Cement substitution by a combination of metakaolin and limestone

This paper reports the partial replacement of Portland cement (PC) by combination of metakaolin (MK) and nanoclay (NC) in sisal fiber-reinforced cement composites by studying the microstructure, mechanical behavior, and the interfacial properties between fiber and cement matrices. The mechanical properties of cement matrix and natural fiber-reinforced composites are studied using compressive strength development and flexural behavior, respectively. The tensile behavior of the natural fiber was also investigated and analyzed by Weibull distribution model. The characteristics of hydration products were analyzed by scanning electron microscope, X-ray diffraction, and thermogravimetry analysis. Our results show that the combination of MK and NC can improve the hydration of cement more effectively, with better microstructure and enhanced mechanical properties, than mixes without them. The calcium hydroxide (CH) contents of matrixes with 50 wt% combined substitutions, containing 1, 3, and 5 wt% of nanoclay, were 58.12, 60.16, and 64.25 % less than that of PC, respectively. The ettringite phase is also effectively removed due to the substitution of MK and NC, which improve both Al/Ca and Si/Ca ratios of calcium silicate hydrates (C–S–H) due to the high content of SiO2 and Al2O3. The interfacial bond between fiber and cement matrix and flexural properties of sisal fiber-reinforced cement composites are also significantly improved. The optimum interface adhesion between sisal fiber and matrix was achieved by replacing cement by 27 % MK and 3 % NC, which increased the bond strength and pull-out energy by 131.46 and 196.35 %, respectively.

Four new calcined kaolinitic clays as source of supplementary cementitious materials to production of cement with high level of clinker replacement were assessed in this research. Anhydrous cements were characterized by particle size distribution (PSD), specific surface (BET) and thermogravimetric analysis (TGA). The pastes were assessed by X ray diffraction (XRD), TGA and mercury intrusion porosimetry (MIP). The behavior of blends was too assessed by compressive strength in standard mortars. The specific surface of LC3 cement depends mainly on the specific surface of calcination product of clay, which depend as well on the mineralogical composition of the raw material and the calcination temperature. Results indicated an agreement with the kaolinite content in the original clay, pozzolanic reactivity and the performances of blended cements. The research showed the potentialities of cuban clay deposits from different geologic origin to be used in the production of ternary blended cements with similar performances to the Portland cement.

Supplementary cementitious materials (SCMs) are commonly used in the manufacture of commercial cements with lower clinker content and carbon footprints, enabling environmental and performance improvements. The present article evaluated a ternary cement combining 23% calcined clay (CC) and 2% nanosilica (NS) to replace 25% of the Ordinary Portland Cement (OPC) content. For this purpose, a series of tests were performed, such as compressive strength, isothermal calorimetry, thermogravimetry (TG/DTG), X-ray diffraction (XDR), and mercury intrusion porosimetry (MIP). The ternary cement studied, 23CC2NS, presents a very high surface area, which influences hydration kinetics by accelerating silicate formation and causes an undersulfated condition. The pozzolanic reaction is potentialized by the synergy between the CC and NS, resulting in a lower portlandite content at 28 days in the 23CC2NS paste (6%) compared with the 25CC paste (12%) and 2NS paste (13%). A significant reduction in total porosity and conversion of macropores in mesopores was observed. For example, 70% of pores in OPC paste were macropores that were converted in the 23CC2NS paste into mesopores and gel pores.

The aim of this work is to investigate the carbonation resistance of limestone and calcined clay blended cement-based concrete. Two limestone and calcined clay concretes with an average 28 d compressive strength of about 36 MPa were considered. Limestone and calcined clay (with a ratio of 2 : 1) were blended with a general purpose (GP) cement. The GP cement substitution rates considered were 30% and 45%.A low-grade calcined clay was used with about 50% amorphous phase. Accelerated and natural carbonation tests were performed. Mercury intrusion porosimetry and X-ray diffraction were carried out, to assist in the analysis of the experimental results. Results show that the early-age compressive strength is only marginally affected by the limestone and calcined clay substitution up to 45% and a significant refinement of the pore structure was observed compared to the reference GP cement concrete. The resistance of concrete against carbonation reduces with increase in the GP cement substitution rate. Overall, this study shows that a limestone and calcined clay blend used as a simple substitution for GP cement in concrete can provide adequate protection against carbonation-induced steel reinforcement corrosion if the ordinary Portland cement content in the mix is at least 60%.

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.

