Setting Of Cement Is Which Type Of Reaction?

Setting Of Cement Is Which Type Of Reaction
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.

Is setting of cement an endothermic reaction?

Setting of cement is an endothermic process.

Is setting of cement is exothermic process?

During the setting of cement heat is liberated as water reacts with cement so it is an exothermic process.

What is the process of setting of cement?

The setting of cement and hardening of concrete – planete-tp : All about public works Cement hardens when it comes into contact with water. This hardening is a process of crystallization. Crystals form (after a certain length of time which is known as the initial set time) and interlock with each other.

  1. Concrete is completely fluid before the cement sets, then progressively hardens.
  2. The cement and water mixture that has crystallized in this way encloses the aggregate particles and produces a dense material.
  3. The concrete continues to harden over several months.
  4. Hardening is not a drying process and can very well take place in water.

Heat speeds up the setting and hardening of cement, and cold slows it down and can even completely stop the processes. Setting Of Cement Is Which Type Of Reaction In order to crystallize or hydrate) cement requires a quantity of water equal to 25% of its weight. But in order for it to be laid and remain sufficiently workable, twice this amount is usually required. However, too much water can reduce the strength and durability of the concrete.

What is the reaction in 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
Gypsum 5% CaSO4 H2O

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.

  1. In addition to these major compounds, several other play important roles in the hydration process.
  2. Different types of cement contain the same four major compounds, but in different proportions.
  3. The cement in concrete needs water to hydrate and harden.
  4. 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.

  1. Tricalcium silicates are responsible for most of the early strength (first seven days).
  2. The original dicalcium silicate hydrates, which form more slowly, contribute to the strength of concrete at later stages.
  3. The following word equations describe the production of concrete.
  4. 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.

  1. The engineering properties of concrete—setting and hardening, strength and dimensional stability—depend primarily on calcium silicate hydrate gel.
  2. It is the heart of concrete.
  3. 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.

Which cement has an exothermic reaction?

Hydraulic cements, including Portland cement, set and harden when mixed with water at room temperature Cement Hydration Hydraulic cements, including portland cement, set and harden when mixed with water at room temperature. The reactions that cause setting and hardening are collectively described as exothermic hydration reactions.

The reaction of portland cement compounds with water is exothermic; that is, heat is generated from the reaction. The average is 120 calories per gram during complete hydration of the cement. In normal construction, structural members have relatively high surface-to-volume ratios such that the dissipation of the heat generated is not a problem.

By insulating the forms, this heat can be used as an advantage during cold weather to maintain proper curing temperatures. However for dams, massive foundations, and other mass concrete structures, measures must be taken to reduce or remove heat by proper design and construction methods.

  • This may involve circulating cold water in embedded pipe coils or other cooling means.
  • Another method of controlling heat evolution is to reduce the percentage of compounds with high heat of hydration, such as C 3 A and C 3 S, and use a coarser fineness to produce a Type IV low heat of hydration cement.

Since Type IV is generally no longer available in most locations, Type II and pozzolans or slag are used as a substitute. The use of large aggregate (nominal diameter greater than 150 millimeters) also helps to reduce the cement requirement and consequent heat by reducing the water demand, hence, less cement at the same water cement ratio.

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Which reactions are endothermic reactions?

What are Endothermic Reactions? (with Examples & Video) Endothermic reactions are chemical reactions in which the reactants absorb heat energy from the surroundings to form products. These reactions lower the temperature of their surrounding area, thereby creating a cooling effect. Setting Of Cement Is Which Type Of Reaction When a chemical bond is broken, it is usually accompanied by a release of energy. Similarly, the formation of chemical bonds requires an input of energy. The energy supplied/released can be of various forms (such as heat, light, and electricity). Endothermic reactions generally involve the formation of chemical bonds through the absorption of heat from the surroundings.

Which process is exothermic reaction?

(i) Reaction of water with quicklime. (ii) Dilution of an acid. (iii) Evaporation of water. (iv) Sublimation of camphor (crystals)

Which process is called exothermic?

