The search for eco-friendly Supplementary cementitious Materials
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Supplementary cementitious materials (SCMs) are powders used to replace clinker in cement or portland cement in concrete. Many SCMs are by-products of other industries, such as fly ash from coal-fired power plants. Some SCMs are natural minerals with lower energy input during processing compared to portland cement. The use of SCMs significantly reduces energy consumption and greenhouse gas emissions in concrete production.
SCMs are considered a primary tool for reducing carbon dioxide emissions in the concrete industry. While various strategies have been identified to reduce emissions, using SCMs as clinker substitutes has the lowest economic and performance impacts. Studies commissioned by the United Nations have also recognized clinker substitution as a favorable carbon reduction method. SCMs not only reduce emissions but also improve the mechanical performance and durability of concrete, leading to more efficient designs and longer lifetimes.
However, there is a challenge in the industry’s heavy reliance on industrial waste streams as the main source of SCMs, particularly fly ash. The closure of coal-fired power plants threatens the supply chain of fly ash, and the increasing demand for cement further complicates the situation. To address this, the industry is exploring new sources of SCMs and developing new test methods to qualify these materials for use.
Research on SCMs is gaining importance as the industry seeks to expand its available material resources. Traditional test methods are not suitable for screening alternative sources with different properties, necessitating the development of new testing approaches. This paper discusses the exploration of new SCMs, advancements in materials testing, and the impact of SCMs on concrete’s long-term durability. The goal is to comprehensively evaluate and incorporate new materials into concrete production, expanding the available options for sustainable construction.
A paper pages based off Maria C.G. Juenger, Ruben Snellings, Susan A. Bernal on cement and concrete research
Created by: Yufrizal. Ahmad A
Various new sources of supplementary cementitious materials (SCMs) are available for use in modern concrete production. These include natural minerals, biomass ashes, and industrial by-products. Natural SCMs, such as volcanic materials like tuffs, ashes, pumicites, perlites, and zeolites, have high amorphous silica and alumina contents and can be excellent natural pozzolans. While natural SCMs have not been widely available or cost-competitive in the past, there has been a resurgence of interest in them, with increased production capacity and ongoing research.
Some recent studies have highlighted the potential of natural SCMs like pumice and perlite, which performed similarly to fly ash in terms of pozzolanic activity. The particle size distribution of these materials affects their reactivity and water demand, but this can be balanced through grinding or the use of chemical admixtures. Zeolites, which have high internal surface area, can also impact workability and water demand in concrete. Modifying zeolites through calcination, chemical treatments, or milling can enhance their reactivity and provide unique benefits like improved concrete performance and self-healing of cracks.
Calcined natural materials, such as metakaolin and calcined clays, are another valuable source of SCMs. Metakaolin, which is commonly used as an SCM, can be costly due to limited availability. However, calcined smectite clays, like bentonite or montmorillonite, can be pozzolanic and offer a cost-effective alternative. Impure clays and clay-rich dredged sediments from industrial waste streams have also shown promise as SCMs. These materials can be calcined and used effectively in cementitious mixtures, providing
similar or better pozzolanic activity than fly ash.
The development of a binder system called LC3 (limestone-calcined clay cement) has gained significant attention. LC3 utilizes impure clays and limestone to form additional reaction products, resulting in improved mechanical properties and permeability compared to ordinary Portland cement (OPC) with fly ash. Industrial trials of LC3 have been successful, and there is increasing interest in changing standards to allow for the implementation of this material.
By-product materials like fly ash and slag, which have been commonly used as SCMs, continue to be researched for their performance and interaction with fillers and additives. Fly ash acts as both a filler and a pozzolan, influencing strength and setting in complex ways. Understanding the contributions of fly ash’s hydraulic and pozzolanic properties requires further research and improved test methods. Additionally, the reactivity of glasses present in fly ashes and slag is being investigated, as their structural properties impact their dissolution rate.
Ternary Diagram of Cementitious Materials
In recent years, there have been advancements in the characterization and testing of supplementary cementitious materials (SCMs) to better understand their behavior and predict their performance and durability. These advancements can be categorized into two levels: (1) in-depth characterization techniques and (2) reliable and rapid screening tests.
