Gypsum is called the retarding agent of cement which is mainly used for regulating the setting time of cement and is an indispensable component. Without gypsum, cement clinker can condense immediately by mixing with water and release heat.
Which equipment is used to test the setting time of cement Examveda?
Basic Civil Engineering Questions and Answers – Properties of Cement This set of Basic Civil Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Properties of Cement”.1. Why is natural cement used very limitedly? a) Brown in Colour b) Standard consistency is not met with c) Sets too quickly d) Particle size is too fine View Answer Answer: c Explanation: Natural cement sets very quickly after the addition of water and hence it is not quite workable.
- Artificial cement is preferred over this.2.
- Who invented Portland cement and in which year? a) William Aspdin, 1824 b) William Aspdin, 1840s c) Joseph Aspdin, 1840s d) Joseph Aspdin, 1824 View Answer Answer: b Explanation: Joseph Aspdin patented Portland cement in 1824.
- William Aspdin, his son is regarded as the inventor of modern Portland cement due to his developments in 1840s.3.
What is the average particle size of cement? a) 15 microns b) 45 microns c) 75 microns d) 100 microns View Answer Answer: a Explanation: Approximately 95% of cement particles are smaller than 45 microns and the average particle size is 15 microns. Note: Join free Sanfoundry classes at or 4.
- What is the meaning of soundness of cement? a) Ability to flow when mixed b) Ability to make ringing noise when struck c) Ability to form strong and sound structure d) Ability to retain volume after setting.
- View Answer Answer: d Explanation: When cement paste hardens and sets, it should not undergo any volume change.
Soundness ensures this and is tested using Autoclave expansion test.5. Time elapsed from the instance of adding water until paste ceases to behave as fluid is called: a) Initial setting time b) Final setting time c) Intermediate setting time d) Absolute setting time View Answer Answer: a Explanation: Final setting time is the time required for cement paste to reach a certain state of hardness.
- Option c and d does not exist.
- Take Now! 6.
- Which of the below mentioned is not a result of field test performed on cement? a) There should not be any lumps b) It should feel cold when you put your hand in bag of cement c) The colour should be blackish grey d) It should not be gritty when rubbed with finger View Answer Answer: c Explanation: The colour of cement is normally grey with a greenish tint.
There are different shades – lighter and darker, but it does not go as dark as blackish grey.7. Which equipment is used to test the setting time of cement? a) Core cutter b) Vibrator c) Universal testing machine (UTM) d) Vicat apparatus View Answer Answer: d Explanation: Core cutter is used to determine dry density of soil.
- Vibrator is used in sieve analysis.
- UTM can be used to test various parameters – tension, bending, shear of various materials.
- Vicat apparatus consists of a needle, used to penetrate the cement paste sample.8.
- What is the initial setting time of cement? a) 1 hour b) 30 minutes c) 15 minutes d) 30 hours View Answer Answer: b Explanation: As per IS code 4031-part 5, the initial setting time of cement is minimum of 30 minutes.
After this cement will start losing its plasticity and will not be workable.9. Use of coarser cement particles leads to: a) Low durability b) Higher strength c) Low consistency d) Higher soundness View Answer Answer: a Explanation: For coarser particles, hydration starts on the surface of particles, hence, it might not be completely hydrated.
This causes low strength and low durability.10. Wet cement can cause severe skin burns if not washed off with water immediately. a) True b) False View Answer Answer: a Explanation: Cement is highly alkaline and setting process is exothermic. Wet cement is strongly caustic and causes skin burns. Similarly, dry cement causes eye or respiratory irritation, when it comes in contact with mucous membranes.11.
Green cement is: a) Green coloured cement b) Cement mixed with plant products c) Cement mixed with recycled materials d) Cement mixed with green algae View Answer Answer: c Explanation: Green cement is a cementitious material which employs the use of optimized recycled materials.
- These can meet or even exceed the functional performance of Portland cement.12.
- What is the depth the needle in Vicat apparatus should penetrate into the cement paste in consistency test? a) 33-35 cm from bottom of the mould b) 33-35 mm from top of the mould c) 33-35 cm from top of the mould d) 33-35 mm from bottom of the mould View Answer Answer: b Explanation: The best procedure has been clearly mentioned in IS 4031 Part 4.
According to the code, 33-35mm depth of penetration is ideal. Sanfoundry Global Education & Learning Series – Basic Civil Engineering. To practice all areas of Basic Civil Engineering,, Next Steps:
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What is Retarder cement?
