Calcined Clay Research Articles

Effects of mix composition on the mechanical, physical and durability properties of alkali-activated calcined clay/slag concrete cured under ambient condition

Alkali-activated concrete (AAC), a possible alternative to ordinary Portland cement (OPC), provides increased environmental advantages, notably lower carbon emissions. Likewise, similar to OPC, it faces durability problems under severe environments. This study evaluated the effectiveness and durability of alkali-activated concretes containing low-grade calcined clay and granulated blast furnace slag (GGBFS), using NaOH/KOH as activators and MgO and superabsorbent polymer (SAP) as additives. The study's focus was on their physical-mechanical characteristics and performance under accelerated carbonation and chloride diffusion. Findings showed that calcined clay-GGBFS alkali-activated concretes exhibited a compressive strength range of 41.2–53.3 MPa, positioning them as a viable concrete for many structural usages. Nevertheless, the modified-RCPT (10 V) and NT Build 492 tests showed all concretes falling into the "high chloride penetrability" category, with values exceeding 350 coulombs and 13.5 (×10−12 m2/s) for the non-steady-state migration coefficient (Dnssm). NaOH and sodium silicate led to higher compressive strengths compared to KOH. Furthermore, the chloride migration coefficient of alkali-activated concrete blends ranged from 55.68 to 121.14 (× 10−12 m2/s). The incorporation of 0.6 % SAP decreased compressive strength but improved the non-steady-state migration coefficient (Dnssm). Lastly, MgO-enriched samples showed the lowest water absorption and carbonation depth, attributed to the buffering action of magnesium phases, reducing carbonate formations, as revealed by XRD analysis.

Harnessing iron tailings as supplementary cementitious materials in Limestone Calcined Clay Cement (LC3): An innovative approach towards sustainable construction

The increasing demand for sustainable construction materials has led to the exploration of iron tailings (ITs) as supplementary cementitious materials (SCMs) in Limestone Calcined Clay Cement (LC3). This study investigates the effects of varying ITs content in LC3 on compressive strength, microstructure, and environmental impact. Replacing 28 % of LC3 with ITs resulted in a 42 MPa compressive strength after 28 days, comparable to ordinary Portland cement (OPC), while reducing OPC content by 50 %. Microstructural analysis revealed that ITs contributed to the formation of additional C-(A)-S-H gel, enhancing the mechanical properties of the cement matrix. The findings also showed that concentrations of Zn (0.003–0.094 mg/L), Pb (0.002–0.090 mg/L), Cu (0.005–0.018 mg/L), Mn (0.115–0.712 mg/L), Ni (0.011–0.021 mg/L) in the leachates of LC3 containing ITs were below the critical limits for surface water and groundwater. Moreover, the life cycle assessment (LCA) demonstrated significant reductions in global warming potential (43.6 %), energy consumption (37.2 %), and cost (35.5 %). This study provides an innovative solution for waste utilization and environmentally friendly cement production.

The role of low-quality calcined clay in enhancing the performance of cement mortar exposed to normal and aggressive media

This study focuses on the role of low-quality calcined Fanja (FNJ) clay in enhancing the behavior of cured cement mortar (CM) and its resistivity to chloride and sulfuric acid attack. Ordinary Portland cement was replaced with different quantities (10%, 25%, 35%, and 50% by weight) of FNJ, which was calcined at different temperatures (620 °C, 760 °C, and 900 °C). After 28 days of curing, all the hardened mortars were immersed in 5% sulfuric acid for up to 12 weeks. Additionally, a rapid chloride permeability test was conducted on the 91-day cured CM and CM-FNJ samples to evaluate the affinity of calcined FNJs to retard the chloride diffusion into CM. The results showed that all samples containing FNJ900 showed better physical and mechanical properties than the control sample, while CM with NFJ760 recorded nearly similar performance as CM-NFJ900. In contrast, the CM-FNJ620 mixtures showed lower properties than those of other mixtures. In addition to the pozzolanic reactivity of the calcined clay, the presence of hematite in the calcined clay strongly contributed to increasing the mechanical properties of the hardened mortar through forming calcium ferrosilicate hydrate binding phase, as confirmed by X-ray diffraction. Moreover, the existence of hematite increased the resistivity of CM against sulfuric acid attack as it acts as a buffer for an acidic medium. Compared with CM-FNJ900, the CM-FNJ760 is recommended for use as it exhibited a higher strength activity index and comparable resistance to accelerated chloride diffusion and sulfuric acid accompanied by lower energy demand and lower CO2 emission, achieving the concept of ‘sustainability’.

