1. DEFINITION OF SCM

Supplementary Cementitious Materials (SCMs) are materials that, when combined with Portland cement, enhance the properties of concrete through hydraulic or pozzolanic reactions. These materials can be industrial by-products, such as fly ash or slag, or naturally occurring substances, like volcanic ash. SCMs are used to partially replace cement in concrete mixtures, reducing the environmental impact of cement production while improving the performance and durability of concrete.

Standards and guidelines have been established to ensure the quality and consistency of SCMs in concrete applications. For example, ASTM C618 defines the chemical and physical requirements for fly ash and natural pozzolans, while ASTM C989 specifies the standards for ground granulated blast furnace slag (GGBFS). Similarly, EN 450-1 in Europe outlines the requirements for fly ash in concrete. These standards ensure that SCMs meet specific criteria for fineness, chemical composition, and reactivity, guaranteeing their effectiveness in concrete pavements.

The use of SCMs is also supported by building codes and sustainability initiatives. For instance, LEED (Leadership in Energy and Environmental Design) encourages the use of SCMs to reduce the carbon footprint of construction projects. By adhering to these standards, engineers and contractors can confidently incorporate SCMs into concrete pavements, ensuring both performance and sustainability.

2. TRENDS OF USING SCM

The global construction industry is increasingly adopting SCMs to address environmental concerns and improve concrete performance. In North America, fly ash and slag are widely used, driven by stringent environmental regulations and the availability of industrial by-products. The U.S. Environmental Protection Agency (EPA) and the Canadian Standards Association (CSA) have promoted the use of SCMs to reduce greenhouse gas emissions from cement production.

In Europe, the use of SCMs is supported by the European Union’s Circular Economy Action Plan, which emphasizes waste reduction and resource efficiency. Countries like Germany and the Netherlands have incorporated SCMs into their national standards, with a focus on fly ash, slag, and silica fume. Similarly, in Asia, the rapid growth of infrastructure projects has led to the widespread use of SCMs. China and India, for example, have significant coal-fired power plants, producing large quantities of fly ash that are repurposed in concrete.

Emerging economies in Africa and South America are also beginning to adopt SCMs, although their use is limited by the availability of industrial by-products. Overall, the global trend toward sustainable construction practices is driving the increased use of SCMs, with a focus on reducing carbon emissions, improving concrete performance, and repurposing industrial waste.

For supplementary cementitious materials (SCMs), the primary concern is the availability of fly ash. This issue is illustrated in Figure 1. Between 2007 and 2013, overall fly ash production declined by approximately 25%; however, the volume of fly ash utilized in cement and concrete production remained steady at around 10 to 12 million tons (9 to 11 million metric tons) annually. During this same period, the total beneficial use of fly ash saw a significant reduction, leading to the situation in 2013 where the beneficial use of fly ash in concrete accounted for 63% of its total beneficial use. A considerable amount of ash remains unused, but much of it is either not suitable for concrete applications without post-combustion processing or is sourced from distant locations that complicate distribution. In some instances, transportation limitations hinder the market availability of usable ash, contributing to reported shortages.

Figure 1. Trends in fly ash production and its usage (American Coal Ash Association, 2015)

3. HYDRAULIC AND POZZOLANIC ACTIVITY

The effectiveness of SCMs in concrete is largely determined by their hydraulic or pozzolanic activity. Hydraulic materials, such as slag, can react with water to form cementitious compounds independently, even without the presence of Portland cement. This property makes slag particularly valuable in high-performance concrete applications, where early and long-term strength are critical.

Pozzolanic materials, such as fly ash and silica fume, react with calcium hydroxide (a by-product of cement hydration) to form additional cementitious compounds. This reaction, known as the pozzolanic reaction, enhances the long-term strength and durability of concrete. The pozzolanic reaction is slower than the hydration of cement, which means that concrete containing pozzolanic SCMs often gains strength over time, improving its resistance to chemical attacks and reducing permeability.

The combination of hydraulic and pozzolanic activities in SCMs contributes to the densification of the concrete matrix, reducing porosity and enhancing durability. This makes SCMs particularly valuable in concrete pavements, which are exposed to harsh environmental conditions, including freeze-thaw cycles, chemical deicers, and heavy traffic loads.

