Cement: Its Chemistry and Properties - Journal of Chemical Education

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George B. Kauffman California State University Fresno, CA 93740

Cement: Its Chemistry and Properties Douglas 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]

One of the most active areas in scientific research is the development of new and exciting materials for a wide variety of applications. In this context, it could be easy to lose sight of the importance of more common materials that are vitally important in many areas of our lives. Cement is one such material, and its rich chemistry links well with a number of concepts in most undergraduate chemistry curricula. This paper addresses several important questions concerning cement, including: What is its optimal composition and why? Why do cement truck barrels roll? What are the processes involved in cement setting, and how long does it take? How does cement break down? A Brief History of Cement Cements and cement-containing materials comprised some 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 of calcium silicate hydrates polymerized into a densely crosslinked matrix (2). Its most important property is called hydraulicity—the ability to set and remain insoluble under water (3, 4). Cement can be used as a mortar to bind large stones or bricks. When sand and stones are added to cement, the aggregate is called concrete. The word cement comes from the Latin phrase, opus caementum, or chip work, in reference to 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 Pozzolanic cements after the Pozzulana region of Italy, which contained Italy’s chief supply of ash (1, 5 ). Pozzolanic cements derive their strength from rich aluminate phases present in the volcanic ash that promote efficient hydration of the final cement powders (6). Fine grinding and attention to consistency are also fundamental to the success of Roman cement, much of which is still in existence today in structures such as the Pantheon, the Pont du Gard, and the Basilica of Constantinople (2, 5). An example of a structure made with Roman cement is shown in Figure 1. The art of cement production was lost in Europe after the fall of the Roman Empire (2, 5). At that time, the access to volcanic ash was limited and the grinding and heating techniques required for cement precursor production were lost. Cements of this period, if still in existence, are inconsistent in composition and are composed almost exclusively of unreacted starting materials (1, 2, 5). There was no significant breakthrough in the development of cement chemistry until 1756, when Smeaton was commissioned to rebuild the Eddystone lighthouse in Cornwall, England. In contrast to

the methods of his contemporaries, Smeaton found superior results through experimentation by using an impure limestone with noticeable clay deposits. This produced extremely strong cement “that would equal the best merchantable Portland stone in solidity and durability”(5).1 Another major advance came in the early 19th century when the French engineer Vicat performed the first empirical study on the composition of cements. Although crude and incomplete, it was one of the most comprehensive examinations of cement chemistry for the next 80 years (3, 4, 8–10). The term Portland cement did not become officially recognized until 1824 when Aspidin filed the first patent for its production (2, 5). Cement compositions at this time were poorly understood but closely guarded secrets. Portland cement was introduced into the United States by Saylor in 1871 (3, 4). By the start of the 20th century, cement manufacture was common but was still regarded as more of an art than a science. Emphasis was placed on bulk manufacture, not quality control or consistency (10, 11). Early in the 20th century, cement research became more scientific, incorporating the relatively new Gibbs phase rule and Le Châtelier equilibrium principles (3). In 1904 the first set of ASTM standards2 for cement were presented and in 1906 the geophysical laboratory of the Carnegie Institution began an extensive investigation of cement chemistry. These advances resulted in the development of uniformity in the cement industry, allowing a rapid expansion in the application of cement to large construction projects such as skyscrapers, roads, and dams (2, 3, 8, 11).

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

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

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Chemistry for Everyone

More recent advances in materials-characterization techniques, such as X-ray crystallography, electron microscopy, nuclear magnetic resonance spectroscopy, Mössbauer spectroscopy, infrared spectroscopy, and thermal analysis, have allowed the systematic examination of cement’s chemistry and the complex processes surrounding its production and hydration (2, 12). Scientific research has led to a better understanding of the properties of cement, cement production, and cement corrosion. In fact, breakthroughs in cement research have provided us with cements of increasing quality and strength. Cement is prepared in a two-step process. The first step is the high-temperature mixing and processing of limestone, sand, and clay starting materials to produce a cement powder. The second step involves the hydration, mixing, and setting of the cement powder into a final cement product (2, 6, 13). The dry portion of Portland cement is composed of about 63% calcium oxide, 20% silica, 6% alumina, 3% iron(III) oxide, and small amounts of other matter including possibly impurities (7). Calcium silicates and calcium aluminates dominate the structure. The cement literature uses abbreviations for the many calcium oxide, silicate, aluminate, and ferrate compounds important to cement. We have used the same abbreviations here and present the correspondence between the chemical formulas and abbreviations in Table 1 (14).

Cement Formation

Preparation of Cement Precursors: Clinkers The raw materials for cement production are blended in the required proportions, ground, and heated to high temperatures, usually with rotation. Heating first releases H2O and CO2 and then causes other reactions between the solids, including partial melting. Cooling results in clinkers, a term from 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 hard glassy clinker material (which was expensive to grind) as being useless to cement manufacture (8, 11), although we now know that clinkers are essential for good cement production. After heating, cement clinkers are reground for use in the production of cement. Commercial cement manufacture incorporates a wide variety of minerals, including: calcium oxide, silica, alumina, iron oxide, magnesium oxide, titanium dioxide, and many others (5, 14). Of these, three are most important to the final cement product: calcium oxide, silica, and alumina. Consideration of all the possible phases produced by these multicomponent systems is simplified by considering a ternary system of primary importance—the calcium oxide兾silica兾alumina system (14). High-quality cement powders require the presence of two major components, tricalcium silicate, ‘C3S’, and dicalcium

Table 1. Common Cement Component Names, Compositions, Formulae, and Abbreviations C ompone nt N ame

C ompos it ion

Empirical Formula

A bbre v iat ion

C alcium ox ide (lime )

C aO

C aO

‘C ’

Silicon diox ide (s ilica)

SiO2

SiO2

‘S ’

A luminum ox ide (alumina)