Solar-driven calcination of clays for sustainable zeolite production: CO2 capture performance at ambient conditions

This study presents the environmentally sustainable synthesis of zeolites from solar-calcined kaolin and halloysite, emphasizing their application in CO2 capture due to their distinctive porous structures and chemical attributes. Expanding upon prior research that utilized solar energy for kaolin calcination, we now explore halloysite as an alternative clay mineral for zeolite production and CO2 capture. Employing a solar simulator, halloysite was calcined at temperatures ranging from 700 to 1000 °C, resulting in the synthesis of zeolites 4A and 13X via hydrothermal methods. The synthesized zeolites were characterized using X-ray diffraction (XRD), low angle XRD (LA-XRD), transmission electron microscopy (TEM), and field-emission scanning electron microscopy (FE-SEM), and Brunauer–Emmett–Teller (BET) surface area measurements. Notably, the presence of Al-Si spinel, which crystallizes at elevated solar calcination temperatures, persisted within the zeolite 13X matrix, inducing a secondary mesoporous phase. The observed hysteresis in 13X samples, rather than confirming the mesoporous character of zeolite 13X, indicates a tandem effect of mesoporous Al-Si spinel with microporous zeolite 13X, exemplifying systems known as micro/mesoporous zeolitic composites (MZCs). The correlation obtained between the interplanar distances calculated from LA-XRD and pore size distributions acquired from the BJH desorption branches highlights LA-XRD as an alternative analysis method for assessing mesoporosity. While the microporosity of Al-Si spinel possessing 13X samples positively correlates with CO2 capture performance, mesoporosity appears to have minimal impact. Among the zeolites synthesized using solar energy, zeolite 4A (LTA) demonstrates superior CO2 capture capability, achieving an adsorption capacity of 2.15 mmol/g at 25 °C and 1 bar. This study highlights the potential of solar energy in producing eco-friendly zeolites from kaolin and halloysite for improved CO2 capture, advancing sustainable environmental solutions.

Investigating the Interaction of Limestone Calcined Clay and OPC-Based Systems with a Methyl Hydroxyethyl Cellulose-Based Viscosity Modifier Used for 3D Printable Concrete

Investigating the Interaction of Limestone Calcined Clay and OPC-Based Systems with a Methyl Hydroxyethyl Cellulose-Based Viscosity Modifier Used for 3D Printable Concrete

Impact of clay reactivity on the hydration of calcined clay limestone cements

The substitution of Portland cement clinker with a combination of calcium carbonate and calcined clay is a promising binder concept to lower the CO2 footprint of the cement industry due to the synergetic effect of both materials which form carbo-aluminate hydrates. In this study, the reactivity of several calcined clays was tested using the R3 test and mortar strength testing in different combinations with calcium carbonate and Portland cement. A multiple regression analysis revealed that the reactivity in the R3 test depends mainly on the amorphous Al2O3 and to a minor extent on the grain size and the MgO content. It was found that the most reactive calcined clay according to the R3 test led to no significant increase in compressive strength between 7 and 28 days. This clay also shows a slightly lower overall strength after 28 days than a calcined clay that was rated “medium reactive” according to the R3 test. The most reactive calcined clay was further investigated in a hydration study using X-ray diffraction. Based on this, a thermodynamic model was built and revealed that only a comparatively small percentage of the calcined clay reacted after 28 days.