The process in which heat is liberated along with the formation of products is known as exothermic process. Heat is released during respiration and burning.

Which process is an exothermic process?

Summary –

  • The law of conservation of energy states that in any physical or chemical process, energy is neither created nor destroyed.
  • A specific portion of matter in a given space that is being studied during an experiment or an observation is the system,
  • The surroundings is everything in the universe that is not part of the system.
  • A chemical reaction or physical change is endothermic if heat is absorbed by the system from the surroundings.
  • A reaction or change is exothermic if heat is released by the system into the surroundings.

Is setting of cement a physical change?

what chemical change takes place while setting of cement Water and cement initially form a cement paste that begins to react and harden (set). This paste binds the aggregate particles through the chemical process of hydration. In the hydration of cement, chemical changes occur slowly, eventually creating new crystalline products, heat evolution, and other measurable signs.

Which is the process of setting cement under water?

Solution : Process of setting of cement under water is essentially an hydration process.

What is meant by setting time of cement?

FAQ – What is initial & final setting time of cement? Initial setting time of cement is the time at which the cement paste starts losing its plasticity. Final setting time of cement is the time when the cement paste totally loses its plasticity. What is the initial setting time of cement? The time when water is added to cement to the time when the cement starts losing its plasticity is called initial setting time.

Initial setting time is the time when water is added to cement until the moment when the square needle penetrates to a depth of 33-35 mm from the top of the mould. Why is the initial setting of cement 30 minutes? The initial setting time of cement should not be less than 30 minutes because the time is required for handling operations of cement like mixing, placing, finishing, etc.

The cement operations cannot be carried out once the initial setting of cement occurs as cement paste starts losing plasticity which will then affect the strength gaining of the paste. What is cement setting? Upon addition of water, the cement paste formed remains plastic for some time.

What are the 4 types of reactions?

How Many Types of Chemical Reactions Are There? – Setting Of Cement Is Which Type Of Reaction Types of Chemical Reactions Worksheet Technically, there are hundreds or even thousands of different types of chemical reactions. However, chemistry students usually learn to classify them as 4 main types, 5 main types, or 6 main types. The four main types of chemical reactions are synthesis, decomposition, single displacement, and double displacement.

Is concrete setting a chemical change?

Water and cement initially form a cement paste that begins to react and harden (set). This paste binds the aggregate particles through the chemical process of hydration. In the hydration of cement, chemical changes occur slowly, eventually creating new crystalline products, heat evolution, and other measurable signs.

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What is an example of exothermic reaction?

Everyday Examples of Exothermic Reactions – Did you know that several exothermic reactions happen during your morning routine? Brushing your teeth, washing your hair, and lighting your stove are all examples of exothermic reactions. Keep reading to learn about combustion, neutralization, corrosion, and water-based exothermic reactions.

What are 3 examples of exothermic reactions?

Examples of exothermic reaction are: Rusting of iron. Setting of cement. Human body.

What is a common example of an exothermic reaction?

Setting Of Cement Is Which Type Of Reaction An exothermic reaction is defined as a reaction that releases heat and has a net negative standard enthalpy change. Examples include any combustion process, rusting of iron, and freezing of water. Exothermic reactions are reactions that release energy into the environment in the form of heat. Exothermic reactions feel warm or hot or may even be explosive. More energy is released making chemical bonds than is used breaking them. In an exothermic reaction, the enthalpy change has a negative value: ΔH < 0

What is the best example of an endothermic reaction?

What are the two main types of thermodynamic reactions? – Exothermic reactions are reactions that release energy in the form of heat. You are probably familiar with many examples of these reactions. For example, burning gasoline in a car’s engine is an exothermic reaction.

  1. This particular type of exothermic reaction is known as a combustion reaction,
  2. A combustion reaction occurs when a compound, such as the hydrocarbons that make up fuel, react with oxygen to form a new product and produce heat.
  3. Endothermic reactions are the opposite of exothermic reactions.
  4. They absorb heat energy from their surroundings.

This means that the surroundings of endothermic reactions are colder as a result of the reaction. Melting ice is an example of this type of reaction.