A l 2O 3

A l 2O 3

‘A ’

Iron(III) ox ide

F e 2O 3

F e 2O 3

‘F ’

Dicalcium s ilicat e

2C aOSiO2

C a2SiO4

‘C 2 S ’

Tricalcium s ilicat e

3C aOSiO2

C a3SiO5

‘C 3 S ’

Tricalcium aluminat e

3 C a O A l 2 O 3

C a3A l 2O 6

‘C 3 A ’

Te t racalcium aluminof e rrat e (B row nmille rit e )

4C aOA l2O3Fe 2O3

C a4A l 2F e 2O 10

‘C 4 A F ’

C alcium s ilicat e hy drat e ge l

(C aO)xSiO2yH 2O w it h x < 1.5 in s olid s olut ion w it h C a(OH )2

(v ariable )

‘C S H ’

C alcium s ilicat e (W ollas t onit e )

C aOSiO2

C aSiO3

‘C S ’

C alcium s ilicat e (Rankinit e )

3C aO2SiO2

C a3Si2O7

‘C 3 S 2 ’

C alcium aluminum s ilicat e (Ge hle nit e )

2C aOA l2O3SiO2

C a2A l2SiO7

‘C 2 A S ’

A luminium s ilicat e (M ullit e )

3A l2O32SiO2

A l6Si2O13

‘A 3 S 2 ’

C alcium aluminum s ilicat e (A nort hit e )

C aOA l2O32SiO2

C aA l2Si2O8

‘C A S 2 ’

A luminium s ilicat e

2A l2O32SiO2

A l4Si2O10

‘A 2 S 2 ’

C alcium aluminat e

C aOA l2O3

C aA l 2O 4

‘C A ’

C alcium dialuminat e

C aO2A l2O3

C aA l 4O 7

‘C A 2 ’

Dode cacalcium s e pt aluminat e

1 2 C a O 7 A l 2 O 3

C a12A l 14O 33

‘C 1 2 A 7 ’

C alcium he x aluminat e

C aO6A l2O3

C aA l 12O 19

‘C A 6 ’

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


For this discussion, the important region of the CaO兾SiO2兾Al2O3 phase diagram is the ‘C3S’兾‘C2S’兾‘C3A’ phase field, the region close to the CaO vertex in Figure 4. A three-dimensional view of the ternary phase diagram in this region is shown in Figure 5. As the temperature of the sysSiO2 1723 1698

x

x

1500

Cristabolite + L

x ‘A2S2’

bo

1385 1405 1380

Corundum

1475

x 1850

1840

(␣-Al2O3)

x

(CaO)

x

‘C A’

x15423 ‘C3A’

x

‘CA’

x

1415 1605

‘C12A7’

x

‘CA’

x ‘CA6’ 1789

x

x

1860

‘CA2’

‘CA6’ Al2O3

Tridymite + L

‘C 3S2’+

α -‘CS’

CaO + β-‘C 2S’

‘C 3S2’+

‘C 3S’ + β-‘C 2S’

‘C 3S2’+ β -‘CS’ α -‘CS’ + Tridymite β-‘CS’ + Tridymite

1000

‘C 3S’ ‘C 2S’ ‘C 3S2’ 0

x

1553 1547

1590 1552 1380 1512 1500 1455 1350 ‘CA2’ 1335 1470 1335

Lime

x

1512

‘CAS2’ 1265

‘C2AS’

‘C3S’

x

x

Figure 4. The ternary phase diagram of calcium oxide, silica, and alumina. The region nearest the CaO vertex represents the primary phase field for the formation of tricalcium silicate. Temperatures are presented in C (13, 16).

Two Liquids

α-‘CS’+L

C 3S 2+ α-‘C 2S’

β-‘C 2S’

Temperature / °C

‘C 3S’ + α-‘C 2S’ ‘C 3S2’ + L

CaO + ‘C 3S’

CaO

Cristabolite + L

(‘CAS2’)

1545

x

x

’) S2

2000

α-‘C2 S’ + L

␣-‘C2S’

‘C3S’

1170

1318

1315

1345

(‘A 3

x

2570

‘C 3S’ + L

x

2130 2050 2150

ite

llite Mu

‘C3S2’ ‘C2S’

x

1368

Anorthite

x

1544 1460 ‘C3S2’ 1307 1464 1310

2500

CaO + L

␣ -‘CS’

‘CS’

x 1470

ym

d Tri

1470 1436

x

Cr ista

x 1698

1598

lite

Two x liquids

e nit hle S’) Ge(‘C 2A

silicate, ‘C2S’, in the clinkers. These materials react vigorously with water to produce the cement paste formed in the final product. Of the two, tricalcium silicate is the more desirable clinker material because it hydrates and sets much faster than dicalcium silicate (hours for ‘C3S’, days for ‘C2S’) (2, 15). The binary phase diagram of SiO2 and CaO is shown in Figure 2 (16, 17). Most important is the 0–30 mass % SiO2 region. ‘C3S’ is formed at less than 30 mass % SiO2 but 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 between two solid phases. For example, forming ‘C3S’ at temperatures of 1200–1400 C would require heating for days and is not economical. At the other end, production from the melt at 2200 C is also impractical because of the very high temperature. Therefore, the temperature of the ‘C3S’ production for the clinker is lowered by fluxing3 the reaction mixture with a third component, alumina (7, 14, 15). The binary phase diagram of CaO and Al2O3 is shown in Figure 3 (16). Comparison with the ternary CaO兾SiO2兾Al2O3 phase diagram (7, 14, 16), Figure 4, shows that the addition of Al2O3 lowers the preparation temperature of ‘C3S’.

