Abstract
The cement industry is expanding rapidly around the globe, despite its significant contribution to global carbon dioxide emissions. Likewise, in Pakistan, the cement industry produces million tons of cement and has a history of contributing as high as 10% of the country’s carbon dioxide emissions annually. The most effective technique for reducing carbon dioxide emissions is to use supplementary cementitious materials (SCMs). Limestone Calcined Clay Cement (LC3-50) is a recently developed cement blend comprised of 50% clinker, 30% calcined clay, 15% raw limestone, and 5% gypsum by weight. The focus of this research is to assess the mechanical properties and durability performance of LC3-50 cement composition formulated using raw materials from Pakistan. The comprehensive case study would help advance the research and commercialization activities in the country. In this study, compressive strength, split tensile, flexural, and pull-out tests were performed for evaluating the mechanical properties. For the assessment of durability, water permeability, sorptivity, carbonation, and rapid chloride penetration test (RCPT) tests were performed as per the available standards. It is concluded that the use of LC3-50 instead of ordinary Portland cement (OPC) improved the strength and durability of the end concrete.
1. Introduction
The country’s economic development is the reflection of growth in infrastructure and an increase in cement production. Globally, 10 billion cubic metres of concrete are produced each year, and this figure is unlikely to decrease. However, it is crucial to look at different options for producing cement because of environmental degradation. Around 0.8 tonnes of carbon dioxide (CO2) are released into the atmosphere for every ton of ordinary Portland cement (OPC) produced. Due to the mass production of concrete, it accounts for about 5–8% of the total carbon dioxide emissions worldwide [1, 2]. The world is on track to exceed carbon dioxide emissions by 2028 because of which the temperature may rise by up to 1.5°C above preindustrial levels as a result. Hence, we have to work aggressively in all areas of life to slow down the temperature rise. Else, we will have to spend in the future to address problems [3].
Ecologically sound (or eco-efficient) cement-based materials have been developed and extensively researched in recent years. The most successful and cost-effective option to attain green objectives in the development of cementitious materials appears in the use of supplementary cementitious materials (SCMs) partly replacing OPC [4, 5]. Subsequently, the main concern is to explore abundant SCM resources to meet the massive demand for cement and concrete production. The overall production of usual SCMs such as fly ash and silica fumes is limited and nonpredictive. Calcined clay plays an essential role in rendering as supplemental material for cement because of its low-cost, widely available, and less energy usage in grinding processes and among other things. It is hard to reduce the clinker factor below 70% without the combinations of SCMs [6].
Calcined clay together with raw limestone in 2 : 1 proportion can reduce clinker as low as 50% or even more. The low carbon cement blend was named Limestone Calcined Clay Cement (LC3-50). LC3-50 cement blend mixture of 30% calcined clay, 15% limestone, 5% gypsum, and 50% clinkers. LC3-50 has been tested using calcined clay containing different proportions of kaolinite content. However, the 40%–75% kaolinite concentration in clay was determined to be preferable to all other LC3-50 blends in compression and durability testing. LC3-50 is the most effortless alternative to replace clinkers in cement, resulting in a significantly lower carbon dioxide emission. The main goal of LC3-50 was to lower clinker content to reduce carbon dioxide emissions. Clinker proportion in OPC was around 95%, LC3-50 lowered the clinker percentage to 50% due to replacements [6]. LC3-50 has numerous advantages, and it is more effective than OPC in terms of durability. It gives subtle resistivity to the intrusion of chloride, steel corrosion, alkali-silica reaction (ASR), and sulphate attack. The use of LC3-50 cement can permanently eliminate these significant problems in concrete [7].
Pakistan is rich in mineral resources. Pakistan has large kaolin clay reserves in Shah Dehrai, Swat (34°53′30″N; 72°53′30″E), and Nagar Parkar District TharParkar, Sindh (24°15′54″ to 24°30′28″N, 70°37′34″ to 71°07′50″E). The estimated kaolin clay reserves at Shah Dehrai and Nagar Parkar are 2.7 million tons and 3.6 million tons, respectively [8]. The clay extracted from these resources is mainly used in the ceramic and sanitary industry [9]. These deposits can be utilized to produce LC3-50 cement and thus lower the carbon dioxide footprint of the cement industry’s sector.