Use of spent fluid catalytic cracking catalyst (FCC) in Limestone Calcined Clay Cement (LC3) systems: Studies in pastes and mortars

Assessment of pore structure evolution in the limestone calcined clay cementitious system and its implications for performance

The substitution of Ordinary Portland Cement (OPC) with Supplementary Cementitious Materials (SCMs) is a key strategy for reducing the carbon footprint of construction materials. In this study, LC 3 (Limestone Calcined Clay Cement) binders were developed using a calcined clay rich in kaolinite and illite, sourced from a naturally occurring deposit, offering an alternative to conventional high-purity kaolinite clays. The primary objective was to investigate the impact of this clay composition on the hydration behaviour, phase composition and mechanical properties of LC 3 systems. The performance of the designed composite (with the clay dosage from 20% to 40%) was assessed through isothermal calorimetry, simultaneous thermal analysis, and basic physical and mechanical properties determination. The binders exhibited relatively high hydration heat over 200 J.g −1 at 7 days, but a modified course of hydration. Consequently, they showed noticeably lower bulk density (by 7 to 16%), slightly improved flexural strength (by 3 to 27%) and varying compressive strength (ranging from -26% to 13% depending on composition). These findings demonstrate the potential of kaolinite-illite clays as viable SCM sources in LC 3 formulations, broadening the raw material base and promoting regional material utilisation in sustainable cement production.

This paper has investigated the properties of mortars made from binary and ternary blends of metakaolin (MK), palm oil fuel ash (POFA), and ordinary Portland cement (OPC). A total of 17 different mortar mixtures were produced. The OPC in the mixtures was partially replaced by MK, POFA, or a combination of MK and POFA at different replacement levels of (0–30%) by weight of the binder. At the fresh state, the flow (workability) of mortar mixtures was determined, while at the hardened state, the compressive strength and porosity at the ages of 7, 28, and 90 days were evaluated. The results showed that the flow of mortar is boosted with the combined use of MK and POFA compared to when MK is separately used. Besides, improvement in low early compressive strength development and reduction in high porosity from use of POFA occurred with the addition of up to 10% MK content. Therefore, the combination of POFA and MK could be used as a supplementary cementitious material to produce cement-based material of higher quality than OPC.

Recent research has recognized limestone calcined clay cement (LC3) a promising sustainable cementitious material. While LC3 could be used in a wide range of applications, it is important to explore its feasibility in developing (ultra-)high performance concrete. In this paper we attempt to produce ultrahigh performance LC3 concrete (UHP-LC3). With 20%, 30%, and 40% OPC replaced by coal gangue waste calcined clay and limestone (LC2), the mechanical, hydration, and microstructural properties were extensively studied. The mechanical properties were investigated by compressive and flexural tensile tests. The hydration behavior of UHP-LC3 was evaluated by thermogravimetric analysis (TGA), quantitative X-ray powder diffraction (QXRD) analysis, 29Si nuclear magnetic resonance (NMR) spectroscopy, and scanning electron microscope (SEM). Finally, the microstructure of UHP-LC3 was studied by mercury intrusion porosimetry (MIP). Results show that the substitution of LC2 improve the early strength by quickly pozzolanic reaction to form denser hydration products as well as the synergistic interaction between silica fume and limestone/calcined clay in the system. However, the long-term strength would drop as LC3 caused insufficient hydration reaction due to the high water-demand of calcined clay that reduced the amount of free water. Also, there are abundant unreacted calcined clay particles in the hydrated system. Meanwhile, LC3 substitution of OPC seems to improve the polymerization of C-A-S-H gel and densify the pore structure, thus display excellent carbonation resistance. This paper explores new options to prepare more sustainable and cost-effective UHPC.

The cement industry is a major source of global CO₂ emissions, and limestone calcined clay cement (LC³) is a promising low-carbon alternative, but its large-scale application is limited by the dependence on high-cost commercial calcined clays. Coal gangue (CG) is a massive industrial solid waste with potential pozzolanic activity after thermal activation, yet its reaction behavior, synergistic mechanisms, and performance equivalence to commercial calcined clays in LC³ systems remain unclear, with a lack of quantitative assessment of its carbon emission reduction potential. To address these gaps, this study conducts a multi-scale investigation on thermally activated coal gangue (CCG)-based LC³ cement, combining experimental characterization including X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, mercury intrusion porosimetry, isothermal calorimetry and life cycle assessment. The results show that the optimal mass ratio of CCG to limestone is 2:1, and the LC³ formulation with 20% CCG replacement achieves a 28-day compressive strength comparable to that of commercial calcined clay-based LC³ and ordinary Portland cement. A synergistic reaction mechanism involving sulfate, carbonate, and aluminosilicate is clarified: gypsum regulates early aluminate hydration, while limestone reacts with reactive aluminates derived from CCG to form stable carboaluminate phases, which refine the pore structure and densifying the microstructure. LCA results indicate that the optimal CCG-based LC³ formulation reduces the global warming potential by 24.4% compared to ordinary Portland cement. This study realizes the high-value utilization of CG and provides a low-cost, sustainable alternative to commercial calcined clays for LC³ technology, offering technical support for the decarbonization of the cement industry and the disposal of industrial solid waste.

