Contents
Basic Cement Technology
Q: What is cement?
A: Cement is a fine, soft, powdery substance, made from a mixture of elements found in natural materials such as limestone, clay, sand and/or shale. When cement is mixed with water, it can bind sand and gravel into a hard, solid mass called concrete. Cement is usually grey. White cement is also available, but is usually more expensive.
- Cement mixed with water, sand and gravel, forms
- Cement mixed with water and sand, forms cement
- Cement mixed with water, lime and sand, forms
Cement powder is extremely fine; one kilo (2.2lbs) contains over 300 billion grains. The powder is so fine it will pass through a sieve capable of holding water.
In India, Ordinary Portland Cement (OPC) is manufactured in three grades, viz. 33 grade, 43 grade and 53 grade. The numbers indicate the compressive strength obtained after 28 days, when tested as per the stipulated procedure.
Apart from OPC, there are several other types of cement, mostly meant for special purposes, e.g. sulphate resistant cement, coloured cement, oil well cement etc. However, there are some general-purpose cements, the commonest one being Portland Pozzolana Cement (PPC).
Q: What is natural cement?
A: Natural cements are hydraulic cements, produced by mining natural deposits of limestone and clay with a specific chemical composition within a narrow range. When heated in a kiln and ground to a fine powder, a type of cement is produced, which through chemical reactions sets and hardens when mixed with water. The strength and uniformity of natural cements are lower than those of Portland cements; but these are more historically accurate materials for restoration projects, which is their primary application. Natural cements were extensively used in 19th and early 20th century construction in several historic structures. However, with improved technology for producing Portland cements, sales of natural cements began to decline in the late 1800s, stopping entirely by the mid 1970s.
Q: How is cement made?
A: 1) Limestone, the major ingredient needed for making cement is quarried. Small quantities of sand and clay are required as well. Limestone, sand and clay contain the four essential elements required to make cement: calcium, silicon, aluminium and iron.
- Boulder-size limestone rocks are transported from the quarry to the cement plant and fed into a crusher, which crushes the boulders into marble-size
- The limestone pieces then go through a blender where they are mixed with the other raw materials in the right
- Raw materials are then ground to a This is sometimes done with rollers that crush the materials against a rotating platform.
- This mixture then goes into a huge, extremely hot, rotating furnace to undergo a process called ‘sintering’. Sintering means: to cause to become a coherent mass by heating without melting. In other words, the raw materials become partially molten. The raw materials reach about 2700° F (1480°C) inside the furnace. This causes chemical and physical changes to the raw materials and they come out of the furnace as large, glassy, red-hot cinders called ‘clinker’.
- This clinker is cooled and ground into a fine grey powder. A small amount of gypsum is added during the final grinding. The finished product is Portland
The cement is then stored in silos (large holding tanks) where it awaits distribution.
The cement is usually shipped in bulk in purpose-made trucks, by rail or even by barges and ships. Some is bagged for those who want small quantities.
Q: What are the different types of Cements?
A: Portland cement: Portland cement is made by heating limestone with small quantities of other materials (such as clay) to 1450°C in a kiln, in a process known as calcination. The resulting hard substance, called ‘clinker,’ which is then ground with a small amount of gypsum into a powder to make ‘Ordinary Portland Cement,’ the most commonly used type of cement (often referred to as OPC).
Portland cement is the basic ingredient of concrete, mortar and most non-speciality grout. Its most common use is in the production of concrete. Concrete is a composite material consisting of aggregate (gravel and sand), cement, and water. As a construction material, concrete can be cast in almost any shape desired, and once hardened can become a structural (load bearing) element. Portland cement may be grey or white.
Portland cement blends: These are often available as inter-ground mixtures from cement manufacturers, but similar formulations are often also mixed from ground components at the concrete mixing plant.
Portland Blastfurnace Cement contains up to 70% ground granulated blast furnace slag, Portland clinker and a little gypsum. All compositions produce high ultimate strength, but as the slag content is increased, the early strength is reduced, while the sulphate resistance increases and heat evolution diminishes. Portland Blastfurnace Cement is used as an economic alternative to Portland sulphate-resisting and low-heat cements.
Portland Flyash Cement contains up to 30% fly ash. The fly ash is pozzolanic, so that ultimate strength is maintained. Because fly ash addition allows for lower concrete water content, early strength can be maintained. This can be an economic alternative to ordinary Portland cement where good quality, cheap fly ash is available.
