Questions & Answers about Cement 

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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.
1. Cement mixed with water, sand and gravel, forms concrete.
2. Cement mixed with water and sand, forms cement plaster.
3. Cement mixed with water, lime and sand, forms mortar.
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.
2) 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 pieces.
3) The limestone pieces then go through a blender where they are mixed with the other raw
materials in the right proportion.
4) Raw materials are then ground to a powder. This is sometimes done with rollers that crush the
materials against a rotating platform.
5) 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’.
6) 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 cement.
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 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 periodfrom,
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?

A. 1. The concrete mix is extremely workable. It can be placed and consolidated properly.
2. 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.
3. 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:
1. 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 amounts.
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