Tricalcium Aluminate Tricalcium Aluminate : C 3 A is formed within 24 hours of the addition of water in the cement and is responsible for maximum evolution of heat of hydration. It is the first compound that is formed after addition of water and sets early.
- 0.1 What is the mixture of cement and water called?
- 1 What type of chemical reaction is cement?
- 2 What is the most important hydration product of Portland cement?
- 3 What starts the hydration process in Portland cement?
- 4 Which of the ingredient is added in the concrete immediately before or during mixing?
What are the main compounds in Portland cement?
Chemical composition – Portland cement is made up of four main compounds: tricalcium silicate (3CaO · SiO 2 ), dicalcium silicate (2CaO · SiO 2 ), tricalcium aluminate (3CaO · Al 2 O 3 ), and a tetra-calcium aluminoferrite (4CaO · Al 2 O 3 Fe 2 O 3 ).
What is the mixture of cement and water called?
A mixture of cement, gravel, coarse sand and water is called:A. mortarB. concreteC. slurryD. hardener Answer Verified Hint: All the options given are building materials used in some way or the other. The answer can be found by knowing the compositions of these materials.
- Gravel is aggregation of rock fragments, coarse sand has higher grit than fine washed sand and cement is a binder material.
- Complete step by step answer: Let us discuss the options and understand their composition, to get a clear figure:A.
- Mortar: ‘Mortar’ word is derived from a Latin word which means crushed.
Mortar includes pitch and soft mud or clay like used between mud bricks. It is a paste which binds blocks of the building such as bricks and stones to fill the irregular gaps between the units and to add decorative patterns to masonry walls. Bricklayers make mortars using a mixture of sand, a binder and water.B.
Concrete: Concrete is a composite material which is composed of coarse and fine aggregate bounded together with cement paste and mixed with water that hardens with time. Also used sometimes with other hydraulic cements like with Portland cement to form Portland cement concrete.C. Slurry: Slurry is a mixture of solids denser than water suspended in aqueous solvents.
The size of solid particles varies from 1 micron to hundreds of millimeters. It is composed of glass beads in silicone oil which are flowing down an inclined plane.D. Hardener: It is the one that hardens a substance. A hardener is a component of certain types of mixtures and as composition is different, the property becomes different.
|Type of mixture||Applications of the mixtures|
|Mortar||Can be used as a glue-like structure for projects that require stone or ceramic installation.|
|Concrete||Used to provide tensile strength. It is one of the most frequently used building materials.|
|Slurry||Slurry is used as a means of transporting solids, the liquid being a carrier that is pumped on a device such as a centrifugal pump.|
|Hardner||Hardener is used as a curing component. A hardener can be a reactant and a catalyst in the chemical reaction that occurs during the mixing process.|
ul> So, the correct answer is “Option B”. Note: Concrete is different from mortar where concrete is itself a building material but mortar is a bonding agent that typically holds tiles, bricks and other masonry units together.
Concrete and cement are distinct terms; cement makes up from 10% to 15% of the total mass of concrete. Concrete includes cement and other things, like aggregates (sand, water) and paste. : A mixture of cement, gravel, coarse sand and water is called:A. mortarB. concreteC. slurryD. hardener
What type of chemical reaction is cement?
Setting, hardening and curing – Cement starts to set when mixed with water, which causes a series of hydration chemical reactions. The constituents slowly hydrate and the mineral hydrates solidify and harden. The interlocking of the hydrates gives cement its strength.
- Contrary to popular belief, hydraulic cement does not set by drying out — proper curing requires maintaining the appropriate moisture content necessary for the hydration reactions during the setting and the hardening processes.
- If hydraulic cements dry out during the curing phase, the resulting product can be insufficiently hydrated and significantly weakened.
A minimum temperature of 5 °C is recommended, and no more than 30 °C. The concrete at young age must be protected against water evaporation due to direct insolation, elevated temperature, low relative humidity and wind. The interfacial transition zone (ITZ) is a region of the cement paste around the aggregate particles in concrete,
- In the zone,a gradual transition in the microstructural features occurs.
- This zone can be up to 35 micrometer wide.
- 351 Other studies have shown that the width can be up to 50 micrometer.
- The average content of unreacted clinker phase decreases and porosity decreases towards the aggregate surface.
- Similarly, the content of ettringite increases in ITZ.
What compound reacts fastest with water?
- Predict the products of the following reactions:
- \(Be_ +2H_ O_ \longrightarrow\)
- \(Ne_ +2H_ O_ \longrightarrow\)
- \(Cl_ +2H_ O_ \longrightarrow\)
- \(Li_2O_ +2H_ O_ \longrightarrow\)
- Metal oxides form basic solutions in water
- Difluorine does not react with water.
