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H.J.H. Brouwers

Chemical Reactions in hydrated Ordinary Portland

Cement based on the Work by Powers and Brownyard

In a pioneering work, Powers and Brownyard (1948) were the first to systematicallyinvestigate the reaction of cement and water and the formation of cement paste. In the late1940s they presented a thorough model of the cement paste, in which unreacted water andcement, the hydration product, and (gel and capillary) porosity were considered. Viaextensive and carefully executed experiments major paste properties were determined suchas the amount of retained water and the chemical shrinkage associated with the hydrationreaction. These properties were furthermore related to the content of the four mostimportant clinker phases, viz. alite, belite, aluminate and ferrite, in the cement. Locher(1975), Hansen (1986), Taylor (1997), Neville (2000) and Brouwers (2003) summarise the

most important features of their work.Here, it will be demonstrated that their results enable the study of the reactions of the four clinker phases and their reaction products, which is a principal innovation. In thepast, the water binding of OPC as predicted by their model has been compared to pure C3Shydration by Locher (1966) and Young and Hansen (1987). This approach is permitted, asC3S is the major constituent of OPC. The model of Powers and Brownyard (1948),however, contains valuable specific information in regard to the reaction of each individualclinker phase, such as C3S. This aspect of their model and experiments has beenoverlooked hitherto. This will be presented here, and will result in a new reaction model.

Water retention measured by Powers and Brownyard (1948)

Powers and Brownyard (1948) executed and reported numerous experiments with cementsof different compositions, with neat cement and with mortars, and at various water/cementratios (w0 /c0) and various hardening times at ambient temperature. Their cements had awide variation in composition and the specific surface was 1610-2045 cm2 /g.

The water retention of the hardened cement was measured both at saturated stateand at P-dried state. The water that can be removed by P-drying (using vacuum andmagnesium perchlorate hydrate at 23 oC) was named evaporable water, the remainingwater non-evaporable or chemically bound water. The evaporable water comprises bothabsorbed water (“gel”) and capillary water.

Assuming that all cement particles smaller than 50 µm have reacted after a longtime (e.g. 1 year or even longer) and at high w0 /c0 (more than 0.44), Powers andBrownyard (1948) fitted the following relation between wn /c and the clinker composition:

wn /c = 0.187 SC3x + 0.158 SC2

x + 0.665 AC3x + 0.213 AFC4

x (1)

This equation (mass of water retained in P-dried state) can be transferred in moles of retained water per mole of reacted clinker phase via



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S C m3

= SC3x c; S C m

2= S C x

2c; etc. (2)

w = H M H n ;S C m

3=

S C M 3 S C n

3;

S C m2

=S C M

3 S C n2

; etc. (3)

yielding

nH,n = 2.37S C n

3+ 1.51

S C n2

+ 9.97 AC n

3+ 5.74

AF C n4

(4)

whereby the molecular masses of water and the clinker phases (and most hydrationproducts) can among others be found in Brouwers (2003).

Powers and Brownyard (1948) used the P-dried samples to perform sorptionexperiments using water vapour. They found that at relative water vapour pressures below45%, the amount of water held is proportional to the amount of cement reacted and hence,was assigned absorbed or “gel” water. Above this relative vapour pressure, the water alsocondenses in the larger capillary pores. Furthermore, using the B.E.T. theory, a quantity

Vm was measured and seen as the mass of water to cover the P-dried hydration productwith one layer of water. It was observed that Vm is linearly proportional to the amount of non-evaporable water. This was explained with the fact that the internal surface of thehydration product is proportional to the cement reacted. In later work Brunauer and Kantro(1964) experimentally confirmed for C2S and C3S hydration that the surface developmentclosely follows the degree of hydration indeed. Accordingly, the amount of gel water wasrelated to the non-evaporable water. Furthermore, it was recognised that the amount of internal surface depends on the composition of the cement as each clinker phase produces ahydration product with its specific internal surface. From the experiments it wasfurthermore concluded that the maximum amount of water that can be retained by thehydration product, the i.e. gel water, corresponds to 4Vm per mass of reacted cement.

Water in surplus of 4Vm is capillary water. This result was explained with the concept thatVm is the mass of water to cover the hydrated cement in the hydration product with onelayer of water, and that with 4 layers the hydration product (gel space) is saturated. Powersand Brownyard (1948) therefore recommended the following empirical fit:

wg /wn = 4Vm = 4 (0.230 SC3x + 0.320 SC2

x + 0.317 AC3x + 0.368 AFC4

x ) (5)

The total water retained by the reaction products (non-evaporable + gel water) thus reads:

wd /c = (wn + wg)/c = (1 + 4Vm) wn /c (6)

The total retained water could also depend on the S C content, so this mass fraction shouldbe included in a fit as well. Accordingly, a least squares method is employed to theexperimental data of Powers and Brownyard (1948). Details of this approach can be foundin Brouwers (2003). The fitting yields

wd /c = 0.334 SC3x + 0.374 SC2

x + 1.410 AC3x + 0.471 AFC4

x + 0.261S C

x (7)



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Using eqs. (2) and (3) the water retention can now be written in moles as:

nH,d = 4.23 S C n3

+ 3.58 S C n2

+ 21.1 AC n3

+ 12.7 AF C n4

+ 1.97S C

n (8)

Eqs. (7) and (8) have as advantage that they provide the mass and moles of totally retained

water as a function of each clinker phase and that it includes calcium-sulphate. Here, theinformation on water retention in P-dried and saturated paste will be used to investigate thereactions of the four clinker phases.