What is endothermic reaction give example?

What are endothermic reactions ? Give an example. Text Solution Solution : Exothermic reactions : Those chemical reactions which occur by the evolution of hert are called exothemic reactions, For example, when one mole of solid carbon burns in excess of oxygen at constant temperature and at constant vlome, 393.5 kj heat is given out.

  • `C(s) + O_(2) (g) to CO_(2) (g),Delta H = – 393.5 kj` In exothermic reactions, the heat liberated is given by negative sign.
  • Endothermic reactions : Those chemical reactions which occur by absorption of heart are know as endothermic reactions.
  • For example, when one mole of nitrogen reacts with one mole of `O_(2)` at constant volume 180 kj heat is absorbed.

`therefore` Reaction is represented as `N_(2) (g) + O_(2)(g) to 2NO(g),Delta H = + 180 kj` In eddothermic reaction, heat absorted by the reacting substance is given by porisitive sign. : What are endothermic reactions ? Give an example.

Is concrete hardening an exothermic reaction?

The heat produced by concrete during concrete curing is called heat of hydration. This exothermic reaction occurs when water and cement react. The amount of heat produced during the reaction is mostly related to the composition and fineness of the cement.

Is concrete setting a chemical reaction?

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.
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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.

  1. Building of the molds and casting can occur on the work-site which reduces costs.
  2. Concrete is a non-combustible material which makes it fire-safe and able withstand high temperatures.
  3. It is resistant to wind, water, rodents, and insects.
  4. Hence, concrete is often used for storm shelters.
  5. 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.

  1. The dormancy period can last from one to three hours.
  2. During this period, the concrete is in a plastic state which allows the concrete to be transported and placed without any major difficulty.
  3. This is particularly important for the construction trade who must transport concrete to the job site.
  4. It is at the end of this stage that initial setting begins.
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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.

TYPE FUNCTION
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.

How does temperature affect setting of cement?

Setting Time of Concrete Setting Of Cement Is Which Type Of Reaction The practical use of concrete as a construction material depends upon the fact that it is “plastic” in the freshly mixed state and subsequently becomes hard, with considerable strength. This change in its physical properties is due to the chemical reaction between cement and water, a process known as hydration.

Hydration involves chemical changes, not just a drying out of the material. Hydration is irreversible. The reaction is gradual, first causing stiffening of the concrete, and then development of strength, which continues for a very long time. Under certain ideal conditions it is probable that concrete would continue to increase in strength indefinitely.

Air temperature, ground temperature and weather conditions all play major roles in the rate with which cement hydrates. The setting time of concrete decreases with a rise in temperature, but above 30 o C a reverse effect could be observed. At low temperatures setting time is retarded.

  • Proper curing techniques and site preparation will aid the setting time.
  • However when concrete is being used in times of temperature extremes, i.e.
  • Colder weather or in the middle of summer, several admixtures may be used in the concrete mix to aid in the placement of the final product.
  • These admixtures are Accelerators and Retarders.

Accelerators have been designed to significantly boost the early setting times and increase the early age strengths of concrete. Setting times of non accelerated concrete are significantly slower as temperatures get colder. This obviously affects finishing times.

The action of the accelerator counters this set retardation and shortens setting times back to what is considered a normal set time. Retarders are designed for use in areas where early setting of concrete is not preferable, e.g. high ambient temperatures, long travel times between concrete plant and job site, large slow pours – to prevent formation of cold joints etc.

The chemical composition of the retarder is formulated to temporarily stop the action of hydration, delaying the initial set of the concrete. This delay is proportional to the dose of retarder used. Once the effect of the retarder wears off initial set will take place and hardening will develop at an accelerated rate.

The two graphs below demonstrate the effects of accelerators and retarders on the setting time of concrete. Allied Concrete staff are more than happy to help you with any problems or enquiries.

For more information or assistance, please don’t hesitate to call. Your call will be automatically connected to our nearest plant. (Calls from mobile phones will be directed for Auckland, Wellington or Christchurch.)