20

‘CS’ 60

40

100

80

(CaO)

(SiO2 )

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

‘CA 6’+ L ‘CA 2’+ L

CaO + L

1800

Al 2O3 +L

‘CA’ + L ‘C 3A’ + L

‘C 12A 7’ + L

1400

CaO + ‘C 3A’

‘C 3A’ ‘C12 A 7’ + ‘C 12A 7’ + ‘CA’

‘CA’ + ‘CA 2’

‘CA 2’ + ‘CA 6’

‘CA 6’ + Al 2O3

Temperature / °C

2200

1000 0

20

‘C3 A’

40

‘C12 A 7’

‘CA’ 60

(CaO)

‘CA 2 ’ 80

‘CA 6 ’

100

(Al 2O3)

mass % Al2O3

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

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


625


tem decreases from about 2100 C, the composition of the liquid goes into the ‘C3S’兾‘C2S’兾‘C3A’ phase field (5). Adding 20 mass % alumina to a silica兾lime system lowers the liquidus into the region of stable ‘C3S’ formation, from the reaction 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 to 1450–1500 C results in a liquid phase that can be quenched to form the final ‘C3S’-rich cement clinker. Although neither Smeaton nor the Romans fully realized the chemistry, it was the addition of rich aluminate matter in the form of volcanic ash or clay impurities that allowed their production of strong cement precursors (5). The total process of cement-clinker formation is summarized in Figure 6, which shows the main components as a function of temperature (18). Calcium carbonate (limestone), quartz, clay (primarily Al2O3), and water are combined and heated. (Iron oxide, clay, and other minor components are neglected in this discussion.) As the temperature rises, first water is lost, and then above 700 C, the limestone decomposes forming CaO and carbon dioxide. CaO reacts with silica to form ‘C2S’ and with the aluminate phases to form a calcium aluminate phase (an Ettringite phase4), which melts at about 1450 C (18). The formation of this liquid phase is associated with the rapid production of tricalcium silicate. The final mixture at 1500 C is primarily tricalcium silicate with smaller portions of dicalcium silicate, aluminate, and aluminoferrate phases. The minor components present in cement paste (e.g., iron oxide) have only subtle effects on the properties of the final cement properties (5, 14). One of the reasons for using them is that they also help to flux the system to a lower temperature. Table 2 lists a group of multicomponent clinker materials in the ‘C3S’ phase field. It is apparent that adding small amounts of other minerals can lower the temperature at which a liquid phase is formed (5). After quenching, the resulting clinker is milled and ground into a fine powder. At this stage various other materials can be added to the cement powder prior to packaging.

Cement Hydration Cement hydration is a familiar process. The cement powder is mixed with water and then is poured for the desired application. The final cement product generally contains about 30–40 mass % water after hydration, and this value varies little with the composition of the cement clinker. Although it might appear simple, cement hydration consists of a complex series of chemical reactions, which are still not completely understood (13). Cement hydration rates can be affected by a variety of factors, including: the phase composition of the clinker, the presence of foreign ions, the specific 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 clinker is shown in Figure 7. Immediately upon contact with water ‘C3S’ undergoes an intense, short-lived reaction, the pre-induction period (I). The rate (dα/dt, where α is the degree of hydration or the fraction of cement precursor material that has been hydrated) is as high as 5 day1. This process begins with the dissolution of ‘C3S’. Oxygen ions on the sur626

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

O2(lattice) + H(aq) 2OH (aq) + Ca2(aq)

OH(aq)

(1)

Ca(OH)2(aq)

(2)

Figure 6. A schematic view of the components of cement-clinker formation, their reactions, and the products formed as the temperature of the mixture is raised. Calcium carbonate decomposes to form calcium oxide and carbon dioxide. Calcium oxide reacts with silica 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 silicate (18).

Table 2. Clinker Components in the ‘C3S’ Cement Phase Field and Their Temperatures of Liquid Formation (5) Components

Temperature of Liquid Formation /C

CaO–SiO2

2065

CaO–SiO2–Al2O3

1455

CaO–SiO2–Al2O3–Na2O

1430

CaO–SiO2–Al2O3–MgO

1375

CaO–SiO2–Al2O3–Fe2O3

1340

CaO–SiO2–Al2O3–Na2O–MgO

1365

CaO–SiO2–Al2O3–Na2O–Fe2O3

1315

CaO–SiO2–Al2O3–MgO–Fe2O3

1300

CaO–SiO2–Al2O3–Na2O–MgO–Fe2O3

1280



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

SiO44(lattice) + n H(aq)

HnSiO4(4-n)(aq) (3)

The dissolved components combine to form the calcium silicate hydrate ‘CSH’ gel, an amorphous two-component solid solution composed of Ca(OH)2 and a calcium silicate hydrate of low Ca:Si ratio, hydrated as in this example (13, 19, 20): 2 (3CaOSiO2)(s) + 6 H2O(l) 3 CaO2SiO23 H2O(s) + 3 Ca(OH)2(aq)

(4)

However, the reaction would not likely be of this exact stoichiometry. Most cement powders have gypsum (CaSO4) added prior to packaging. Gypsum acts to slow down the pre-induction period to avoid rapid setting of the cement (3, 8). It reacts with tricalcium aluminate (‘C3A’) to form various aluminate and sulfoaluminate phases, collectively referred to as Ettringite phases (7, 13, 15, 19). Some examples are: 3CaOAl2O3(s) + 3 CaSO4(s) + 32 H2O(l) 3CaOAl2O33CaSO432 H2O(s)

(5)

A

Fraction of 'C3S' Hydrated

0.25

IV 0.20

III

0.15 0.10

II I

0.05 0.00 0

5

10

15

Hydration Time / h

B Fraction of 'C3S' Hydrated

1.0 0.8

IV

0.6 0.4 0.2 0.0 0

50

100

150

Hydration Time / days Figure 7. A graphic representation of the rate of consumption of tricalcium silicate (‘C3S’) as a function of hydration time: (A) changes in hydration rates in the first few hours as a result of (I) pre-induction, (II) induction, (III) acceleration, and (IV) deceleration processes. (B) an expanded view showing the length of time required for complete cement hydration (13).