- Beryllium has a large atomic radius.
- Sodium is the alkali element that reacts most violently with water.
- Why are do we called Group 1 and 2 metals “alkali” and “alkaline”?
- How is aluminum affected by water?
- Will the following reaction create an acidic or basic solution?
\(NaH +2H_ O_ \longrightarrow\)
Which Bogue’s compounds react fast with water for early strength of cement *?
Which of the following is an important and major ingredient Option 1 : Tricalcium silicate Free 60 Questions 60 Marks 60 Mins Explanation:
- Portland cement (OPC) consists of tri and dicalcium silicates, t ricalcium aluminate, and tetra calcium alumino ferrite, and calcium sulfate as gypsum.
- There are four compounds (Called Bogue’s Compounds) formed as a result of the hydration of cement:
- Alite : C 3 S, or Tricalcium Silicate
- Belite : C 2 S, or Dicalcium Silicate
- Aluminate phase : C 3 A, or Tricalcium Aluminate
- Ferrite phase : C 4 AF, or Tetracalcium Aluminoferrit
in Portland Cement dicalcium silicate (26 %), tricalcium silicate (51 %) and tricalcium aluminate (11 %) are present.
- The setting and hardening of cement after the addition of water is due to the hydration of some of the constituent compounds of cement such as Tricalcium aluminate, Tricalcium silicate, Dicalcium silicate, and Tetra calcium aluminoferrite. These compounds are known as Bogue’s Compounds.
- Hydration of Bogues Compounds
- Tricalcium aluminate (C3A): Celite is the quickest one to react when water is added to the cement. It is responsible for the flash setting. The increase of this content will help in the manufacture of Quick Setting Cement. The heat of hydration is 865 J/Cal,
- Tricalcium silicate (C3S): This is also called Alite, This is also responsible for the early strength of the concrete. The cement that has more C3S content is good for cold weather concreting. The heat of hydration is 500 J/Cal,
- Dicalcium Silicate (C2S): This compound will undergo a reaction slowly. It is responsible for the progressive strength of concrete. This is also called Belite, The heat of hydration is 260 J/Cal.
- Tetra calcium Alumino ferrite (C4AF): This is called Felite, The heat of hydration is 420 J/Cal, It has the poorest cementing value but it is responsible for the long-term gain of strength of the cement.
- ∴ Tricalcium silicate is supposed to be the best cementing material as C 3 S will result in the flash settings and C2S & C4AF are not strong enough binding agents.
- Important Points
- The following table shows different Bogue’s compounds and their properties.
|Sr no.||Bogue’s compound||Composition(%)||Properties|
|1.||Tricalcium silicate (C3S)||40 – 50||
|2.||Dicalcium Silicate (C2S)||25 – 40||Later strength Less heat of hydration|
|3.||Tricalcium Aluminate (C3A)||11 – 25||Causes Initial setting A high amount of heat of hydration.|
|4.||Tetracalcium alumina ferrite (C4AF)||9 – 11||Poor cementing value. Less heat of hydration|
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Which compound is responsible for quick setting of cement?
Silica is responsible for strength of cement, alumina imparts quick setting property, Calcium Sulphate delays setting of cement, Magnesium Oxide in large quantity-causes delayed expansion.Q.
What is the most important hydration product of Portland cement?
6.1 Introduction: general characteristics of cementitious gels – Most of the physical, chemical and mechanical properties of conventional cements and concretes can be attributed to C-S-H (hydrated calcium silicate) gel, the main reaction product of Portland cement hydration,
- The ‘CaO-SiO 2 -H 2 O’ system in which the gel is described was the subject of a substantial number of studies conducted in the twentieth century under ambient temperature and equilibrium conditions.
- Over 30 C-S-H crystalline phases have been identified ( Taylor 1990 ).
- Taylor (1986), who fathered the chemistry of cement, suggested in 1986 that the C-S-H gel forming as a result of the hydration of Ca 3 SiO 5 contained two types of local structures, one similar to 1.4 nm tobermorite and the other jennite-like.
In tobermorite, two silica chains flank a central Ca-O layer. The chains have a ‘dreierketten’ structure ( Taylor 1992 ), in which two tetrahedra – called paired tetrahedra – share an oxygen atom, while the second member of the pair also shares one of its O atoms with a third (‘outer’) tetrahedron.
- The latter is known as the bridging tetrahedron because it shares a second oxygen with the first tetrahedron of the following pair in the chain.
- Finally, all the paired tetrahedra are linked to the inner Ca-O layer across their two remaining oxygens.
- Water molecules and additional calcium cations occupy the interlayer.
