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.