Cement Its Chemistry and Properties

Chemistry for Everyone

JChemEd.chem.wisc.edu • Vol. 80 No. 6 June 2003 • Journal of Chemical Education 623

One of the most active areas in scientific research is thedevelopment of new and exciting materials for a wide vari-ety of applications. In this context, it could be easy to losesight of the importance of more common materials that arevitally important in many areas of our lives. Cement is onesuch material, and its rich chemistry links well with a num-ber of concepts in most undergraduate chemistry curricula.

This paper addresses several important questions con-cerning cement, including: What is its optimal compositionand why? Why do cement truck barrels roll? What are theprocesses involved in cement setting, and how long does ittake? How does cement break down?

A Brief History of Cement

Cements and cement-containing materials comprisedsome of the first structural materials exploited by humanity(1), as cement’s components are common materials: sand,lime, and water. On a molecular level, cement is a paste ofcalcium silicate hydrates polymerized into a densely cross-linked matrix (2). Its most important property is calledhydraulicity—the ability to set and remain insoluble underwater (3, 4). Cement can be used as a mortar to bind largestones or bricks. When sand and stones are added to cement,the aggregate is called concrete. The word cement comes fromthe Latin phrase, opus caementum, or chip work, in referenceto the aggregate often used in applications (3).

Cement production dates back to the ancient Romans,who produced mortars using a mixture of lime, volcanic ash,and crushed clay. These cements are referred to as Pozzolaniccements after the Pozzulana region of Italy, which containedItaly’s chief supply of ash (1, 5 ). Pozzolanic cements derivetheir strength from rich aluminate phases present in the vol-canic ash that promote efficient hydration of the final ce-ment powders (6). Fine grinding and attention to consistencyare also fundamental to the success of Roman cement, muchof which is still in existence today in structures such as thePantheon, the Pont du Gard, and the Basilica ofConstantinople (2, 5). An example of a structure made withRoman cement is shown in Figure 1.

The art of cement production was lost in Europe afterthe fall of the Roman Empire (2, 5). At that time, the accessto volcanic ash was limited and the grinding and heating tech-niques required for cement precursor production were lost.Cements of this period, if still in existence, are inconsistentin composition and are composed almost exclusively of un-reacted starting materials (1, 2, 5). There was no significantbreakthrough in the development of cement chemistry until1756, when Smeaton was commissioned to rebuild theEddystone lighthouse in Cornwall, England. In contrast to

the methods of his contemporaries, Smeaton found superiorresults through experimentation by using an impure lime-stone with noticeable clay deposits. This produced extremelystrong cement “that would equal the best merchantable Port-land stone in solidity and durability”(5).1

Another major advance came in the early 19th centurywhen the French engineer Vicat performed the first empiri-cal study on the composition of cements. Although crudeand incomplete, it was one of the most comprehensive ex-aminations of cement chemistry for the next 80 years (3, 4,8–10).

The term Portland cement did not become officially rec-ognized until 1824 when Aspidin filed the first patent for itsproduction (2, 5). Cement compositions at this time werepoorly understood but closely guarded secrets. Portland ce-ment was introduced into the United States by Saylor in 1871(3, 4).

By the start of the 20th century, cement manufacturewas common but was still regarded as more of an art than ascience. Emphasis was placed on bulk manufacture, not qual-ity control or consistency (10, 11). Early in the 20th cen-tury, cement research became more scientific, incorporatingthe relatively new Gibbs phase rule and Le Châtelier equi-librium principles (3). In 1904 the first set of ASTM stan-dards2 for cement were presented and in 1906 the geophysicallaboratory of the Carnegie Institution began an extensive in-vestigation of cement chemistry. These advances resulted inthe development of uniformity in the cement industry, al-lowing a rapid expansion in the application of cement to largeconstruction projects such as skyscrapers, roads, and dams(2, 3, 8, 11).

Cement: Its Chemistry and PropertiesDouglas C. MacLaren and Mary Anne White*Department of Chemistry and Institute for Research in Materials, Dalhousie University, Halifax, Nova Scotia B3H 4J3,Canada; *[email protected]

Products of Chemistryedited by

George B. KauffmanCalifornia State University

Fresno, CA 93740

Figure 1. Roman aqueduct in Segovia, Spain, from the first cen-tury C.E. Courtesy Stephen L. Sass. Reproduced, with permission,from ref 1.

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624 Journal of Chemical Education • Vol. 80 No. 6 June 2003 • JChemEd.chem.wisc.edu

More recent advances in materials-characterization tech-niques, such as X-ray crystallography, electron microscopy,nuclear magnetic resonance spectroscopy, Mössbauer spec-troscopy, infrared spectroscopy, and thermal analysis, haveallowed the systematic examination of cement’s chemistry andthe complex processes surrounding its production and hy-dration (2, 12). Scientific research has led to a better under-standing of the properties of cement, cement production, andcement corrosion. In fact, breakthroughs in cement researchhave provided us with cements of increasing quality andstrength.

Cement is prepared in a two-step process. The first stepis the high-temperature mixing and processing of limestone,sand, and clay starting materials to produce a cement pow-der. The second step involves the hydration, mixing, and set-ting of the cement powder into a final cement product (2, 6,13). The dry portion of Portland cement is composed ofabout 63% calcium oxide, 20% silica, 6% alumina, 3%iron(III) oxide, and small amounts of other matter includ-ing possibly impurities (7). Calcium silicates and calcium alu-minates dominate the structure.

The cement literature uses abbreviations for the manycalcium oxide, silicate, aluminate, and ferrate compoundsimportant to cement. We have used the same abbreviationshere and present the correspondence between the chemicalformulas and abbreviations in Table 1 (14).