Use of Water Reducers, Retarders, and Superplasticizers. Introduction Many important characteristics of concrete are influenced by the ratio (by weight) of water to cementitious materials (w/cm) used in the mixture. By reducing the amount of water, the cement paste will have higher density, which results in higher paste quality.
An increase in paste quality will yield higher compressive and flexural strength, lower permeability, increase resistance to weathering, improve the bond of concrete and reinforcement, reduce the volume change from drying and wetting, and reduce shrinkage cracking tendencies (PCA, 1988). Reducing the water content in a concrete mixture should be done in such a way so that complete cement hydration process may take place and sufficient workability of concrete is maintained for placement and consolidation during construction.
The w/cm needed for cement to complete its hydration process ranges from 0.22 to 0.25. The existence of additional water in the mixture is needed for ease of concrete placing and finishing (workability of concrete). Reducing the water content in a mixture may result in a stiffer mixture, which reduces the workability and increases potential placement problems.
- Water reducers, retarders, and superplasticizers are admixtures for concrete, which are added in order to reduce the water content in a mixture or to slow the setting rate of the concrete while retaining the flowing properties of a concrete mixture.
- Admixtures are used to modify the properties of concrete or mortar to make them more suitable to work by hand or for other purposes such as saving mechanical energy.
Water reducing admixtures (WRA) The use of WRA is defined as Type A in ASTM C 494, WRA affects mainly the fresh properties of concrete by reducing the amount of water used by 5% to 12% while maintaining a certain level of consistency, measured by the slump as prescribed in ASTM C 143-90.
The use of WRA may accelerate or retard the initial setting time of concrete. The WRA that retards the initial setting time more than three hours later is classified as WRA with retarding effect (Type D). Commonly used WRA is lignosulfonates and hydrocarboxylic (HC) acids. The use of HC acids as WRA requires higher water content compared to the lignosulfonates.
Rapid bleeding is a problem for concrete treated with HC acids. Increase of slump is different according to its type and dosage. Typical dosage rate is based upon the cementitious material content (milliliters per hundred of kilograms). The figure below illustrates the influence of dosage of Lignosulfonates and HC acid on slump. Figure 1 Influence of Dosage of Retarders on Slump (Neville, 1995). WRA has been used primarily in hot weather concrete placing, pumping, and tremie. Careful concrete placement is required, as the initial setting time of concrete will take place an hour earlier.
It is also shown that the use of WRA will give a higher initial concrete compressive strength (up to 28 days) by 10% compared to the control mixture. Other benefit of using WRA is that higher concrete density is achieved which makes the concrete less permeable and have a higher durability. Retarding admixtures The use of this admixture is defined in ASTM C494,
There are two kinds of retarders, defined as Type B (Retarding Admixtures) and Type D (Water Reducing and Retarding Admixtures). The main difference between these two is the water-reducing characteristic in Type D that gives higher compressive strengths by lowering w/cm ratio.
Complex concrete placement or grouting Special architectural surface finish Compensating the accelerating effect of high temperature towards the initial set Preventing cold joint formation in successive lifts.
Retarder can be formed by organic and inorganic material. The organic material consists of unrefined Ca, Na, NH 4, salts of lignosulfonic acids, hydroxycarboxylic acids, and carbohydrates. The inorganic material consists of oxides of Pb and Zn, phosphates, magnesium salts, fluorates, and borates.
- Commonly used retarders are lignosulfonates acids and hydroxylated carboxylic (HC) acids, which act as Type D (Water Reducing and Retarding Admixtures).
- The use of lignosulfonates acids and hydroxylated carboxylic acids retard the initial setting time for at least an hour and no more than three hours when used at 65 to 100 o F.
A study performed on the influence of air temperature over the retardation of the initial set time (measured by penetration resistance as prescribed in ASTM C 403 92) shows that decreasing effect with higher air temperature (Neville1995). The table below describes the effect of air temperature on retardation of setting time: Table 1 Air Temperature and Retardation of Initial Setting Time
|Admixture Type||Description||Retardation of initial setting time (h:min) at temperature of|
|30 o C||40 o C||50 o C|
The use of retarding admixture has the main drawback of the possibility of rapid stiffening, where rapid slump loss will result in difficulty of concrete placement, consolidation, and finishing. An extended-set admixture has been developed as another retarding admixture.
The advantages of this admixture compared to the conventional one is the capability to react with major cement constituents and to control hydration and setting characteristics of concrete while the conventional one will only react with C 3 A. Careful usage of retarder is required to avoid excessive retardation, rapid slump loss and excessive plastic shrinkage.