3CaOAl2O3(s) + 3CaSO4(s) + 12H2O(s) 3CaOAl2O33CaSO312H2O(s)

(6)

‘C3A’ and ‘C4AF’ can also hydrate independently of calcium sulfate: 3CaOAl2O3(s) + 6H2O(l)

3CaOAl2O36 H2O(s) (7)

4CaOAl2O3Fe2O3(s) + 2Ca(OH)2(aq) + 10 H2O(l) 3CaOAl2O36 H2O(s) + 3CaOFe2O36 H2O(s)

(8)

During the pre-induction period about 5–25% of the ‘C3A’ and ‘C4AF’ undergoes hydration, causing a saturation of Ettringite in the solution (13). After a few minutes of hydration an induction period (II in Figure 7) begins where the reaction slows significantly, dα/dt = 0.01 day1. The exact reason for this induction period is not known. Several theories have been proposed that involve some sort of mixture saturation from the intense burst of hydration in the pre-induction period (13). One theory states that the ‘CSH’ layer quickly covers the surface of dissolving ‘C3S’, slowing the reaction. As time passes, the ‘CSH’ becomes more permeable and the reaction accelerates. Another theory states that the solution may become supersaturated with Ca(OH)2 because the surfaces of Ca(OH)2 crystal nuclei are poisoned by silicate ions. The high concentration of aqueous Ca(OH)2 limits the rate of dissolution of the silicate species to negligible rates. Eventually the level of aqueous Ca(OH)2 becomes too high and calcium hydroxide crystallizes, allowing the hydration reactions to continue. Another theory speculates that two types of ‘CSH’ are formed. The rate of “first-stage” ‘CSH’ is dependent on the concentration of aqueous Ca(OH)2. As the concentration of aqueous Ca(OH)2 decreases, the production of “first-stage” ‘CSH’ stops, causing induction. Hydration resumes later when the thermodynamic barrier for the nucleation of “second-stage” ‘CSH’ is overcome (13). At any rate, an induction period occurs and varies in time depending on the type of cement and the desired application, usually lasting several hours. This property of cement hydration is what makes it easy to use as a construction material—it is a semi-solid that can be easily poured into desired shapes for application. Aqueous gels are often semisolid owing to interaction between water molecules and the surfaces of the particles. Mixing of the system provides energy to overcome these interactions and allows the gel to become more fluid. In the case of cement mixtures, constant mixing is required to keep the material in a fluid state (15). This is why wet cement is often stored in large rotating drums until it is poured. During this induction time, so long as it is continuously mixed, the cement can be held ready for pouring. Following induction, the reaction rate accelerates to approximately dα/dt = 1 day1. At this point the hydration processes are limited by the nucleation and growth of the hydration products. This acceleration stage (III in Figure 7) is characterized by rapid hydration of ‘C3S’, followed slowly by the hydration of ‘C2S’ (13):


627


2(2CaOSiO2)(s) + 4H2O(aq) 3CaO2SiO23H2O(s) + Ca(OH)2(aq)

(9)

During this process, calcium hydroxide reaches its maximum concentration in the solution and then begins to precipitate out as crystalline calcium hydroxide, referred to as Portlandite by cement chemists (7, 13, 15). As the solution becomes concentrated with solid product the rate of hydration slows and becomes diffusion controlled. The reactions slow to nearly negligible rates but continue for weeks as the ‘CSH’ gel continues to form.

Calcium Silicate Hydrate (‘CSH’) Gel Formation: NMR Studies In its final form, cement is a suspension of calcium hydroxide, Ettringite, and unreacted clinker materials in a solid solution of mineral glue called ‘CSH’ gel (13, 15). The formation of ‘CSH’ gel is vital to the understanding of cement hydration processes. One of the most powerful tools for studying the reactions of cement hydration is solid-state nuclear magnetic resonance spectroscopy (21–23). Cements are rich in several NMR active isotopes: 1H, 29Si, 27Al, and 23Na. 29Si magic angle spinning (MAS) NMR can be used to examine the silicon–oxygen bonding in a cement sample as a function of hydration 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 to in this field as a Q0 unit, where the superscript on Q refers to the number of (SiO4)4 units attached to the central (SiO4)4 unit. Q1 represents a dimer and Q2 corresponds to silicon atoms within a polymeric chain of (SiO4)4 units. Q3 and Q4 correspond to silicon centers from which increasingly complex degrees of chain branching occur, as shown in Figure 8 (25). 29 Si NMR is especially useful for examining Si–O bonding because an increase in the number of (SiO4)4 units bonded to each Si center produces an increase in the average electron density around the central Si atom. This leads to a more negative chemical shift, relative to tetramethylsilane (TMS), for successively increasing n values in Qn (see Figure 8 for typical values). In the pre-induction period of cement hydration, 29Si MAS NMR shows the presence of monomeric (SiO4)4 units, Q0. 1H NMR shows that in the first few minutes protonation of the (SiO4)4 units also occurs, an indication that the surface hydroxylation mentioned previously (eqs 1–3) is probably the first step of the reaction (13, 24). As the reaction continues, signals corresponding to Q1 units become predominant, indicating a dimerization of (SiO4)4 units. As time passes the intensities of the Q1 signals decrease, and signals corresponding to polymerization of the dimers, Q2, increase. Crystallographic and NMR studies have shown that the primary species formed are pentamer (Si5O16)12 and octamer (Si8O25)18 units (13). It is interesting to note that Q3 and Q4 signals are not observed for silicon in the hydration of Portland cement, indicating that polymerization takes place predominantly in a linear fashion without branching. 29Si MAS NMR spectra 628

of pure ‘C2S’ and pure ‘C3S’ in comparison with a Portland cement (PC) sample that had been hydrated for 28 days are shown in Figure 9. Broad Q1 and Q2 signals in the 75 to 88 ppm region of the cement sample show the presence of dimer and linear polymer units. However, the signal corresponding to ‘C2S’ in the hydrated cement sample remains essentially unchanged after 28 days, which shows the slow hydration rates of ‘C2S’ relative to ‘C3S’. This is why the production of ‘C3S’ in the cement clinker is so vital for effective cement hydration (24). Data from NMR experiments such as these, combined with X-ray crystallography and microscopy, can be used to postulate a general structure for the cement paste. ‘CSH’ gel has structural features similar to that of two naturally occurring minerals: Tobermorite and Jennite (13, 20). In fact, ‘CSH’ gel is often referred to as Tobermorite gel in the cement literature. These minerals, shown schematically in Figure 10, are characterized by linear Q2 type OSiO bonding and are formed as multiple layers separated by layers of Ca2 or Ca(OH)2.