It is a systematic study conducted to evaluate the mechanical and long-term durability of the LC3-50 blend prepared from Pakistani raw materials and to determine the future scope of the research and the pros and cons of using it as a structural building material in Pakistan. The focus of this study is to compare the mechanical properties of LC3-50 and OPC concretes. The mechanical tests which were conducted in this research include compressive strength, splitting tensile strength, flexural strength, and pull-out tests. In addition to mechanical characterization, this study compares the durability of LC3-50 cement concrete to that of OPC concrete. Four tests have been conducted to ascertain the durability of the LC3-50 blend and OPC. These tests include water permeability, sorptivity, carbonation, and rapid chloride penetration.
2. Materials and Methods
2.1. Raw Materials
The OPC used in this study was a general-purpose Portland cement classified as 53 grade, considering it has a higher strength. For the production of LC3-50, clay containing 75% kaolinite content was utilized. The protocol for finding the kaolinite content in clay is given by [10]. In this method, take three crucibles and add 10 g of kaolinitic clay. First, check the weight of empty crucibles and then check the mass burn of clay at 200°C, 400°C, and 600°C and put the acquired value in the following formula:
An industrial factory in Nagarparkar, Tharparkar, and Sindh provided calcined clay for this project. Studies show that kaolinite is partially dehydroxylated at 600°C, and the dehydroxylation is complete from 700°C onward. Hence, the clay becomes reactive after calcining it at 800°C [11]. The clay calcined at 800°C for 1 hour in an electric furnace. The main component of limestone constitutes calcium carbonate (CaCO3). Limestone that is not suitable for clinker production can be used in LC3-50. LC3-50 can also prepare with low-calcite limestone that contains impurities such as dolomite and quartz. The content of limestone preserves 15% in LC3-50. The content of gypsum in the LC3-50 blend sustains at 2.5%. The chemical composition of the LC3-50 cement blend is presented in Table 1. Figure 1 shows the particle size distribution of OPC and LC3-50. The coarse aggregate and fine aggregate used in this project were extracted from a local source. The mix design used a coarse aggregate blend of 10 mm and 16 mm aggregates to circumvent voids in concrete. The fine aggregate was sieved at a size of 4.75 mm. Additional reactivity of the pozzolana action occurs in calcined clay, requiring more water to create the secondary gel phase [12]. A high-performance superplasticizer based on polycarboxylic ether (MasterGlenium SKY8233) is used. It assists in the manufacturing of high-performance concrete with improved workability and early strength [13]. Because calcined clay fineness necessitates the addition of water, the superplasticizer is required in the correct proportions to fulfill the optimum hydration content in concrete.
2.2. Mix Design
Concrete is with a target compressive strength of 30 MPa after 28 days, as determined by a series of mixed design trials. The water-cement ratio retained at 0.416 throughout the casting. The remaining amount is divided into a 40 : 60 ratio of fine and coarse aggregate proportions, respectively. Additionally, the coarse aggregate constituted of a 30 : 70 mix of 10 mm and 16 mm aggregates, respectively. Superplasticizer (SP) with dosage 1.5% by the amount of cement was added to the mixture to improve concrete workability and achieve a target slump of 180 mm in the mix. Table 2 shows the mix design used in the project to examine the testing properties of OPC and LC3-50 concrete. The raw materials were carefully mixed in a pan mixer at 25 revolutions per minute.
2.3. Experimental Method
2.3.1. Compressive Strength
For evaluating the evolution of the compressive strength of the OPC and LC3-50 concrete mixtures, 150 mm size concrete cubes were made as per BS EN 12390 [14]. In addition, 100 mm diameter and 200 mm height cylindrical specimens were tested as per ASTM C39-14 [15]. The compression testing of cubes and cylinders was conducted on a 2000 KN compression testing machine with a ±1% accuracy. Three specimens for each day were kept in a damp room for 3, 7, 14, 28, and 90 days. Results were averaged from the three samples.