Plastic shrinkage is the change in fresh concrete volume as surface water evaporates. The amount of water evaporation is influenced by temperature, ambient relative humidity, and wind velocity. Proper concrete curing and adequate water supply for surface evaporation will prevent plastic shrinkage cracking. Figure 2 Rate of Surface Moisture Evaporation The extended-set admixture is widely used as a stabilizing agent for wash water concrete and fresh concrete. Addition of extended-set admixture enables the reuse of wash water to the next batch without affecting concrete properties.
This admixture can also be used for long haul concrete delivery and to maintain slump. Factors affecting the use of this admixture include the dosage rate and the ambient temperature of the concrete. Superplasticizers (High Range Water reducer) ASTM C494 Type F and Type G, High Range Water Reducer (HRWR) and retarding admixtures are used to reduce the amount of water by 12% to 30% while maintaining a certain level of consistency and workability (typically from 75 mm to 200 mm) and to increase workability for reduction in w/cm ratio.
The use of superplasticizers may produce high strength concrete (compressive strength up to 22,000 psi). Superplasticizers can also be utilized in producing flowing concrete used in a heavy reinforced structure with inaccessible areas. Requirement for producing flowing concrete is defined in ASTM C 1017. Figure 3 Relation between Flow Table and Water Content of Concrete with and without Plasticizers (Neville, 1995). Another benefit of superplasticizers is concrete early strength enhancement (50 to 75%). The initial setting time may be accelerated up to an hour earlier or retarded to be an hour later according to its chemical reaction.
- Retardation is sometimes associated with range of cement particle between 4 30 m m.
- The use of superplasticizers does not significantly affect surface tension of water and does not entrain a significant amount of air.
- The main disadvantage of superplasticizer usage is loss of workability as a result of rapid slump loss and incompatibility of cement and superplasticizers.
Superplasticizers are soluble macromolecules, which are hundreds of times larger than water molecule (Gani, 1997). Mechanism of the superplasticizers is known as adsorption by C 3 A, which breaks the agglomeration by repulsion of same charges and releases entrapped water.
- The adsorption mechanism of superplasticizers is partially different from the WRA.
- The difference relates to compatibility between Portland Cement and superplasticizers.
- It is necessary to ensure that the superplasticizers do not become fixed with C 3 A in cement particle, which will cause reduction in concrete workability.
Typical dosage of superplasticizers used for increasing the workability of concrete ranges from 1 to 3 liters per cubic meter of concrete where liquid superplasticizers contained about 40 % of active material. In reducing the water cement ratio, higher dosage is used, that is from 5 to 20 liters per cubic meter of concrete.
Dosage needed for a concrete mixture is unique and determined by the Marsh Cone Test. There are four types of superplasticizers: sulfonated melamine, sulfonated naphthalene, modified lignosulfonates and a combination of high dosages of water reducing and accelerating admixtures. Commonly used are melamine based and naphthalene based superplasticizers.
The use of naphthalene based has the advantage of retardation and affecst slump retention. This is due to the modified hydration process by the sulfonates Admixtures Dispensers The basic function of a dispenser as defined in ACI Bulletin E4-95 is:
To transport the admixture from storage to batch To measure the quantity of the admixtures required To provide verification of the volume dispensed To inject the admixture into the batch.
Admixtures have been dispensed in liquid form to ensure proper dispersion in the concrete mixture. WRA should be dispensed with the last water batch. Proper timing is very important, as any delay ranges between one to five minutes after the water addition will result in excessive retardation of setting time.
The Superplasticizers should be dispensed on to the batch immediately before discharge for placement (Type F) or with the last portion of the water (Type G). References: Chemical Admixtures for Concrete, ACI Committee 212.3R-91 Report. Chemical and Air Entraining Admixtures for Concrete, ACI Education Bulletin No.
E4-95. Dodson, Vance, Concrete Admixtures, VNR, 1990. Gani, M.J., Cement and Concrete, Chapman & Hall, 1997. Komatska, S.H. and Panarese, W.C., Design and Control of Concrete Mixtures, PCA, 1988. Ramachandran, V.S., Concrete Admixtures Handbook, Properties, Sciences, and Technology, 2 nd edition, 1995.
How can we reduce the setting time of cement?
The setting time of cement decreases by adding Calcium Chloride.
Is fly ash used in cement?
Mix Design and Specification Requirements – Procedures for proportioning fly ash concrete (FAC) mixes necessarily differ slightly from those for conventional PCC. Basic guidelines for selecting concrete proportions are contained in the American Concrete Institute (ACI) Manual of Concrete Practice, Section 211.1.