Portland Pozzolan Cement includes fly ash cement, since fly ash is a pozzolan, in addition to cements made from other natural or artificial pozzolans. In countries where volcanic ashes are available (e.g. Italy, Chile, Mexico, the Philippines) these cements are often the most common form in use.
Portland Silica Fume Cement is produced by the addition of silica fume to cement, and exceptionally high strength substance. Cements containing 5–20% silica fume are occasionally produced. However, silica fume is more usually added to Portland cement at the concrete mixer. Masonry Cements are used for preparing bricklaying mortars and stuccos, and must not be used in concrete. They are usually complex proprietary formulations containing Portland clinker and a number of other ingredients that may include limestone, hydrated lime, air-entrainers, retarders, waterproofers and colouring agents. They are formulated to yield workable mortars that allow rapid and consistent masonry work. Subtle variations of Masonry cement in the US are Plastic Cements and Stucco Cements. These are designed to produce controlled bonds with masonry blocks.
Expansive Cements contain, in addition to Portland clinker, expansive clinkers (usually sulfoaluminate clinkers) and are designed to offset the effects of drying shrinkage that is normally encountered with hydraulic cements. This allows large floor slabs (up to 60m2) to be prepared without contraction joints.
White blended cements may be made using white clinker and white supplementary materials such as high-purity metakaolin.
Coloured cements are used for decorative purposes. Some standards allow the addition of pigments to produce ‘coloured Portland cement’. In other standards (e.g. ASTM), pigments are not allowed constituents of Portland cement, and coloured cements are sold as ‘blended hydraulic cements’.
Very finely ground cements are made from mixtures of cement with sand or slag or other pozzolan type minerals, which are finely ground together. Such cements can have the same physical characteristics as normal cement but with 50% less cement, particularly due to their increased surface area for the chemical reaction. Even with intensive grinding they can use up to 50% less energy for fabrication than ordinary Portland cements.
Non-Portland hydraulic cements
Pozzolan-lime cements: Mixtures of ground pozzolan and lime were the cements used by the Romans, and are found in Roman structures still standing (e.g. the Pantheon in Rome). They develop strength slowly, but their ultimate strength can be very high. The hydration products that produce strength are essentially the same as those of Portland cement.
Slag-lime cements: Ground granulated blast furnace slag is not hydraulic on its own, but is ‘activated’ by the addition of alkalis, most economically using lime. They are similar to pozzolan lime cements in their properties. Only granulated slag (i.e. water-quenched, glassy slag) is effective as a cement component.
Supersulphated cements: These contain about 80% ground granulated blast furnace slag, 15% gypsum or anhydrite and small quantities of Portland clinker or lime as an activator. They produce strength by formation of ettringite, with strength growth similar to a slow Portland cement. They exhibit good resistance to aggressive agents, including sulphates.
Calcium aluminate cements are hydraulic cements made primarily from limestone and bauxite. The active ingredients are monocalcium aluminate CaAl2O4 (CA in Cement chemist notation) and Mayenite Ca12Al14O33 (C12A7 in CCN). Strength forms by hydrating calcium aluminate hydrates. They are well adapted for use in refractory (high-temperature resistant) concretes, e.g. furnace linings.
Calcium sulfoaluminate cements are made from clinkers that include ye’elimite (Ca4(AlO2)6SO4 or C4A3 in CCN) as a primary phase. They are used in expansive cements, in ultra-high early strength cements, and in ‘low-energy’ cements. Hydration produces ettringite, and specialised physical properties (such as expansion or rapid reaction) are obtained by adjustment of the availability of calcium and sulphate ions. Their use as a low-energy alternative to Portland cement has been pioneered in China, where several million tonnes per year are produced. Energy requirements are lower because of the lower kiln temperatures required for reaction and the lower
amount of limestone (that has to be endothermically decarbonised) in the mix. In addition, the lower limestone content and lower fuel consumption leads to a CO2 emission around half that associated with Portland clinker. However, SO2 emissions are significantly higher.
‘Natural’ Cements correspond to certain cements of the pre-Portland era, produced by burning argillaceous limestone at moderate temperatures. The level of clay components in the limestone (around 30–35%) is so that large amounts of belite (the low-early strength, high-late strength mineral in Portland cement) are formed without the formation of excessive amounts of free lime. As with any natural material, such cements have highly variable properties.
Geopolymer cements are made from mixtures of water-soluble alkali metal silicates and aluminosilicate mineral powders such as fly ash and metakaolin.