Cement Formation

Preparation of Cement Precursors: ClinkersThe raw materials for cement production are blended

in the required proportions, ground, and heated to high tem-peratures, usually with rotation. Heating first releases H2Oand CO2 and then causes other reactions between the solids,including partial melting. Cooling results in clinkers, a termfrom the coal industry in the 19th century to describe stony,heavily burnt materials that were left after the burning of coal(7). Ironically, Aspiden and Vicat both dismissed the hardglassy clinker material (which was expensive to grind) as be-ing useless to cement manufacture (8, 11), although we nowknow that clinkers are essential for good cement production.After heating, cement clinkers are reground for use in theproduction of cement. Commercial cement manufacture in-corporates a wide variety of minerals, including: calcium ox-ide, silica, alumina, iron oxide, magnesium oxide, titaniumdioxide, and many others (5, 14). Of these, three are mostimportant to the final cement product: calcium oxide, silica,and alumina. Consideration of all the possible phases pro-duced by these multicomponent systems is simplified by con-sidering a ternary system of primary importance—the calciumoxide�silica�alumina system (14).

High-quality cement powders require the presence of twomajor components, tricalcium silicate, ‘C3S’, and dicalcium

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silicate, ‘C2S’, in the clinkers. These materials react vigorouslywith water to produce the cement paste formed in the finalproduct. Of the two, tricalcium silicate is the more desirableclinker material because it hydrates and sets much faster thandicalcium silicate (hours for ‘C3S’, days for ‘C2S’) (2, 15).

The binary phase diagram of SiO2 and CaO is shownin Figure 2 (16, 17). Most important is the 0–30 mass %SiO2 region. ‘C3S’ is formed at less than 30 mass % SiO2but is not stable below about 1250 �C or above about 2200�C. In the low end of this temperature range ‘C3S’ will form,but extremely slowly because it involves a reaction betweentwo solid phases. For example, forming ‘C3S’ at temperaturesof 1200–1400 �C would require heating for days and is noteconomical. At the other end, production from the melt at2200 �C is also impractical because of the very high tempera-ture.

Therefore, the temperature of the ‘C3S’ production forthe clinker is lowered by fluxing3 the reaction mixture with athird component, alumina (7, 14, 15). The binary phase dia-gram of CaO and Al2O3 is shown in Figure 3 (16). Com-parison with the ternary CaO�SiO2�Al2O3 phase diagram (7,14, 16), Figure 4, shows that the addition of Al2O3 lowersthe preparation temperature of ‘C3S’.

For this discussion, the important region of theCaO�SiO2�Al2O3 phase diagram is the ‘C3S’�‘C2S’�‘C3A’phase field, the region close to the CaO vertex in Figure 4. Athree-dimensional view of the ternary phase diagram in thisregion is shown in Figure 5. As the temperature of the sys-

Figure 3. The binary phase diagram of calcium oxide and alumina(Al2O3). The temperature of the liquidus of the binary system de-creases significantly as Al2O3 is added to the mixture (13).

Al 2O3

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mass % Al2O3

Tem

pera

ture

/ °C

(CaO) ( )

Figure 2. The binary phase diagram of calcium oxide and silicondioxide. The region of interest is 0–30 mass % SiO2 wheretricalcium silicate (‘C3S’) is formed (14).

1500

2500

1000

2000

20 40 60 80(CaO) SiO2( )

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Figure 5. Three-dimensional view of the CaO-rich portion of theternary phase diagram of calcium oxide, silica, and alumina em-phasizing the tricalcium silicate primary phase field. The composi-tion of the liquid will follow the minimum path along the liquidus,which deeply slopes into the tricalcium silicate phase field as thetemperature of the system is lowered from 2150 �C to 1450 �C (5).

Figure 4. The ternary phase diagram of calcium oxide, silica, andalumina. The region nearest the CaO vertex represents the primaryphase field for the formation of tricalcium silicate. Temperaturesare presented in �C (13, 16).

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tem decreases from about 2100 �C, the composition of theliquid goes into the ‘C3S’�‘C2S’�‘C3A’ phase field (5). Add-ing 20 mass % alumina to a silica�lime system lowers theliquidus into the region of stable ‘C3S’ formation, from thereaction of ‘C2S’ and CaO in the liquid phase. This, of course,is much faster than the solid–solid reaction (5, 7, 15, 18).Therefore, heating a composition in the ‘C3S’ phase field to1450–1500 �C results in a liquid phase that can be quenchedto form the final ‘C3S’-rich cement clinker. Although nei-ther Smeaton nor the Romans fully realized the chemistry, itwas the addition of rich aluminate matter in the form of vol-canic ash or clay impurities that allowed their production ofstrong cement precursors (5).

The total process of cement-clinker formation is sum-marized in Figure 6, which shows the main components as afunction of temperature (18). Calcium carbonate (limestone),quartz, clay (primarily Al2O3), and water are combined andheated. (Iron oxide, clay, and other minor components areneglected in this discussion.) As the temperature rises, firstwater is lost, and then above 700 �C, the limestone decom-poses forming CaO and carbon dioxide. CaO reacts withsilica to form ‘C2S’ and with the aluminate phases to form acalcium aluminate phase (an Ettringite phase4), which meltsat about 1450 �C (18). The formation of this liquid phase isassociated with the rapid production of tricalcium silicate.The final mixture at 1500 �C is primarily tricalcium silicatewith smaller portions of dicalcium silicate, aluminate, andaluminoferrate phases.

The minor components present in cement paste (e.g.,iron oxide) have only subtle effects on the properties of thefinal cement properties (5, 14). One of the reasons for usingthem is that they also help to flux the system to a lower tem-perature. Table 2 lists a group of multicomponent clinker ma-terials in the ‘C3S’ phase field. It is apparent that adding smallamounts of other minerals can lower the temperature at whicha liquid phase is formed (5).

After quenching, the resulting clinker is milled andground into a fine powder. At this stage various other mate-rials can be added to the cement powder prior to packaging.

Cement HydrationCement hydration is a familiar process. The cement pow-

der is mixed with water and then is poured for the desiredapplication. The final cement product generally containsabout 30–40 mass % water after hydration, and this valuevaries little with the composition of the cement clinker. Al-though it might appear simple, cement hydration consists ofa complex series of chemical reactions, which are still notcompletely understood (13). Cement hydration rates can beaffected by a variety of factors, including: the phase compo-sition of the clinker, the presence of foreign ions, the spe-cific surface of the mixture, the initial water:cement ratio,the curing temperature, and the presence of additives (13,18).

