What is in This Stuff? The importance of concrete in modern society cannot be overestimated. Look around you and you will find concrete structures everywhere such as buildings, roads, bridges, and dams. There is no escaping the impact concrete makes on your everyday life.
So what is it? Concrete is a composite material which is made up of a filler and a binder. The binder (cement paste) “glues” the filler together to form a synthetic conglomerate. The constituents used for the binder are cement and water, while the filler can be fine or coarse aggregate. The role of these constituents will be discussed in this section.
Cement, as it is commonly known, is a mixture of compounds made by burning limestone and clay together at very high temperatures ranging from 1400 to 1600 ]C. Although there are other cements for special purposes, this module will focus solely on portland cement and its properties.
The production of portland cement begins with the quarrying of limestone, CaCO 3, Huge crushers break the blasted limestone into small pieces. The crushed limestone is then mixed with clay (or shale), sand, and iron ore and ground together to form a homogeneous powder. However, this powder is microscopically heterogeneous.
(See flowchart.) Figure 1: A flow diagram of Portland Cement production. The mixture is heated in kilns that are long rotating steel cylinders on an incline. The kilns may be up to 6 meters in diameter and 180 meters in length. The mixture of raw materials enters at the high end of the cylinder and slowly moves along the length of the kiln due to the constant rotation and inclination. Figure 2: Schematic diagram of rotary kiln. As the mixture moves down the cylinder, it progresses through four stages of transformation. Initially, any free water in the powder is lost by evaporation. Next, decomposition occurs from the loss of bound water and carbon dioxide.
This is called calcination, The third stage is called clinkering. During this stage, the calcium silicates are formed. The final stage is the cooling stage. The marble-sized pieces produced by the kiln are referred to as clinker, Clinker is actually a mixture of four compounds which will be discussed later.
The clinker is cooled, ground, and mixed with a small amount of gypsum (which regulates setting) to produce the general-purpose portland cement. Water is the key ingredient, which when mixed with cement, forms a paste that binds the aggregate together.
The water causes the hardening of concrete through a process called hydration. Hydration is a chemical reaction in which the major compounds in cement form chemical bonds with water molecules and become hydrates or hydration products. Details of the hydration process are explored in the next section. The water needs to be pure in order to prevent side reactions from occurring which may weaken the concrete or otherwise interfere with the hydration process.
The role of water is important because the water to cement ratio is the most critical factor in the production of “perfect” concrete. Too much water reduces concrete strength, while too little will make the concrete unworkable. Concrete needs to be workable so that it may be consolidated and shaped into different forms (i.e.
Walls, domes, etc.). Because concrete must be both strong and workable, a careful balance of the cement to water ratio is required when making concrete. Aggregates are chemically inert, solid bodies held together by the cement. Aggregates come in various shapes, sizes, and materials ranging from fine particles of sand to large, coarse rocks.
Because cement is the most expensive ingredient in making concrete, it is desirable to minimize the amount of cement used.70 to 80% of the volume of concrete is aggregate keeping the cost of the concrete low. The selection of an aggregate is determined, in part, by the desired characteristics of the concrete.
For example, the density of concrete is determined by the density of the aggregate. Soft, porous aggregates can result in weak concrete with low wear resistance, while using hard aggregates can make strong concrete with a high resistance to abrasion. Aggregates should be clean, hard, and strong. The aggregate is usually washed to remove any dust, silt, clay, organic matter, or other impurities that would interfere with the bonding reaction with the cement paste.
It is then separated into various sizes by passing the material through a series of screens with different size openings. Refer to Demonstration 1 Table 1: Classes of Aggregates
|class||examples of aggregates used||uses|
|ultra-lightweight||vermiculite ceramic spheres perlite||lightweight concrete which can be sawed or nailed, also for its insulating properties|
|lightweight||expanded clay shale or slate crushed brick||used primarily for making lightweight concrete for structures, also used for its insulating properties.|
|normal weight||crushed limestone sand river gravel crushed recycled concrete||used for normal concrete projects|
|heavyweight||steel or iron shot steel or iron pellets||used for making high density concrete for shielding against nuclear radiation|
Refer to Demonstration 2 The choice of aggregate is determined by the proposed use of the concrete. Normally sand, gravel, and crushed stone are used as aggregates to make concrete. The aggregate should be well-graded to improve packing efficiency and minimize the amount of cement paste needed.