Q: How is Portland cement made?
A: Materials that contain appropriate amounts of calcium compounds like silica, alumina and iron oxide are crushed, screened and placed in a rotating cement kiln. Ingredients used in this process are typically materials such as limestone, marl, shale, iron ore, clay and fly ash.
The kiln resembles a large horizontal pipe with a diameter of 10–15ft (3–4.1m) and a length of 300ft (90m) or more. One end is raised slightly and the raw mix is placed in the high end; as the kiln rotates, the materials move slowly toward the lower end. Flame jets are at the lower end and all the materials in the kiln are heated to high temperatures that range between 2700 and 3000°F (1480 and 1650°C). This high heat drives off, or calcines, the chemically combined water and carbon dioxide from the raw materials and forms new compounds (tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite). For each ton of material that goes into the feed end of the kiln, two thirds of a ton of clinker comes out the discharge end. This clinker is in the form of marble sized pellets. The clinker is very finely ground to produce Portland cement. A small amount of gypsum is added during the grinding process to control the cement’s set or rate of hardening.
Q: What is Fibre Reinforced Concrete?
A: Low Fibre volume composite concrete contains less than 1% fibre. It is used for field applications involving large volumes of concrete. The fibres do not significantly increase the strength of the concrete. Low fibre volume concrete is used for paving roads.
High Fibre Volume Concrete: Typically used for thin sheets with cement mortar mix. The fibre volume in this mix ranges from 5% to 15%.
High Fibre Volume Composite: The fibre volume in this mix can be as high as 40%. This significantly increases the strength and toughness of the mix. The reinforcement in High Fibre Volume Composite concrete is usually in sheet form. This reinforced concrete type is used in roof and wall panels.
Q: What is the difference between cement and concrete?
A: Concrete should not be confused with cement because the term cement refers only to the dry powder substance used to bind the aggregate materials of concrete. Upon the addition of water and/or additives the cement mixture is referred to as concrete, especially if aggregates have been added.
Q: What is concrete?
A: Concrete is a mixture of cement, water, sand and gravel (stones, crushed rock). The mixture eventually hardens into a stone-like material. Cement and water are the two ingredients that chemically react; the gravel and sand give strength.
Q. How was concrete made in the earlier times?
A: During the Roman Empire, Roman concrete (or Opus caementicium) was made from quicklime, pozzolanic ash/pozzolana and an aggregate of pumice. Its widespread use in many Roman structures, a key event in the history of architecture termed the Concrete Revolution, freed Roman construction from the restrictions of stone and brick material and allowed for revolutionary new designs both, in terms of structural complexity and dimension. Concrete, as the Romans knew it, was in effect a new and revolutionary material. Laid in the shape of arches, vaults and domes, it quickly hardened into a rigid mass, free from many of the internal thrusts and strains, which troubled the builders of similar structures in stone or brick.
Q: How is modern structural concrete different from the earlier form of concrete?
A: Modern structural concrete differs from Roman concrete in two important details. First, its mix consistency is fluid and homogeneous, allowing it to be poured into forms rather than requiring hand layering together with the placement of aggregate, which in Roman practice often consisted
of rubble. Second, integral reinforcing steel gives modern concrete assemblies great tensile strength, whereas Roman concrete could depend only upon the strength of the concrete bonding to resist tension.
Q: What does ‘curing’ concrete mean?
A: Curing is one of the most important steps in concrete construction, because proper curing greatly increases concrete strength and durability. Concrete hardens as a result of hydration: the chemical reaction between cement and water. However, hydration occurs only in the presence of water and if the concrete’s temperature stays within a suitable range. During the curing period- from, five to seven days after placement for conventional concrete, the concrete surface needs to be kept moist to permit the hydration process. New concrete can be wet with soaking hoses, sprinklers or covered with wet burlap, or can be coated with commercially available curing compounds, which seal in moisture.
Q: What is Reinforced concrete?
A: Reinforced concrete contains steel reinforcing that is designed and placed in structural members at specific positions to cater for the stress conditions that the member is required to accommodate.
Q. What is Prestressed concrete?
A: The principle behind Prestressed concrete is that compressive stresses induced by high-strength steel tendons in a concrete member before loads are applied will balance the tensile stresses imposed in the member during service.
For example a horizontal beam will tend to sag down. However, if the reinforcement along the bottom of the beam is prestressed, it can counteract this.