Highway agencies generally use variations to this procedure, but the basic concepts recommended by ACI are widely acknowledged and accepted. There is very little on proportioning in ACI 232.2. Fly ash is used to lower the cost and to improve the performance of PCC. Typically, 15 percent to 30 percent of the portland cement is replaced with fly ash, with even higher percentages used for mass concrete placements.
An equivalent or greater weight of fly ash is substituted for the cement removed. The substitution ratio for fly ash to portland cement is typically 1:1 to 1.5:1. A mix design should be evaluated with varying percentages of fly ash. Time versus strength curves can be plotted for each condition.
To meet specification requirements, curves are developed for various replacement ratios and the optimum replacement percentage ratio is selected. A mix design should be performed using the proposed construction materials. It is recommended that the fly ash concrete being tested incorporates local materials in performance evaluation.
Cement Factors. Because fly ash addition contributes to the total cementitious material available in a mix, the minimum cement factor (portland cement) used in the PCC can be effectively reduced for FAC. The ACI acknowledges this contribution and recommends that a water/ (cement plus pozzolan) ratio be used for FAC in lieu of the conventional water/cement ratio used in PCC.
Which of the following is added for quick setting of cement Mcq?
Free 10 Questions 30 Marks 10 Mins Quick setting cement means the cement which sets early. Quick setting cement is prepared by adding small amount of alumina in finely grinded cement clinker and reducing the proportion of calcium sulphate (Gypsum). Applications: 1.
- Grouting 2.
- Under water concreting.
- Important point: Quick setting cement should not be confused with rapid hardening cement.
- Rapid hardening cement develops higher rate of gain of strength while quick setting cement sets quickly only and its rate of gain of strength is similar to ordinary Portland cement.
Note: This is SSC JE Official question and answer. Option 1 and 3 both can be correct as per concept but, Option 3 is more appropriate. Last updated on Sep 22, 2022 UPPSC AE Final Result Out on 1st December 2022. The candidates appeared for the interview from 17th October to 15th November 2022.
Why fly ash is used?
Fly ash in concrete; benefits and types Fly ash improves concrete’s workability, pumpability, cohesiveness, finish, ultimate strength, and durability as well as solves many problems experienced with concrete today–and all for less cost. Fly ash, sometimes called flue ash, has been a popular supplementary cementitious material (SCM) since the mid-1900s.
For most concrete producers, fly ash is an important ingredient in concrete mix designs. Depending on the application, the type of fly ash, specification limits, geographic location, and climate, fly ash can be used at levels ranging from 15% to 25% (most common) to 40% to 60% (when rapid setting time is not required), reducing emissions by roughly the same amount—and helping to keep concrete products at an affordable price.
Fly ash is produced by coal-fired electric and steam generating plants. Typically, coal is pulverized and blown with air into the boiler’s combustion chamber where it immediately ignites, generating heat and producing a molten mineral residue. Boiler tubes extract heat from the boiler, cooling the flue gas and causing the molten mineral residue to harden and form ash. Fly ash Fly ash benefit in concrete Fly ash is a pozzolanic material. It is a finely-divided amorphous alumino-silicate with varying amounts of calcium, which when mixed with portland cement and water, will react with the calcium hydroxide released by the hydration of portland cement to produce various calcium-silicate hydrates (C-S-H) and calcium-aluminate hydrates.
Some fly ashes with higher amounts of calcium will also display cementitious behavior by reacting with water to produce hydrates in the absence of a source of calcium hydroxide. These pozzolanic reactions are beneficial to the concrete in that they increase the quantity of the cementitious binder phase (C-S-H) and, to a lesser extent, calcium-aluminate hydrates, improving the longterm strength and reducing the permeability of the system.
Both of these mechanisms enhance the durability of the concrete. The performance of fly ash in concrete is strongly influenced by its physical, mineralogical and chemical properties. The mineralogical and chemical composition are dependent to a large extent on the composition of the coal and since a wide range of domestic and imported coals.
- For mix design purposes, fly ash itself should be considered like portland cement, except that the specific gravity for fly ash is different.
- The specific gravity of portland cement is typically 3.15, while the specific gravity of fly ash may range from 2.2 to 2.8, depending on fly ash composition.
- Therefore, if a certain percentage of cement is replaced with fly ash on a mass basis, simply multiply the initial portland cement quantity by the percent replacement.