The rate of hydration of ‘C3S’ in a Portland cement clin-ker is shown in Figure 7. Immediately upon contact withwater ‘C3S’ undergoes an intense, short-lived reaction, thepre-induction period (I). The rate (dα/dt, where α is the de-gree of hydration or the fraction of cement precursor mate-rial that has been hydrated) is as high as 5 day�1. This processbegins with the dissolution of ‘C3S’. Oxygen ions on the sur-

face of the ‘C3S’ lattice react with protons in the water andform hydroxide ions, which in turn combine with Ca2� toform Ca(OH)2 (13):

OH�(aq)O2�(lattice) + H�(aq) (1)

Ca(OH)2(aq)2OH�(aq) + Ca2�(aq) (2)

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OiS–OaC 2 lA– 2O3 OgM– 5731

OiS–OaC 2 lA– 2O3 eF– 2O3 0431

OiS–OaC 2 lA– 2O3 aN– 2 OgM–O 5631

OiS–OaC 2 lA– 2O3 aN– 2 eF–O 2O3 5131

OiS–OaC 2 lA– 2O3 eF–OgM– 2O3 0031

OiS–OaC 2 lA– 2O3 aN– 2 eF–OgM–O 2O3 0821

Figure 6. A schematic view of the components of cement-clinkerformation, their reactions, and the products formed as the tempera-ture of the mixture is raised. Calcium carbonate decomposes toform calcium oxide and carbon dioxide. Calcium oxide reacts withsilica to form dicalcium silicate at temperatures below 1250 �C,which converts to tricalcium silicate at temperatures above 1250�C. Formation of a liquid aluminate, Ettringite, phase at about 1450�C facilitates the conversion of dicalcium silicate to tricalcium sili-cate (18).






At the same time, silicate material from the ‘C3S’ lattice sur-face enters the liquid phase (13):

HnSiO4(4-n)�(aq)SiO4

4�(lattice) + nH�(aq) (3)

The dissolved components combine to form the calciumsilicate hydrate ‘CSH’ gel, an amorphous two-componentsolid solution composed of Ca(OH)2 and a calcium silicatehydrate of low Ca:Si ratio, hydrated as in this example (13,19, 20):

3CaO�2SiO2�3H2O(s) + 3Ca(OH)2(aq)

2(3CaO�SiO2)(s) + 6H2O(l) (4)

However, the reaction would not likely be of this exact sto-ichiometry.

Most cement powders have gypsum (CaSO4) added priorto packaging. Gypsum acts to slow down the pre-inductionperiod to avoid rapid setting of the cement (3, 8). It reactswith tricalcium aluminate (‘C3A’) to form various aluminateand sulfoaluminate phases, collectively referred to as Ettringitephases (7, 13, 15, 19). Some examples are:

3CaO�Al2O3�3CaSO4�32H2O(s)

3CaO�Al2O3(s) + 3CaSO4(s) + 32H2O(l) (5)

3CaO�Al2O3�3CaSO3�12H2O(s)

3CaO�Al2O3(s) + 3CaSO4(s) + 12H2O(s) (6)

‘C3A’ and ‘C4AF’ can also hydrate independently of calciumsulfate:

3CaO�Al2O3�6H2O(s)3CaO�Al2O3(s) + 6H2O(l) (7)

3CaO�Al2O3�6H2O(s) + 3CaO�Fe2O3�6H2O(s)

4CaO�Al2O3�Fe2O3(s) + 2Ca(OH)2(aq) + 10H2O(l) (8)

During the pre-induction period about 5–25% of the ‘C3A’and ‘C4AF’ undergoes hydration, causing a saturation ofEttringite in the solution (13).

After a few minutes of hydration an induction period(II in Figure 7) begins where the reaction slows signifi-cantly, dα/dt = 0.01 day�1. The exact reason for this in-duction period is not known. Several theories have beenproposed that involve some sort of mixture saturation fromthe intense burst of hydration in the pre-induction period(13). One theory states that the ‘CSH’ layer quickly cov-ers the surface of dissolving ‘C3S’, slowing the reaction.As time passes, the ‘CSH’ becomes more permeable andthe reaction accelerates. Another theory states that the so-lution may become supersaturated with Ca(OH)2 becausethe surfaces of Ca(OH)2 crystal nuclei are poisoned by sili-cate ions. The high concentration of aqueous Ca(OH)2limits the rate of dissolution of the silicate species to neg-ligible rates. Eventually the level of aqueous Ca(OH)2 be-comes too high and calcium hydroxide crystallizes,allowing the hydration reactions to continue. Anothertheory speculates that two types of ‘CSH’ are formed. Therate of “first-stage” ‘CSH’ is dependent on the concentra-tion of aqueous Ca(OH)2. As the concentration of aque-ous Ca(OH)2 decreases, the production of “first-stage”‘CSH’ stops, causing induction. Hydration resumes laterwhen the thermodynamic barrier for the nucleation of“second-stage” ‘CSH’ is overcome (13).

At any rate, an induction period occurs and varies intime depending on the type of cement and the desiredapplication, usually lasting several hours. This property of ce-ment hydration is what makes it easy to use as a construc-tion material—it is a semi-solid that can be easily poured intodesired shapes for application. Aqueous gels are often semi-solid owing to interaction between water molecules and thesurfaces of the particles. Mixing of the system provides energyto overcome these interactions and allows the gel to becomemore fluid. In the case of cement mixtures, constant mixingis required to keep the material in a fluid state (15). This iswhy wet cement is often stored in large rotating drums untilit is poured. During this induction time, so long as it iscontinuously mixed, the cement can be held ready forpouring.