2.3.2. Splitting Tensile Test
The split tensile test was conducted according to the ASTM C496 [16]. The cylindrical samples were cast for split tensile strength dimensions (200 mm height and 100 mm diameter). After unmoulding, the concrete specimens immersed in the curing tank for the split tensile strength test on 7 and 28 days. The results from the three samples were summed. The test was performed on the universal testing machine, having a limit of 500 kN.
2.3.3. Flexural Strength Test
The flexural strength of LC3-50 cement concrete is determined by ASTM C78 [17]. The load was applied to the specimen surface at the loading points to push the block. Three concrete beams were used as the specimen for this test and had a cross-section (150 mm height and 150 mm width) with a span length of 500 mm. The test was carried out on the universal testing machine (UTM) after 7 and 28 days of curing, and an average was taken out from three beams. This UTM had a loading limit of 500 kN and a loading rate of 0.5 mm/min was used in this experiment.
2.3.4. Pull-Out Test
It measures the force required to pull an embedded rod out of hardened concrete to evaluate its pull-out strength. Concrete cylinders with dimensions of 100 mm diameter and 200 mm height were used, with steel bars with a rod length of 850 mm embedded 100 mm in the specimen. Three samples were cured for 7 and 28 days, and the results were averaged. The displacement control with a loading rate of 0.5 mm/min was performed using the UTM having a capacity of 500 kN according to the ASTM C900 [18].
2.3.5. Water Permeability Test
Durability is the criterion for making quality concrete. In terms of durability, water permeability is a concern. It is determined by the German Standard DIN 1048 [19]. Three concrete cylinders of dimensions (100 mm diameter and 200 mm height) were assembled in the apparatus after 28 days of curing. The water pressure of 500 kPa (5 bar) was applied for 72 hours constantly in the water permeability apparatus. The test result was the average maximal penetration achieved from the three specimens examined.
2.3.6. Sorptivity Test
The absorption rate was determined in a homogeneous material using water as the test fluid and measuring the capillary rate of absorption. The test was performed according to ASTM C1585 [20]. Three cylindrical specimens of 100 mm diameter and 200 mm height were cast and cured in a damp room for 28 days and calculated an average of three. After curing, the samples were split into 50 mm thick slices. To prevent the end of each specimen from being exposed to water, epoxy was applied on the side cylindrical surface of each slice. These slices were stored in a pot at a height of no more than 5 mm above the specimen’s base and submerged 2 mm in water. The flow from the peripheral surface was stopped by adequately sealing it with a nonabsorbent coating. The quantity of water absorbed in the time period of 30 minutes was measured by weighting the specimen on a balance weight, water on the sample surface was wiped off with a dampened tissue, and each weighting operation was completed within 30 seconds. After vacuum saturation, the porosity was calculated by multiplying the difference between the starting and final dry weights by the volume of the concrete cylinders. The sorptivity index was calculated by standardizing the plot pitch of the capillary mass increase with the square root of time to the cumulative water absorption by the specimen unit thickness [21].
2.3.7. Carbonation
The accelerated carbonation test was performed in this investigation with a carbon dioxide concentration of 5%; cylindrical specimens were placed in an accelerated carbonation chamber for the desired exposure period. The temperature was 23°C, and the relative humidity was 55 percent. It is reasonable to assume fifteen months of natural exposure to the environment is equivalent to that one week of accelerated exposure to concrete in the carbonation chamber [22]. The effect of temperature is significant as the structure of hydration products lessens detrimental pores [23]. Both OPC and LC3-50 concrete samples were exposed for 34 hours and 68 hours in an accelerated carbonation chamber, equivalent to 3 and 6 months of natural exposure, respectively. After the exposure period was completed, three specimens were split into two halves for each month in UTM, and their inner portion was sprayed with a phenolphthalein indicator (0.2%) to indicate the carbonation depth. The part which was colourless or light pink was considered as affected by carbonation [24].
