Introduction
People have used cement for thousands of years to construct infrastructure and buildings because it is an essential building material.
Cement is one of the Major component of Concrete which change the face of the world
Despite its environmental impact, its unique properties and versatility make it a valuable tool for creating structures that are safe, durable, and long-lasting. In this blog post, we will take an in-depth look at cement, exploring its history, chemical composition and characteristics.
What is Cement and How is it Used?
Heating a mixture of limestone and clay to a high temperature produces cement, a fine powder.
During this process, the formation of cement involves grinding the material called clinker into a fine powder.
Cement is an essential component of concrete, which is a mixture of water, cement, sand, and aggregate that forms a paste that hardens into a solid structure. Hydraulic and non-hydraulic cement are two broad categories of cement, each with different properties and uses.
History of Cement
One of the first civilizations to use clay as a cementing material were the Assyrians and Babylonians.
Ancient builders used lime as the binder to bind stones together in monuments.
The Egyptians used both lime and gypsum as cementing materials in the construction of their famous pyramids.
A Roman named Vitruvius was the first recorded scientist to have a deeper understanding of the chemistry of cementitious lime.
The Roman Empire’s one of famous construction featuring cements is the Pantheon, with its 43.43m concrete dome.
Following Vitruvius, M. Vicat of France conducted further research on cement. Joseph Aspedin of Yorkshire, England was the first to introduce Portland cement in 1824.
He formed it by heating a mixture of limestone and finely divided clay in a furnace to a temperature high enough to drive off carbonic acid gas.
In 1845, Isaac C. Johnson enhanced this process by elevating the temperature at which he burned the limestone and clay mixture to form clinker.
This cement became the prototype for modern Portland cement.
In the centuries that followed, researchers in the USA, UK, France, and Germany made gradual improvements in the properties and qualities of cement.
Today, cement is an adhesive and cohesive material that is capable of bonding together solid matter into a compact, durable mass.
For civil engineering works, the most commonly used type of cement is calcareous cement, containing compounds of lime as its chief constituent.
Hydraulic Vs Non-hydraulic Cement
Non-hydraulic cements do not set or harden in water, such as non-hydraulic lime or plaster of Paris.
On the other hand, hydraulic cements set and harden in water, producing a stable end product. Portland cement is an example of a hydraulic cement.
Non-hydraulic cement is a type of cement that does not harden when it comes into contact with water.
Instead, it requires air and heat to set and harden.
Construction projects in environments without moisture concerns utilize non-hydraulic cement, which is less resistant to moisture compared to hydraulic cement.
Some common types of non-hydraulic cement include masonry cement and lime mortar.
The main advantage of non-hydraulic cement is that it does not set and harden in the presence of water.
This makes it ideal for use in dry conditions, such as in the construction of walls and chimneys.
Additionally, non-hydraulic cement has a lower carbon footprint compared to hydraulic cement, as it does not require the high temperatures and energy-intensive processes needed to produce hydraulic cement.
Construction projects exposed to moisture benefit from hydraulic cement, while non-hydraulic cement is best suited for dry conditions.
Specific project requirements and usage conditions determine the choice between hydraulic and non-hydraulic cement.
Portland cement
Portland cement is a widely used cementing material in the construction industry.
The production involves finely pulverizing clinker obtained through calcining an intimate and properly proportioned mixture of argillaceous and calcareous materials.
It is named “Portland” because it resembles a natural stone quarried from Portland in the UK.
To produce high-quality Portland cement, it is crucial to exercise care in proportioning the raw materials to obtain clinker of the proper constitution after burning.
The raw materials used for Portland cement production include limestone, clay, shale, iron ore, and sand.
The raw materials are first crushed and then proportioned in the correct ratios to form a mixture.
Grinding the mixture into a fine powder is followed by heating it to a temperature of 1450°C to 1500°C in a rotary kiln, causing it to fuse and form small lumps known as clinkers.
Grinding the clinkers with gypsum produces the final cement product.
Manufacturing Process of Hydraulic and non-hydraulic cement:
The manufacturing process for hydraulic cement involves the mixing of raw materials such as limestone, clay, iron ore, and sand.
Grinding these raw materials into a fine powder and blending them together create a mixture known as the “raw mix.”