Advances in SCM Characterization Techniques:
To develop microstructural models of blended cements, it is essential to have detailed knowledge of the fundamental material properties of SCMs. This includes information about their chemical and phase composition, as well as physical properties such as particle size distribution, density, and specific surface area. Commonly used techniques for SCM characterization include X-ray fluorescence (XRF) spectrometry, X-ray powder diffraction (XRD), thermogravimetry, laser diffractometry, He pycnometry, and BET N2 adsorption. These techniques provide valuable data, but it is crucial to use well-calibrated measurement procedures and careful interpretation to obtain accurate results. Adjustments in analytical procedures may be required depending on the type and properties of the SCM.
Solid-state nuclear magnetic resonance (NMR) spectroscopy has contributed significantly to understanding cement hydration and SCM reactivity by providing quantitative structural information on cement hydrates. Recent advancements in solid-state NMR, such as 27Al and 29Si magic-angle spinning (MAS) NMR, have shed light on the relationships between composition, atomic structure, and reactivity of SCMs. Additionally, scanning electron microscopy (SEM) combined with energy-dispersive X-ray (EDX) microchemical analysis allows for the determination of physicochemical properties at the individual component level of SCMs. Advanced imaging techniques like transmission electron microscopy (TEM) and X-ray tomography provide nanoscale characterization of SCMs and their reaction products.
Advances in SCM Reactivity Testing:
While in-depth characterization techniques are crucial for understanding SCM behavior, practical and reproducible tests are needed to evaluate SCM reactivity and performance. Existing test methods often fall short in terms of covering a wide range of SCMs and correlating with performance. Some methods measure “reactive silica” or the consumption of Ca(OH)2 as a proxy for SCM reactivity, but the correlation with compressive strength development is not clear. On the other hand, strength-based tests like the ASTM C 311 strength activity index require a longer time to observe the contribution of SCMs and cannot differentiate between physical packing effects and chemical reactivity. Efforts have been made to improve SCM reactivity test methods. Initiatives by organizations like RILEM and ASTM have evaluated standardized and newly proposed tests based on criteria such as correlation to strength development and interlaboratory reproducibility. One promising test method is the R3 model paste, which measures heat release or bound water in SCM pastes to quantify both pozzolanic and latent hydraulic reactivity. This test method eliminates the interference between clinker and SCM reactions and provides a direct measurement of SCM reactivity. The R3 test has shown good correlation with compressive strength for various SCMs.
In conclusion, advancements in SCM characterization techniques, such as advanced analytical methods and imaging techniques, have provided a better understanding of SCM properties and reactivity at different scales. Moreover, progress has been made in developing more practical and reliable SCM reactivity tests that can evaluate a wide range of SCMs and correlate with performance. These advancements contribute to the development of high-performance and durable cementitious materials.
3D X-ray nanotomography reconstruction of C3A particles hydrated in the presence of gypsum for 143 min
Understanding the impact of SCMs on concrete durability
Concrete durability is significantly influenced by the use of supplementary cementitious materials (SCMs) in blended cements. SCMs such as blast furnace slag, fly ash, and natural SCMs can enhance the long-term performance and durability of concrete. The impact of SCMs on concrete durability is attributed to several factors, including modification of hydration products, improved particle packing, pore refinement, and reduced pore connectivity.
Concrete containing natural SCMs, such as calcined clays and volcanic ashes, has been extensively studied and found to exhibit improved durability. These materials contribute to the long-term compressive strength development of concrete and can enhance properties such as low chloride permeability, high sulfate resistance, and reduced alkali-silica reaction. However, the effects of natural SCMs on cement and concrete properties can vary depending on their chemical composition and reactivity.
There is limited research on the durability of concrete containing new sources of natural SCMs, as these materials have often been used for other purposes or their potential as SCMs has only recently been identified. More studies are needed to evaluate the microstructural features, phase assemblage evolution, pore structure, and chemical resistance of concrete containing these new SCMs to determine their performance in service. Additionally, the corrosion mechanism of steel reinforcing bars embedded in concrete with natural SCMs, particularly under carbonation and chloride exposure, requires further investigation.
Natural zeolites, such as clinoptilolite, have been commonly used as SCMs in concrete production. Addition of clinoptilolite improves chloride chemical binding and reduces chloride penetration in concrete. It also enhances water penetration resistance and freeze-thaw resistance when used in combination with chemical admixtures. Concrete containing clinoptilolite exhibits good chemical resistance to various aggressive agents, although its carbonation resistance may vary depending on the zeolite content.
