. This implies that small amounts of activated carbon can effectively adsorb contaminants from the solution. Activated carbon is available in two different forms: granular activated carbon (GAC) and powdered activated carbon (PAC). GAC is generally used as filter media infiltration, and PAC is typically mixed in water in reactors and then settled or filtered out downstream [9
]. However, the production of activated carbon from typical carbonaceous sources such as natural coals and petroleum residues is complex and expensive, demanding considerable energy consumption, so there exists a need to explore alternative options. One possible alternative is using coffee waste as a base material for activated carbon. Several studies reported that spent coffee grounds proved to be a suitable base material for activated carbon [10
]. In this paper, we investigated a novel, sustainable cementitious composite that utilized activated carbon reinforcement manufactured from waste coffee grounds. The surface morphology of lab-manufactured activated carbon and the effect of activated carbon, particularly on the compressive strength of normal cement mortar, are discussed.
The compressive strength test results revealed that a small amount of activated carbon up to 1.5 wt% increased the compressive strength of normal cement mortar, whereas the excessive amount equal to or greater than 5% substantially decreased the strength. Similar results have been reported by other studies, although the number of studies pertinent to activated carbon as an additive to cementitious materials is limited. Ersan et al. [29
] found that the addition of granular activated carbon increased the 7 day and 28 day compressive strengths by 9% and 13%, respectively. Justo-Reinoso et al. [23
] replaced a small portion of fine aggregate from 1% up to 10% by mass with powdered activated carbon in cement mortar. They investigated the effect of activated carbon on both the compressive and tensile strengths of cement mortar and found that increased strength was observed at 1% and 2%, while the highest strength was measured at 1%. However, higher contents exceeding 4% gradually decreased the strength.
Understanding the mechanism that causes the change of strength would gain insight into the effect of activated carbon on the strength of normal cement mortar. Justo-Reinoso et al. [23
] proposed that porous activated carbon acts as micro-reservoirs. Some amount of water in normal cement mortar would be lost to bulk evaporation during the process of hydration reaction. In activated carbon cement mortar, however, water adsorbed to activated carbon granules is redistributed and delivered to cement particles during the progression of the curing process. This microscale curing process caused by local hydration contributes to an increase in the global strength of the cement mortar.
It should be noted that they used activated carbon as a substitute for a portion of fine aggregates. Thus, blending a large amount of activated carbon resulted in strength reduction due to the intrinsic compressive strength of activated carbon granules, which are weaker than fine aggregates. Zheng et al. [30
] reported that calcium hydrate (CSH) and ettringite populated by activated carbon filled the hydration gap in fly ash cement composites, resulting in an increase in strength. The type and composition of the cement mortar used in this study were different from other studies. However, it can be assumed that the enhanced strength observed in this study may be attributed to the holding water capacity of activated carbon and the microscale curing process. In addition, the excessive addition of activated carbon above the threshold might hinder the hydration of cement due to a lack of water in the cement.
The effect of activated carbon manufactured from waste coffee grounds on the strength of normal cement mortar was studied. The results revealed that adding activated carbon smaller than or equal to 1.5% (by weight to cement) increased the compressive strength of the cement mortar. Furthermore, activated carbon could also accelerate the strength hardening process, resulting in the reduction of curing time. Particularly, a small amount of the content (0.5%) had more meaningful increases in strength at the early stage, while a higher content (1.5%) had better increases at 28 days. However, a large amount of activated carbon exceeding 5% substantially decreased the strength. This study concludes that activated carbon recycled from waste coffee grounds can expect performances similar to those reported in other studies that used commercially available activated carbon products. Therefore, the recycled one can be also used as a potential additive to enhance the compressive strength of cement mortar.