Feeding the raw mix into a kiln and heating it to temperatures between 1400°C and 1500°C occurs in the next step.
The high temperatures induce a reaction in the raw materials, leading to the formation of clinker. Subsequently, grinding the clinker into a fine powder results in the production of Portland cement.
The manufacturing process for non-hydraulic cement is relatively simple and involves the burning of limestone and clay to produce lime and carbon dioxide.
To create a mortar, first mix lime with water to form a slaked lime paste, then combine it with sand and other aggregates.
Ordinary Portland Cement
The construction industry commonly uses Ordinary Portland Cement, abbreviated as OPC.
The production of this binding material involves finely grinding clinker, which results from burning a mixture of limestone and clay in a kiln, along with a small amount of gypsum.
The proportion of the raw materials is important to ensure that the resulting clinker has the desired chemical composition.
OPC is classified into three grades: 33 grade, 43 grade, and 53 grade.
Compressive strength, measured in megapascals (MPa), determines the grade of OPC.The higher the grade, the higher the compressive strength.
33 grade OPC has a minimum compressive strength of 33 MPa after 28 days of curing. Non-structural applications, like plastering and flooring, generally use it.
43 grade OPC has a minimum compressive strength of 43 MPa after 28 days of curing. Structural applications, including the construction of buildings, bridges, and roads, use it.
53 grade OPC has a minimum compressive strength of 53 MPa after 28 days of curing. High-strength applications, like precast concrete elements and high-rise buildings, use it.
chemical composition of Portland cement
Hydraulic cements, such as Portland cement, consist primarily of three main constituents: lime (CaO), silica (SiO2), and alumina (Al2O3). These oxides combine with other minor components, such as iron oxide (Fe2O3), magnesia (MgO), sulphur trioxide (SO3), and alkalis (primarily sodium oxide (Na2O) and potassium oxide (K2O)) to produce cement clinker through a process called fusion.
The lime (60-65%)
It plays a critical role in controlling the strength and soundness of the cement. However, too much lime can result in the formation of free lime, which can cause the cement to be unsound.
The silica (17-25%)
It is responsible for its strength, but excessive amounts can cause the cement to set slowly.
Alumina (3-8%)
It is responsible for quick setting, but excessive amounts can lower the strength of the cement.
Iron oxide (0.5-6%)
It gives colour to the cement and helps in the fusion of different ingredients.
Magnesia (0.5-4%)
if it present in excess, can cause cracks in the mortar and concrete and lead to unsoundness.
Alkalis
The presence of alkalis can accelerate the setting of the cement paste.
The chemical composition of Portland cement has changed over the years, with an increase in lime content and a slight decrease in silica content.
However, the chemical composition of cement has limits, and excessive amounts of any constituents can make fusing and forming clinker difficult. The SiO2/(Al2O3 + Fe2O3) ratio regulates the cement paste’s setting rate.
Increasing the silica content to around 21% and limiting the alumina and iron oxide contents to 6% each helps control the heat of hydration and enhances resistance to the action of sulfate waters.
Conversely, raising the silica content to 24% and reducing the alumina and iron contents to 4% each can further increase resistance to sulfate waters.
Iron oxide, in small percentages, can make highly siliceous raw materials easier to burn, but excessive amounts can result in a hard clinker that is difficult to grind.
When combined with lime and alumina to form C4AF, iron oxide can neutralize some of the undesirable properties contributed by alumina.
Bogue’s compounds in cement
Burning a mixture of limestone, clay, and other materials in a kiln at a high temperature produces the main component of cement, which is cement clinker.
The burning process produces compounds that have the properties of setting and hardening in the presence of water.
Bogue, who identified these compounds, and they are now known as Bogue compounds.
Le-Chatelier and Tornebohm have referred to these compounds as Alite (C3S), Belite (C2S), Celite (C3A), and Felite (C4AF).
The proportions of these four compounds determine the composition of cement clinker. The principal minerals and their symbols in Portland cement are as follows:
- Tricalcium silicate (3CaO-SiO2) – Alite (C3S)
- Dicalcium silicate (2CaO-SiO2) – Belite (C2S)
- Tricalcium aluminate (3CaO-Al2O3) – Celite (C3A)
- Tetracalcium alumino ferrite (4CaO-Al2O3-Fe2O3) – Felite (C4AF)
Considering its well-burnt nature, Tricalcium Silicate (C3S), also known as Alite, typically constitutes 25-50% (usually around 40%) of cement and is regarded as the best cementing material.