O O

Si

Q0= -70 ppm O O

O

O

Si

Si

O

O O

Si

O O

Si

O

O

O

Q2 = -88 ppm

O

Si

Q1 = -80 ppm

O

O

O

O

O

O

O O

O

Si

O

O

O O

Si

O O

O

Si

Q3 = -98 ppm O

O

Si

O

O

O O O

Si

O

O

O O

Si

O O

O

Si

O O

Si

Si

O

O

O O

O

Q4 = -110 ppm

O

O Figure 8. Various arrangements of silicon–oxygen bonding are expressed using a superscripted Q, where the superscript refers to the number of (SiO4)4 units bound to the central (SiO4)4 unit in the cluster. The average 29Si NMR signals (relative to TMS) are also shown for each unit. For simplicity, charges are omitted.



Cement Degradation Crumbling cement, rust stains, and cracks in reinforced concrete are commonly observed. These are a few examples of a serious problem that costs North Americans nearly a billion dollars a year—cement corrosion (26). Corrosion in cement and concrete materials is a twofold problem because the cement material and the steel reinforcement are both susceptible to corrosion, and the weakening of one generally accelerates the degradation of the other. Although cement corrosion is complicated, the action of water is a common factor (27). Cement is a porous material containing a dual network of pores. The capillary pore system, with a distribution of diameters that range from 50 to 1000 nm, extends throughout the system, acting as channels between various components of the system. The cement gel itself contains a network of gel pores, with diameters on the order of 10–50 nm (19, 28).

Physical properties of cement such as its elastic modulus, fire resistance, and durability are directly related to the amount of water present (29). Cement is generally 30–40 mass % water, which is present in three forms: 1. Chemically bound water: Water of hydration chemically bound to the cement precursor materials in the form of hydrates. This comprises more than 90% of the water in the system. 2. Physically bound water: Water adsorbed on the surfaces of the capillaries. This water is most predominant in the small gel pores of the system. 3. Free water: Water within larger pores that is free to flow in and out of the system. The amount of free water depends on the pore structure and volume, the relative humidity, and the presence of water in direct contact with the cement surface, such as in water-bearing cement pipes and marine structures (19, 27, 30). A

−

−

OH

O

O

Si O

O

O

Si

O

O Si

Si

Si

O

O

O

O

O

O

O

O

Ca

Ca

Ca

Ca

Ca

Ca

Ca

Ca

O

O

O

O

O

O

O Si

Si

Si O

O

O Si

O

O

O

O

Si

Si

−

HO

Ca2+

−

O

HO

O

8 H2O

OH

O

Si

Ca2+

8 H2O

Tobermorite [Ca4(Si3O9H)2]Ca2 . 8H2O B

−

−

OH

O

O O

Si O

O

O Si O

Si

O

O

O

O

O

Ca

Ca

Ca

Ca

Ca

Ca

Ca

OH OH

OH OH OH OH

OH OH

OH

OH OH

OH

OH

OH OH

OH

Ca

Ca

Ca

Ca

Ca

Ca

Ca

Ca

O

O

O

O

O

O

O

Ca

O

O Si

Si O

Si O

Si O

Si O

O

O Si

O

O

Si −

O

6 H2O

Figure 9. 29Si NMR examination of cement hydration: (A) pure dicalcium silicate, (B) pure tricalcium silicate, (C) Portland cement sample 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. The slow hydration rate of dicalcium silicate is shown by a large peak of unreacted pure material at about 70 ppm (22).

OH

O

Si

Si OH

Ca2+

−

O 6 H2O

OH Ca2+

Jennite [Ca8(Si3O9H)2(OH)8]Ca2 .6H2O Figure 10. Cement paste is believed to closely resemble the minerals Tobermorite and Jennite. These minerals are characterized by layers of polymerized silicon oxide cross-linked with calcium oxide or calcium hydroxide (13).