Following induction, the reaction rate accelerates to ap-proximately dα/dt = 1 day�1. At this point the hydration pro-cesses are limited by the nucleation and growth of thehydration products. This acceleration stage (III in Figure 7)is characterized by rapid hydration of ‘C3S’, followed slowlyby the hydration of ‘C2S’ (13):

Figure 7. A graphic representation of the rate of consumption oftricalcium silicate (‘C3S’) as a function of hydration time: (A) changesin hydration rates in the first few hours as a result of (I) pre-induc-tion, (II) induction, (III) acceleration, and (IV) deceleration processes.(B) an expanded view showing the length of time required for com-plete cement hydration (13).

Hydration Time / h

Frac

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Hyd

rate

d

50 10 150.00

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0.10

0.15

0.20

0.25

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B






3CaO�2SiO2�3H2O(s) + Ca(OH)2(aq)

2(2CaO�SiO2)(s) + 4H2O(aq) (9)

During this process, calcium hydroxide reaches its maximumconcentration in the solution and then begins to precipitateout as crystalline calcium hydroxide, referred to as Portlanditeby cement chemists (7, 13, 15). As the solution becomes con-centrated with solid product the rate of hydration slows andbecomes diffusion controlled. The reactions slow to nearlynegligible rates but continue for weeks as the ‘CSH’ gel con-tinues to form.

Calcium Silicate Hydrate (‘CSH’) Gel Formation:NMR Studies

In its final form, cement is a suspension of calcium hy-droxide, Ettringite, and unreacted clinker materials in a solidsolution of mineral glue called ‘CSH’ gel (13, 15). The for-mation of ‘CSH’ gel is vital to the understanding of cementhydration processes.

One of the most powerful tools for studying the reac-tions of cement hydration is solid-state nuclear magnetic reso-nance spectroscopy (21–23). Cements are rich in severalNMR active isotopes: 1H, 29Si, 27Al, and 23Na. 29Si magicangle spinning (MAS) NMR can be used to examine the sili-con–oxygen bonding in a cement sample as a function of hy-dration time. This facilitates the understanding of ‘CSH’formation (6, 22).

Various forms of Si–O bonding are shown in Figure 8(24, 25). The basic tetrahedral unit, (SiO4)4�, is referred toin this field as a Q0 unit, where the superscript on Q refersto the number of (SiO4)4� units attached to the central(SiO4)4� unit. Q1 represents a dimer and Q2 corresponds tosilicon atoms within a polymeric chain of (SiO4)4� units. Q3

and Q4 correspond to silicon centers from which increasinglycomplex degrees of chain branching occur, as shown in Fig-ure 8 (25).

29Si NMR is especially useful for examining Si–O bond-ing because an increase in the number of (SiO4)4� unitsbonded to each Si center produces an increase in the averageelectron density around the central Si atom. This leads to amore negative chemical shift, relative to tetramethylsilane(TMS), for successively increasing n values in Qn (see Figure8 for typical values).

In the pre-induction period of cement hydration, 29SiMAS NMR shows the presence of monomeric (SiO4)4� units,Q0. 1H NMR shows that in the first few minutes protona-tion of the (SiO4)4� units also occurs, an indication that thesurface hydroxylation mentioned previously (eqs 1–3) is prob-ably the first step of the reaction (13, 24). As the reactioncontinues, signals corresponding to Q1 units become pre-dominant, indicating a dimerization of (SiO4)4� units. As timepasses the intensities of the Q1 signals decrease, and signalscorresponding to polymerization of the dimers, Q2, increase.Crystallographic and NMR studies have shown that the pri-mary species formed are pentamer (Si5O16)12� and octamer(Si8O25)18� units (13).

It is interesting to note that Q3 and Q4 signals are notobserved for silicon in the hydration of Portland cement, in-dicating that polymerization takes place predominantly in alinear fashion without branching. 29Si MAS NMR spectra

of pure ‘C2S’ and pure ‘C3S’ in comparison with a Portlandcement (PC) sample that had been hydrated for 28 days areshown in Figure 9. Broad Q1 and Q2 signals in the �75 to�88 ppm region of the cement sample show the presence ofdimer and linear polymer units. However, the signal corre-sponding to ‘C2S’ in the hydrated cement sample remainsessentially unchanged after 28 days, which shows the slowhydration rates of ‘C2S’ relative to ‘C3S’. This is why the pro-duction of ‘C3S’ in the cement clinker is so vital for effectivecement hydration (24).

Data from NMR experiments such as these, combinedwith X-ray crystallography and microscopy, can be used topostulate a general structure for the cement paste. ‘CSH’ gelhas structural features similar to that of two naturally occurringminerals: Tobermorite and Jennite (13, 20). In fact, ‘CSH’gel is often referred to as Tobermorite gel in the cement litera-ture. These minerals, shown schematically in Figure 10, arecharacterized by linear Q2 type O�Si�O bonding and areformed as multiple layers separated by layers of Ca2� orCa(OH)2.

Figure 8. Various arrangements of silicon–oxygen bonding are ex-pressed using a superscripted Q, where the superscript refers tothe number of (SiO4)4� units bound to the central (SiO4)4� unit inthe cluster. The average 29Si NMR signals (relative to TMS) are alsoshown for each unit. For simplicity, charges are omitted.

Si

O

O

O

O OSi

O

O

O Si

O

O

O

OSi

O

O

O OSi

O

O

O Si

O

O

O

Si

O

O

O

O

OSi

O

O

O OSi

O

O

O Si

O

O

O

Si

O

O

O

O

Si

O

O

O

O

Q0= -70 ppm Q1 = -80 ppm

Q2 = -88 ppm

OSi

O

O

O Si

O

O

O

Si

O

O

O

O

Q3 = -98 ppm

Q4 = -110 ppm






Cement Degradation

Crumbling cement, rust stains, and cracks in reinforcedconcrete are commonly observed. These are a few examplesof a serious problem that costs North Americans nearly a bil-lion dollars a year—cement corrosion (26). Corrosion in ce-ment and concrete materials is a twofold problem becausethe cement material and the steel reinforcement are both sus-ceptible to corrosion, and the weakening of one generally ac-celerates the degradation of the other. Although cementcorrosion is complicated, the action of water is a commonfactor (27).