Tricalcium Silicate (C3S)
It is responsible for rendering the clinker easier to grind, as well as increasing its resistance to freezing and thawing. Additionally, it hydrates rapidly, generating high heat and developing an early hardness and strength.
However, it’s important to note that raising the C3S content beyond specified limits can have negative effects. This can increase the heat of hydration and solubility of the cement in water.
The hydrolysis of C3S is mainly responsible for the 7-day strength and hardness of the cement paste.
The rate of hydrolysis of C3S and the character of the gel developed are the main causes of the hardness and early strength of cement paste.
The heat of hydration for Tricalcium Silicate is about 500 J / g.
Dicalcium silicate (C2S)
It is also known as Belite, typically comprising 25-40% (usually around 32%) of the total mixture.
Unlike tricalcium silicate, C2S hydrates and hardens slowly, taking a year or more to significantly contribute to the strength of the cement.
However, it does offer resistance to chemical attack, making it an important component for certain applications.
Increasing the C2S content in clinker can have negative effects on the properties of the resulting cement.
It can make the clinker harder to grind, reduce early strength, decrease resistance to freezing and thawing in the early stages, and lower the heat of hydration.
The hydrolysis of C2S occurs more slowly than that of C3S, and in the first month of setting, it has little influence on the strength and hardness of the cement.
However, after a year or more, its contribution to strength and hardness is almost equal to that of C3S.
The heat of hydration for Tricalcium Silicate is about 260 J / g.
Tricalcium Aluminate (C3A)
It is also known as Celite, typically making up about 5-11% (usually around 10.5%).
Combining it with water leads to a rapid reaction, and it is responsible for the initial setting of the cement.
However, this rapid reaction can be a problem, as it can lead to flash set of the cement if not properly controlled.
Adding a small amount of gypsum (typically around 2-3%) during the grinding of the cement helps prevent flash set.
This helps to regulate the rapidity of the reaction, allowing for a more controlled setting time.
Despite this, tricalcium aluminate is still responsible for the high heat of hydration that occurs during the setting and hardening of cement.
While tricalcium aluminate plays an important role in the initial setting of the cement, it can also lead to problems in the long term.
Specifically, it has a greater tendency to cause volume changes, which can result in cracking and other forms of damage over time.
This is especially true if the C3A content is too high. In addition, higher levels of C3A can weaken the resistance of the cement to sulphate attack, and lower the ultimate strength, heat of hydration, and contraction during air hardening.
The heat of hydration for tricalcium aluminate is high, at around 865 J/g.
Tetracalcium Aluminate Ferrite (C₄AF)
It typically comprising about 8-14% (normally about 9%) of the cement composition.
C₄AF is responsible for the early strength development and contributes to the colour of cement.
Compared to tricalcium aluminate, C₄AF generates less heat of hydration and has a lower cementing value.
The reaction of calcium oxide, alumina, and iron oxide during the clinkering process forms C₄AF.
Having a cubic crystal structure, it is typically found in small, isolated crystals within the cement matrix.
Increasing the C₄AF content in cement can slightly reduce its strength, but it also has a positive effect on reducing the alkali-aggregate reaction.
The heat of hydration of C₄AF is 420 J/g.
Hydration of cement
Hydration, the process of combining cement and water, is crucial for the formation of concrete. This reaction occurs between the active components of cement, which include C4AF, C3A, C3S, and C2S, and water.
The physical properties of concrete depend on the extent of hydration of cement and the resulting microstructure of the hydrated cement.
When water comes into contact with cement, the hydration products start depositing on the outer periphery of the nucleus of hydrated cement.
This process, known as the induction or dormant period, takes 2-5 hours.
As the hydration proceeds, the deposit of hydration products on the original cement grain makes the diffusion of water to unhydrated nucleus more and more difficult. This reduce the rate of hydration with time.
At any stage of hydration, the cement paste consists of gel, the unreacted cement, calcium hydroxide, water, and some minor compounds.
The crystals of the various resulting compounds gradually fill the space originally occupied by water. This result in the stiffening of the mass and the subsequent development of strength.