A review on serviceability of foam concrete

In the face of the present global warming crisis, building energy conservation and emissions reduction have gained increasing attention. In the sustainable development of buildings, foam concrete has become increasingly popular in the construction industry due to its excellent thermal insulation, sound insulation and fire resistance. Foam concrete employed in buildingand construction (used as either a structural or non-structural member) must meet certain serviceability criteria. This paper reviews the serviceability of ordinary Portland cement (OPC) foam concrete, geopolymer foam concrete (GFC), sulphoaluminate cement (SAC) foam concrete, magnesium phosphate cement (MPC) foam concrete and limestone calcined clay cement (LC3) foam concrete, to include drying shrinkage, creep, thermal conductivity, water absorption and acoustic properties. Subsequently, it is proposed to change the initial curing conditions to reduce drying shrinkage, add glass fiber and basalt fiber to improve creep performance, increase heat flow direction resistance and extend the thermal conduction path to reduce thermal conductivity, control slump flow to regulate water absorption, and combine sound absorbing foam concrete with sound reflecting materials to achieve a comfortable and low noise living environment. Finally, ideas and suggestions for future work are proposed.

Effects of kaolinite and montmorillonite calcined clays on the sulfate balance, early hydration, and artificial pore solution of limestone calcined clay cements (LC3)

This study investigated the physicochemical effects of kaolinite (CK) and montmorillonite (CM) calcined clays on the sulfate balance, early hydration, and artificial pore solution of limestone calcined clay cement (LC3). The effects of fineness, clay dissolution, and ion-adsorption capacity were evaluated by isothermal calorimetry, compressive strength, ICP-OES, and zeta potential within 72 h, respectively. Increasing the fineness of both calcined clays did not significantly affect the sulfate depletion kinetics or the compressive strength and the adsorption of Ca2+ ions onto the calcined clay’s surface is not the main factor responsible for differences in sulfate demand. The higher dissolution of ions Al in CK provided an intensified and accelerated formation of ettringite that competes for the available sulfate. We demonstrate that the chemical effects have a significant impact on the sulfate balance of LC3, revealing the lesser impact of alternative clays like montmorillonite compared to metakaolin (MK) which can minimize the problem of accelerated sulfate depletion of LC3 mixes with MK.

Optimizing alkali-activated clay with rubber waste for sustainable earthen bricks: A comprehensive study

This study explores the potential of optimizing alkali-activated clay with rubber waste to create sustainable earthen bricks. The research addresses the environmental challenges associated with traditional cement-based construction materials by incorporating locally sourced calcined clay and recycled rubber tire waste. The experimental program involves testing different proportions of sand with calcined clay, varying sodium hydroxide (NaOH) concentrations, and integrating rubber content. The aim is to develop a material that balances environmental sustainability with improved mechanical and durability properties. The mechanical and microstructural properties of the resulting materials were evaluated through compressive and flexural strength tests, wetting and drying cycles, and advanced analysis techniques such as X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results indicate that while the inclusion of rubber enhances certain properties such as reducing the unit weight and Structural Integrity During Wet-Dry Cycles, it adversely affects compressive and flexural strengths. This study highlights the importance of optimizing the balance between durability and mechanical strength to develop effective and sustainable construction materials.

Mechanical properties and chloride resistance of ultra-high-performance seawater sea-sand concrete with limestone and calcined clay cement (UHPSSC-LC3) under various curing conditions

Although the application of ultra-high performance seawater sea-sand concrete with limestone calcined clay cement (UHPSSC-LC3) provides a promising solution for sustainable marine structures, research on the mechanical properties of UHPSSC-LC3 is quite limited, and no research has been conducted on its chloride resistance performance. In this study, the effect of four curing regimes (i.e. standard curing, 20 °C seawater curing, 40 °C fresh water curing, and 80 °C fresh water curing) on the compressive and flexural strength of UHPSSC-LC3 is experimentally investigated. Rapid chloride migration test and chloride titration test are conducted to determine the chloride diffusion and distribution of UHPSSC-LC3, considering the effects of curing conditions and calcined clay (CC) dosage. In addition, XRD analysis is performed to show the differences in the composition of UHPC, UHPSSC, and UHPSSC-LC3. The obtained results demonstrate that UHPSSC-LC3 attains higher mechanical strength than UHPC and UHPSSC under various curing conditions. The incorporation of LC3 makes the free chloride concentration of UHPSSC-LC3 close to that of UHPC, especially when the heated water curing is applied. The free chloride concentrations of UHPSSC- LC3 cured with 40 °C and 80 °C water are only 10.1 % and 2.9 % higher, respectively, than that of UHPC cured under standard condition. Besides, UHPSSC-LC3 achieves the lowest chloride ion diffusion rate, which is 40.4 % lower than that of UHPSSC, due to the denser matrix resulting from the carboaluminate products and the pozzolanic effects of the calcined clay. Furthermore, the application of heated water curing regimes is recommended for UHPSSC-LC3 when the early strength and chloride resistance are the main concerns.