The Ca/Si ratio is generally around 0.83, but this value varies in less crystalline forms (see Fig.6.1 ). Like tobermorite, jennite consists in a central layer of CaO in between two rows of (dreierketten-type) silicates with calcium atoms and water molecules in the interlayer. 6.1, Structural model proposed for C-S-H gel (after Taylor 1990 ). Richardson et al, (1993), in turn, proposed a model for C-S-H similar to Taylor’s, assuming that the aluminium could replace the silicon in the bridging tetrahedra, as 29 Si NMR studies revealed the presence of a signal at –82 ppm attributed to Q 2 (1Al) units.
- The charges would be balanced in this model by alkali cations or Ca 2 + ions in the interlayer region.
- Many factors affect both C-S-H gel composition and structure: temperature, relative humidity, pH, presence of alkalis and so on.
- Many studies have been published on the effects of all these parameters on C-S-H gels ( Glasser and Hong 2003, Richardson 2000, Cong and Kirkpatrick 1995 ).
At the same time, a host of methods have been developed for synthesizing C-S-H gels, ranging from hydrothermal treatments of SiO 2, CaO and H 2 O at high temperatures (the most common in the literature) to calcium silicate reactions (C 3 S or β-C 2 S) at ambient temperature in an aqueous suspension ( Sharara et al,1994, Chen et al,2004),
- Sol-gel procedures can likewise be used to synthesize C-S-H.
- The quest for a new construction material less environmentally detrimental and more durable than traditional Portland cement has generated increasing interest in the study of alkali activation as a procedure for obtaining alternative cementitious systems.
Two major groups of materials are presently under study for the production of alkali-activated cements: calcium-, silicon- and aluminium-rich materials (primarily blast furnace slag) on the one hand, and silica- and alumina-rich materials (such as metakaolin or type F fly ash) on the other.
- The main reaction product generated by the first group is a hydrated calcium silicate or C-S-H gel similar to the gel formed in Portland cement hydration.
- The chemistry and structure of the products generated in the second group of systems, however, vary substantially from Portland cement hydration products in those respects.
In fact, the main reaction product of the alkali activation of both fly ash and metakaolin is an alkaline silicoaluminate (geopolymer) containing silicon and aluminium tetrahedra arranged to form a three-dimensional structure ( Davidovits 1994, Barbosa et al,2000, Palomo et al,2004a, Fernandez-Jimenez et al,2006a, Criado et al,2007b, Duxson et al,2007a), 6.2, Two-dimensional representation of the structural model proposed for N-A-S-H gel. (Data from Criado 2007 ) As in the case of C-S-H gel, many factors affect the structure and chemical composition of alkaline aluminosilicate gel. A wide range of possibilities for producing such gels can be found in the literature.
In some methods the initial reagents are natural raw materials or industrial by-products (slag, metakaolin, fly ash, and others) ( Shi et al,2006, Palomo and Glasser 1992, Duxson et al,2007b, Davidovits 2008, Palomo et al,1999, Fernandez-Jimenez and Palomo 2003 ), while in others synthesis involves the use of laboratory reagents ( Ikeda 1997, Barbosa et al,, 2000, Barbosa and MacKenzie 2003, Fernandez-Jimenez et al,2006b ).
The characteristics of the product formed are directly affected by the characteristics of the prime materials, activator type and concentration, curing time and temperature and so on. Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9781845694494500060
What starts the hydration process in Portland cement?
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.
Which of the ingredient is added in the concrete immediately before or during mixing?
5.10 Role of Admixtures and Supplementary Cementing Materials – Admixtures are ingredients that are added to the concrete batch immediately before or during mixing. They confer certain beneficial effects to concrete, including frost resistance, sulfate resistance, controlled setting and hardening, improved workability, increased strength, etc.
- Special concretes are made with coloring pigments, polymer latexes, expansion producing admixtures, flocculating agents, antifreezing chemicals, corrosion inhibiting formulations, etc.
- Admixtures influence the physical, chemical, surface-chemical, and mechanical properties of concrete and its durability.
Accelerating admixtures reduce the time of setting and increase the rate at which the strength is developed. They are used in cold weather concreting. Examples of accelerators include calcium chloride, formates, carbonates, nitrites, amines, etc. Water reducing admixtures reduce the amount of water (about 8–10%) required for concrete mixing at a given workability.
- These admixtures improve the strength and durability of concrete.
- Refined lignosulfonates, gluconates, hydroxycarboxylic acids, sugar acids, etc., act as water reducers.
- Retarders lengthen the setting times of concrete.
- They are particularly useful for hot weather concrete operations.
- Phosphonates, sugars, unrefined lignosulfonates, carbohydrate derivatives, and borates are some examples of retarders.