2.3.8. Rapid Chloride Penetrability Test
The RCPT calculates the total charge transported through a saturated concrete sample to determine its chloride resistance. The penetration of chloride ingress was assessed using ASTM C1202 [25]. Three cylindrical specimens of 100 mm diameter and 200 mm height were cast and cured in the damp room for 28 days and calculated an average of three. The samples were sliced into 50 mm thick slices, and the cylinders were coated with epoxy around the cylindrical surface. The slices were then conditioning for 24 hours in a dry vacuum consisting of boiled water which was soaked in it. The slices were then placed in the apparatus. One side of the apparatus consists of a 3% salt solution which was connected to the negative terminal of the battery, whereas the other side consists of 0.3 N NaOH solution which was connected to the positive terminal of the battery. The potential difference of 30 V was maintained across the two ends for 6 hours. The test measures the total charge passed across the concrete sample. Read and record the current after at least 30 min. Based on the charge, a qualitative rating can be made of concrete’s permeability.
3. Test Results and Discussion
3.1. Compressive Strength
The compressive strength test was performed on cubical and cylindrical samples (Figure 2(a) and Figure 2(c)) of OPC and LC3-50. The cubical and cylindrical specimens of LC3-50 gain more strength than OPC after 28 days and 14 days of curing, respectively. Early in the curing process, OPC had higher compressive strength than LC3-50. It could be related to the reasons that LC3-50 and OPC contain different amounts of C3S and C2S. OPC has a substantial proportion of C3S than mixed cement, and these proportions are essential for increased early strength OPC. However, after 28 days of curing and afterward, LC3-50 had higher compressive strength than OPC because the extended curing time can be attributable to pozzolana reaction in LC3-50 [26]. The targeted compressive strength for the cylinder is 30 MPa for 28 days of curing that accomplished just in the curing period of 14 days, Figure 2(b), which is slightly more than OPC. Furthermore, samples of LC3-50 have more strength on 28 and 90 days of curing and out powered OPC. Similarly, Figure 2(d) shows the same results as obtained for cubes. The compressive strength of LC3-50 was less than OPC at 3, 7, and 14 days of curing. However, LC3-50 cylinders showed greater strength than OPC after 28 days of curing time and achieved targeted strength, and there was a significant difference in the compressive strength of LC3-50 and OPC after 90 days.
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3.2. Splitting Tensile Test
The split tensile test was performed on OPC and LC3-50 cement in concrete for 7 and 28 days, as shown in Figure 3(a). In one study, LC3-50 concrete specimens have similar strength comparable to OPC-based concrete. In split tensile strength, the OPC and LC3-50 concrete do not have much difference but vary slightly with different binders [27]. This study also demonstrates that LC3-50 concrete has a modest increase in split tensile strength compared to OPC concrete. As shown in Figure 3(b), the test outcomes presented that the percentage improvement for LC3-50 varies from 3% to 8% compared to OPC. The LC3-50 samples have an 8% higher strength value than the OPC specimens after seven days of curing. After 28 days of curing, LC3-50 concrete only demonstrated a 3% increase in split tensile strength compared to OPC. It represents that OPC concrete gains split tensile strength gradually.
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3.3. Flexural Strength Test
A flexural strength test was also conducted on both specimens, LC3-50 and OPC. LC3-50 and OPC beams were tested for 7 and 28 days, as illustrated in Figure 4(a). Two samples of the blend were acquired, as well as an average flexure value. The percentage improvement in flexural strength of the LC3-50 cement blend is consistent with previous research studies. The preceding study shows that the flexural strength of LC3-50 concrete performs better than OPC and averages a 7 percent increase in flexural strength. It happens because of the thick and compact morphology of LC3-50. It forms silicon chains to produce the crack bridge in the inside layer arrangement [27]. Another study demonstrates that adding calcined clay and limestone powder to cementitious materials improved their toughness, increasing flexural strength [28]. After curing of 7 days, the OPC and LC3-50 samples are nearly identical in strength. However, from Figure 4(b), after the curing period of 28 days, the flexural strength of the LC3-50 beam exceeds the OPC sample.