In pre-tensioned concrete, prestressing is achieved by using steel or polymer tendons or bars that are subjected to a tensile force prior to casting; and for post-tensioned concrete, after casting.
Q. What are the sought after properties of concrete?
- 1. The concrete mix is extremely workable. It can be placed and consolidated properly.
- Desired qualities of the hardened concrete are met. For example, resistance to freezing and thawing and deicing chemicals, watertightness (low permeability), wear resistance and strength.
- Economy. Since the quality depends mainly on the water to cement ratio, the water requirement should be minimised to reduce the cement requirement (and thus reduce the cost). The following steps reduce water and cement requirements:
Use the stiffest mix possible
Use the largest size aggregate practical for the job Use the optimum ratio of fine to coarse aggregate
Q: What is the composition of Concrete A: 11% Cement (usually Portland)
16% Water
6% Air
26% Sand
41% Gravel or crushed stone
Q: Descriptive composition of Concrete.
A: There are many types of concrete available, created by varying the proportions of its main ingredients.
The mix design depends on the type of structure being built, how the concrete will be mixed, delivered and how it will be placed to form the structure.
Cement
Portland cement is the most widely used cement. It is the basic ingredient in concrete, mortar and plaster. English engineer, Joseph Aspdin patented Portland cement in 1824; it was named because of its similar colour to Portland limestone, quarried from the Isle of Portland and used extensively in London architecture. It consists of a mixture of oxides of calcium, silicon and aluminium and is manufactured by heating limestone (source of calcium) and clay, then grinding this product (clinker) with a source of sulphate (most commonly gypsum). The manufacturing of Portland cement creates about 5% of human CO2 emissions.
Water
Combining water with a cementitious material forms a cement paste by the process of hydration.
The cement paste glues the aggregate together, fills voids within it and allows it to flow more easily.
Lower amounts of water in the cement paste will yield a stronger, more durable concrete; more water will give an easier-flowing concrete with a higher slump.
Impure water used to make concrete can cause problems when setting or premature failure of the structure.
Hydration involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the cement hydration process gradually bind the individual sand and gravel particles with other components of the concrete to form a solid mass.
Reaction
Cement chemist notation: C3S + H2O → CSH(gel) + CaOH
Standard notation: Ca3SiO5 + H2O → (CaO)•(SiO2)•(H2O)(gel) + Ca(OH)2 Balanced: 2Ca3SiO5 + 7H2O → 3(CaO)•2(SiO2)•4(H2O)(gel) + 3Ca(OH)2
Aggregates
Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel and crushed stone are mainly used for this purpose. Recycled aggregates (from construction, demolition and excavation waste) are increasingly used as partial replacements of natural aggregates, while a number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash are also permitted.
Decorative stones such as quartzite, small river stones or crushed glass are sometimes added to the surface of concrete for a decorative ‘exposed aggregate’ finish, popular among landscape designers.
Reinforcement
Concrete is strong in compression, as the aggregate efficiently carries the compression load. However, it is weak in tension as the cement holding the aggregate in place can crack, allowing the structure to fail. Reinforced concrete solves these problems by adding either metal reinforcing bars, glass fibre or plastic fibre to carry tensile loads.
Chemical admixtures
Chemical admixtures are materials in the form of powder or fluids that are added to the concrete to give it certain characteristics not obtainable with plain concrete mixes. In normal use, admixture dosages are less than 5% by mass of cement, added to the concrete at the time of batching/mixing.
Mineral admixtures and blended cements
There are inorganic materials that also have pozzolanic or latent hydraulic properties. These very fine-grained materials are added to the concrete mix to improve the properties of concrete (mineral admixtures) or as a replacement to Portland cement (blended cements).
A by-product of coal fired electric generating plants, Fly ash is used to partially replace Portland cement (up to 60% by mass). The properties of fly ash depend on the type of coal burnt. In general, silicious fly ash is pozzolanic, while calcareous fly ash has latent hydraulic properties.
Ground granulated blast furnace slag (GGBFS or GGBS), a by-product of steel production, is used to partially replace Portland cement (up to 80% by mass). It has latent hydraulic properties.
Silica fume is one of the by-products of the production of silicon and ferrosilicon alloys. Silica fume is similar to fly ash, but has a particle size 100 times smaller. This results in a higher surface to volume ratio and a much faster pozzolanic reaction. Silica fume is used to increase strength and durability of concrete, but generally requires the use of superplasticisers for workability.
