09 - portland cement

Portland Cement

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ACI Definition

portland cement (n.) a cementitious productmade by heating raw materials containing oxides ofaluminum, silicon, and calcium to temperaturesapproaching 1500°C, then pulverizing the end productwith a small amount of gypsum.

Origins

In 1824, bricklayer Joseph Aspdin patented a materialhe called “Portland cement” because it had a colorsimilar to that of a popular building stone quarriedon the Isle of Portland off the coast of England. Thiswas strictly a marketing move!

He made his cement by grinding up local limestone,adding water and clay to make a slurry, drying it,heating it in a kiln (a process called clinkering) andthen grinding the fired product into a fine powder.

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Limestone QuarryIsle of Portland

Origins

Joseph’s portland cement was mixed with water tocreate a mortar that could be used to stucco buildings.When the mortar cured, it has a color very similar toPortland stone.

Unfortunately, Aspdin’s product wasn’t really muchbetter than other products in use at the time. Joseph’sson William was actually the one who perfected theprocess for creating what we know today as portlandcement.

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William Aspdin

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https://en.wikipedia.org/wiki/William_Aspdin

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Primary Ingredients

lime (CaO) ‒ 61‐67%limestone, marble, chalk, marl, calcite, seashells, blast furnace slag

silica (SiO2) ‒ 19‐23%clay, loess, shale, sand, sandstone, quartzite, fly ash, rice‐hull ash

alumina (Al2O3) ‒ 2‐6%clay, loess, shale, bauxite, fly ash

iron oxide (Fe2O3) ‒ 0‐6%clay, shale, iron ore, pyrite, blast furnace flue dust

Cement Manufacture

Today, there are two general methods used to makeportland cement: the dry process and the wet process.

In the dry process, the raw materials are crushed to anominal size of ¾" then fed into a grinding mill. Theresulting powder is fed into silos where the variousingredients are proportioned and blended by addingcompressed air to “fluidize” the power so it can beintimately mixed. The powder is then fed into thekiln where it is transformed into clinker.

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Cement Manufacture

In the wet process, hard materials like limestone arefirst crushed, then fed into a ball mill along with claydispersed in water to form a slurry. This allows theingredients to be intimately blended into a uniformmixture that is then fed directly into the kiln.

Compared to the dry process, the wet process is asimpler operation, produces much less dust, blendsthe ingredients better to produce a higher quality endproduct, but uses more energy to drive off the water.

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Cement Manufacture

After the raw materials enter the kiln, they go througha series of transformations as they travel along thelength of the kin, getting ever closer to the burnerend of the kiln where the temperatures reach 1500°C.

First, water is driven off to dry the material. Next,the limestone is calcined to produce lime and carbondioxide. Then sintering occurs. This is where theingredients partially melt and recombine into theminerals responsible for cement’s properties.

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Cement Manufacture

As the semi-molten material approaches the end ofthe kiln, it cools into roughly golf-ball-sized, darkgreen lumps of clinker.

The clinker is then fed, along with a small amount(2% to 5%) of gypsum, into a grinding mill where itis reduced to a fine, gray-green powder.

During grinding, certain other materials such as flyash can be added to enhance the cement’s properties.

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Inside a Cement Kiln

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drying

Total time in kiln = 60‐90 minutes

400ºC1500ºC 800ºC

cooling

clinker


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3 2825

heatC

CaCO CaO CO

calcining

400ºC1500ºC 800ºC

cooling

clinker 1 ton of cement ½ ton of CO2


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sintering

C2Sforms

400ºC800ºC1500ºC

2 2 412002 heat

CCaO SiO Ca SiO

cooling

clinker


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sintering

C3Sforms

400ºC800ºC1500ºC

2 4 3 51250

heatC

CaO Ca SiO Ca SiO

cooling

clinker


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sintering

C3Aforms

400ºC800ºC1500ºC

2 3 3 2 613003 Ca Al Oheat

CCaO Al O

cooling

clinker


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C4AFforms

400ºC800ºC1500ºC

3 2 6 2 3 4 2 2 10CaO Ca Al O Fe O Ca Al Fe O

cooling

clinker

Cement Phases

Portland cement is actually a chemically complexmaterial composed of 4 major compounds (phases):

• Tricalcium silicate (Ca3SiO5)• Dicalcium silicate (Ca2SiO4)• Tricalcium aluminate (Ca3Al2O6)• Tetracalcium aluminoferrite (Ca4Al2Fe2O10)

Each contributes different properties to the cement.