Cement is a porous material containing a dual networkof pores. The capillary pore system, with a distribution of di-ameters that range from 50 to 1000 nm, extends throughoutthe system, acting as channels between various componentsof the system. The cement gel itself contains a network of gelpores, with diameters on the order of 10–50 nm (19, 28).

Physical properties of cement such as its elastic modu-lus, fire resistance, and durability are directly related to theamount of water present (29). Cement is generally 30–40mass % water, which is present in three forms:

1. Chemically bound water: Water of hydration chemi-cally bound to the cement precursor materials in theform of hydrates. This comprises more than 90% ofthe water in the system.

2. Physically bound water: Water adsorbed on the sur-faces of the capillaries. This water is most predomi-nant in the small gel pores of the system.

3. Free water: Water within larger pores that is free toflow in and out of the system. The amount of free wa-ter depends on the pore structure and volume, the rela-tive humidity, and the presence of water in directcontact with the cement surface, such as in water-bear-ing cement pipes and marine structures (19, 27, 30).

Figure 9. 29Si NMR examination of cement hydration: (A) puredicalcium silicate, (B) pure tricalcium silicate, (C) Portland cementsample hydrated at 40% by mass of water for 28 days. (A) and(B) show Q0 29Si NMR signals (~ �70 ppm). The addition of Q1

and Q2 29Si signals (�80 to �90 ppm) is seen upon hydration. Theslow hydration rate of dicalcium silicate is shown by a large peakof unreacted pure material at about �70 ppm (22).

Figure 10. Cement paste is believed to closely resemble the miner-als Tobermorite and Jennite. These minerals are characterized bylayers of polymerized silicon oxide cross-linked with calcium oxideor calcium hydroxide (13).

Tobermorite

Jennite

[Ca4(Si3O9H)2]Ca2 . 8H2O

[Ca8(Si3O9H)2(OH)8]Ca2 .6H2O

SiO O

O OSi

O O

CaCaCaCa

SiO O

OSi

O O

OOSi

O

OOSi

OO

OOSi

SiHO

Ca

OHSi

O

OOSi

OO

OOSi

Ca Ca Ca

OO−

−

−

− −

− −

HO

O

O−O

SiO O

OSi

O O

Ca

OH OH OH OH

Ca Ca CaCaCaCa

OHOHOHOH

Ca

OOSi

OO

OOSi

CaCaCa

OHOHOHOH

Ca

OH

SiO

OOSi

OO

OOSi

O OH

SiO

O

SiO O

OSi

O O

Ca

OH OH OH OH

Ca Ca Ca

OHSi

O

O

OHSi

O

O OHSi

O

O

8H2O Ca2+ 8H2O Ca2+

6H2O Ca2+ 6H2O Ca2+

B

A






Corrosion of cement due to water can be discussed in termsof physical and chemical corrosion.

Physical Corrosion of CementPhysical corrosion of cements is attributable to the physi-

cal properties of water, especially its volume change duringfreezing and its ability to dissolve cement components. Themost significant problem concerning degradation of cementsis the free water in the system. When cement is hydrated,most of the water used in the process is taken up as hydrates.If too much water is present, the remaining water is able tomove through the cement causing various problems.

Drying of a cement or concrete paste is an importantfactor in the physical corrosion of cement. As a cement pastehydrates over a period of several months, its porosity de-creases. Initially, the drying process takes place through cap-illary flow of water in the larger pore system. As porositydecreases, the drying process slows and becomes diffusive (13,28).

Higher water:cement ratios in the hydration reactionsresult in larger pore sizes as the cement gel forms, and thesepores contain a larger volume of water. Larger pore sizes alsolead to faster drying rates, which is a serious problem in ar-eas with low humidity. When cement is exposed to low hu-midity, free water in the large pores (> 50-nm diameter)evaporates quickly. This water removal is not serious if thecement is in contact with water periodically because largepores also quickly fill with water. However, if cement is ex-posed to an extended period of low humidity and high tem-peratures, adsorbed water in the gel pores of the cement willevaporate. This process leads to drying shrinkage. Dryingshrinkage is destructive because partially filled gel pores (5–50-nm diameter) contain water menisci that exert consider-able tensile stress on the walls of the pores. This stress leadsto microcracking and eventually weakens the material. Theuse of aggregates minimizes the effect of drying shrinkagebecause aggregates increase the elastic modulus and compres-sive strength of the finished product (28).

Cements in maritime climates at midlatitudes are par-ticularly susceptible to stress owing to a process known asfreeze–thaw cycling (6, 28, 30). Freeze–thaw cycles occur inwinter when ambient temperatures hover near 0 �C. In theseclimates freeze–thaw cycles can occur on nearly a daily basisin a typical winter season. Freeze–thaw cycles are damagingto cements because of the 9% volume increase of water uponfreezing (31). When water in the capillary pores freezes, itexpands and exerts stress on the pore walls. This leads tomicrocracks, which can in turn fill with water during the sub-sequent thaw period. Stress exerted in the microcracks dur-ing further freezing will extend the cracks until macroscopiccracking is observed. While freeze–thaw degradation gener-ally is most serious at the surfaces of the cement structure,extensive cracking will allow the penetration of water deeperinto the structure leading to the eventual failure of the sys-tem (6, 30).

Crystalline calcium hydroxide makes up about 10% ofthe volume of most common cements (5, 13, 15), and seri-ous physical corrosion of cements results from the leachingof calcium hydroxide (15). With a room-temperature solu-bility of 1.7 g�L (15), calcium hydroxide can be easily dis-solved in free water within cement pastes. This is especially

problematic with pure water, for example, rain water, meltedsnow, and condensation within pipes (32). Removal of cal-cium hydroxide leaves void volumes within the cement, caus-ing a loss of strength and allowing the deeper penetration ofleaching waters (15, 27, 30).