High Reactivity Metakaolin (HRM): Metakaolin produces concrete with strength and durability similar to concrete made with silica fume. While silica fume is usually dark grey or black in colour, high reactivity metakaolin is usually bright white, making it the preferred choice for architectural concrete where appearance is important.
Q: What is the moisture content of concrete?
A: The moisture content of concrete is viewed from the context of total water content of the fresh concrete mixture and the available moisture content of the hardened concrete. The total water content of a fresh concrete mixture is a function of the total cementitious materials and water cement ratio (w/cm). Typical fresh concrete mixtures vary in cementitious material content in a
range of 279 kg/m3 to 415 kg/m3 (470 lb/yd3 to 700 lb/yd3). Water cement ratios typically vary from 0.4 to 0.55. To estimate the available moisture content of hardened concrete one must begin with the total water content of the fresh mixture and define the service condition of the hardened concrete with regard to relative humidity (%). In addition, the water that is chemically bound with the cement in the hydration process must be accounted for. The water bound with the cement is in the range of 0.22 to 0.24 of the cement content.
As an example, the moisture content of a concrete mixture with 334 kg/m3 (564 lb/yd3) of cement and a w/c of 0.45 and in a service environment with a 50% relative humidity could be estimated as follows:
Total water content:
334kg cement/m3 times 0.45 w/c ~ 150kg water/m3 (564lb cement/yd3 times 0.45 w/c ~ 254lb water/yd3) Chemically bound water at 0.24 w/c:
334kg cement/m3 times 0.24 w/c ~ 80kg water/m3 (564lb cement/yd3 times 0.24 ~ 135lb water/yd3) Moisture content:
150kg water/m3 – 80kg water/m3times .50 relative humidity ~ 35kg water/m3 (254lb water/yd3 – 135lb water/yd3 times .50 relative humidity ~ 60lb water/yd3)
In reality, the relative humidity of the concrete will only reach 50% at the near surface of the concrete and the moisture gradient with depth will increase toward 100% relative humidity; hence, this method of estimation would typically overstate the quantity of moisture available to leave the concrete due to the initial mixing of water.
This is only an estimate of the moisture available to leave the concrete, but it may help in gaining a perspective to the limited amount of water that the concrete can contribute when considering the drying time of hardened concrete.
Relative Humidity Profile Q: When was concrete first made?
A: 500BC
Q: What is the purpose of cement in concrete?
A: It acts as a primary binder that joins the aggregate into a solid mass.
Q: Why does concrete harden?
A: The chemical process called cement hydration produces crystals that interlock and bind together.
Q: How strong can concrete or cement be (in pounds per square inch (psi))? A: 50,000
Q: How long can concrete last (in years)? A: 50,000
Q: What are Type I/II or Type II/V cements?
A: Type I/II and Type II/V cements simply means that the cement complies with the requirements of ASTM C 150, Standard Specification for Portland Cement. It is quite common to find cements that comply with multiple cement designations such as Type I/II and Type II/V.
Q: How is white cement different and why is it used in decorative concrete?
A: There are only slight chemical and physical differences between grey Portland cement and white Portland cement. This is due to raw material differences and sometimes, though not always, slight differences in manufacturing. White cement has small amounts of the oxides (particularly iron and manganese) that impart the greyish colour normally associated with Portland cement.
Q. What are the decorative finishes that can be applied to concrete surfaces?
A: Adding pigment before or after the concrete is placed and using white cement rather than conventional grey cement, using chemical stains or exposing colourful aggregates at the surface may add colour to concrete. Textured finishes can vary from a smooth polish to the roughness of
gravel.
Geometric patterns can be scored, stamped, rolled, or inlaid into the concrete to resemble stone, brick or tile paving. Other interesting patterns are obtained by using divider strips (commonly redwood) to form panels of various sizes and shapes rectangular, square, circular or diamond.
Special techniques are available to make concrete slip-resistant and sparkling.
Q: What are the different forms of sulphate in Portland cement and how can we analyse cement for SO3?