Also, this makes the concrete more workable. Refer to Demonstration 3 Properties of Concrete Concrete has many properties that make it a popular construction material. The correct proportion of ingredients, placement, and curing are needed in order for these properties to be optimal. Good-quality concrete has many advantages that add to its popularity.
First, it is economical when ingredients are readily available. Concrete’s long life and relatively low maintenance requirements increase its economic benefits. Concrete is not as likely to rot, corrode, or decay as other building materials. Concrete has the ability to be molded or cast into almost any desired shape.
Building of the molds and casting can occur on the work-site which reduces costs. Concrete is a non-combustible material which makes it fire-safe and able withstand high temperatures. It is resistant to wind, water, rodents, and insects. Hence, concrete is often used for storm shelters. Concrete does have some limitations despite its numerous advantages.
Concrete has a relatively low tensile strength (compared to other building materials), low ductility, low strength-to-weight ratio, and is susceptible to cracking. Concrete remains the material of choice for many applications regardless of these limitations.
Hydration of Portland Cement Concrete is prepared by mixing cement, water, and aggregate together to make a workable paste. It is molded or placed as desired, consolidated, and then left to harden. Concrete does not need to dry out in order to harden as commonly thought. The concrete (or specifically, the cement in it) needs moisture to hydrate and cure (harden).
When concrete dries, it actually stops getting stronger. Concrete with too little water may be dry but is not fully reacted. The properties of such a concrete would be less than that of a wet concrete. The reaction of water with the cement in concrete is extremely important to its properties and reactions may continue for many years.
|Cement Compound||Weight Percentage||Chemical Formula|
|Tricalcium silicate||50 %||Ca 3 SiO 5 or 3CaO, SiO 2|
|Dicalcium silicate||25 %||Ca 2 SiO 4 or 2CaO, SiO 2|
|Tricalcium aluminate||10 %||Ca 3 Al 2 O 6 or 3CaO, Al 2 O 3|
|Tetracalcium aluminoferrite||10 %||Ca 4 Al 2 Fe 2 O 10 or 4CaO, Al 2 O 3, Fe 2 O 3|
|Gypsum||5 %||CaSO 4,2H 2 O|
Table 2: Composition of portland cement with chemical composition and weight percent. When water is added to cement, each of the compounds undergoes hydration and contributes to the final concrete product. Only the calcium silicates contribute to strength. Tricalcium silicate is responsible for most of the early strength (first 7 days). Dicalcium silicate, which reacts more slowly, contributes only to the strength at later times. Tricalcium silicate will be discussed in the greatest detail. The equation for the hydration of tricalcium silicate is given by: Tricalcium silicate + Water->Calcium silicate hydrate+Calcium hydroxide + heat 2 Ca 3 SiO 5 + 7 H 2 O -> 3 CaO,2SiO 2,4H 2 O + 3 Ca(OH) 2 + 173.6kJ Upon the addition of water, tricalcium silicate rapidly reacts to release calcium ions, hydroxide ions, and a large amount of heat. The pH quickly rises to over 12 because of the release of alkaline hydroxide (OH – ) ions. This initial hydrolysis slows down quickly after it starts resulting in a decrease in heat evolved. The reaction slowly continues producing calcium and hydroxide ions until the system becomes saturated. Once this occurs, the calcium hydroxide starts to crystallize. Simultaneously, calcium silicate hydrate begins to form. Ions precipitate out of solution accelerating the reaction of tricalcium silicate to calcium and hydroxide ions. (Le Chatlier’s principle). The evolution of heat is then dramatically increased. The formation of the calcium hydroxide and calcium silicate hydrate crystals provide “seeds” upon which more calcium silicate hydrate can form. The calcium silicate hydrate crystals grow thicker making it more difficult for water molecules to reach the unhydrated tricalcium silicate. The speed of the reaction is now controlled by the rate at which water molecules diffuse through the calcium silicate hydrate coating. This coating thickens over time causing the production of calcium silicate hydrate to become slower and slower. Figure 3: Schematic illustration of the pores in calcium silicate through different stages of hydration. The above diagrams represent the formation of pores as calcium silicate hydrate is formed. Note in diagram (a) that hydration has not yet occurred and the pores (empty spaces between grains) are filled with water. Diagram (b) represents the beginning of hydration. In diagram (c), the hydration continues. Although empty spaces still exist, they are filled with water and calcium hydroxide. Diagram (d) shows nearly hardened cement paste. Note that the majority of space is filled with calcium silicate hydrate. That which is not filled with the hardened hydrate is primarily calcium hydroxide solution. The hydration will continue as long as water is present and there are still unhydrated compounds in the cement paste. Dicalcium silicate also affects the strength of concrete through its hydration. Dicalcium silicate reacts with water in a similar manner compared to tricalcium silicate, but much more slowly. The heat released is less than that by the hydration of tricalcium silicate because the dicalcium silicate is much less reactive. The products