Calcined Clays as Concrete Additive in Structural Concrete: Workability, Mechanical Properties, Durability, and Sustainability Performance.

Calcined clays (CCs) as supplementary cementitious materials (SCMs) can be a promising option to reduce clinker content and CO2 emissions in eco-friendly concretes. Although CCs as components of composite cements in combination with Ordinary Portland Cement (OPC) and limestone powder (LSP) have attracted industry interest, their use as concrete additives is limited. This study investigates the effects of the addition of CCs on the fresh and hardened properties of industry-standard ready-mixed concretes. Four concrete mix designs, each with three superplasticizer dosages, were tested, resulting in 12 variations. The CCs used, which are typical of 2:1 bentonite clays with low metakaolin content, reflect the clays available in Germany. The results showed that CCs significantly influenced the workability, which could be controlled with a high superplasticizer dosage. Increased CC contents reduced bleeding tendencies, which was beneficial for certain structural applications. Early age strength decreased with CCs, but the 28-day strength exceeded that of pure OPC concretes up to 30 wt% CCs. Resistance to CO2-induced carbonation decreased with higher levels of CCs but was comparable up to 15 wt%. Freeze-thaw damage decreased, and chloride migration resistance improved due to a denser microstructure. The global warming potential (GWP) of the concretes tested is in line with that reported in the literature for concretes made from highly blended cements, suggesting that CCs can improve the sustainability of concrete production.

Comparative study of limestone calcined clay cement produced with mechanically activated kaolin and calcined kaolin

Limestone calcined clay cement (LC3) is a promising solution for mitigating CO2 emissions in cement production by substantially replacing clinker with widely available supplementary cementitious materials. While calcination is the conventional method for activating clay, there is a growing interest in mechanical activation. Incorporating mechanically activated kaolin into LC3 formulation would offer a novel approach to producing this cement. This study aims to assess the impact of replacing calcined clay (CC) with mechanically activated clay (MC) in LC3 properties and relevant features. Accordingly, LC3 with MC substitutions ranging from 0 to 100 wt% were formulated. The resulting cements were characterised to evaluate their physicochemical properties, microstructure, strength, and pore distribution. LC3 incorporating MC exhibited comparable crystalline and amorphous phases to LC3 containing CC. However, the incorporation of MC accelerated hydration, leading to the earlier formation of carboaluminates and consumption of portlandite, alongside enhanced compressive strength at early curing stages. At 28 days, LC3 with 100 wt% of MC displayed similar compressive strength (42 MPa) to LC3 with 100 wt% of CC (40 MPa) with comparable pore distribution and microstructure. These findings validate MC's potential to substitute CC in LC3, offering an alternative for activating clay.