4. TYPES OF SCM

4.1. Fly Ash

Fly ash is a by-product of coal combustion in power plants and is one of the most widely used SCMs. Coal fly ash has been incorporated into concrete since the 1930s, with initial findings on its use published in 1937 (Davis et al. 1937). Today, the annual production of pulverized coal combustion fly ash in the US exceeds 53 million tons, of which approximately 44% is utilized beneficially (American Coal Ash Association 2015).

Fly Ash can be classified into two types: Class F and Class C. Class F fly ash is low in calcium and exhibits pozzolanic properties, while Class C fly ash has higher calcium content and can exhibit both pozzolanic and hydraulic properties. Fly ash improves workability, reduces water demand, and enhances the long-term strength and durability of concrete.

The specification characterizes Class F ash as having a total oxide content of 70% or higher, while Class C ash has an oxide content of 50% or more. At the two ends of this classification, Class F ash is primarily pozzolanic, whereas Class C ash is mainly hydraulic but also possesses some pozzolanic characteristics. Coal ash with an oxide sum ranging from 50% to 70% generally displays a mix of both hydraulic and pozzolanic properties.

4.2. Slag (Ground Granulated Blast Furnace Slag - GGBFS)

Slag is a by-product of iron and steel production and is known for its hydraulic activity. When finely ground, it reacts with water to form cementitious compounds, contributing to the early and long-term strength of concrete. Slag also improves resistance to sulfate and chloride attacks, making it suitable for use in aggressive environments, such as coastal areas and regions with high sulfate concentrations in the soil.

Slag cement is defined in the ASTM C989-15 (AASHTO M 302-15) Standard Specification for Slag Cement in Concrete and Mortars (ASTM 2015). This specification primarily focuses on material performance and categorizes the material into three grades: Grade 80, Grade 100, and Grade 120.

Slag cement impacts the characteristics of both fresh and hardened concrete. In terms of fresh properties, concrete with slag cement consolidates more readily under vibration than ordinary Portland cement (OPC) concrete (ACI 2011). Additionally, due to the slower reaction of slag cement, the setting time can be considerably extended when compared to OPC concrete, which can give rise to other challenges.

4.3. Silica Fume

Silica fume is a by-product of silicon and ferrosilicon alloy production and is characterized by its extremely fine particles. These particles fill the voids in the concrete matrix, increasing density and strength. Silica fume is particularly effective in high-performance concrete, where it enhances resistance to chemical attacks, abrasion, and bond strength between concrete and reinforcement.

Silica fume is governed by the ASTM C1240 Standard Specification for Silica Fume Used in Cementitious Mixtures. In terms of chemical classification, this specification mandates a minimum SiO2 content of 85%, along with specific limits on moisture content and loss on ignition (LOI). For physical characteristics, it stipulates a maximum amount retained on a 45 μm sieve and requires an accelerated pozzolanic strength activity index of at least 105% of the control sample at 7 days when 10% of ordinary Portland cement (OPC) is replaced with silica fume.

Another key factor contributing to enhanced concrete strength and durability is that silica fume can effectively pack around aggregate particles, react with calcium hydroxide (CH) in the aggregate-paste interfacial zone, and significantly enhance the strength and impermeability of the interfacial transition zone. As a highly effective pozzolan, silica fume greatly reduces permeability, making it particularly effective in mitigating alkali-silica reaction (ASR) and sulfate attack.

5. BENEFITS

5.1. Fly Ash

- Workability: Improves the flowability of concrete, making it easier to place and finish. 

- Durability: Enhances resistance to sulfate attacks and reduces permeability. 

- Sustainability: Reduces the carbon footprint of concrete by replacing cement.

Due to the broad spectrum of chemical and physical properties encountered, various sources of fly ash can exhibit significantly different performance characteristics. Table 1 offers an overview of the general property variations linked to the substitution of fly ash for Ordinary Portland Cement (OPC).

Table 1. Benefits of using Fly Ash Class C and Class F (Kosmatka and Wilson, 2011)

5.2. Slag

- Strength: Contributes to both early and long-term strength development. 