For some fly ashes, particularly low calcium Class F fly ashes, higher replacement rates (1.2:1 up to 2:1) are required to maintain equivalent early concrete strength. The next modification to the concrete mix design involves the water content. Due to the particle shape of fly ash, the water demand is typically reduced, up to 5% less with Class F fly ash, and up to 10% less with Class C fly ash (this may also be accomplished by a partially lowered chemical admixture dosage). Advantages of Fly Ash in Concrete The most important benefit is reduced permeability to water and aggressive chemicals. Properly cured concrete made with fly ash creates a denser product because the size of the pores are reduced. This increases strength and reduces permeability.
- Today, there are at least two ways to make fly ash more beneficial: a dry process that involves triboelectric static separation and a wet process based on froth flotation.
- These procedures generally lower the carbon content and the LOI of fly ash.
- The cost of an additional storage bin should be easily covered by the reduction in the cost of the concrete and the added benefits to the concrete.
Low-carbon fly ash or the use of a better air-entraining agent at a higher-than-usual addition rate can control the problem of freeze-thaw durability. Advantages in Fresh Concrete Since fly ash particles are spherical and in the same size range as portland cement, a reduction in the amount of water needed for mixing and placing concrete can be obtained.
- In precast concrete, this can be translated into better workability, resulting in sharp and distinctive corners and edges with a better surface appearance.
- This also makes it easier to fill intricate shapes and patterns.
- Fly ash also benefits precast concrete by reducing permeability, which is the leading cause of premature failure.
The use of fly ash can result in better workability, pumpability, cohesiveness, finish, ultimate strength, and durability. The fine particles in fly ash help to reduce bleeding and segregation and improve pumpability and finishing, especially in lean mixes.
Advantages in Hardened Concrete Strength in concrete depends on many factors, the most important of which is the ratio of water to cement. Good quality fly ash generally improves workability or at least produces the same workability with less water. The reduction in water leads to improved strength. Because some fly ash contains larger or less reactive particles than portland cement, significant hydration can continue for six months or longer, leading to much higher ultimate strength than concrete without fly ash.
Different types of Flyash Fly-ash categories Two classes of fly ash are defined by American Society for Testing and Materials (ASTM) C618: Class F fly ash and Class C fly ash. The chief difference between these classes is the amount of calcium, silica, alumina, and iron content in the ash.
The chemical properties of the fly ash are largely influenced by the chemical content of the coal burned (i.e., anthracite, bituminous, and lignite). Not all fly ashes meet ASTM C618 requirements, although depending on the application, this may not be necessary. Fly ash used as a cement replacement must meet strict construction standards, but no standard environmental regulations have been established in the United States.
Seventy-five percent of the fly ash must have a fineness of 45 µm or less, and have a carbon content, measured by the loss on ignition (LOI), of less than 4%. In the US, LOI must be under 6%. The particle size distribution of raw fly ash tends to fluctuate constantly, due to changing performance of the coal mills and the boiler performance.
This makes it necessary that, if fly ash is used in an optimal way to replace cement in concrete production, it must be processed using beneficiation methods like mechanical air classification. But if fly ash is used as a filler to replace sand in concrete production, unbeneficiated fly ash with higher LOI can be also used.
Especially important is the ongoing quality verification. This is mainly expressed by quality control seals like the Bureau of Indian Standards mark or the DCL mark of the Dubai Municipality. Class “F” The burning of harder, older anthracite and bituminous coal typically produces Class F fly ash.
This fly ash is pozzolanic in nature, and contains less than 7% lime (CaO). Possessing pozzolanic properties, the glassy silica and alumina of Class F fly ash requires a cementing agent, such as Portland cement, quicklime, or hydrated lime-mixed with water to react and produce cementitious compounds.
Alternatively, adding a chemical activator such as sodium silicate (water glass) to a Class F ash can form a geopolymer. Class “C” Fly ash produced from the burning of younger lignite or sub-bituminous coal, in addition to having pozzolanic properties, also has some self-cementing properties.
- In the presence of water, Class C fly ash hardens and gets stronger over time.
- Class C fly ash generally contains more than 20% lime (CaO).
- Unlike Class F, self-cementing Class C fly ash does not require an activator.
- Alkali and sulfate (SO 4 ) contents are generally higher in Class C fly ashes.
- The primary difference between Class C and Class F fly ash is the chemical composition of the ash itself.
While Class F fly ash is highly pozzolanic, meaning that it reacts with excess lime generated in the hydration of portland cement, Class C fly ash is pozzolanic and also can be self cementing. : Fly ash in concrete; benefits and types