Calcium hydroxide leaching can be observed in a spec-tacular effect: opaque white material appears to ooze out ofconcrete walls or hang in a stalactite formation from con-crete ceilings. In this case, water containing dissolved calciumhydroxide has leached out of the concrete and evaporated,leaving behind a layer of calcium hydroxide that reacts withcarbon dioxide to form calcium carbonate (15)

CaCO3(s) + H2O(l)Ca(OH)2(s) + CO2(g) (10)

in a process known as efflorescence.5 Efflorescence is often asign of water seepage problems in the concrete or cementstructure.

Chemical Corrosion of CementWater also carries chemical agents into cement pastes that

react to destroy various components of the cement. A seri-ous problem is the action of acidic waters from acid precipi-tation, industrial effluent, or the decay of organic matter (6,15, 32). Acids also lower the pH of the pore water withincement pastes, which otherwise has a pH of 11–13 owing tothe large amount of calcium hydroxide present (5, 15, 30).Lowering the pH will also increase the rate of the corrosionof the iron in iron-reinforced cement.

The conversion of calcium hydroxide to calcium carbon-ate through the action of carbon dioxide in the atmosphereis a problem for all types of cement. This can take place di-rectly on the surface (efflorescence), or as the CO2 diffusesinto the cement (5, 15, 30):

H2CO3(aq)CO2(g) + H2O(l) (11)

CaCO3(s) + 2H2O(l)H2CO3(aq) + Ca(OH)3(s) (12)

Ca(HCO3)2(aq)H2CO3(aq) + CaCO3(s) (13)

2CaCO3(s) + 2H2O(l)Ca(HCO3)2(aq) + Ca(OH)2(s) (14)

This process, called carbonation, depletes the cement of cal-cium hydroxide and leaves CaCO3 deposits inside the cement.

Another problem, particularly in marine environments,is the action of corrosive sulfates such as ammonium sulfateand magnesium sulfate on cement. These salts react with cal-cium hydroxide to form calcium sulfate (12, 15):

CaSO4(s) + 2NH3(aq) + 2H2O(l)

(NH4)2(SO4)(aq) + Ca(OH)2(s) (15)

CaSO4(s) + Mg(OH)2(aq)

Mg(SO4)(aq) + Ca(OH)2(s)

(16)

Reactions that deplete cement pastes of calcium hydroxideare particularly destructive because the products are usuallymaterials with significantly larger volumes. For example, thevolume of calcium sulfate, 74.2 mL�mol, formed in eqs 15






and 16 is more than twice the volume of the calcium hy-droxide removed, 33.2 mL�mol (5). This volume changeleads to stresses and cracks that further accelerate the pro-cesses discussed above.

Behavior of Water in CementUnderstanding the behavior of water in porous cement

is central to the understanding of cement corrosion. Varioustheoretical and statistical-mechanical approaches have beenused to try to describe the movement and distribution ofwater in the pores of cement (27, 33–35). However, for manyyears examination of water in cement pastes was hinderedby the absence of viable experimental techniques for observ-ing its presence. Recent developments in nuclear magneticresonance imaging have provided valuable experimental data(6).

Magnetic resonance imaging (MRI) is a common tech-nique used in imaging materials, especially biological mate-rials. MRI is typically used to measure the spatial distributionof water in a material (21, 36, 37). It is based on the prin-ciple that the nuclear magnetic resonance frequency of anucleus, such as 1H, in a magnetic field gradient is propor-tional to its spatial position in the magnetic field gradient

B G zz

kzz= − + = +ν γ α ν( )( ( ))1

20

0 (17)

where ν is the observed NMR frequency, γ is the magneto-gyric ratio of the nucleus, α is the chemical shielding of thenucleus, B0 is the magnetic field associated with a static fieldmeasurement, Gz(z) is the magnetic field gradient (dB/dz),ν0 is the NMR frequency in the static field (B0), k is a scal-ing constant for the signal, and z is the position of the nucleusin the field (21). Figure 11(A) shows two nuclei in a mag-netic field gradient Gz = dBz�dz. In Figure 11(B) an inter-ferogram is produced when a 90� radio frequency pulse isapplied. Fourier transformation of the signal in (B) producestwo peaks separated by ∆ν = γGzdz , shown in Figure 11(C).The width of the individual peaks is proportional to (πT2)�1,which gives a resolution, dz,

dzG Tz

∝π1

2(18)

where T2 is the spin–spin relaxation time of the nucleus (22,23). The shift in the NMR frequency of a nucleus is propor-tional to the position of the nucleus in the sample, while thesignal intensity corresponds to the amount of that nucleuspresent.

Water in gel pores is tightly confined and is susceptibleto the effects of various paramagnetic species in the sampleincluding iron and aluminum (6). These factors combine tomake the spin–spin relaxation times short, dramatically de-creasing the effective resolution (6, 37). For example, usingconventional MRI techniques, a field gradient of about 10T�m is necessary for a resolution of 10 mm in a cement paste(6). This is much greater than the normal field gradients usedin MRI, but is on the order of the stray fields associated withthe superconducting magnets of high-resolution NMR in-struments.6 In 1988, a MRI technique known as stray fieldimaging (STRAFI) was developed (38). In this experiment a

sample is moved through a stationary field gradient of a su-perconducting magnet. STRAFI experiments have allowedthe detailed examination of water in solid cement samples(35).

Imaging techniques such as STRAFI are useful for ex-amining the effectiveness of waterproof coatings. One of theeasiest ways to prevent cement corrosion is to prohibit themovement of water in and out of the material by establish-ing a waterproof barrier on the exposed surfaces (6, 30). Awide variety of surface coatings are used in the waterproof-ing of cements; for example, a common class of waterproof-ing agents is silanes (22). The rate and depth of surface waterabsorption into the cement surface can be compared for aseries of coatings and treatments. The depth and durabilityof the surface treatment can also be examined for various ap-plications (6, 22). A STRAFI image of a Portland cementsample coated with methyltrialkoxysilane is shown in Figure12. The images show penetration of the silane coating as itis repeatedly applied to the surface. After 24 hours the coat-ing penetrates to a depth of about 2.5 mm. A comparison ofthe water penetration in treated and untreated Portland ce-ment is shown in Figure 13. The treated sample shows wateron the surface (intense surface signal) and the silane coating

Figure 11. Schematic of conventional MRI experiment. (A) Two nu-clei situated in a magnetic field gradient, Gz, are separated bydz. (B) A 90� RF pulse is used to obtain an interferogram of thenuclei in the sample. (C) Fourier transformation of (B) gives twopeaks separated by a frequency proportional to their separationin the sample (21).