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Cement Phases

Dicalcium silicateC2S (Ca2SiO4)

Tricalcium silicateC3S (Ca3SiO5)

Tricalcium aluminateC3A (Ca3Al2O6)

Tetracalcium aluminoferriteC4AF (Ca4Al2Fe2O10)

Cement Phases

Tricalcium silicate (C3S) hydrates and hardens fairlyquickly and is largely responsible for initial settingand early strength gain.

Dicalcium silicate (C2S) hydrates and hardens slowlyand is largely responsible for long-term strength gain.

Tricalcium aluminate (C3A) hydrates and hardensthe quickest, liberating a large amount of heat in theprocess. It is primarily responsible for setting.

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Cement Phases

Gypsum is added to portland cement to retard C3Ahydration. Without the gypsum, C3A hydration wouldcause the portland cement to set almost immediatelyafter adding water.

C3A reacts poorly when exposed to sulfates (MgSO4and NaSO4 salts) that naturally occur in groundwater,seawater, and some clayey soils. The reaction causesthe concrete to expand and crack. Sulfate resistancecement has a low C3A concentration.

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Cement Phases

Tetracalcium aluminoferrite (C4AF) hydrates rapidlybut contributes very little to setting or strength gain.Its presence allows for lower kiln temperatures in themanufacturing process, which is why ferrous materialsare added to the raw ingredients.

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Cement Phases(Taken from Cement and Concrete by M.S.J. Gani)

Cement Phases

Characteristic C3S C2S C3A C4AFRate of hydration Med Slow Fast FastHeat of hydration Med Low High LowEarly strength High Low Med LowUltimate strength High High Low LowSulfate resistance Good Good Poor Good

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Responsiblefor

Short‐termHardening

Responsiblefor

Long‐termHardening

Responsiblefor

InitialSetting

Setting = transformation of cement paste from fluid to gel to solid

Hardening = gain in strength after concrete gel has become a solid

Rate of Strength Gain

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Cement Composition

A typical portland cement contains around 50% C3S,25% C2S, 12% C3A, 8% C4AF, 4% gypsum, and 1%other compounds.

By varying the proportions of the raw ingredientsand things like the temperatures and dwell times inthe various areas of the kiln, we can create portlandcements with different properties.

ASTM C150 recognizes eight basic types of portlandcement: Types I, IA, II, IIA, III, IIIA, IV, and V.CIVL 3137 27

Cement Composition

TricalciumSilicate

50%

DicalciumSilicate

25%

Tricalcium Aluminate

12%

Tetracalcium Aluminoferrite

8%

Gypsum4%

Other1%

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Typical Percentages by Weight(Mindess and Young, 1981)

Types of Portland Cement

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Type Name C3S C2S C3A C4AFI Normal 50 24 11 8II Modified 42 33 5 13III High early 60 13 9 8IV Low heat 26 50 5 12V Sulfate-resistant 40 40 3.5 9

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Responsiblefor

Short‐termHardening

Responsiblefor

Long‐termHardening

Responsiblefor

InitialSetting

Setting = transformation of cement paste from fluid to gel to solid

Hardening = gain in strength after concrete gel has become a solid


Changing the proportions of the various phases is azero-sum game because they always total 100%.

For example, if you increase the C3A and C3S toincrease the early strength gain, there will be lessC2S available to provide late strength gain. As aresult, Type III cement gains a lot of strength in thefirst few days, but then the strength gain slows to acrawl. Also, because C3A and C3S hydrate rapidly,Type III cement produces more heat of hydration.

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If you want to produce low-heat cement, you need tolimit the phases with the highest heat of hydration(C3A and C3S), but those phases are responsible forsetting and early strength gain, so Type IV cementdevelops strength more slowly than the other types.However, removing C3A and C3S means the C2S ismore prevalent, leading to better long-term strengthdevelopment.

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Rate of Strength Gain

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“High Early”

“Low Heat”

Rate of Heat Generation

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Time (days)

“High Early”

“Low Heat”

Cement Hydration

When cement comes into contact with water, anexothermic chemical reaction called hydration occurs.

The hydration of the C3S and C2S is responsible forthe strength of concrete. When both compounds aremixed with water, they create calcium silicate hydrategel (CSH) and calcium hydroxide crystals (CH). TheCSH is what glues the aggregate particles together toform concrete.