A: Sulphates in Portland cement can be broadly categorised as:
- Added sulphates – gypsum, hemihydrates, anhydrite, several synthetic forms of sulphates (typically by-products like flue gas desulphurisation materials). Clinker sulphates include arcanite, aphthitalite, calcium langbeinite and thenardite. Although normally reported as SO3 (% by mass) for consistency, sulphur can be found in any combination of forms. Elemental sulphur is almost never found in Portland cement, except in trace
Added sulphates are blended with clinker during the final grinding of the cement, in amounts needed to control early setting properties as well as shrinkage and strength development. The amount needed varies depending on the chemistry and fineness of the cement, but is typically on the order of 5% by mass. The most common form of sulphate added to Portland cement is gypsum, some of which is intentionally dehydrated by the heat of grinding to form hemihydrates, which are more soluble and therefore available to control early hydration reactions.
Clinker sulphates form naturally during clinker production. These sulphates tend to volatilise at the temperatures of cement kilns (up to about 1450ºC) and condense on the outer surface of clinker nodules as alkali sulphates, during the last stage of clinker production (rapid cooling). Again, the amount depends on the chemistry of the raw materials and kiln operating conditions, making the cement somewhat unique. These alkali sulphates also are soluble enough to help control early hydration reactions. Some clinker sulphate is also incorporated into other cement phases.
Since cement is unique, chemical analyses are the best method of determining the SO3 content of cements. Typically the total SO3 content is measured (or elemental S measured and converted to SO3) through methods in ASTM C 114 (or AASHTO T 105). XRF analysis is probably the most common technique.
Q: What is air-entrained concrete?
A: Air-entrained concrete contains billions of microscopic air cells per cubic foot. These air pockets relieve the internal pressure on the concrete by providing tiny chambers for water to expand into when it freezes. Air-entrained concrete is produced through the use of air-entraining Portland cement, or by the introduction of air-entraining agents, under careful engineering supervision. The amount of entrained air is usually between 4% and 7% of the volume of the concrete, but may be varied as required by special conditions.
Q: What are recommended mix proportions for good concrete?
A: Good concrete can be obtained by using a wide variety of mix proportions if proper mix design procedures are used. The general custom is the rule of 6’s:
A minimum cement content of 6 bags per cubic yard of concrete A maximum water content of 6 gallons per bag of cement
A curing period (keeping concrete moist) a minimum of 6 days
An air content of 6% (if concrete will be subject to freezing and thawing)
Q: Will concrete harden under water?
A: Portland cement is a hydraulic cement, which means that it sets and hardens due to a chemical reaction with water. Consequently, it will harden under water.
Q: What does 28 -day strength mean?
A: Concrete hardens and gains strength as it hydrates. The hydration process continues over a long period of time; beginning rapidly and progressively slowing down. To measure the ultimate strength of concrete would require a wait of several years. This would be impractical, so a time period of 28 days was selected, by specification writing authorities, as the age that all concrete should be tested. At this age, a substantial percentage of the hydration has taken place.
Q: What is 3,000 pound concrete?
A: Concrete that is strong enough to carry a compressive stress of 3,000psi (20.7MPa) at 28 days is 3,000 pound concrete. Concrete may be specified at other strengths as well. Conventional
concrete has strengths of 7,000psi or less; concrete with strengths between 7,000 and 14,500psi is considered high-strength concrete.
Q: How do you control the strength of concrete?
A: The easiest way to add strength is to add cement. The factor that most predominantly influences concrete strength is the water to cement ratio in the cement paste that binds the aggregates together. The higher this ratio is, the weaker the concrete will be and vice versa. Every desirable physical property will be adversely affected by adding more water.
Q: What is alkali-silica reactivity (ASR)?
A: Alkali-silica reactivity is an expansive reaction between reactive forms of silica in aggregates and potassium and sodium alkalis, mostly from cement, but also from aggregates, pozzolans, admixtures and mixing water. External sources of alkali from soil, deicers and industrial processes can also contribute to ASR. The reaction forms an alkali-silica gel that swells as it draws water from the surrounding cement paste, thereby inducing pressure, expansion and cracking of the aggregate and surrounding paste. This often results in map-pattern cracks, sometimes referred to as alligator pattern cracking. ASR can be avoided through
- Proper aggregate selection
- Use of blended cements
- Use of proper pozzolanic materials
- Contaminant-free mixing water
Q. What are Supplementary Cementations Materials (SCM)?
A: Supplementary Cementations Materials (SCM) like silica fumes, meta-kaolin, fly ash, slag are the substances which improve the properties of concrete and enhance its durability, by reducing pore size in concrete through better particle distribution and through increased packing density of the concrete.
Q: Are there different types of Portland cement?