A

B

C

0ν

M

t

Gz =

90o

pul

se

∆ν = γGz dz

z

dBz

dz

dz

1width ~

πT2






penetrating to about 2–3 mm. The untreated sample showslittle surface water but significant water penetration to 8–9mm after 24 hours (22).

While STRAFI is a powerful technique for examiningthe water content of a cement paste, it is limited to relativelysmall sample sizes (∼ 2-cm diameter) because of magneticfield constraints. Studies of concrete are limited to those withvery small aggregates such as fine gravels and sands (28).

Another way of alleviating the problem of short T2 whileavoiding enormous field gradients is through the use of singlepoint imaging (SPI; ref 21, 39). SPI is an MRI techniquethat uses an oscillating field gradient in which signals are mea-sured at a constant encoding time, tp, following a radio fre-quency (RF) pulse. A recent variation on SPI developed byBalcom et al. (40) has proven useful in the examination ofwater in cement samples. This technique, called SPRITE(single point ramped imaging with T1 enhancement), uses a

ramped magnetic field gradient that is much easier to con-trol than the oscillating gradients used in conventional SPI(28, 29, 40). While conventional MRI, including STRAFI,measures all resonance frequencies simultaneously anddeconvolutes using a Fourier transform, SPRITE uses a pro-cess called position encoding where only one frequency, cor-responding to a particular encoding time, tp, is measured. Thespatial position, z, of the analyte nucleus is encoded in re-ciprocal space such that the signal, S(k), is proportional to k(40):

k == 1

2πγG tz,max p(19)

With a constant encoding time, k is inversely proportionalto the maximum field gradient, Gz,max. A schematic SPRITEimaging sequence and resulting image are shown in Figure14. Following an RF pulse a single frequency is measuredafter a desired encoding time, tp. Next, the field gradient isramped and the sequence is repeated. Each sequence givesthe nuclear density at a particular point in the sample.Through repetition of the sequences at varying Gz,max an im-

Figure 13. One-dimensional STRAFI of Portland cement samples incontact with water for 24 hours. Sample A was treated with methyl-trialkoxysilane. Sample B was untreated and shows deep penetra-tion of water into the cement surface (22).

Figure 14. Schematic of the SPRITE imaging sequence: (A) the sig-nal is measured after a time, tp, has elapsed from an RF pulse; (B)the ramping of the field gradient that accompanies the measure-ment of signal; (C) each successive sequence will show the densityof the analyte nucleus at a particular position, dictated by the gra-dient used during that sequence (40).

z

tp tp tp tp tp

RF RF RF RF RF

Gz

Time

Time

z1 z2 z3 z4 z5

Sig

nal

z1 z2 z3 z4 z5

A

B

C

Figure 12. One-dimensional STRAFI image of a Portland cementsample coated with methyltrialkoxysilane. The coating is appliedevery 30 minutes during the analysis. The signals show the ingressof the polymer coating into the cement to a depth of about 2.5 mmafter 24 hours (22).






age is produced. SPRITE and other SPI techniques takelonger than conventional MRI techniques, but are less sus-ceptible to noise and magnetic inhomogeneities in the samplebecause only one frequency is analyzed at a time (21, 28, 29,39, 40). SPRITE is also useful because signal resolution de-pends only on the size of Gz,max and the ramping sequenceused, not on the T2 of the analyte nucleus (40).

SPRITE can be used to examine the behavior of pro-tons in a concrete sample as a function of physical param-eters such as temperature. From eq 19 it can be shown thatkeeping Gz,max and tp constant results in repeated measure-ments of protons in a defined position, z. The intensity ofthe signal can be observed as a function of temperature,shown in Figure 15. Through an adjustment of parameterssuch as Gz,max, RF flip angles, and tp, the experiment can betailored to be sensitive to a nucleus of defined T2. This al-lows the ability to differentiate between the protons of freeliquid water (T2 ≈ 200 µs) and ice (T2 ≈ 10 µs) in a cementsample. By following the appearance and disappearance offree water at various regions of a cement sample in the freeze–thaw cycle, characteristics of the material can be examined(29, 40).

In cement gels, MRI has shown that water freezes in twosteps. The first step occurs between 0 and �2 �C, where freebulk water and water in the capillary pores freeze (29). Thisfreezing-point variation is due to a freezing-point depressionphenomenon caused by vapor pressure lowering in the cap-illaries and related to the pore size of the capillaries by theKelvin equation (29),

=2 0γ

ρT

MT

r H∆

∆ (20)

where ∆T is the freezing-point depression, γ is the surfacetension of the liquid, M is the molecular weight, T0 is thenormal freezing point, r is the pore radius, ρ is the densityof the absorbate, and ∆H is the molar enthalpy of fusion (41).Information on the freezing-point depression of water in acement sample is valuable for the determination of pore dis-tributions in these materials.

As ice forms in a cement sample the internal pressure ofthe closed system increases owing to the volume expansionof water. The resulting pressure increase once freezing beginsin the gel pores forces the migration of water from the gelpores to larger pore regions where ice will form immediately.This results in a secondary freezing point at about �40 to �45�C (29). Figure 16 shows a measurement of these two freez-ing phenomena for a cement sample measured using an SPItechnique (29), in which the evaporable water content of aconcrete sample is measured as a function of temperature asthe sample is slowly cooled (2 K�hr). The first freezing eventis seen between 0 and �1.6 �C. As the sample temperature islowered the amount of evaporable water decreases slowly atfreezing temperatures corresponding to the respective poresizes present. The large change at 0 to �1.6 �C shows that themajority of evaporable water present is contained in largepores. The second major freezing event, associated with thedesorption and freezing of water from the gel pores, is shownat �45 �C. Other studies have shown that evaporable watercan still exist in cement samples at temperatures as low as�90 �C (29).