Superplasticizing admixtures are capable of reducing water requirement by about 30%. The most popular formulations are based on sulfonated naphthalene formaldehyde and sulfonated melamine formaldehyde. Figure 8 shows the effect of dosage of superplasticizer on the slump increase of concrete.
- Air entraining agents incorporate minute bubbles in concrete.
- Such a concrete exhibits good frost resistance.
- Salts of wood resins, synthetic detergents, salts of sulfonated lignin, salts of proteinaceous materials, fatty and resinous acids and their salts, and organic salts of sulfonated hydrocarbons, are air entraining agents.
There are many other admixtures used for special purposes. They include polymers, antifreezing admixtures, alkali-aggregate expansion reducing admixtures, corrosion inhibitors, expansion reducing admixtures, pigments, fungicidal admixtures, flocculators, permeability reducers, shotcreting admixtures, and damproofing admixtures. Figure 8, Effect of dosage of superplasticizer on slump of concrete. Supplementary cementing materials are finely divided and are added to concrete in relatively large amounts (20–100%) by weight of cement. Granulated blast furnace slag and high calcium fly ash are cementitous and pozzolanic whereas condensed silica fume and rice husk ash are highly active pozzolans.
- Low calcium fly ash and naturally occurring materials (derived mainly from volcanic eruptions and calcined clays) are normal pozzolans.
- Weak pozzolanic materials include slowly cooled slag, bottom ash, boiler slag, and field burnt rice husk ash.
- Low Ca fly ash contains mainly aluminosilicate glass, sillimanite, and mullite.
The glass content may be as high as 80%. Hematite, quartz, and magnetite are also found in low Ca fly ashes. The glassy phase in the high Ca fly ash is different from that in the low Ca fly ash. The principal phase in the high Ca fly ash is tricalcium aluminate.
- The crystalline phases in high calcium fly ash are much more reactive than those in low Ca fly ash.
- In general, in both fly ashes the spherical sizes of the glassy phase vary between 1 μm and 100 μm, most of the material being under 20 μm.
- Granulated blast furnace slag is essentially glassy, having a chemical composition corresponding to melilite, a solid solution phase between gehlenite (C 2 AS) and akermanite (C 2 MS 2 ).
In slag-cement mixtures, hydration of cement provides alkali and sulfate for activating the glass. Slags cooled from a high temperature at a faster rate are likely to contain more reactive glass than those cooled slowly. Silica fume and rice husk ash, produced by controlled combustion contain essentially silica in a noncrystalline form.
They have a high surface area (20–25 m 2 /g for condensed silica fume and 50–60 m 2 /g for rice husk ash). Addition of mineral admixtures (supplementary materials) can influence concrete mix proportions, rheological behavior of plastic concrete, degree of hydration of cement, strength and permeability of concrete, resistance to thermal cracking, alkali-silica expansion, and sulfate attack.
These aspects are discussed in many books and notably in proceedings of the conferences organized by CANMET/American Concrete Institute, in 1983, 1986, 1989, 1992, and 1995, under the title “Fly Ash, Silica Fume, Slag, and Other Pozzolans in Concrete.” A bibliography of references to many publications related to supplementary materials is to be found in the book edited by Malhotra.
Which chemical is accelerator for hydration of cement?
Abstract – Cold weather concreting often requires the use of chemical accelerators to speed up the hydration reactions of the cement, so that setting and early-age strength development will occur in a timely manner. While calcium chloride (dihydrate – CaCl 2 ·2H 2 O) is the most commonly used chemical accelerator, recent research using fine limestone powders has indicated their high proficiency for physically accelerating early-age hydration and reducing setting times.
This paper presents a comparative study of the efficiency of these two approaches in accelerating hydration (as assessed via isothermal calorimetry), reducing setting times (Vicat needle), and increasing early-age mortar cube strength (1 d and 7 d). Both the CaCl 2 and the fine limestone powder are used to replace a portion of the finest sand in the mortar mixtures, while keeping both the water-to-cement ratio and volume fractions of water and cement constant.
Studies are conducted at 73.4 °F (23°C) and 50 °F (10 °C), so that activation energies can be estimated for the hydration and setting processes. Because the mechanisms of acceleration of the CaCl 2 and limestone powder are different, a hybrid mixture with 1 % CaCl 2 and 20 % limestone powder (by mass of cement) is also investigated.
Both technologies are found to be viable options for reducing setting times and increasing early-age strengths, and it is hoped that concrete producers and contractors will consider the addition of fine limestone powder to their toolbox of techniques for assuring performance in cold weather and other concreting conditions where acceleration may be needed.
Keywords: Acceleration, calcium chloride, cement hydration, early-age strength, limestone powder, setting time