The combined effect of temperature and vapor pressure on hydration reactions of three different types of Portland cements was studied using a laboratory autoclave. Oil well Portland cement Class G high sulfate resistant (HSR), Dyckerhoff Portland cement and Portland cement CEM I 42.5 R Extra were cured under hydrothermal conditions (165 °C–0.5 MPa and 220 °C–2.0 MPa) up to 7 days. In parallel, hydration reactions at laboratory conditions (25 °C–0.1 MPa) of these samples were also studied. Simultaneous thermogravimetric and differential thermal analysis measurements (TG/DTA) were mostly used to characterize the course of hydration under different curing conditions. X-ray diffraction, scanning electron microscopy and mercury intrusion porosimetry were used to identify the hydration products and to characterize the coupled effect of temperature and vapor pressure on microstructure and pore structure development. Also, mechanical properties were correlated with pore structure and scanning electron microscopy analysis. Different hydrothermal curing regimes resulted in sequential and overlapped hydration reactions with products including portlandite, ettringite, poorly crystalline C–S–H, hydrogarnet (C–A–S–H), α-C2SH, jaffeite (C6S2H3), scawtite (C7S6$${\bar{\text{C}}}$$H2) and reinhardbraunsite (C5S2H). Calcium silicate hydrate underwent systematic changes starting with the transformation of C–S–H gel formed during the non-equilibrium phases or under low-pressure hydrothermal conditions to α-C2SH, jaffeite and reinhardbraunsite with increasing hydrothermal temperatures. The gradual transition of amorphous C–S–H phases to α-C2SH, C6S2H3, C7S6$${\bar{\text{C}}}$$H2 and C5S2H has caused the deterioration of pore structure with corollaries of the increase in permeability and the decrease in mechanical properties. Moreover, different temperature peaks from 600 to 1000 °C denoting thermal decomposition of different calcium carbonate species were depicted at DTG curves. These are ranged from low to well-crystallized CaCO3.

The use of natural calcined clay as supplementary cementitious materials for the replacement of a portion of cement has become increasingly interesting. It is used for reducing the cement content in mortars and concrete production and also for improving strength and durability of concrete construction. In this context, the objective of this study was to make calcined clay in laboratory using two Tunisian clays burnt at 600, 700 and 800 °C and to study the effect of temperature on Pozzolanic reaction in terms of physical, mechanical and microstructure properties. Here, we studied the thermal and chemical properties, mechanical compressive strength of mortars, electrical conductivity as well as the physical properties, namely Blaine specific surface area, average particle diameter, pycnometer density, to evaluate its pozzolanic effect by substitutions of cement (5, 10, 15 and 20%). The results show the beneficial effect of adding calcined clay at different temperatures of calcinations on these properties. Besides, the average particle diameter increased with temperatures which agglomerate the particles. The Blaine specific surface area of calcined clay exceeded that of cement. Furthermore, the substitution of cement by calcined clay enhanced the compressive strength. All samples showed the development of strength between 28 and 90 days.

Understanding the hydration of dolomite in cementitious systems with reactive aluminosilicates such as calcined clay

The use of calcined clays providing from low grade kaolinitic clays combined with the limestone filler in ternary blended cement formulation has received considerable attention in recent years. This paper describes the results of a research project to study the behavior of kaolinitic calcined clays (CC) in combination with limestone filler (F). Blended cements were obtaining replacing CC (0–30 %) and F (0–10 %) by mass by Portland cement (PC). The pozzolanicity of blended cement was assessed by the Frattini tests at 2, 7 and 28 days. The response of the system was evaluated in terms of flow, and the compressive strength at 2, 7 and 28 days. The hydration progress was determined by the type and amount of hydration compounds at 2, 7 and 28 days using the Rietveld method. The change in pore size distribution was determined by mercury intrusion porosimetry (MIP). Hydrated phases obtained correspond to the pozzolanic reaction (contribution CC) and phase stabilization (contribution F) modifying the pore structure and all factors contribute to develop acceptable mechanical properties with a large reduction of energy consumption and CO2 emission.