from the hydration of dicalcium silicate are the same as those for tricalcium silicate: Dicalcium silicate + Water->Calcium silicate hydrate + Calcium hydroxide +heat 2 Ca 2 SiO 4 + 5 H 2 O-> 3 CaO,2SiO 2,4H 2 O + Ca(OH) 2 + 58.6 kJ The other major components of portland cement, tricalcium aluminate and tetracalcium aluminoferrite also react with water. Their hydration chemistry is more complicated as they involve reactions with the gypsum as well. Because these reactions do not contribute significantly to strength, they will be neglected in this discussion. Although we have treated the hydration of each cement compound independently, this is not completely accurate. The rate of hydration of a compound may be affected by varying the concentration of another. In general, the rates of hydration during the first few days ranked from fastest to slowest are: tricalcium aluminate > tricalcium silicate > tetracalcium aluminoferrite > dicalcium silicate. Refer to Demonstration 4 Heat is evolved with cement hydration. This is due to the breaking and making of chemical bonds during hydration. The heat generated is shown below as a function of time. Figure 4: Rate of heat evolution during the hydration of portland cement The stage I hydrolysis of the cement compounds occurs rapidly with a temperature increase of several degrees. Stage II is known as the dormancy period. The evolution of heat slows dramatically in this stage.
The dormancy period can last from one to three hours. During this period, the concrete is in a plastic state which allows the concrete to be transported and placed without any major difficulty. This is particularly important for the construction trade who must transport concrete to the job site. It is at the end of this stage that initial setting begins.
In stages III and IV, the concrete starts to harden and the heat evolution increases due primarily to the hydration of tricalcium silicate. Stage V is reached after 36 hours. The slow formation of hydrate products occurs and continues as long as water and unhydrated silicates are present.
Refer to Demonstration 5 Strength of Concrete The strength of concrete is very much dependent upon the hydration reaction just discussed. Water plays a critical role, particularly the amount used. The strength of concrete increases when less water is used to make concrete. The hydration reaction itself consumes a specific amount of water.
Concrete is actually mixed with more water than is needed for the hydration reactions. This extra water is added to give concrete sufficient workability. Flowing concrete is desired to achieve proper filling and composition of the forms, The water not consumed in the hydration reaction will remain in the microstructure pore space. Figure 5: Schematic drawings to demonstrate the relationship between the water/cement ratio and porosity. The empty space (porosity) is determined by the water to cement ratio. The relationship between the water to cement ratio and strength is shown in the graph that follows. Figure 6: A plot of concrete strength as a function of the water to cement ratio. Low water to cement ratio leads to high strength but low workability. High water to cement ratio leads to low strength, but good workability. The physical characteristics of aggregates are shape, texture, and size.
These can indirectly affect strength because they affect the workability of the concrete. If the aggregate makes the concrete unworkable, the contractor is likely to add more water which will weaken the concrete by increasing the water to cement mass ratio. Time is also an important factor in determining concrete strength.
Concrete hardens as time passes. Why? Remember the hydration reactions get slower and slower as the tricalcium silicate hydrate forms. It takes a great deal of time (even years!) for all of the bonds to form which determine concrete’s strength. It is common to use a 28-day test to determine the relative strength of concrete.
- Concrete’s strength may also be affected by the addition of admixtures.
- Admixtures are substances other than the key ingredients or reinforcements which are added during the mixing process.
- Some admixtures add fluidity to concrete while requiring less water to be used.
- An example of an admixture which affects strength is superplasticizer.
This makes concrete more workable or fluid without adding excess water. A list of some other admixtures and their functions is given below. Note that not all admixtures increase concrete strength. The selection and use of an admixture are based on the need of the concrete user.
|AIR ENTRAINING||improves durability, workability, reduces bleeding, reduces freezing/thawing problems (e.g. special detergents)|
|SUPERPLASTICIZERS||increase strength by decreasing water needed for workable concrete (e.g. special polymers)|
|RETARDING||delays setting time, more long term strength, offsets adverse high temp. weather (e.g. sugar )|
|ACCELERATING||speeds setting time, more early strength, offsets adverse low temp. weather (e.g. calcium chloride)|
|MINERAL ADMIXTURES||improves workability, plasticity, strength (e.g. fly ash)|
|PIGMENT||adds color (e.g. metal oxides)|
Table 3: A table of admixtures and their functions. Durability is a very important concern in using concrete for a given application. Concrete provides good performance through the service life of the structure when concrete is mixed properly and care is taken in curing it.