A: Though all Portland cement is basically the same, eight types of cement are manufactured to meet different physical and chemical requirements for specific applications:
Type I is a general purpose Portland cement suitable for most uses.
Type II is used for structures in water or soil containing moderate amounts of sulphate, or when heat build-up is a concern.
Type III cement provides high strength at an early state, usually in a week or less.
Type IV moderates heat generated by hydration that is used for massive concrete structures such as dams.
Type V cement resists chemical attacks by soil and water high in sulphates.
Types IA, IIA and IIIA are cements used to make air-entrained concrete. They have the same properties as types I, II and III, except that they have small quantities of air-entrained materials combined with them.
White Portland cement is made from raw materials containing little or no iron or manganese.
Q. Is there any shelf life of cement?
A: Cement is a hygroscopic material, meaning that in presence of moisture it undergoes chemical reaction termed as hydration. Therefore cement remains in good condition as long as it does not come in contact with moisture. If cement is more than three months old then it should be tested for its strength before being employed.
Q. How fineness of cement affects strength gain?
A: Finer cement particles imply more particles in unit weight. This enhances the reaction rate, which in turn will result in faster gain of strength at earlier stages.
Q: Why do concrete surfaces flake and spall?
A: Concrete surfaces can flake or spall for one or more of the following reasons:
In areas subjected to freezing and thawing, the concrete should be air-entrained to resist flaking and scaling of the surface. If air-entrained concrete is not used, there will be subsequent damage to the surface.
The water/cement ratio should be as low as possible to improve durability of the surface. Too much water in the mix will produce a weaker, less durable concrete, in turn leading to early flaking
and spalling of the surface.
The finishing operations should not begin until the water sheen on the surface is gone and excess bleed water on the surface has had a chance to evaporate. If this excess water is worked into the concrete because the finishing operations are begun too soon, the concrete on the surface will have too high a water content and will be weaker and less durable
Q: How do you remove stains from concrete?
A: Stains can be removed from concrete with dry or mechanical methods, or by wet methods using chemicals or water.
Common dry methods include sandblasting, flame cleaning, shotblasting, grinding, scabbing, planning and scouring. Steel-wire brushes should be used with care because they can leave metal particles on the surface that later rust and stain the concrete.
Wet methods involve the application of water or specific chemicals according to the nature of the stain. The chemical treatment either dissolves the staining substance so it can be blotted up from the surface of the concrete or bleaches the staining substance so it will not show.
To remove bloodstains, for example, wet the stains with water and cover them with a layer of sodium peroxide powder. Let stand for a few minutes, rinse with water and scrub vigorously. Follow with an application of a 5% solution of vinegar to neutralise any remaining sodium peroxide.
Q: What is Self-Consolidating concrete (SCC)?
A: SCC is a high-performance concrete that can flow easily into tight and constricted spaces without segregating and without requiring vibration. The key to creating SCC, also referred to as self-compacting, self-levelling, or self-placing concrete, is a mixture that is fluid, but also stable to prevent segregation.
To achieve the desired flowability a new generation of superplasticisers based on polycarboxylate ethers works best. Developed in the 1990s, they produce better water reduction and slower slump loss than traditional superplasticisers. The required level of fluidity is greatly influenced by the particular application under consideration. Obviously the most congested structural members demand the highest fluidity. However, element shape, desired surface finish, and travel distance can also determine the required fluidity.
Generally, the higher the required flowability of the SCC mix, the higher the amount of fine material needed to produce a stable mixture. However, in some cases, a viscosity-modifying admixture (VMA) can be used instead of, or in combination with, an increased fine content to stabilize the concrete mixture.
Q: The size of concrete cube is 150mm x 150mm x 150mm as per Indian Standards. Why?
A: Because the shape effect is the least for the 15cm cube and we get a fairly accurate idea of the strength of the concrete as such.
Q: How do you protect a concrete surface from aggressive materials like acids?
A: Many materials have no effect on concrete. However, there are some aggressive materials, such as most acids, that can have a deteriorating effect on concrete. The first line of defence against chemical attack is to use quality concrete with maximum chemical resistance, followed by the application of protective treatments to keep corrosive substances from contacting the concrete. Principles and practices that improve the chemical resistance of concrete include using a low water-cement ratio, selecting a suitable cement type (such as sulphate-resistant cement to prevent sulphate attacks), using suitable aggregates, water- and air-entrainment. A large number of chemical formulations are available as sealers and coatings to protect concrete from a variety of environments; detailed recommendations should be requested from manufacturers, formulators or material suppliers.