Sustainable Concreting: Optimization Modelling of the Strength Properties of Bio-Self Compacting Concrete Incorporating Sporosarcina Pasteurii, Calcined Clay and Limestone Powder

Sustainable concreting is prerequisite for infrastructural development in developing countries so as to meet up with the sustainable development goal of adequate mass housing and other critical infrastructure. Thus, research is ever ongoing aimed at developing cheaper and more durable concrete via the incorporation of bio-based by-products in concrete to improve its properties, as well as optimizing the quantities of these secondary materials for maximum and optimal concrete production. One such revolutionary concrete that is yet to find full application in the developing world is self-compacting concrete, because of the cost and attendant environmental effects. There is thus a need to arrive at optimal materials quantities that can maximize concrete properties without recourse to many trial and error experimentations that are both time and resources consuming. The application of modelling tools in concrete technology aids in the optimization of concrete constituents for optimal self-compacting concrete performance. This research uses optimization techniques to optimize the bacteria dosage as well as model the Compressive and Tensile strength properties of a calcined clay and Limestone powder blended ternary self-compacting concrete using sporosarcina pasteurii as Microbial induced calcite precipitation agent and calcium lactate as nutrient source. The Bacteria was incorporated into the concrete at a bacterial content of 1.5x108cfu/ml, 1.2x10 cfu/ml and 2.4x109cfu/ml corresponding to the McFarland turbidity scale of 0.5, 4 and 8 while the nutrient (calcium lactate) content was 0.5, 1.0 and 2.0% by weight of cement for each bacterial content. The Compressive strength and tensile strengths at 28 days were determined and the results used for both the model development, strength optimization and model validation, with the strengths as the dependent variable (y) and the bacterial content corresponding to a McFarland scale of and calcium lactate content as the independent variables, X1 and X2 respectively. The results show an improvement in the compressive strength from 32N/mm2 to 45.2N/mm2 at the optimal bacterial and nutrient content of 1.2x10 cfu/ml and 0.5% respectively, and tensile strength from 4.01N/mm2 to 5.0N/mm2. Also, the non-linear regression models proved adequate for optimizing the bacterial content for optimal self-compacting concrete performance. Journal of Engineering Science 15(1), 2024, 11-19

Target dispersion of pozzolanic materials nearby hydration-released calcium hydrates to improve the pozzolanic reaction degree

The random distribution of pozzolanic materials (PM) using direct mixing strategy reduces their contact possibility with hydration-released calcium hydroxide (CH), and limits the corresponding reinforcing efficiency to cement-based materials via pozzolanic reaction. In this study, a target dispersion strategy of PM nearby CH was proposed using super absorbent polymers (SAPs) as the carrier. On the one hand, nano silica (NS) and calcined clay (CC), two typical PM, can be encapsulated into SAPs through the hydrogen bonding interaction; on the other hand, the region around SAPs was enriched with more water and higher Ca2+ concentration, which triggered the in situ crystallization of CH. In this case, the contact possibility between the NS or CC (NS/CC) and hydration-released CH was improved by the target dispersion strategy, and resulted in a 60 %–253 % increase in pozzolanic reaction degree (PRD) of NS/CC. Furthermore, the reinforcing effect of NS/CC on compressive strength of cement paste was enhanced by 25 % through this strategy. Therefore, the current study introduces a novel method for increasing the pozzolanic reinforcing efficiency of cement-based materials by turning the random to target dispersion of PM nearby CH.

Performance of limestone calcined clay cement (LC3) made with CO2-mineralized concrete slurry waste

Concrete slurry waste (CSW) is an under-recycled material with great potential to be substitute ordinary Portland cement (OPC) after proper treatment. In this study, dehydrated CSW (DCSW) was used to mineralize CO2 and the carbonated product was used to prepare limestone calcined clay cement (LC3). The results showed that the CaCO3 content of carbonated DCSW (CDCSW) reached ∼57.7% after mineralization for 8.5 h. During carbonation the Portlandite peak disappeared after 1.5 h, and the Mc_AFm peak disappeared after 3.5 h. The alite and belite peaks gradually diminished, with a weak diffraction remaining at 8.5 h. Replacing limestone with CDCSW led to a decrease of 14.7% in fluidity, at a plasticizer dosage of 1.3%. The compressive strength of LC3-cdcsw mortar specimens was higher than those of LC3-limestone specimens at all ages. The effective chloride diffusion coefficient of LC3-cdcsw was also promisingly small (1.08 × 10−11 m2/s). The comprehensive properties (grey correlation degree) of LC3-cdcsw were 17.82% lower than that of OPC, but 17.73% higher than that of ordinary LC3. At 28 d, the contents of monocarboaluminate for LC3-cdcsw and LC3-limestone are 11.2% and 9.5%, respectively. The fourth thermal decomposition peak of LC3 system is higher than that of OPC, and the peak intensity of LC3-cdcsw is lower than that of LC3-limestone. Our work explores the pathway of carbon sequestration in concrete slurry waste for the preparation low-carbon and durable cementitious materials.