The Chemical reactions of the compounds and their products
The product C–S–H gel represents the calcium silicate hydrate also known as Tobermorite Gel, which is the gel structure.
The C–S–H phase makes up 50–60% of the volume of solids in a completely hydrated Portland cement paste.
The Ca(OH)2 liberated during the silicate phase crystallizes in the available free space.
The calcium hydroxide crystals, also known as portlandite, make up 20-25% of the volume of solids in the hydrated paste.
Crystals with lower surface area have limited potential to contribute to strength. For hydration to continue, it is necessary to saturate the gel with water.
The calcium hydroxide crystals formed in the process dissolve in water, providing hydroxyl (OH–) ions, which are important for the protection of reinforcement in concrete.
As hydration proceeds, the two crystal types become more heavily interlocked, increasing the strength.
However, the main cementing action is provided by the gel, which occupies two-thirds of the total mass of hydrate.
Comparatively, the hydration of C3S produces less calcium silicate hydrate and more Ca(OH)2 than the hydration of C2S.
Since Ca(OH)2 is soluble in water and leaches out, making the concrete porous, especially in hydraulic structures, experts recommend using a cement with a lower percentage of C3S and a higher percentage of C2S for hydraulic structures.
It’s important to note that the setting and hardening of cement is a chemical reaction, wherein water plays an important role, and is not just a matter of drying out.
Rate of hydration
Chemistry behind Hydration
Upon the reaction of the powder mix of the cement with water, a solid, structural material is formed. The key products formed along with calcium silicate hydrate (C-S-H) gel are calcium hydroxide and other by-products that set the scene for hardening and strength development in concrete.
Factors Affecting Hydration
The major factors that influence the rate of hydration, setting times, and ultimate strength include cement composition, the water-to-cement ratio, and the environment of curing. These factors help in optimum performance of cement in construction works.
Role of Cement Compounds in Hydration
- C3A: C3A induces rapid setting; its hydration can partly reduce the hydration of C3S and C2S. On the reverse, this occurs during cement manufacture where calcium sulfate is added to the barrier created to limit reactivity of C3A and ensure controlled setting.
- C3S: Its main strength contributor is within the first 28 days, in particular with the C-S-H gel. The interlocking growth ensures the early gain in strength; hence, this is the highlight of cement chemistry.
- C2S: Although the hydration of C2S is slower than most other substances, the function of C2S in long-term development of strength becomes pronounced, showing the importance of balance of compound ratios in obtaining the desired performance outcomes.
Hydration and Heat Evolution
The heat evolved from hydration provides a clue to the reactivity of each compound. In the generation of heat, C3A predominates over C3S, C4AF, and C2S in succession. This thermal footprint guides in anticipating the setting times and the early strength indicators.
Water requirement for hydration
For Portland cement, about 23% of water by weight of cement is required for complete hydration and is known as bound water.
About 15% of water by weight of cement is needed to fill the pores in the cement gel, and this is known as gel water.
Therefore, a total of 38% of water by weight of cement is required to complete the chemical reaction.
It is a common belief that a water/cement ratio of less than 0.38 should not be used in concrete because the gel pores should be saturated for the process of hydration to occur.
In addition to the amount of water required, the quality of water used is also important.
Water used in concrete mixtures must be clean, free of impurities, and suitable for drinking.
Impurities in water can affect the setting and hardening of the concrete and may even cause corrosion of the reinforcement.
The temperature and humidity of the environment can also affect the water requirement for hydration.
In hot and dry weather conditions, the water in the mix can evaporate quickly, leading to insufficient hydration.
This can cause the concrete to crack and weaken over time.
In contrast, in cold and wet weather conditions, the water in the mix can freeze and expand, causing the concrete to crack.
Conclusion:
Cement is an essential component of modern construction, and its history can be traced back to ancient times.
Portland cement is the most commonly used type of cement in construction today and is made by heating a mixture of limestone and clay or shale.
The chemical composition of Portland cement determines its properties, and the hydration process is crucial for the hardening of cement.
The use of cement has revolutionized the construction industry, allowing for the construction of structures that are stronger, more durable, and longer-lasting than ever before.
We hope this blog post has provided you with a deeper understanding of cement and its chemical composition.