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3.4. Pull-Out Test
Both the LC3-50 and OPC concrete samples were subjected to the pull-out test (Figure 5(a)). Two specimens of each concrete type were collected, and the average value is considered as the resultant. The specimens were cured for 28 days before testing. Previous research has revealed that the microstructure of LC3-50 concrete is thicker, with fewer pores and a better ratio of splitting to compressive strength than OPC concrete. Among the concrete and steel bar, it has a greater elastic modulus and higher bond stress. LC3-50 concrete has a higher bond-slip stiffness than compared to OPC concrete [29]. The UTM (UH 500 KNI) was used to check the pull-out strength of the 28 days cured samples. Figure 5(b) shows that LC3-50 shows greater reluctance to the applied load compared to OPC. All the specimens of LC3-50 show greater strength.
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3.5. Water Permeability Test
The permeability test was performed on three concrete specimens of dimensions (100 mm diameter and 200 mm height) at 28 days of curing. Then, the sample was placed in a test machine (Figure 6(a)). The test result is the average of the highest penetration depth acquired from three specimens. From Figure 6(b), the average depth of OPC is 62 mm, while the average penetration of the LC3-50 is 15.5 mm from the three specimens considered. In comparison to the OPC, the LC3-50 cement blend concrete exhibited a reduced water penetration depth of about 75%. Keeping the water-to-cement ratio low resulted in a low porosity for the concrete [30]. These findings show that the LC3-50 blend can be used in other applications where the concrete is likely to come into contact with water or be laid underwater. Other materials used to protect the concrete from water, such as crystalline waterproofing, will be used in smaller amounts than the OPC cement, making the project more cost-effective.
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3.6. Sorptivity Test
Figure 7(a) shows the testing sample and equipment of sorptivity for OPC and LC3-50 concrete. The test was carried out on LC3-50 and OPC cylindrical specimens after a 28-day curing time. Sorptivity measures in mm show the water absorption rates for the two samples. The fewer values of sorptivity indicate better resistance to water absorption. An Indian study reveals that the combination of pozzolanic and filler impact caused by SCM addition lowers the adsorption rate due to the contraction in pore diameter that enhances the durability performance of LC3 concrete [31]. The following result from Figure 7(b) shows the excellent performance of LC3-50 in sorptivity compared to OPC. The rate of sorptivity in OPC is higher than LC3-50 because the improvement of pores water absorption rate (sorptivity) is reduced due to the usage of limestone and calcined clay in LC3-50 samples.
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3.7. Carbonation
The phenolphthalein indicator sprayed over the split specimens that had previously been exposed to 5% carbon dioxide (CO2) were used to calculate the accelerated carbonation depth. In general, the results determined that the resistance of LC3-50 concrete decreases with the OPC replacement rate to carbonation. In supplementary cementitious material (SCM), the rate of carbonation is often higher than OPC cement-based concrete. As a result, this case also applies to LC3-50 cement-based concrete, which has a higher carbonation rate than OPC [11]. A Chinese study suggested that a low amount of calcium in LC3-50 cement comparatively OPC can be the reason for the lower binding capacity of carbon dioxide and having more carbonation depth in the LC3-50 system [32]. Figure 8(a) shows the LC3-50 and OPC samples kept in the carbonation chamber for exposure to 5% carbon dioxide up to 24 weeks. Ultimately, the susceptibility of LC3-50 concrete to carbonation is more than OPC. Figure 8(b) shows that LC3-50 underperforms compared to OPC as the penetration depth is higher than OPC. LC3-50 shows 60% and 45% more vulnerability than OPC at 3 and 6 months, respectively.