- Chemical Resistance: Improves resistance to sulfate and chloride attacks. 

- Permeability: Reduces water and chloride ion penetration, enhancing durability. 

5.3. Silica Fume

- Density: Fills voids in the concrete matrix, increasing density and strength. 

- Abrasion Resistance: Enhances resistance to wear and tear, making it suitable for high-traffic pavements. 

- Bond Strength: Improves the bond between concrete and reinforcement. 

In summary, the effects of using SCM can be found in this table below.

Table 2. Effect of SCM usage (Kosmatka and Wilson, 2011).

6. BEST PRACTICES

This comprehensive review highlights the critical role of SCMs in enhancing the performance and sustainability of concrete pavements, providing a foundation for their effective use in modern construction practices. A few reasons on why SCM are used in many civil infrastructures including:

• Proportioning: Optimize the replacement ratio of cement with SCMs based on project requirements and material properties. For example, fly ash can replace up to 30% of cement, while slag can replace up to 50%. 
• Mix Design: Conduct trial mixes to ensure compatibility and desired performance. Consider factors such as workability, strength, and durability. 
• Curing: Ensure proper curing to maximize the pozzolanic and hydraulic reactions. Extended curing periods are often beneficial for concrete containing SCMs. 
• Quality Control: Use SCMs from reliable sources and adhere to relevant standards (e.g., ASTM, EN). Conduct regular testing to ensure consistency and performance. 
• Environmental Considerations: Prioritize locally available SCMs to reduce transportation emissions and support circular economy principles. 

Here are few examples of using SCM in civil infrastructures.

The One World Trade Center

The One World Trade Center building consists of 104 floors, and its height, including the antenna, reaches 540 meters. The specified compressive strength of the concrete at the core is 97 MPa. The concrete mixture uses a quaternary blend containing 52% slag cement combined with Portland cement, fly ash, and silica fume.

Figure 2. The One World Trade Center (source: Aecom)

432 Park Avenue

432 Park Avenue has a building height of 425 meters. A compressive strength of 97 MPa is specified for the foundation and part of the upper structure. The concrete mixture used is a quaternary blend composed of 55% slag cement, 30% Portland cement, 11% fly ash, and 4% silica fume. In laboratory testing, a compressive strength of up to 124 MPa can be achieved.

Figure 3. 432 Park Avenue (source: Town and Country Magazine)

Patimban Port

The environmentally friendly cement product Green Maxstrength, developed by PT Semen Indonesia, has been successfully utilized in the development of the largest port project in West Java, which is also a National Strategic Project, namely the Patimban Port. By employing the concepts of reduce, reuse, and recycle, PT Semen Indonesia has created a cement product made from slag or by recycling waste from steel manufacturing as a substitute for traditional raw materials like limestone and clay used in cement production.

Figure 4. Patimban Port (source: Suryacipta)

7. REFERENCES

1. ACI Committee 232. (2018). Report on the Use of Fly Ash in Concrete. American Concrete Institute.
2. Mehta, P. K., & Monteiro, P. J. M. (2014). Concrete: Microstructure, Properties, and Materials. McGraw-Hill Education.
3. Malhotra, V. M., & Mehta, P. K. (2005). High-Performance, High-Volume Fly Ash Concrete. Supplementary Cementing Materials for Sustainable Development Inc.
4. ASTM C618. (2023). Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete. ASTM International.
5. Neville, A. M. (2011). Properties of Concrete*. Pearson Education Limited.
6. Thomas, M. (2013). Supplementary Cementing Materials in Concrete. CRC Press.
7. Suryacipta (2024). Invest in Patimban Top Opportunity Ports in Indonesia.
8. Aecome (2024). One World Trade Center.

ABOUT AUTHOR

ALI ARYO BAWONO, Dr.-Ing. is an esteemed civil engineering and urban transport infrastructure specialist with a doctoral degree and 15 years of experience with specialization in pavement and highway design and construction. He holds a Ph.D. in Material Science from Nanyang Technological University (NTU) in Singapore, an M.Sc. in Transportation System from Technische Universität München (TUM) in Germany and bachelor’s degree in Civil Engineering from the Bandung Institute of Technology.

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