Q: Why does concrete crack?
A;Concrete, by nature, shrinks as it hardens. When concrete is placed on supporting soil or around steel reinforcement, the concrete mass is prevented from shrinking. This restraint creates internal forces exceeding the strength of concrete; cracks form to relieve these forces.
Q: Does the presence of cracks indicate a structural problem?
A: In most instances, the answer is no. Very narrow ‘hairline’ cracks are aesthetic in nature and do not indicate any structural problem. Cracks that have movement, i.e. where one side of the crack moves relative to the opposite side, should be investigated by a professional engineer.
Q: Why does concrete harden?
A: Concrete solidifies and hardens after mixing with water and placement due to a chemical process known as hydration. The water reacts with the cement, which bonds the other components together, eventually creating a stone-like material.
Q: What is concrete used for?
A: Concrete is used to make pavements, pipe, architectural structures, foundations, motorways/roads, bridges/overpasses, parking structures, brick/block walls and footings for gates, fences and poles.
Concrete is used more than any other man-made material in the world. As of 2006, about 7.5km3 of concrete is made each year—more than 1m3 for every person on earth.
Q: What are the more popular types of concrete in use?
A: Reinforced concrete and prestressed concrete are the most widely used modern kinds of functional concrete extensions.
Q: What evidence is there for the long life of concrete?
A: The widespread use of concrete in many Roman structures has ensured that many of them have survived. The Baths of Caracalla is just one example of the longevity of concrete, which allowed the Romans to build this and similar structures across their Empire. Many Roman aqueducts and Roman bridges have masonry cladding to a concrete core, a technique they used in structures such as the Pantheon, the dome of which is concrete.
Q: Who discovered concrete?
A: The Romans used concrete in their structures but the secret had been lost for 13 centuries until 1756, when the British engineer John Smeaton pioneered the use of hydraulic lime in concrete, using pebbles and powdered brick as aggregate. Portland cement was first used in concrete in the early 1840s. This version of history has been challenged however, as the Canal du Midi was constructed using concrete in 1670.
Q: What is the role of water in concrete mix?
A: Combining water with cementitious material forms a cement paste by the process of hydration. The cement paste glues the aggregate together, fills voids within it and allows it to flow more easily.
Less water in the cement paste will yield a stronger, more durable concrete; more water will give an easier-flowing concrete with a higher slump.
Impure water used to make concrete can cause problems when setting or in causing premature failure of the structure.
Hydration involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the cement hydration process gradually bind the individual sand and gravel particles with other components of the concrete, to form a solid mass.
Q: How do aggregates affect the strength of concrete?
A: Concrete has a high compressive strength, as the aggregate efficiently carries the compression load. However, it is weak in tension as the cement holding the aggregate in place can crack, allowing the structure to fail. Reinforced concrete solves these problems by adding either metal reinforcing bars, steel fibres, glass fibre or plastic fibre to carry tensile loads.
Q. What are the reasons for slow or fast setting of concrete or mortar?
A: the rate of setting normally depends on the nature of the cement. It could also be due to extraneous factors not related to the cement. Ambient conditions also play an important role. In hot weather, concrete sets faster, whereas in cold weather, setting is delayed. Some salts, chemicals, clay etc., if inadvertently mixed with the sand, aggregate and water could accelerate or delay the setting of concrete.
Q: What do grade numbers indicate?
A: The grade number indicates the minimum compressive strength of cement sand mortar in N/mm2 at 28 days.
Q. What is slag?
A: Slag is a non-metallic product, essentially consisting glass containing silicates, alumino-silicates
of lime and other bases, and is obtained as a by-product in the manufacture of pig iron in blast or electric furnaces. Granulated slag is used in the manufacture of Portland Slag Cement (PSC).
Q. How is PSC made?
A: PSC is made by intergrading clinker, granulated blast furnace slag and gypsum or by blending ground slag with Portland cement.
Q. Where can PSC be used?
A: Slag cement can be used for all plain and reinforced concrete constructions and mass concreting structures such as dams, reservoirs, swimming pools, river embankments, bridge piers etc. It is used with advantage where low heat of hydration and resistance to alkali-silica reactions are desired; for structures in aggressive environments where chemical and mildly acidic waters are encountered (where the use of OPC is not recommended) and for marine constructions, dykes, wharves, etc where sulphuric water is encountered. In short, PSC can be used wherever OPC is used.