629


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

Physical Corrosion of Cement Physical corrosion of cements is attributable to the physical properties of water, especially its volume change during freezing and its ability to dissolve cement components. The most significant problem concerning degradation of cements is 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 to move through the cement causing various problems. Drying of a cement or concrete paste is an important factor in the physical corrosion of cement. As a cement paste hydrates over a period of several months, its porosity decreases. Initially, the drying process takes place through capillary flow of water in the larger pore system. As porosity decreases, the drying process slows and becomes diffusive (13, 28). Higher water:cement ratios in the hydration reactions result in larger pore sizes as the cement gel forms, and these pores contain a larger volume of water. Larger pore sizes also lead to faster drying rates, which is a serious problem in areas with low humidity. When cement is exposed to low humidity, free water in the large pores (> 50-nm diameter) evaporates quickly. This water removal is not serious if the cement is in contact with water periodically because large pores also quickly fill with water. However, if cement is exposed to an extended period of low humidity and high temperatures, adsorbed water in the gel pores of the cement will evaporate. This process leads to drying shrinkage. Drying shrinkage is destructive because partially filled gel pores (5– 50-nm diameter) contain water menisci that exert considerable tensile stress on the walls of the pores. This stress leads to microcracking and eventually weakens the material. The use of aggregates minimizes the effect of drying shrinkage because aggregates increase the elastic modulus and compressive strength of the finished product (28). Cements in maritime climates at midlatitudes are particularly susceptible to stress owing to a process known as freeze–thaw cycling (6, 28, 30). Freeze–thaw cycles occur in winter when ambient temperatures hover near 0 C. In these climates freeze–thaw cycles can occur on nearly a daily basis in a typical winter season. Freeze–thaw cycles are damaging to cements because of the 9% volume increase of water upon freezing (31). When water in the capillary pores freezes, it expands and exerts stress on the pore walls. This leads to microcracks, which can in turn fill with water during the subsequent thaw period. Stress exerted in the microcracks during further freezing will extend the cracks until macroscopic cracking is observed. While freeze–thaw degradation generally is most serious at the surfaces of the cement structure, extensive cracking will allow the penetration of water deeper into the structure leading to the eventual failure of the system (6, 30). Crystalline calcium hydroxide makes up about 10% of the volume of most common cements (5, 13, 15), and serious physical corrosion of cements results from the leaching of calcium hydroxide (15). With a room-temperature solubility of 1.7 g兾L (15), calcium hydroxide can be easily dissolved in free water within cement pastes. This is especially 630

problematic with pure water, for example, rain water, melted snow, and condensation within pipes (32). Removal of calcium hydroxide leaves void volumes within the cement, causing a loss of strength and allowing the deeper penetration of leaching waters (15, 27, 30). Calcium hydroxide leaching can be observed in a spectacular effect: opaque white material appears to ooze out of concrete walls or hang in a stalactite formation from concrete ceilings. In this case, water containing dissolved calcium hydroxide has leached out of the concrete and evaporated, leaving behind a layer of calcium hydroxide that reacts with carbon dioxide to form calcium carbonate (15) Ca(OH)2(s) + CO2(g)

CaCO3(s) + H2O(l)

(10)

in a process known as efflorescence.5 Efflorescence is often a sign of water seepage problems in the concrete or cement structure.

Chemical Corrosion of Cement Water also carries chemical agents into cement pastes that react to destroy various components of the cement. A serious problem is the action of acidic waters from acid precipitation, industrial effluent, or the decay of organic matter (6, 15, 32). Acids also lower the pH of the pore water within cement pastes, which otherwise has a pH of 11–13 owing to the large amount of calcium hydroxide present (5, 15, 30). Lowering the pH will also increase the rate of the corrosion of the iron in iron-reinforced cement. The conversion of calcium hydroxide to calcium carbonate through the action of carbon dioxide in the atmosphere is a problem for all types of cement. This can take place directly on the surface (efflorescence), or as the CO2 diffuses into the cement (5, 15, 30): CO2(g) + H2O(l) H2CO3(aq) (11) H2CO3(aq) + Ca(OH)3(s)

CaCO3(s) + 2H2O(l) (12)

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

Ca(HCO3)2(aq)

(13)

2CaCO3(s) + 2H2O(l) (14)

This process, called carbonation, depletes the cement of calcium hydroxide and leaves CaCO3 deposits inside the cement. Another problem, particularly in marine environments, is the action of corrosive sulfates such as ammonium sulfate and magnesium sulfate on cement. These salts react with calcium hydroxide to form calcium sulfate (12, 15):

(NH4)2(SO4)(aq) + Ca(OH)2(s) CaSO4(s) + 2NH3(aq) + 2H2O(l)

(15)

Mg(SO4)(aq) + Ca(OH)2(s) CaSO4(s) + Mg(OH)2(aq)

(16)

Reactions that deplete cement pastes of calcium hydroxide are particularly destructive because the products are usually materials with significantly larger volumes. For example, the volume of calcium sulfate, 74.2 mL兾mol, formed in eqs 15



and 16 is more than twice the volume of the calcium hydroxide removed, 33.2 mL兾mol (5). This volume change leads to stresses and cracks that further accelerate the processes discussed above.

Behavior of Water in Cement Understanding the behavior of water in porous cement is central to the understanding of cement corrosion. Various theoretical and statistical-mechanical approaches have been used to try to describe the movement and distribution of water in the pores of cement (27, 33–35). However, for many years examination of water in cement pastes was hindered by the absence of viable experimental techniques for observing its presence. Recent developments in nuclear magnetic resonance imaging have provided valuable experimental data (6). Magnetic resonance imaging (MRI) is a common technique used in imaging materials, especially biological materials. MRI is typically used to measure the spatial distribution of water in a material (21, 36, 37). It is based on the principle that the nuclear magnetic resonance frequency of a nucleus, such as 1H, in a magnetic field gradient is proportional to its spatial position in the magnetic field gradient (17)

A

where ν is the observed NMR frequency, γ is the magnetogyric ratio of the nucleus, α is the chemical shielding of the nucleus, B0 is the magnetic field associated with a static field measurement, Gz(z) is the magnetic field gradient (dB/dz), ν0 is the NMR frequency in the static field (B0), k is a scaling constant for the signal, and z is the position of the nucleus in the field (21). Figure 11(A) shows two nuclei in a magnetic field gradient Gz = dBz兾dz. In Figure 11(B) an interferogram is produced when a 90 radio frequency pulse is applied. Fourier transformation of the signal in (B) produces two 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, dz ∝