Good concrete can have an infinite life span under the right conditions. Water, although important for concrete hydration and hardening, can also play a role in decreased durability once the structure is built. This is because water can transport harmful chemicals to the interior of the concrete leading to various forms of deterioration.
Such deterioration ultimately adds costs due to maintenance and repair of the concrete structure. The contractor should be able to account for environmental factors and produce a durable concrete structure if these factors are considered when building concrete structures.
- 1 What is meant by pozzolanic reaction?
- 2 Which chemical is used in making cement?
- 3 What chemical reactions cause the setting and hardening of cement?
- 4 Why pozzolana is used in cement?
- 5 What are 3 types of reactions?
What is the chemical reaction of cement?
Background – Concrete is made by the combination of cement, water, and aggregate of various sizes to make a workable slurry that has the consistency of a thick milk shake.
|Name||Percent by Weight||Chemical Formula|
|Tricalcium silicate||50%||3Ca0 SiO2|
|Dicalcium silicate||25%||2Ca0 SiO2|
|Tricalcium aluminate||10%||3Ca0 Al2 O3|
|Tetracalcium aluminoferrite||10%||4Ca0 Al2 Fe2 O3|
The binding quality of portland cement paste is due to the chemical reaction between the cement and water, called hydration. Portland cement is not a simple chemical compound, it is a mixture of many compounds. Four of these make up 90% or more of the weight of portland cement: tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite.
In addition to these major compounds, several other play important roles in the hydration process. Different types of cement contain the same four major compounds, but in different proportions. The cement in concrete needs water to hydrate and harden. Even though the chemical reactions may be complete at the surface of the concrete, the chemical reactions at the interior of the concrete take much longer to complete.
The strength of the concrete keeps growing as long as the chemical reactions continue. When water is added to cement, the chemical reaction called hydration takes place and contributes to the final concrete product. The calcium silicates contribute most to the strength of concrete.
- Tricalcium silicates are responsible for most of the early strength (first seven days).
- The original dicalcium silicate hydrates, which form more slowly, contribute to the strength of concrete at later stages.
- The following word equations describe the production of concrete.
- Tricalcium silicate + Water (yields) Calcium silicate hydrate + Calcium hydroxide + heat Dicalcium silicate + Water (yields) Calcium silicate hydrate + Calcium hydroxide + heat Of the five chemical reactions important for providing the strength for concrete the above reactions are the most important.
The two calcium silicates, which constitute about 75 percent of the weight of portland cement, react with water to form two new compounds: calcium hydroxide and calcium silicate hydrate. The latter is by far the most important cementing component in concrete.
- The engineering properties of concrete—setting and hardening, strength and dimensional stability—depend primarily on calcium silicate hydrate gel.
- It is the heart of concrete.
- When concrete sets, its gross volume remains almost unchanged, but hardened concrete contains pores filled with water and air that have no strength.
The strength is in the solid part of the paste, mostly in the calcium silicate hydrate and crystalline phases. The less porous the cement paste, the stronger the concrete. When mixing concrete, therefore, use no more water than is absolutely necessary to make the concrete plastic and workable.
What type of reaction is setting of cement?
Setting of cement is:(A) exothermic reaction (B) endothermic reaction(C) neither endothermic nor exothermic (D) none of these. Answer Verified Hint: Setting is called as the action of changing from a fluid state to a solid state. During setting when water reacts with cement it liberates heat.
Complete step by step answer: So, the correct answer is Option A. Note: Wet cement is strongly corrosive and can cause severe skin burns and even if they come in contact with mucous membranes it can cause severe eye or respiratory irritation.
Cement is known to a binder that is used in construction sites.Depending upon the ability of the cement to set in the presence of water, cement is divided into:(1). Non-hydraulic cement(2). Hydraulic cementNon – hydraulic cement is that which does not set in wet conditions or under water.
It sets as it dries and reacts with carbon dioxide in the air. It does not get attacked by the chemicals after setting. Hydraulic cement is that which sets and becomes adhesive due to a reaction with dry ingredients and water. Hydraulic cements consist of a mixture of silicates and oxides. The process of formation of cement includes the calcination process of limestone that is calcium carbonate.