Novel low-carbon eco-concrete filled-FRP tubes under axial compression: Experimental behavior and analytical model

Concrete is essential in global construction but significantly contributes to carbon emissions, primarily due to cement production. The cement industry is adopting low-carbon practices, such as using supplementary cementitious materials (SCMs) to reduce emissions. Concrete prepared with calcined clay can achieve a carbon reduction effect of approximately 30 % compared to ordinary cement. Current studies highlight the reuse of kaolin clay, the predominant constituent of engineering sediment in Shenzhen, China, providing raw materials for sustainable low-carbon and eco-friendly concrete. However, high substitution rates of calcined clay can compromise concrete strength, posing a challenge for the application in load-bearing structures. This study proposed a novel low-carbon eco-friendly concrete (i.e., using calcined clay as supplementary cementitious materials) filled into FRP tubes (i.e., Low-Carbon Eco-Concrete-Filled FRP Tubes, termed LCE-CFFTs) aiming to enhance substitution rates of calcined clay and to improve compressive strength and ductility. This paper provides a detailed description of the preparation method, chemical reaction process, chemical composition, and microstructure of calcined clay. Experimental tests investigated the axial compressive performance of LCE-CFFTs, considering calcined clay substitution rates and FRP thickness as key parameters. The experimental work and theoretical analysis indicated that: the inclusion of 10 % and 40 % calcined clay led to a reduction in carbon emissions, resulting in a decrement of 5.8 % and 23.3 %, respectively; confinement provided by FRP tubes mitigated the reduction in axial compressive strength resulting from calcined clay inclusion, particularly evident at higher substitution rates (40 %); the effect of FRP tube confinement in improving peak stress and axial performance of LCE-CFFTs became more pronounced with higher calcined clay substitution rates; to more accurately predict the axial compressive behavior of LCE-CFFTs of this study, it is essential to consider the biaxial stress-strain state of filament-wound FRP tubes.

On the hydration of limestone calcined kaolinitic clay cement and energy-efficient production

Understanding hydration kinetics of cement composite incorporating calcined clay is crucial in further progressing low-carbon binder designs. However, due to diversity of clays and complex synergy with further limestone addition, it is challenging to predict the hydration process. In this study, a coupled kinetic-thermodynamic model is developed based on unified functional form to generate comprehensive hydration studies on cement binders including calcined kaolinitic clay and limestone. Dedicated model parameterisation analyses are performed, following available rapid, relevant, and reliable (R3) tests, to determine the kinetic models. Moreover, a numerical framework is proposed to describe the synergy in hydration of limestone calcined clay cement (LC3) utilising clays of diverse kaolinite contents. The model is demonstrated to be capable of generating robust hydration assessments on both binary and ternary systems. In enhancing the overall sustainability, the proposed hydration model is leveraged to gauge the energy-efficient production of LC3 binders utilising local resources.

Influence of microwave curing on the early performance of heat-stored LC3 composites

Limestone calcined clay cement (LC3), as a new green cementitious material, has a wide range of functional applications in cement-based composite materials. Heat-stored LC3 effectively achieve energy-saving and carbon reduction in buildings. However, the addition of phase change material (PCM) particles leads to a significant decrease in strength, which imposes higher requirements on early performance. To address this issue, this study adopts microwave curing technology to rapidly enhance the early performance of heat-stored LC3, with a specific focus on the influence of microwave curing on the early mechanical properties, hydration products, and microstructure of heat-stored LC3. The results showed that microwave curing at a power of 320 W for 5 min increased the 3-day compressive strength of heat-stored LC3 by 87.3 %, without adversely affecting the later-stage strength, thus compensating for the deficiency in early strength. Microstructure and pore structure analysis revealed that the enhancement of early strength by microwave curing was mainly attributed to the promotion of early hydration of clinker and the activation of pozzolanic reaction occurred calcined clay. The additional formation of C-(A)-S-H gel reduced the matrix porosity, improved overall integrity, and enhanced the microstructure. Additionally, the temperature regulation capability of heat-stored LC3 was verified, ensuring its auxiliary temperature regulation function while maintaining reliable mechanical properties and excellent early strength.

