One of the core ingredients of cement is pozzolan. It is a natural material derived from volcanic ash and contains silica, which gives it cementitious properties. This means that volcanic ash can react with certain elements to form a hard, binding material. The ancient Greeks first used volcanic ash as a building material as early as 500 BC. However, it was the Romans who really pioneered the use of volcanic ash as a binder to hold stones together. This allowed the Romans to build high-rise buildings and underwater structures, some of which are still standing today, more than 2,100 years later. Pozzolanic materials are widely used in cement production. Pozzolanic materials have a variety of physical properties that make them ideal for the production of cement. Natural volcanic ash has been used as a cementitious material for thousands of years. The earliest use was when ancient civilizations near the Persian Gulf used lime and diatomaceous earth together as a cementitious binder. The invention of "modern" Portland cement in the early 1800s - nearly 7,000 years after the first recorded use of natural volcanic ash - did not end its use as a concrete binder. Instead, the U.S. Bureau of Reclamation began extensive research to better understand the effects of natural volcanic ash on heat of hydration and concrete durability in mass concrete applications. The use of natural pozzolans in North America expanded in the early to mid-20th century with the construction of a number of public works projects, including numerous dams and the Los Angeles Aqueduct. Unlike early civilizations that relied on raw natural pozzolans, most modern applications require calcined or heat-treated natural pozzolans.
Definition and composition of pozzolanic cement
Pozzolanic cement is a mixture of Ordinary Portland Cement (OPC) and pozzolanic materials such as fly ash, silica fume or natural pozzolans. The pozzolanic materials react with calcium hydroxide (Ca(OH)_2) released during the hydration of OPC to form additional calcium silicate hydrate (CSH) gel, which is the main binding phase of the cement paste. The reaction is as follows:
Ca(OH)2+SiO2→C−S−H
The composition of pozzolanic cement varies depending on the type and proportion of pozzolanic materials used. Typically, the content of pozzolanic materials accounts for 15% to 50% of the total cementitious material content.
Pozzolan cement ingredients
Pozzolanic cement is a blended cement made by synthesizing and modifying OPC cement in a certain proportion using pozzolanic materials as raw materials. It is also commonly known as PPC cement. Pozzolanic cement is a type of Portland cement, which is characterized by the presence of pozzolanic particles such as volcanic ash, which ranges from 15% to 35% of OPC. Pozzolanic cement is available in various grades, but it is finer and has a lower density than OPC. The addition of pozzolanic particles helps in the production of cement, which consumes less OPC but has a longer service life and higher strength. PPC grade cement has a longer initial settling time and lower compressive strength. Even so, it is still considered to achieve the same effect as OPC in the long run. It is often used in the construction of marine systems, masonry mortar (including hydraulic structures). They are widely used in large-volume concrete projects such as embankments, cesspools, dams, etc. In some cases, pozzolanic cement grades must be used to replace OPC cement grades.
Types of pozzolanic materials
Pozzolanic materials are siliceous and aluminous materials which have very little cementitious properties in themselves, but which, in finely divided form, react chemically with calcium hydroxide produced by a hydration process at room temperature, even in the presence of water, to form compounds with cementitious properties. The finely divided siliceous or aluminous compounds react with calcium hydroxide to form extremely stable cementitious materials of various structures containing calcium, silicon dioxide and water. In general, amorphous silicates react faster than crystalline silicates. Calcium hydroxide is a water-soluble substance which is converted from insoluble cement or pozzolanic materials. Initially, the pozzolanic reaction occurs, and therefore the generation of hydration heat and the formation of powder are also slow. This reaction requires the absorption of Ca(OH)2, so there is no output of Ca(OH)2. The reduction of Ca(OH)2 makes the cement paste dense and impermeable, thereby increasing the toughness of the cement paste.
Pozzolanic materials can be divided into two categories
1.Natural volcanic ash materials
Natural pozzolans are natural materials that have not been processed or calcined and have the properties of a volcanic ash. Examples of natural pozzolans include volcanic ash or pumice, tuff, shale, opal flint and diatomaceous earth.
- Calcined diatomaceous earth.
- Volcanic ash, tuff and pumice.
- Opal flint.
- Clay and shale.
Natural volcanic ash, which must be more deeply ground and calcined to be useful, has also lost popularity today as more effective artificial volcanic ash has become available.
2.Artificial volcanic ash
- Silica ash: This is an artificial pozzolanic product. It is also produced by reducing high-quality quartz in an electric arc furnace with coal during the production of silicon or ferrosilicon alloys. Silica ash rises in the process as oxidizing gases. It is melted, compressed, and collected in bags. The silica ash thus obtained is further processed to extract impurities and monitor particle size.
