Use of Ceramic Tile Wastes as Raw Substitution Material in the Production of Blended Cement

Clinker is a key component of many conventional cements. The production process for clinker is energy-intensive, consuming approximately 850 kcal/kg, and contributes to greenhouse gas emissions, releasing about 0.85 tons of CO₂ per ton of clinker (1,2). Various studies have been conducted over the years to address these issues and develop greener cements. One of the possible solutions is using waste materials as a partial replacement for clinker in cement production, as mentioned earlier by several studies (1,3,4). This approach not only reduces the demand for clinker but also promotes an environmentally friendly strategy through the utilization of waste materials in the cement industry.

Portland-limestone cement is a blended cement produced by replacing up to 15% of the clinker with limestone, resulting in an average reduction in the carbon footprint of 10% while maintaining the intended performance in concrete production. The use of Portland-limestone cement began in the 1960s and rapidly became widespread with its adoption into the EN 197-1 (5) standard in the 2000s. According to the statistical reports of the Turkish Cement and Cement Products Assembly, the market share of Portland-limestone cements in Turkey has increased from 0.07% to 12.20% of the total cement market between 2000 and 2023 (Figure 1). However, the use of limestone in the production of blended cement continues to have environmental impacts due to the quarrying of natural limestone resources, transportation, machinery, milling, etc., which contribute to additional greenhouse gas emissions.

The ceramic industry plays a significant role in construction, with ceramic tiles being among the most commonly produced materials for use in flooring and wall coverings. The global production volume of ceramic tiles was reported as 18.2 million m² in 2021 (6). Turkey is among the top 10 countries in the world in ceramic coating material production. In 2022, Turkey ranked ninth in the world with a production share of 2.3%. Turkey’s ceramic tile production is expected to be 461 million m² by 2025, according to the World Production and Consumption of Ceramic Tiles Report (7). It has been reported that approximately 30% of the ceramic products produced are released as waste. The accumulation of these wastes is expected to lead to environmental and health problems in the long term.

Many studies in the past have attempted using ceramic tile wastes in concrete as an alternative to cement. Alsaif (4) reviewed studies that investigated the use of ceramic waste as a replacement material for cement in mortar and concrete production. The findings indicated that substituting up to 20% of cement with ceramic waste can have beneficial effects on rapid chloride permeability, electrical resistivity, and sulfate resistance. Additionally, improvements in compressive strength were observed, particularly in the medium term (56 and 90 days). El-Dieb and Kanaan (8) investigated the use of ceramic waste powder as an alternative to cement at 10, 20, 30, and 40% substitutions by mass in three different concrete grades (25, 50, and 75 MPa). The rate of compressive strength development decreased at 7 and 28 days; however, the target compressive strengths were achieved for all substitution rates by 90 days. Raval et al. (9) replaced cement with ceramic waste powder at substitution percentages of 10, 20, 30, 40, and 50% in concrete production. Optimum results were reported at a 30% substitution, with a compressive strength of 22.98 MPa and a 12.67% reduction in cost. Similarly, in a recent study by Faldessai et al. (10), cement was substituted with ceramic waste powder in concrete production at percentages of 10, 20, 25, and 30% by mass and achieved a compressive strength of 41.22 MPa and a 14.89% cost reduction. Samadi et al. (11) prepared mortars by replacing cement with ground ceramic waste at percentages of 20, 40, and 60%. The results showed that at up to a 40% replacement ratio, compressive strengths were enhanced at 28 and 90 days, and energy consumption was expected to decrease from 3.02 GJ/m³ to 2.13 GJ/m³, with significant cost savings. Hoppe Filho et al. (12) used ceramic waste as a supplementary cementitious material. In this study, a reference cement and a cement incorporating ceramic waste (30% of cement in mass replaced by ceramic waste) were prepared, and their physical and chemical properties were analyzed. The mechanical performance of the prepared mortars was evaluated at three different water-to-cement ratios over curing periods between 7 and 182 days. They reported that the ettringite phases were intensified and mono-carbo-aluminates were formed when using ceramic wastes, although compressive strengths generally decreased. Mohit et al. (13) replaced ceramic waste powder and limestone powder at various ratios by weight of cement in the production of ternary cement. They concluded that the ternary cement mortar with 10% ceramic waste powder and 15% limestone powder had the highest compressive strength. Samples containing 30% ceramic waste powder by mass of cement showed approximately 51% less alkali–silica reaction compared to the reference. The initial and final setting times of the samples increased compared to the reference sample as the ceramic waste powder content was increased up to 30%.

Puertas et al. (14) indicated that some ceramic wastes could be utilized in cement manufacturing due to their suitable chemical and mineralogical properties. However, there are relatively few studies in the literature on the use of ceramic wastes in cement production. Puertas et al. (1) published an article on the use of ceramic waste in the production of cement clinker. They prepared raw cement mixes using limestone, clay, Fe₂O₃, and ground ceramic wastes. The mixes were burned at temperatures up to 1500 °C, and their chemical, mineralogical, and morphological properties were investigated. They reported that the mineral composition and phase distribution in the resulting clinker were comparable to those of conventional reference clinker. Mas et al. (2) conducted a study on the use of ceramic wastes in the production of pozzolanic cement. They found that the workability of mortars incorporating ceramic waste was similar to the control mortar, and the addition of ceramic waste to between 15 and 35% of cement met the strength activity index requirements established by fly ash regulations. Abdul-Wahab et al. (15) notably highlighted that industrial wastes, such as ceramic, concrete, marble, etc., have the potential to serve as sources of calcium, silica, and alumina, which also contribute to the sustainability of raw materials and waste management.