Ground pumice, another natural SCM, reduces early age compressive strength but improves properties such as water absorption, sorptivity, and volume of permeable voids at extended curing times. The addition of low contents of silica fume further enhances the chloride resistance of pumice-blended concrete. Pumice-containing systems also show increased durability against magnesium sulfate and sulfuric acid attacks. However, the carbonation resistance of pumice-containing concrete decreases with higher pumice contents.
Non-kaolinitic clays and sediments have been studied mainly for their reactivity and phase assemblages when blended with cement. Limited durability studies have focused on their effects as SCMs in concrete. Non-kaolinitic clays have been found to reduce water and chloride permeability in self-consolidating concrete. Calcined dredged sediments, when used as an SCM, exhibit comparable freeze-thaw resistance, chloride permeability, and sulfate attack performance to materials without SCMs at moderate replacement levels. However, higher contents of calcined sediment may lead to reduced durability due to increased porosity and water permeability.
Portland cement-limestone-SCM systems, including limestone powder, have been widely used. The filler, dissolution, and chemical effects of limestone powder depend on various factors such as particle size, dosage, and mineral composition. Concrete with high limestone content may be susceptible to sulfate attack, particularly thaumasite formation. However, low-water concrete with higher limestone content can exhibit good resistance to chloride penetration and sulfate attack when designed with low water contents and optimized packing density. The addition of other SCMs, such as metakaolin, further enhances the chloride binding capacity and sulfate resistance of limestone-blended concrete. The durability of limestone calcined clay cement (LC3) systems shows excellent resistance to chloride penetration, alkali-silica reaction, and sulfates due to high pore refinement. LC3 materials also demonstrate good carbonation resistance.
In conclusion, the use of natural SCMs in concrete can significantly improve its durability. Each natural SCM offers unique properties that can enhance specific aspects of concrete performance, such as reducing chloride permeability, enhancing sulfate resistance, improving freeze-thaw resistance, and refining the pore structure.
However, it is important to consider the specific characteristics and potential limitations of each SCM to optimize their use in concrete mixtures.
Insights, recommendations, and conclusions
SCMs have been widely used in concrete mixtures for several decades to improve performance, reduce costs, and minimize environmental impact. The cement and concrete industries heavily rely on SCMs, particularly those sourced from low-cost and readily available industrial waste streams like fly ash and slag. However, with increasing demand and decreasing supply of conventional SCMs, there is a pressing need to explore new materials that offer comparable or superior properties. This exploration should prioritize resource efficiency and sustainability by utilizing waste materials from industries that currently have little commercial value.
Research into new sources of SCMs, their reactivity, and their impact on concrete properties has been on the rise. However, it is apparent that new research is necessary, especially in addressing barriers such as reduced reactivity, slower production speed, resource scarcity, and increased quality control requirements compared to conventional SCMs. Further studies are required to overcome these barriers, quantify the environmental and economic benefits of using new SCMs, and support policy changes to incentivize their adoption in infrastructure projects. It is crucial to incorporate new SCMs into existing or new standards to enable their widespread use in the future. Natural SCMs like pumices and calcined clays may have a faster implementation process due to their homogeneity and known chemistry.
For some SCMs, particularly impure calcined clays, a better understanding of their reactivity is necessary. Complex and heterogeneous materials require improved knowledge of the minerals’ reactivity within these SCMs. Additionally, the combination of calcined clays and other SCMs with limestone and cement requires further investigation into the resulting reactions, microstructural development, and their impact on durability. Systems like LC3 (low-carbon cementitious materials) show promise as low-cost, readily available SCMs, and more research will facilitate their implementation. It is important to study the potential mobility of heavy metals in SCMs derived from wastes like municipal solid waste bottom ashes or non-ferrous slags to ensure their safe use in infrastructure materials.
Alongside the exploration of new materials, there is a need for new standardized tests and standards governing their use. Existing test methods for evaluating pozzolanic reactivity have proven ineffective or lacking robustness. The development of new test methods, which are currently being researched, is expected to find their way into practice in the coming years. Similarly, methods to characterize the impact of SCMs on durability, such as resistance to carbonation, need to be revisited and revised in the context of new materials and systems. Further research on test methods for SCM characterization and concrete durability will contribute to the development of new standards, allowing for a greater variety of SCMs in evolving concrete mixtures.