Enhancing fluidity and mechanical properties in Limestone Calcined Clay cements with one-third Portland clinker content

Limestone Calcined Clay Cements (LC3) are attracting considerable attention due to their low CO2 footprints and relatively fast hydration kinetics. LC3-50 formulations (approximately 50 % Portland clinker, 30 % kaolinitic calcined clay, 15 % limestone and 5 % gypsum) are being extensively investigated and the hydration chemistry, rheology, mechanical strength developments and durability performances are starting to be well established. LC3 binders with lower clinker factors are possible, offering an opportunity for further CO2 footprint reduction. However, these blends have been much less studied. This work contributes to fill this gap. Here, a LC3-35 binder (i.e. 35 % Portland clinker content) is investigated and its performances are compared to those of the original Portland cement (PC) and a standard LC3-50 binder. The effects of two superplasticizers and a strength-enhancing admixture are thoroughly investigated in mortars and pastes. Larger calcined clay contents result in an exacerbation of the slump retention problem, but this issue is solved by the use of a new superplasticizer. Moreover, the low mechanical strengths at early ages can be partly mitigated with a C-S-H nucleation seeding admixture. Employing the right admixtures, LC3-35 binders can develop strength values similar to those of neat PC and LC3-50 at 7 and 28 days. The laboratory X-ray powder diffraction results for LC3-35 pastes, when using the strength-enhancing admixture, show that C4AF exhibits an accelerated reaction rate, leading to the formation of larger amounts of AFt and AFm phases. Approximate mass balance calculations are carried out to estimate the metakaolin reaction degree. Finally, an environmental performance indicator is used to place the footprints of the reported binders within the LC3 landscape.

Assessment of fracture toughness in fibrous concrete via Brazilian test: Experimental study with low-clinker cementitious binder

As the predominant and cost-effective material in the construction industry, concrete is susceptible to tension weaknesses, leading to significant flaws such as cracking and brittle fracturing. Recent advancements in fibrous concrete have emerged as a solution to mitigate these issues. Incorporating steel fibers in concrete has emerged as a promising solution to improve crack resistance and structural integrity. This study focuses on developing eco-friendly concrete using a low-clinker cementitious binder and short steel fibers to enhance fracture toughness and mitigate the environmental impact by effectively utilizing lime sludge. Concrete specimens were prepared with three binder types: treated lime sludge (TLS) at 15 % and 30 % and calcined clay (CC) at 15 % and 30 %, replacing conventional clinker. Short steel fibers were added at 1.5 % by volume to enhance mechanical properties. Fracture toughness was evaluated using notched Brazilian disc specimens, assessing mode I, II, and mixed-mode (I/II) fracture at multiple notch orientations (β = 0°, 15°, 28.83°, 45°, 60°, 75°, and 90°). Microstructural analysis during strength development was performed using scanning electron microscopy and X-ray diffraction. The findings revealed that specimens containing 15 % TLS, 30 % CC, and 1.5 % steel fibers exhibited the highest fracture toughness. Mode II fracture toughness exceeded mode I, indicating improved resistance to crack propagation. The addition of fibers to the specimens under mode II demonstrated improved fracture toughness, ranging from 0.44 to 0.53 MPa·m^1/2 compared to the corresponding non-fibrous specimens. The fibrous specimens showed significantly higher ultimate loads at β = 90° compared to β = 0°, indicating superior crack resistance and structural integrity under perpendicular loading conditions. The Brazilian disc specimens demonstrated variability in fracture toughness across different loading orientations, highlighting their suitability for mixed-mode fracture assessment.