1 Gz π T2

Gz =

dBz dz

z dz

B

M

γ (1 − α)( B0 + Gz ( z )) = ν0 + kz 2z

90o pulse

ν =

sample is moved through a stationary field gradient of a superconducting magnet. STRAFI experiments have allowed the detailed examination of water in solid cement samples (35). Imaging techniques such as STRAFI are useful for examining the effectiveness of waterproof coatings. One of the easiest ways to prevent cement corrosion is to prohibit the movement of water in and out of the material by establishing a waterproof barrier on the exposed surfaces (6, 30). A wide variety of surface coatings are used in the waterproofing of cements; for example, a common class of waterproofing agents is silanes (22). The rate and depth of surface water absorption into the cement surface can be compared for a series of coatings and treatments. The depth and durability of the surface treatment can also be examined for various applications (6, 22). A STRAFI image of a Portland cement sample coated with methyltrialkoxysilane is shown in Figure 12. The images show penetration of the silane coating as it is repeatedly applied to the surface. After 24 hours the coating penetrates to a depth of about 2.5 mm. A comparison of the water penetration in treated and untreated Portland cement is shown in Figure 13. The treated sample shows water on the surface (intense surface signal) and the silane coating

t

(18)

where T2 is the spin–spin relaxation time of the nucleus (22, 23). The shift in the NMR frequency of a nucleus is proportional to the position of the nucleus in the sample, while the signal intensity corresponds to the amount of that nucleus present. Water in gel pores is tightly confined and is susceptible to the effects of various paramagnetic species in the sample including iron and aluminum (6). These factors combine to make the spin–spin relaxation times short, dramatically decreasing the effective resolution (6, 37). For example, using conventional MRI techniques, a field gradient of about 10 T兾m is necessary for a resolution of 10 mm in a cement paste (6). This is much greater than the normal field gradients used in MRI, but is on the order of the stray fields associated with the superconducting magnets of high-resolution NMR instruments.6 In 1988, a MRI technique known as stray field imaging (STRAFI) was developed (38). In this experiment a

C

width ~

0

1 πT2

ν

∆ν = γ Gz dz Figure 11. Schematic of conventional MRI experiment. (A) Two nuclei situated in a magnetic field gradient, Gz, are separated by dz. (B) A 90 RF pulse is used to obtain an interferogram of the nuclei in the sample. (C) Fourier transformation of (B) gives two peaks separated by a frequency proportional to their separation in the sample (21).


631


penetrating to about 2–3 mm. The untreated sample shows little surface water but significant water penetration to 8–9 mm after 24 hours (22). While STRAFI is a powerful technique for examining the water content of a cement paste, it is limited to relatively small sample sizes (∼ 2-cm diameter) because of magnetic field constraints. Studies of concrete are limited to those with very small aggregates such as fine gravels and sands (28). Another way of alleviating the problem of short T2 while avoiding enormous field gradients is through the use of single point imaging (SPI; ref 21, 39). SPI is an MRI technique that uses an oscillating field gradient in which signals are measured at a constant encoding time, tp, following a radio frequency (RF) pulse. A recent variation on SPI developed by Balcom et al. (40) has proven useful in the examination of water 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 control than the oscillating gradients used in conventional SPI (28, 29, 40). While conventional MRI, including STRAFI, measures all resonance frequencies simultaneously and deconvolutes using a Fourier transform, SPRITE uses a process called position encoding where only one frequency, corresponding to a particular encoding time, tp, is measured. The spatial position, z, of the analyte nucleus is encoded in reciprocal space such that the signal, S(k), is proportional to k (40): 1 2 πγGz,max tp

k =

(19)

With a constant encoding time, k is inversely proportional to the maximum field gradient, Gz,max. A schematic SPRITE imaging sequence and resulting image are shown in Figure 14. Following an RF pulse a single frequency is measured after a desired encoding time, tp. Next, the field gradient is ramped and the sequence is repeated. Each sequence gives the nuclear density at a particular point in the sample. Through repetition of the sequences at varying Gz,max an im-

A RF

RF tp

z1

RF tp

RF tp

z2

RF tp

z3

z4

tp

z5

Time

B Figure 12. One-dimensional STRAFI image of a Portland cement sample coated with methyltrialkoxysilane. The coating is applied every 30 minutes during the analysis. The signals show the ingress of the polymer coating into the cement to a depth of about 2.5 mm after 24 hours (22).

Gz

Time

Signal

C

z1

z2

z3

z4

z5

z

Figure 13. One-dimensional STRAFI of Portland cement samples in contact with water for 24 hours. Sample A was treated with methyltrialkoxysilane. Sample B was untreated and shows deep penetration of water into the cement surface (22).

632

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



2 γ MT0 r ρ ∆H

A RF

RF tp

T1

RF tp

RF tp

T2

RF tp

T3

tp

T4

T5

Time

B

Gz

Time

C

(20)

where ∆T is the freezing-point depression, γ is the surface tension of the liquid, M is the molecular weight, T0 is the normal freezing point, r is the pore radius, ρ is the density of the absorbate, and ∆H is the molar enthalpy of fusion (41). Information on the freezing-point depression of water in a cement sample is valuable for the determination of pore distributions in these materials. As ice forms in a cement sample the internal pressure of the closed system increases owing to the volume expansion of water. The resulting pressure increase once freezing begins in the gel pores forces the migration of water from the gel pores 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 freezing phenomena for a cement sample measured using an SPI technique (29), in which the evaporable water content of a concrete sample is measured as a function of temperature as the sample is slowly cooled (2 K兾hr). The first freezing event is seen between 0 and 1.6 C. As the sample temperature is lowered the amount of evaporable water decreases slowly at freezing temperatures corresponding to the respective pore sizes present. The large change at 0 to 1.6 C shows that the majority of evaporable water present is contained in large pores. The second major freezing event, associated with the desorption and freezing of water from the gel pores, is shown at 45 C. Other studies have shown that evaporable water can still exist in cement samples at temperatures as low as 90 C (29).

T1

T2

T3

T4

T5

Temperature Figure 15. Schematic of SPRITE used for temperature dependent measurements. (A) The normal SPRITE sequence from Figure 14 is used again, but (B) the field gradient is kept constant. (C) This allows for a repeated measurement of the signal density for analyte nuclei at a particular position in the sample as a function of temperature (40).