The reaction of burning of limestone is:$ } }_ }} \to } }_ }}$From this reaction carbon is removed from limestone and the formation of lime occurs. Then, after this lime reacts with silicon dioxide to produce dicalcium silicate and tricalcium silicate.
The reaction can be written as:$ } }_ }} \to } } }_ }} \\ } }_ }} \to } } }_ }} \\ $Then, lime reacts with aluminium oxide to form tricalcium aluminate.$ } }_ }} }_ }} \to } } }_ }} }_ }}$At last, calcium oxide, aluminium oxide and ferric oxide react together to form cement.$ } }_ }} }_ }} } }_ }} }_ }} \to } } }_ }} }_ }} } }_ }} }_ }}$When cement is mixed with water it starts to set and causes hydration chemical reactions.
The hydration of the constituents occurs slowly and the material solidly and hardens.$CaO.A + 6 O \to 3CaO.A,6 O + \operatorname $ During setting and hardening of cement, some amount of heat is liberated due to hydration and the chemical reactions that occur.As the release of heat takes place.
What is meant by pozzolanic reaction?
Pozzolanic reaction is defined as the chemical reaction between reactive silica or alumina present in the FA particles and portlandite formed during the cement hydration in the presence of water at ambient temperature. From: Handbook of Fly Ash, 2022.
What is the chemical equation for making cement?
Chemical Formulas of Cement Materials
|C 3 S||3CaO·SiO 2 = tricalcium silicate = alite|
|C 2 S||2CaO·SiO 2 = dicalcium silicate = belite|
|C 3 A||3CaO·Al 2 O 3 = tricalcium aluminate|
Which chemical is used in making cement?
Visit ShapedbyConcrete.com to learn more about how cement and concrete shape the world around us. Portland cement is the basic ingredient of concrete. Concrete is formed when portland cement creates a paste with water that binds with sand and rock to harden.
- Cement is manufactured through a closely controlled chemical combination of calcium, silicon, aluminum, iron and other ingredients.
- Common materials used to manufacture cement include limestone, shells, and chalk or marl combined with shale, clay, slate, blast furnace slag, silica sand, and iron ore.
These ingredients, when heated at high temperatures form a rock-like substance that is ground into the fine powder that we commonly think of as cement. Bricklayer Joseph Aspdin of Leeds, England first made portland cement early in the 19th century by burning powdered limestone and clay in his kitchen stove.
With this crude method, he laid the foundation for an industry that annually processes literally mountains of limestone, clay, cement rock, and other materials into a powder so fine it will pass through a sieve capable of holding water. Cement plant laboratories check each step in the manufacture of portland cement by frequent chemical and physical tests.
The labs also analyze and test the finished product to ensure that it complies with all industry specifications. The most common way to manufacture portland cement is through a dry method. The first step is to quarry the principal raw materials, mainly limestone, clay, and other materials.
- After quarrying the rock is crushed.
- This involves several stages.
- The first crushing reduces the rock to a maximum size of about 6 inches.
- The rock then goes to secondary crushers or hammer mills for reduction to about 3 inches or smaller.
- The crushed rock is combined with other ingredients such as iron ore or fly ash and ground, mixed, and fed to a cement kiln.
The cement kiln heats all the ingredients to about 2,700 degrees Fahrenheit in huge cylindrical steel rotary kilns lined with special firebrick. Kilns are frequently as much as 12 feet in diameter—large enough to accommodate an automobile and longer in many instances than the height of a 40-story building.
- The large kilns are mounted with the axis inclined slightly from the horizontal.
- The finely ground raw material or the slurry is fed into the higher end.
- At the lower end is a roaring blast of flame, produced by precisely controlled burning of powdered coal, oil, alternative fuels, or gas under forced draft.
As the material moves through the kiln, certain elements are driven off in the form of gases. The remaining elements unite to form a new substance called clinker. Clinker comes out of the kiln as grey balls, about the size of marbles. Clinker is discharged red-hot from the lower end of the kiln and generally is brought down to handling temperature in various types of coolers.
The heated air from the coolers is returned to the kilns, a process that saves fuel and increases burning efficiency. After the clinker is cooled, cement plants grind it and mix it with small amounts of gypsum and limestone. Cement is so fine that 1 pound of cement contains 150 billion grains. The cement is now ready for transport to ready-mix concrete companies to be used in a variety of construction projects.