- Fly ash: This is the finely crushed residue produced after the combustion of pulverized coal. It is a waste product from coal-fired power plants and railway locomotives, respectively. It used to be the most commonly used artificial pozzolanic material. Fly ash crystals are spherical and have the same fineness as cement. Silica also reacts easily.
The pozzolanic activity of fly ash is pleasing, but it is important that it has a stable carbon content and constant fineness. Using fly ash as an admixture in concrete not only adds technical advantages, but also improves the performance of concrete
Production of pozzolana cement
Calcination vs. Raw Material
The process of converting clay or shale into calcined clay, metakaolin, or calcined shale requires the application of a significant amount of heat to the material. This process causes a reaction that introduces oxygen into the material's structure, effectively changing its composition and properties and converting it into a pozzolan. After calcination, the calcined material can be further processed and ultimately ground into a fine powder by vertical roller mill suitable for use as a supplementary cementitious material. Metakaolin is considered a unique type of calcined clay. Metakaolin is produced using a high temperature calcination process that is specific to kaolin and then ground into finer particles than conventional calcined clays. The fineness of the material and the different calcination process cause metakaolin to exhibit different properties than regular calcined clays. Some materials, such as pozzolans, exhibit the properties of pozzolanes in their raw state without calcination or extensive processing. These materials are sometimes referred to as true natural pozzolans.
Manufacturing Process
Pozzolan cement is made by mixing and grinding Portland cement clinker, pozzolan materials and gypsum.
Its production process is roughly the same as that of ordinary Portland cement, which can be divided into four steps: raw material crushing, raw material grinding, clinker calcination and cement grinding.
- Raw material crushing: Limestone and clay are the main raw materials for Portland cement production. After these raw stones are mined, they are dumped by trucks and sent to crushers for crushing to reduce their particle size. Then, they are piled in pre-homogenization yards and wait for recycling.
- Raw material grinding: Fine-grained raw materials are fed into raw material mills in the required proportions to further reduce the particle size, and then stored in silos to complete the mixing and homogenization process of materials.
- Calcination of clinker: Cement raw materials are fed into cement rotary kilns for high-temperature calcination, and after a series of chemical reactions, some spherical gray particles are generated, which we call clinker. These hot clinkers are cooled to a certain temperature in cement coolers.
- Cement grinding: After freezing, the clinker is mixed with an appropriate amount of volcanic ash materials and gypsum, and then sent to cement mills for actual grinding. Cement materials are usually processed in cement silos and packaged into
Characteristics of pozzolana cement
Pozzolan cement has several properties and benefits that make it an ideal choice for construction projects.
1.Improved durability and chemical resistance
Due to the reduced Ca(OH)_2 content, pozzolan cement has improved durability and chemical resistance. The reaction between pozzolan materials and Ca(OH)_2 reduces the amount of Ca(OH)_2 that can react with corrosive chemicals such as sulfates and acids. This improves chemical resistance and reduces the risk of concrete damage.
2.Reduced heat of hydration and lower carbon footprint
Due to the lower OPC content, pozzolan cement has a lower heat of hydration. This reduces the risk of thermal cracking and improves the overall quality of concrete. In addition, the use of pozzolan materials can also reduce the carbon footprint associated with cement production. According to a study by the Portland Cement Association (PCA), the use of fly ash in cement can reduce carbon dioxide emissions by up to 20%.
3.Enhanced workability and pumpability
Due to the spherical shape of fly ash particles, the workability and pumpability of pozzolan cement are improved. The spherical shape of fly ash particles reduces friction between particles, making it easier to pump and pour concrete. This not only improves production efficiency but also reduces labor costs.
Application of pozzolanic cement in construction
Pozzolan cement has a wide range of applications in the construction sector, including infrastructure projects, housing construction and special purposes
Use in Infrastructure Projects
Pozzolan cement is widely used in infrastructure projects such as roads, bridges and dams. Its excellent durability and resistance to chemical attack make it ideal for construction in harsh environments. Pozzolan cement is well suited for underground, underwater or wet concrete works, especially for projects that require impermeability, resistance to freshwater and sulfate corrosion, including hydraulic systems, marine structures, dam construction, offshore construction, waste disposal, etc.
Applications in Building Construction
Pozzolan cement is also used in building construction, including high-rise buildings and residential buildings. Its improved workability and pumpability make it ideal for high-rise buildings, which require concrete to be pumped to great heights. In addition, the reduced heat of hydration of pozzolan cement also reduces the risk of thermal cracking, which is critical for high-rise buildings.