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3.8. Rapid Chloride Penetration Test
ASTM Standard C 1202 [25] provides remarks regarding the passage of charge to categorize concrete according to its permeability. This standard is used to determine if the charged particles passes from LC3-50 to OPC samples (Figure 9(a)). The charged particles of chloride (measured in coulombs) indicate the penetrability of the concrete specimen. The more the charged particles of chloride passed through the sample, the less durable it is. The LC3-50 concrete specimen, in comparison to OPC concrete, shows very little chloride penetration. The quick reactivity of the limestone and calcined clay benefaction in the LC3-50 systems, which boosts the degree of hydration, causes the pore diameter to shrink immediately. Because of the better effect of this early pore size reduction, the concrete can achieve a reliable degree of durability without requiring more curing time [33]. The greater the pore size in OPC samples (Figure 9(b)), the more the current (charge) passes through them. Because of the increased pore diameter and a lower pore threshold radius value, LC3-50 concrete has a higher resistance to chloride incursion.
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4. Conclusion
The rapidly growing population of Pakistan causes exponential growth in urbanization which makes it the top concrete consuming country in the world. Pakistan can alone become an extensive polluter globally due to its use of cement for concrete production. Pakistan urgently needs an environmentally friendly building material to meet its expanding demands. This comprehensive experimental study on concrete concludes that LC3-50 performs similarly to OPC, and in certain circumstances, it performs better than OPC. The first ever study on LC3 will be used as a benchmark for advancing research and commercialization activity in the country. The following conclusions have been reached after thorough experimental research into LC3-50 concrete with OPC-based concretes.(i)LC3-50 concrete specimens surpassed OPC concrete specimens in terms of mechanical properties. In the early stages, LC3-50 achieves compressive strength slowly compared to OPC. However, after 28 days, LC3-50 outperformed OPC concrete, and after 90 days, LC3-50 had a 14 percent higher compressive strength than OPC. Other mechanical tests show that LC3-50 concrete performs better than OPC concrete.(ii)LC3-50 has a higher pull-out strength than OPC. When compared to OPC, LC3-50 exhibits more resistance to the imposed load. LC3-50 concrete has a thicker microstructure than the OPC concrete, with lower permeability and a better splitting to compressive strength ratio.(iii)The performance against water absorption LC3-50 concrete specimen resulted in a low percentage compared to OPC. Similarly, it also shows improved performance for the sorptivity test. Consequently, concretes using LC3-50 binder may be suitable for use in the marine environment.(iv)LC3-50 concrete could not perform better for the carbonation test. The concentration of CaO content in LC3-50 concrete is the significant element that binds carbon dioxide in concrete from the environment, which controls the carbonation rate. The carbonation rate of blended cement concrete is often more than the OPC concrete.(v)In the chloride ion penetration test, LC3-50 concrete shows better resistance. It is clear from the RCPT data that LC3-50 concrete specimens had less chloride ion ingress than OPC concrete specimens.(vi)According to the findings, LC3-50 concrete specimens had improved mechanical properties, and LC3-50 outperformed OPC in all durability tests except carbonation.(vii)This study demonstrates that LC3-50 has the potential to use as a substitute for OPC and lessen carbon footprints. It is a new and extensive area of research, and additional research needs to adapt to the market.
Notations
| : | Kaolinite content in percentage present in clay |
| : | At 200 degrees Celsius, the weight of the crucible and the clay |
| : | At 400 degrees Celsius, the weight of the crucible and the clay |
| : | At 600 degrees Celsius, the weight of the crucible and the clay |
| : | The weight of the empty crucible. |
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
The authors are grateful and acknowledge the support from the China National Key R&D Program‐International Scientific and Technological Innovation Cooperation Key Project (Grant no.: 2018YFE0106300). The research was mainly supported by the Higher Education Commission of Pakistan, National Research Program for Universities (NRPU) Project no. 14074 entitled “Development of Cost-Effective Structural Concrete Formulation using Limestone Calcined Clay-Based LC3 Cement Blend with Domestic Resources and its Application in a Pilot Project.” The authors appreciate the helping hands of undergraduate students Muhammad Sharjeel Afzal, Syed Ali Zamin, Zohaib Saleem, Abdul Basit, and Muhammad Yahya in laboratories during experimentations. Moreover, the authors are grateful for the valuable inputs from Prof. Dr. Karen Scrivener, Head of Laboratory of Construction Materials, Swiss Federal Institute of Technology (EPFL) based in Lausanne, Switzerland.