Although the dry process is the most modern and popular way to manufacture cement, some kilns in the United States use a wet process. The two processes are essentially alike except in the wet process, the raw materials are ground with water before being fed into the kiln.
What chemical reactions cause the setting and hardening of cement?
The setting and hardening of cement is due to the formation of inter locking crystals reinforced by rigid gels formed by the hydration and hydrolysis of the constitutional compounds.
What are the types of reaction?
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- Classify a reaction as combination, decomposition, single-replacement, double-replacement, or combustion.
- Predict the products and balance a combustion reaction.
Many chemical reactions can be classified as one of five basic types. Having a thorough understanding of these types of reactions will be useful for predicting the products of an unknown reaction. The five basic types of chemical reactions are combination, decomposition, single-replacement, double-replacement, and combustion.
Why pozzolana is used in cement?
What does a pozzolan do in the concrete? – Q. What does a pozzolan do in the concrete? A. As the definition implies, a pozzolan combines with calcium hydroxide in the concrete to form calcium silicate hydrate, similar to that produced by hydration of portland cement.
This adds to the strength, impermeability, and sulfate resistance, and reduces expansion from the alkali-silica reaction that might otherwise take place. Use of pozzolans may increase or decrease water demand depending on the particle shape, surface texture, and fineness. Fly ash usually decreases water demand.
Most of the other pozzolans increase the water demand. Pozzolans reduce bleeding because of fineness; reduce the maximum rise in temperature when used in large amounts (more than 15% by mass of cementitious material) because of the slower rate of chemical reactions; which reduce the rise in temperature.
How is pozzolana formed?
Pozzolana, also spelled pozzuolana, hydraulic cement perfected by the Romans and still used in some countries, traditionally made by grinding a material of volcanic origin (the pozzolan) with powdered hydrated lime.
What is the process of making of cement?
Manufacture of cement – There are four stages in the manufacture of portland cement: (1) crushing and grinding the raw materials, (2) blending the materials in the correct proportions, (3) burning the prepared mix in a kiln, and (4) grinding the burned product, known as ” clinker,” together with some 5 percent of gypsum (to control the time of set of the cement).
- The three processes of manufacture are known as the wet, dry, and semidry processes and are so termed when the raw materials are ground wet and fed to the kiln as a slurry, ground dry and fed as a dry powder, or ground dry and then moistened to form nodules that are fed to the kiln.
- It is estimated that around 4–8 percent of the world’s carbon dioxide (CO 2 ) emissions come from the manufacture of cement, making it a major contributor to global warming,
Some of the solutions to these greenhouse gas emissions are common to other sectors, such as increasing the energy efficiency of cement plants, replacing fossil fuels with renewable energy, and capturing and storing the CO 2 that is emitted. In addition, given that a significant portion of the emissions are an intrinsic part of the production of clinker, novel cements and alternate formulations that reduce the need for clinker are an important area of focus.
What is cement hydration reaction?
18.1.2 Cement hydration – The addition of water to OPC powder commences immediately with the cement hydration reactions, These series of chemical reactions result in the subsequent setting and hardening of the cement paste. Needle-like crystals of calcium sulfoaluminate hydrate, namely ettringite, are formed within a few minutes.
- The ettringite subsequently transforms to monosulfate hydrate after a time.
- Two hours after the start of the cementation process, large prismatic crystals of calcium hydroxide (CH) and very small crystals of calcium silicate hydrates (C–S–H) begin to fill the empty pores previously occupied by water and the hydrated cement particles.
Therefore the major components of the hydrated cement paste are: Calcium silicate hydrate : This is the most important hydration product and forms nearly 60% of the volume of solids. It is formed by a layer of sponge-like structures with a large high surface area (~ 500 m 2 /g).
The end product strength is largely contributed by the C–S–H formation and is attributed mainly by their van der Waals physical adhesion forces. Calcium hydroxide : This is the second most abundant component with respect to the volume of solids and constitutes about 25%. It is formed of large plate-like crystals with a smaller surface area compared to C–S–H.
It contributes to limited van der Waal binding forces and is relatively highly soluble compared to C–S–H, and renders the concrete reactive to acidic solutions. Calcium sulfoaluminate : This has a minor role in the cementitious structure properties, and forms almost ~15% of the volume of solids.
|Component||Estimated reaction rate|
|3CaO·SiO 2||Fast relative to 2CaO·SiO 2|
|2CaO·SiO 2||Slow compared to 3CaO·SiO 2|
|3CaO·Al 2 O 3||Fastest, but addition of gypsum retards the rate|
|4CaO·Al 2 O 3 ·Fe 2 O 3||Relatively slow|
|CaSO 4 ·2H 2 O (gypsum)||Retardant|
The approximate hydration reactions between water and the cement components can be represented for each of the Portland cement major ingredient as follows. These equations are not stoichiometrically balanced due to the variations in the products formed and their compositions.