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Hydration Chemistry

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C‐S‐HCH

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Hydration Chemistry

2 4 2 2 2 22 4 3 2 3Ca SiO H O CaO SiO H O Ca OH

dicalciumsilicate

water

calcium silicate hydrate

calciumhydroxidecrystals

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Hydration Chemistry

3 4 2 2 2 22 6 3 2 3 3Ca SiO H O CaO SiO H O Ca OH

tricalciumsilicate

water



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Hydration Chemistry

CS H CSH CH

calciumsilicate

water



Cement Hydration

The hydration process is actually quite complex.

Soon after mixing, the C3A reacts with the water toform an aluminate-rich gel (Stage I). The gel reactswith sulfate in solution to form small needle-likecrystals of ettringite. The C3A reaction with water isstrongly exothermic but does not last long, typicallyonly a few minutes, and is followed by a period of afew hours of relatively low heat generation (Stage II).

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Cement Hydration

The first half of the dormant period corresponds towhen concrete can be placed. After that, the pastebecomes too stiff to be workable.

At the end of the dormant period, the C3S and C2Sstart to react, forming calcium silicate hydrate andcalcium hydroxide (Stage III). This stage is when theconcrete turns from a paste into a solid and starts togain strength.

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Cement Hydration

During the next stage (Stage IV) the reaction rateslows as the C3S continues to hydrate and the concretegains more strength.

As the individual cement grains hydrate from thesurface inward, C3A hydration also resumes as freshcrystals become exposed to water.

After about 12-24 hours, the rate of heat generationtapers off to a low steady state (Stage V).

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Cement Hydration

Initial Set(2-4 hours)

Final Set(6-8 hours)

C3 S & C3 Ahydration

(renewed)

(log scale)

(15 minutes)

Steady State(12-24 hours)

Initial and Final Set

Shortly after the end of Stage II, the concrete reachesinitial set. Up until this point, the concrete is actuallya gel and, if remixed, will return to its original fluidconsistency. This is known as thixotropy. It is thereason why ready-mix trucks have revolving drums:they keep the concrete fluid until it is ready to beplaced. Several hours later (near the end of Stage III)the concrete reaches final set. At this point it is solidenough to resist penetration, but it doesn’t have anyappreciable structural strength.CIVL 3137 45

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Setting Time Factors

amount of C3Aamount of CaSO4cement fineness } cement type

Effects of Fineness

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Source: www.theconstructioncivil.com

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Setting Time Factors

amount of C3Aamount of CaSO4cement fineness

amount of mixing wateramount of sun/wind

ambient temperature } placing conditions

} cement type

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Hardening Factors

amount of C3S

amount of C2S

ambient temperature

ambient humidity } curing conditions

} cement type

Effects of Water Loss

If there is not enough water present for the hydrationreaction to occur, or the cement is robbed of waterduring hydration, the concrete may gain little or nostrength.

It takes roughly 25 g of water to completely hydrate100 g of cement. For each 1% decrease in the wateravailable to hydrate the cement, there is a 15% lossin the 28-day compressive strength.

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Relative effects of water lossduring the first three days on28-day compressive strength


That’s why it’s important to make sure the concretedoesn’t lose water while it is curing. In the field, thisis accomplished by doing things like covering theconcrete with plastic sheeting or wax-based spray-oncoatings, or spraying it with water, or ponding wateron top of the concrete.

In the laboratory, concrete specimens can be put intowater baths or stored in a humid room that keeps therelative humidity around the concrete near 90%.

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Concrete that has a high water/cement ratio is lesssusceptible to strength loss from improper curingbecause the amount of water is more than is neededfor hydration.

Every day the concrete spends protected from waterloss adds to the final strength of the concrete. If youcan keep the concrete covered for at least 7 days, youcan achieve the 28-day compressive strength calledfor in the specifications.

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Stored in laboratory air after 3 days

Stored in laboratory air after 7 days

Stored continuously in laboratory air

Effects of Temperature

Like many chemical reactions, ambient temperaturewill affect the speed of the hydration reaction. Up toa point, the rate of hydration (and therefore strengthgain) increases with increasing temperature. Beyondabout 60°C (140°F), though, increasing temperatureshurt the strength.

On the other end of the scale, at low temperatures, thehydration reaction slows to a crawl, making winterconcreting difficult.

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Effects of Temperature

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Effects of Steam-Curing

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Effects of Steam-Curing

09 - portland cement

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