1.0

Magnetization (arb. u)

∆T =

Corrosion of Steel Reinforcement Reinforced concrete is often used in bridge decks, roads, and sidewalks. One of the most serious threats to concrete in cold climates is the use of deicing salts in the winter to ensure safe conditions for motor vehicles and pedestrians

Signal at z 1

age is produced. SPRITE and other SPI techniques take longer than conventional MRI techniques, but are less susceptible to noise and magnetic inhomogeneities in the sample because only one frequency is analyzed at a time (21, 28, 29, 39, 40). SPRITE is also useful because signal resolution depends only on the size of Gz,max and the ramping sequence used, not on the T2 of the analyte nucleus (40). SPRITE can be used to examine the behavior of protons in a concrete sample as a function of physical parameters such as temperature. From eq 19 it can be shown that keeping Gz,max and tp constant results in repeated measurements of protons in a defined position, z. The intensity of the signal can be observed as a function of temperature, shown in Figure 15. Through an adjustment of parameters such as Gz,max, RF flip angles, and tp, the experiment can be tailored to be sensitive to a nucleus of defined T2. This allows the ability to differentiate between the protons of free liquid water (T2 ≈ 200 µs) and ice (T2 ≈ 10 µs) in a cement sample. By following the appearance and disappearance of free 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 two steps. The first step occurs between 0 and 2 C, where free bulk water and water in the capillary pores freeze (29). This freezing-point variation is due to a freezing-point depression phenomenon caused by vapor pressure lowering in the capillaries and related to the pore size of the capillaries by the Kelvin equation (29),

0.8

0.6

0.4

- center - drying face

0.2

0.0 -50

-40

-30

-20

-10

0

10

T / °C Figure 16. Magnetization signal for evaporable water in a sample of Portland cement mixed with 14-mm diameter graded quartz aggregate measured using SPRITE as a function of temperature. Freezing of water is associated with a decrease in signal intensity (29).


633


(6, 15, 26, 42, 43). Chloride ions, when transported by water, attack steel reinforcement (rebar) of these structures causing them to weaken from within. High pH is important for minimizing the rate of steel rebar corrosion because it allows the formation of a passive oxide layer on the surface of the metal (6, 30). Low pH aqueous states caused by the leaching of calcium hydroxide from the cement, combined with chloride ion ingress, causes extensive rebar corrosion in short periods of time. Chloride ions set up redox reactions along the rebar, as shown by the following equations (26):

2 Fe(s) O2(g) + 2 H2O(l) + 4e 2 Fe2(aq) + 4 Cl(aq)

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

2 Fe(s) +

1

/2 O2(g)

3

/2O2(g)

2 Fe2(aq) + 4 e

(21)

4 OH(aq)

(22)

2 FeCl2 (aq)

(23)

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

(24)

Fe2O3(s) + 2 H2O(l)

(25)

Fe2O3(s)

(26)

Chemical attack of chloride ions is destructive because it not only reduces the amount of hydroxide ion and iron, but it also acts in a catalytic manner. Transport of chloride ions throughout the system is also increased as cracks form as a result of the other decay processes. Furthermore, patching can create localized corrosion cells between the rebar in the

existing chloride-contaminated concrete and in the new chloride-free patch, accelerating the concrete corrosion (43). Waterproof coatings will stop the introduction of new chloride ions, but will not remove the chloride ions already present in the system. Coatings also become ineffective if the concrete surface is cracked or damaged (26, 43). A particularly interesting approach to treating chloride ion ingress is electrochemical chloride extraction (ECE), in which chloride ions are effectively pulled from the concrete. A dc circuit, shown in Figure 17, is set up using rebar as the anode and an electrolyte gel packed on the concrete surface as the cathode. When a dc potential of 10,000–30,000 V is applied, water hydrolyzes at the anode, replenishing the hydroxide content of the system. The negatively charged steel rebar repels chloride ions to the surface of the concrete and into the electrolyte gel. After 4–8 weeks the process is complete, at a fraction of the cost of replacement (43). After sealing with a waterproof coating, the concrete is effectively protected against further rebar corrosion. Structures with extremely problematic chloride ion problems, such as ocean piers, can be cathodically protected by constant maintenance of the rebar at a negative potential of about 10,000–30,000 V (43). Concluding Remarks The study of cement offers an opportunity to explore the chemistry of earth materials, their preparation, and resulting properties. Furthermore, examination of cement degradation comprises an extensive part of modern cement chemistry. Recent innovations in research techniques have made the study of cement preparation and degradation behavior more accessible. Improvement of corrosion resistance in cement and concrete structures would significantly lengthen the lifetime of applications using these materials, potentially saving billions of dollars worldwide. Acknowledgments

DC power supply

steel rebar

ⴚ Cl ⴙ

ⴚ

ⴙ

ⴚ

H2O

H + OH

H2O

H + OH

ⴚ

ⴚ

Cl

Notes

+

ⴚ

electrolyte gel

concrete sample 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 concrete. Hydrolysis of water takes place at the rebar and chloride ions are repelled from the concrete into the electrolyte gel (43).

634

This work was supported by the Natural Sciences and Engineering Research Council of Canada and the Izaak Walton Killam Trusts.

1. Portland stone, a gray stone quarried from the Dorset region of England, was a commonly used building material in Europe in the 16th–19th centuries (2, 7). 2. Founded in 1898, ASTM International is a nonprofit organization that provides a global forum for the development and publication 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, in this case by lowering of the melting point of a mixture by adding another component (7). 4. Ettringite is a collective term referring to the various aluminate and sulfoaluminate phases present in the clinker material. 5. Efflorescence is the “blossoming” to a powdery substance on exposure to air. 6. The stray field near a 9.4 T (400 MHz) NMR magnet is on the order of 60 T兾m.



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Cement: Its Chemistry and Properties - Journal of Chemical Education

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