For C 3 S : C 3 S + 6 H 2 O → C 3 S 2,3 H 2 O + 3 Ca ( HO ) 2 For C 2 S : C 2 S + 4 H 2 O → C 3 S 2,3 H 2 O + 3 Ca ( HO ) 2 For C 3 A : C 3 A + 6 H 2 O → C 3 A,6 H 2 O For C 4 AF : 4 C 4 AF + 2 Ca ( OH ) 2 + 10 H 2 O → C 3 A,6 H 2 O + C 3 F,6 H 2 O It is clear that both silicates, C 3 S and C 2 S, need almost the same mass of water for hydration.
However, calcium hydroxide resulting from C 3 S hydration is more than twice that obtained from C 2 S hydration. It is worth stating that the reaction rate of C 3 A is quicker than that of calcium silicates, Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780128189610000181
What are 3 types of reactions?
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- Differentiate between substrate and product, and define chemical equation
- Define metabolism, synthesis (anabolic), decomposition (catabolic), and exchange reactions
- Differentiate between reversible reactions and irreversible reactions
- Explain dehydration synthesis and hydrolysis reactions
- Explain the relationship between monomers and polymers
Chemical reactions begin with one or more substances that enter into the reaction. The substances in our cells and body tissues that enter into the reaction are called substrates, The one or more substances produced by a chemical reaction are called products,
Chemical reactions are represented by chemical equations by placing the substrate(s) on the left and the product(s) on the right. Substrate(s) and product(s) are separated by an arrow (\(\rightarrow\)) which indicates the direction and type of the reaction. For example, lactose, the sugar found in milk, is broken down by our digestive system into two smaller sugars, glucose and galactose.
In this reaction, lactose is the substrate, and glucose and galactose are the products. The chemical equation for this reaction is: Lactose \(\rightarrow\) Glucose + Galactose Concepts, terms, and facts check Study Questions Write your answer in a sentence form (do not answer using loose words) 1.
- In a synthesis reaction (syn- = together; -thesis = “put, place, set”), two or more substrates molecules covalently bond to form a larger product molecule. Synthesis reactions require energy to form the bond(s). A synthesis reaction is often symbolized as A + B \(\rightarrow\) AB, where A and B are the substrates, and AB is the product. Synthesis reactions can also be called anabolic or constructive activities in a cell.
- In a decomposition reaction (de- off, away= -composition = “putting together, arranging”), covalent bonds between components of a larger substrate molecule are broken down to form smaller product molecules. Decomposition reactions release energy when covalent bonds in the substrate are broken down. A decomposition reaction is often symbolized as AB \(\rightarrow\) A + B; where AB is the substrate, and A and B are the products. Different types of decomposition reactions may also be referred to as digestion, hydrolysis, breakdown, and degradation reactions. Decomposition reactions are the basis of all catabolic, or breakdown activities in a cell.
- In an exchange reaction, covalent bonds are both broken down and then reformed in a way that the components of the substrates are rearranged to make different products. An exchange reaction is often symbolized as AB + CD \(\rightarrow\) AC + BD. In this exchange reaction, the covalent bonds between A and B, and between C and D were broken; and new covalent bonds between A and C, and B and D were formed.
Figure \(\PageIndex \) Representation of three types of chemical reactions. From top to bottom: synthesis, decomposition, and exchange. Concepts, terms, and facts check Study Questions Write your answer in a sentence form (do not answer using loose words) 1.
What is a synthesis reaction? 2. How can a synthesis reaction be represented by using letters? 3. What is an anabolic reaction? 4. What is a decomposition reaction? 5. How can a decomposition reaction be represented by using letters? 6. What is a catabolic reaction? 7. What is an exchange reaction? 8. How can an exchange reaction be represented by using letters? 9.
What is a metabolism reaction? 10. What is metabolism? Some metabolic reactions are called irreversible reactions, This means that the product(s) cannot be changed or “reversed” back into substrates. These reactions are represented with a single arrow as in A+B \(\rightarrow\) C.