Clinker, limestone, pozzolana, and gypsum were used to produce a reference limestone blended cement. Then five different ceramic waste-blended cements were formulated by replacing limestone with ceramic waste at percentages of 5, 10, 15, 20 and 28%. According to the recent EN 197-1 (5) standard, cement types containing limestone are classified as “CEM II/A-L”, “CEM II B-L”, “CEM II A-LL”, and “CEM II B-LL”. The proportion of limestone used in the grinding process ranges between 21% and 35% by weight for the “CEM II A-LL” and “CEM II B-LL” cements. The 28% ratio was selected as it represents the average value between 21% and 35%.

The design and mix ratios of the materials are presented in Table 1. The materials used in the preparation of the cements are shown in Figure 2. Clinker, limestone, pozzolana, and gypsum were obtained from BAŞTAŞ (Baştaş Başkent Cement Industry and Trade Inc., Ankara, Turkey), while the waste ceramics were obtained from local ceramic manufacturer (Ece Bathroom Equipment Industry and Trade Inc., Çorum, Turkey). Note that all materials were initially crushed into clinker size using a laboratory-type mill (Figure 3). In the production stage of all types of cement, the ingredients were ground together in the mill for 30 min.

After the preparation of each cement, its chemical, physical, and mechanical properties were determined. For the chemical analysis, a FLUXANA X-ray fluorescence (XRF) (FLUXANA® GmbH & Co., Bedburg-Hau, Germany) analyzer conforming to ISO 29581-2 (16) was used (Figure 4a). Loss on ignition values was determined using a ProTherm (Alser Technical Ceramic Co. Inc., Ankara, Turkey) high temperature furnace conforming to ASTM C114 (17) (Figure 4b). Blaine specific surface areas of cements were calculated via an Atom Technic Blaine test device conforming to ASTM C 204 (18), automatically (Figure 4c). The chemical structure of each cement was analyzed through X-ray diffraction analysis using a PANALYTICAL Empyrean (Malvern Panalytical Ltd., Malvern, UK) brand test device.

After the determination of basic cement characteristics, water consistency demand and setting properties of cements were assessed using a Vicat apparatus in accordance with ASTM C 187 (19) and ASTM C 191 (20), respectively (Figure 5). Cement mortars were prepared with a cement–sand–water ratio of 1:3:0.50, and spread flow diameters were determined conforming to ASTM C230 (21). A total of 18 pieces of 40 × 40 × 160 mm³ prismatic specimens were cast from each mortar made with produced cements. Mechanical tests were conducted using a ToniTechnic convertible test device. The flexural strengths of mortars were determined at 2, 7, 28, 56, 90, and 365 days following the ASTM C348 (22) (Figure 6a). All specimens were water-cured until the test day. The compressive strength test was performed on both sections separated during the flexural strength tests, conforming to the ASTM C349 (23) standard (Figure 6b).

Approximately 0.85 tons of CO₂ are released for every 1 ton of clinker produced. Therefore, the CO₂ emissions of the reference cement, due to the clinker, were calculated to be approximately 0.54 tons of CO₂ per ton of cement. According to Shan et al. (2016) (28), the emission factor for lime was reported as 0.683 tons of CO₂ per ton of lime. By factoring in the emissions from the added limestone, the estimated total CO₂ emissions of the reference cement were calculated to be 0.72 tons of CO₂ per ton of cement. During the preparation of blended cements in this study, only the amount of limestone additive was reduced. Therefore, the estimated CO₂ reduction due to the decreased limestone additive ranged between 4.70% and 26.32% (Figure 15). It should be noted that this calculation does not include the CO₂ emissions from waste ceramics due to a lack of available data; however, it may provide insight for future studies, particularly with regard to the conservation of resources.

• The chemical analysis showed that the SiO₂ content of cements was increased with an increased ceramic waste substitution ratio, while the CaCO₃ contents were decreased. Similarly, the peak intensity of SiO₂ was increased and the peak intensity of CaCO₃ was decreased in the XRD analysis of the cements.

• Since the hardness of ceramic wastes was higher than of limestone, the grindability of cements was decreased with the increased ceramic waste percentage, which decreased the Blaine specific surface area values.

• The consistency of water for cements was accepted as 28%, where all the cements achieved the standard limitations. However, the Vicat probe penetrated the cement pastes more easily at the same water consistency level, related to the decreased Blaine values.

• The spread diameters obtained for all types of cements were similar and practically usable in terms of workability.

• The cements including ceramic waste up to 10% maintained the setting time; however, when the ceramic waste substitution percentage increased to 15% and above, the setting time was prolonged.

• When the mechanical performances were taken into account, all the ceramic waste-included cements demonstrated higher flexural and compressive strength compared to the reference cement, regardless of the substitution percentage.

• The highest flexural strengths were obtained when the ceramic waste substitution was 28%, for all the curing ages.

• In the case of compressive strengths, all the cements exhibited higher compressive strength than 10 MPa at 2 days and 32.5 MPa at 28 days, which classified them as 32.5 R-type blended cements. The 10% ceramic waste-substituted cement exhibited the highest compressive strength; however, it is remarkable that over this replacement percentage, the compressive strengths were found to be similar.

• When the medium- and long-term compressive strengths were compared, the highest strength values were obtained from the cement with a 28% ceramic waste substitution.