Corrosion of Steel ReinforcementReinforced concrete is often used in bridge decks, roads,

and sidewalks. One of the most serious threats to concretein cold climates is the use of deicing salts in the winter toensure safe conditions for motor vehicles and pedestrians

Figure 16. Magnetization signal for evaporable water in a sampleof Portland cement mixed with 14-mm diameter graded quartz ag-gregate measured using SPRITE as a function of temperature. Freez-ing of water is associated with a decrease in signal intensity (29).

0-10 10-20-30-40-50

0.0

0.2

0.4

0.6

0.8

1.0

- center

Ma

gn

etiz

atio

n (

arb.

u)

- drying face

T / °C

Figure 15. Schematic of SPRITE used for temperature dependentmeasurements. (A) The normal SPRITE sequence from Figure 14 isused again, but (B) the field gradient is kept constant. (C) This al-lows for a repeated measurement of the signal density for analytenuclei at a particular position in the sample as a function of tem-perature (40).

tp tp tp tp tp

RF RF RF RF RF

Gz

Time

Time

T1 T2 T3 T4 T5

Temperature

Sig

nal

at z

1

T1 T2 T3 T4 T5

A

B

C






(6, 15, 26, 42, 43). Chloride ions, when transported by wa-ter, attack steel reinforcement (rebar) of these structures caus-ing them to weaken from within. High pH is important forminimizing the rate of steel rebar corrosion because it allowsthe formation of a passive oxide layer on the surface of themetal (6, 30). Low pH aqueous states caused by the leachingof calcium hydroxide from the cement, combined with chlo-ride ion ingress, causes extensive rebar corrosion in short pe-riods of time. Chloride ions set up redox reactions along therebar, as shown by the following equations (26):

2Fe2�(aq) + 4e�2Fe(s)

4OH�(aq)O2(g) + 2H2O(l) + 4e�

2FeCl2 (aq)2Fe2�(aq) + 4Cl�(aq)

2Fe(OH)2 + 4Cl�(aq)2FeCl2(aq) + 4OH�(aq)

Fe2O3(s) + 2H2O(l)2Fe(OH)2 + 1/2O2(g)

Fe2O3(s)2Fe(s) + 3/2O2(g)

(21)

(22)

(23)

(25)

(24)

(26)

Chemical attack of chloride ions is destructive because it notonly reduces the amount of hydroxide ion and iron, but italso acts in a catalytic manner. Transport of chloride ionsthroughout the system is also increased as cracks form as aresult of the other decay processes. Furthermore, patchingcan create localized corrosion cells between the rebar in the

existing chloride-contaminated concrete and in the newchloride-free patch, accelerating the concrete corrosion (43).

Waterproof coatings will stop the introduction of newchloride ions, but will not remove the chloride ions alreadypresent in the system. Coatings also become ineffective if theconcrete surface is cracked or damaged (26, 43).

A particularly interesting approach to treating chlorideion ingress is electrochemical chloride extraction (ECE), inwhich chloride ions are effectively pulled from the concrete.A dc circuit, shown in Figure 17, is set up using rebar as theanode and an electrolyte gel packed on the concrete surfaceas the cathode. When a dc potential of 10,000–30,000 V isapplied, water hydrolyzes at the anode, replenishing the hy-droxide content of the system. The negatively charged steelrebar repels chloride ions to the surface of the concrete andinto the electrolyte gel. After 4–8 weeks the process is com-plete, at a fraction of the cost of replacement (43). After seal-ing with a waterproof coating, the concrete is effectivelyprotected against further rebar corrosion.

Structures with extremely problematic chloride ion prob-lems, such as ocean piers, can be cathodically protected byconstant maintenance of the rebar at a negative potential ofabout 10,000–30,000 V (43).

Concluding Remarks

The study of cement offers an opportunity to explorethe chemistry of earth materials, their preparation, and re-sulting properties. Furthermore, examination of cement deg-radation comprises an extensive part of modern cementchemistry. Recent innovations in research techniques havemade the study of cement preparation and degradation be-havior more accessible. Improvement of corrosion resistancein cement and concrete structures would significantlylengthen the lifetime of applications using these materials,potentially saving billions of dollars worldwide.

Acknowledgments

This work was supported by the Natural Sciences andEngineering Research Council of Canada and the IzaakWalton Killam Trusts.

Notes

1. Portland stone, a gray stone quarried from the Dorsetregion of England, was a commonly used building material inEurope in the 16th–19th centuries (2, 7).

2. Founded in 1898, ASTM International is a nonprofit orga-nization that provides a global forum for the development and pub-lication of voluntary consensus standards for materials, products,systems, and services. See http://www.astm.org (accessed Mar 2003).

3. Fluxing is a process that promotes fusing of materials, inthis case by lowering of the melting point of a mixture by addinganother component (7).

4. Ettringite is a collective term referring to the various alu-minate and sulfoaluminate phases present in the clinker material.

5. Efflorescence is the “blossoming” to a powdery substanceon exposure to air.

6. The stray field near a 9.4 T (400 MHz) NMR magnet ison the order of 60 T�m.

Figure 17. Schematic of electrochemical chloride extraction (ECE).A dc voltage of 10–30 000 V is applied between the steel rebar(anode) and an electrolyte gel (cathode) on the surface of the con-crete. Hydrolysis of water takes place at the rebar and chlorideions are repelled from the concrete into the electrolyte gel (43).

Cl ��

Cl �

+

�

�H2O H + OH

H + OH�

��

steel rebar

concrete sample

electrolyte gel

DC power supply

2H O




http://www.astm.org



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Cement Its Chemistry and Properties

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