For example: Glucose + Oxygen \(\rightarrow\) Carbon dioxide + Water Note: This is a type of catabolic reaction (the larger glucose molecule is broken down to smaller carbon dioxide molecules) related to cellular energy production. In animal cells, such as humans, this is an irreversible reaction. Other metabolic reactions are called reversible reactions,
This means that the reaction can proceed from substrates to product(s) or from product(s) back to substrates. The product(s) can be changed back into or “reversed” into substrates. They are represented with a double arrow as in A+B \(\leftrightarrow\) C+D.
- For example: Glycogen + Water \(\leftrightarrow\) Glucose Note: When cells need energy, glycogen (a larger molecule used as an energy store in some cells) can be catabolized to smaller glucose molecules, which can then be further catabolized to provide energy for cell functions.
- When cells do not need as much energy, or when glucose levels are very high, glycogen is synthesized from the smaller glucose molecules.
For example, muscle cells synthesize glycogen when resting and catabolize glycogen when contracting. Which way this reversible reaction proceeds depends on body needs. concepts,,terms, and facts check Study Questions Write your answer in a sentence form (do not answer using loose words) 1.
What is a reversible reaction? 2. What is an irreversible reaction? In the body, synthesis reactions (smaller molecules to larger molecule, requires energy) and decomposition reactions (larger molecule to smaller molecules, releases energy) are often associated with the formation and breakdown of water molecules, respectively.
A dehydration synthesis reaction is a type of synthesis reaction that makes water as a byproduct. A hydrolysis reaction is a type of decomposition reaction that uses water, In the dehydration synthesis (de- = “off, remove”; hydrate = “water”) shown in figure \(\PageIndex \), two monomers are covalently bonded in a reaction in which one gives up a hydroxyl ion (-OH – ) and the other a hydrogen ion (-H + ).
- Monomer 1 and monomer 2 are the substrates on the left, and the “monomers linked by a covalent bond” is the product on the right.
- The product shown here is also called a dimer (di- = two, mer = part).
- OH- and H+ combine to form a molecule of water, which is released as a byproduct.
- This can be confusing because water is made during dehydration synthesis.
The larger product has been dehydrated (lost the water). Figure \(\PageIndex \) Example of dehydration synthesis: two glucose molecules (substrates on the left of the arrow) form a covalent bond to form a maltose molecule (product on the right of the arrow). The OH- and H+ shown in red combine with each other to form H2O (shown in red too) In the hydrolysis reaction shown in figure \(\PageIndex \), (hydro- = “water”; -lysis = “breakingdown, a loosening, a dissolution”) the covalent bond between two monomers is split by the addition of a hydrogen ion (H + ) to one and a hydroxyl ion (OH – ) to the other.
These two ions come from splitting a water molecule, H 2 O, into H + and OH -, The dimer (monomers linked by a covalent bond on the left) is the substrate, and monomer 1 and monomer 2 on the right are the products. Concepts, terms, and facts check Study Questions Write your answer in a sentence form (do not answer using loose words) 1.
What is a dehydration synthesis reaction? 2. What is a hydrolysis reaction? Large molecules composed of hundreds or thousands of atoms are called macromolecules. Many macromolecules are composed of repetitive units of the same building block, similar to a pearl necklace that is composed of many pearls.
Polymers (poly- = “many”; meros = “part”) are long chain, large organic molecules (macromolecules) assembled from many covalently bonded smaller molecules called monomers, Polymers consist of many repeating monomer units in long chains, sometimes with branching or cross-linking between the chains. Three of the four classes of organic molecules previously identified, i.e.
carbohydrates, lipids, proteins, and nucleic acids are often polymers made of smaller monomer subunits (lipids are not). For example, proteins are polymers made of many covalently bonded smaller molecules, monomers, called amino acids. Each of these classes is considered in more detail below. Figure \(\PageIndex \) Two-dimensional view of the protein insulin. Insulin is a polymer made of covalently linked monomers called amino acids (shown as green balls). Concepts, terms, and facts check Study Questions Write your answer in a sentence form (do not answer using loose words) 1. What is a polymer? 2. What is a monomer?
What is chemical reaction example?
A chemical reaction is a process in which reactants react chemically and convert into products by chemical transformation. For example, Respiration – we inhale oxygen which reacts with glucose and produces carbon dioxide, water and energy. The reaction is given below. C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O + Energy
Is making cement a chemical change?
Chemical. Water evaporates as bonds form, heat is evolved, and cement crystals form to harden the concrete, thus both new physical and chemical properties form. The changes which can not be undone are called permanent changes. Cement, when it comes in contact with moisture, it gets hardened.