Use in Specialty Applications
Pozzolan cement is used for special purposes, such as marine and offshore construction. Its excellent resistance to chemical attack and durability make it ideal for structures exposed to seawater and harsh marine environments. For example, the Troll A platform in Norway was built using a pozzolan cement containing silica fume.
Advantages of pozzolana cement
- It is an environmentally friendly cement as the ingredients used in the manufacturing process are all made from renewable recycled waste.
- It is a very fine cement, so it is very suitable for plastering.
- Pozzolanic materials are composed of siliceous materials, so they are cheap, which reduces the cost of cement and makes it more economical to use.
- Pozzolanic cement has a strong resistance to sulfate attack, so it can be used in hydraulic structures, coastal buildings, seaside buildings, dam construction, etc.
- Pozzolanic cement is used in prestressed and post-stressed concrete elements.
- It eliminates carbon monoxide emissions in concrete, making it safe for the environment.
- Since pozzolanic materials are very fine, they can fill the pores between steel bars and aggregates, reduce shrinkage, minimize the formation of honeycombs, reduce leakage, and thus improve the strength and toughness of concrete.
Disadvantages of pozzolan cement
- The initial power gained is less, which has an early impact on the disengagement of the support.
- The PPC setting time is shorter compared to the OPC setting time.
- The reduction in alkalinity reduces the corrosion susceptibility of the reinforcement.
- If the strength of the concrete is to continue to increase, then the repair process is crucial. Any defects may affect its service life.
- Performance improvement
Volcanic ash plays a key role in cement
The key to pozzolanic effectiveness is the thoroughness of the reaction it initiates in hydrated concrete. The pozzolanic reaction is a secondary, supercharged reaction of the primary reaction of pozzolanic cement with water. Pozzolanic ash improves concrete by converting problematic calcium hydroxide (CH), a byproduct of the hydraulic reaction between water and cement powder, into additional calcium silicate hydrate (CSH), the glue that binds concrete aggregates.
The truly amazing thing about the pozzolanic reaction is that it both consumes the source of the problem and converts it into the needed CSH. Even after the cement-water hydration process has waned, the pozzolanic reaction process continues until the CH or pozzolanic ash is consumed.
The pozzolanic reaction has many benefits:
Reduced Hydration Heat Damage and Higher Strength
Natural pozzolans reduce the heat of hydration by 10% to 40% in the first 100 hours, depending on the final mix design, which reduces the risk of hot cracking and allows for a cooler, more controlled setting. After 100 hours, the cement-water hydration process tapers off, while the pozzolan mixture continues to hydrate until one of the two remaining hydrating agents (calcium hydroxide or natural pozzolan) is consumed. This slow pozzolan hydration process can continue for months or even years, giving natural pozzolan-based concrete long-term strength far superior to that of ordinary Portland cement concrete.
Prevents Chemical Attack
When methane migrates out of concrete through capillary action, it leaves behind a maze of pores that not only weakens the concrete but also facilitates future water infiltration. This infiltrating water may contain sulfates, chlorides, and other harmful chemicals. In cold climates, this infiltrating water can freeze and cause freeze-thaw damage to the concrete. Not all methane migrates out of concrete, the remaining methane combines or reacts with other chemicals that may be present in the concrete matrix, or penetrates the concrete through the pathways mentioned above. Natural pozzolans are able to effectively absorb problematic methane, and as a result, concrete with natural pozzolans added generally has a high resistance to sulfates.
Enhances corrosion resistance
Resistance to chloride penetration is a key factor in protecting the steel reinforcement embedded in concrete from corrosion, and ultimately preventing concrete failure due to expansion of iron oxide hydrates (rust). Natural pozzolans make the concrete matrix dense and impermeable to liquids and/or gases, thereby preventing steel reinforcement corrosion.
Mitigates Alkali-Silica Reaction (ASR)
As concrete hardens, natural pozzolans react rapidly with calcium hydroxide, trapping any alkali metals present in a dense cement paste, thereby mitigating capillary action and virtually eliminating ASR and white bloom. Research has clearly shown that natural pozzolan blended cements are highly resistant to damage from ASR.
Prevents white bloom
Natural pozzolans can mitigate or eliminate unsightly white bloom in concrete. White bloom refers to a white powdery substance that forms on the surface of some concrete when no pozzolana is added to the mix formula.
Conclusion
Pozzolan cement is a versatile and sustainable material that offers many benefits, including improved durability, reduced environmental impact, and enhanced workability. Its applications range from infrastructure projects to building construction to specialty applications. As the construction industry continues to grow, the use of pozzolan cement is likely to become more widespread.