Reactions of calcium silicate phases

Powers and Brownyard (1948) presented a literature review of the reaction products andwere aware that the products of the clinker phases C3S and C2S were “microcrystalline”CH and a “colloidal gel”, also named “colloidal hydrous silicate” and ”calcium silicatehydrate” (p. 106-132, p. 260, p. 488). In later work this product was called “tobermorite

gel” (Brunauer et al. (1958), Brunauer and Greenberg (1960), Kantro et al. (1960),Brunauer and Kantro (1964), Kantro et al. (1966)) and nowadays it is generally called C-S-H (Taylor (1997)). It is known to be a poorly crystalline to almost amorphous material.

Both clinker phases and said reaction products are most abundant in cement paste,and can react independently from the other clinker phases. Accordingly, the followingrelations are put forward:

C2S + (2 – x + y) H → CxSHy + (2 – x) CH (9)C3S + (3 – x + y) H → CxSHy + (3 – x) CH (10)

Note that at that time x, i.e. the C/S ratio, was unknown, though it was known that the C2S

reaction hardly produces CH (from Powers and Brownyard (1948), p. 488). In other words,x should be close to 2, being in agreement with the present knowledge of x being in therange 1.5 to 2. The C/S ratio depends among others on the hydration conditions (bottle,paste), w0 /c0, particle size, age and the analytical method employed (Taylor (1997)).

Eq. (10) was proposed by Locher (1966), who was the first to couple the overallnon-evaporable water of Portland cement (thus not C3S in particular) as determined byPowers and Brownyard (1948), to the hydration of the single clinker phase C3S. Note that(2 – x + y) and (3 – x + y) correspond to

H n , and that 12=S C n and 1

3=S C n in eqs. (9) and

(10), respectively. They depend on the drying conditions: see eqs. (4) and (8) for theirvalues in P-dried and saturated state, respectively.

P-dried stateBased on the findings of Brunauer et al. (1958), Brunauer and Greenberg (1960) andKantro et al. (1966), P-drying only removes water from the C-S-H (and not from thecrystalline CH) and it does not affect the C/S (x) ratio of the C-S-H.

X-ray analysis on P-dried C2S and C3S/alite by Kantro et al. (1966) revealed that (x– y) amounts to 0.5 and 0.4 in C-S-H formed by C2S and C3S/alite, respectively. Thesevalues are stable in time (up to 1700 days) and are found for water-cement ratios ranging



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from 0.45 to 0.7. Using 12=S C n in eq. (4) yields 51.1, =n H n , and using 1

3=S C n in eq. (4)

yields 37.2, =n H n . So it follows that (x – y) equals 0.49 and 0.63 for the C2S and C3S/alite

reactions, respectively. For C2S the agreement with the aforesaid values of Kantro et al.(1966) is excellent, for C3S/alite it is very good as well. Note that Locher (1966)experimentally found 66.2

,

=n H n (i.e. the coefficient of C3S in eq. (1) being 0.21 instead

of 0.187). This value yields (x – y) = 0.34, this figure is in better agreement with theaforementioned value in Kantro et al. (1966).

To quantify the two reaction products, a value for the C/S ratio, x in eqs. (9) and(10), now needs to be specified. An average value of 1.7 is often found and generallyaccepted (Brunauer et al. (1958) and Brunauer and Greenberg (1960), Copeland andKantro (1964), Locher (1966), Young and Hansen (1987), Taylor (1997)) and also takenhere, yielding the following approximate reactions

C2S + 1.51 H → C1.7SH1.21 + 0.3 CH (11)C3S + 2.37 H → C1.7SH1.07 + 1.3 CH (12)

Saturated state

The next step is to study the hydration of C2S and C3S in saturated conditions. From eq. (8)it readily follows that (2 – x + y = 3.58) and (3 – x – y = 4.23) for the C2S and C3Sreactions, respectively. Again invoking x = 1.7, eqs. (9) and (10) yield the followingapproximate reactions

C2S + 3.58 H → C1.7SH3.28 + 0.3 CH (13)C3S + 4.23 H → C1.7SH2.93 + 1.3 CH (14)

In Brouwers (2003) the model of Powers and Brownyard and the reactions (11)-(14) are

furthermore used to derive the volume fraction of C-S-H in the hydrated product, thedensity of C1.7SH3.2 (2.27 g/cm3), and the porosity of C-S-H (33%).

Reactions of aluminate and sulphate phases

In contrast to the calcium silicate phases, which basically form CH and C-S-H, thealuminate phase can react in several ways and to more different hydration products. Insome reactions calcium sulphate and carbon dioxide may also be involved. In contrast tothe C-S-H gel, which is poorly crystalline to amorphous, the water content of mostconceivable hydration products, which are (quasi-)crystalline, are known, as well as their

specific volumes and other data. The retained water of these crystalline reactions productsis therefore of other nature than the gel water of the C-S-H. Likewise the CH, thesecrystalline hydration products are impermeable and in the paste their gel water (theydehydrate upon drying) can, unlike the C-S-H gel space, not be considered as gel space. Inthis Section the reaction of aluminate and sulphate phases is discussed, and it will beassume a priori that the ferrite phase does not react with the sulphate phase. In thefollowing Section the reason of this major assumption is motivated. The results of thisSection also support this hypothesis.



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In Taylor (1997) a comprehensive review of the hydration of C3A is presented,which is briefly summarised here. In water, the pure compound will mainly hydrate tohydrogarnet (C3AH6). In a real water-cement system at room temperature, in the presenceof calcium sulphate and calcium hydroxide, the aluminate hydrate C4AH13-19 (formed from

C3A, CH and H), and aluminate sulphate phases are formed, such as C4A S H12-14 (mono-

sulphate) and C6A 3S H32-36 (ettringite). Firstly, ettringite is formed, which is then partlyconverted into mono-sulphate. This mono-sulphate is very susceptible to carbonation,

resulting in the formation of ettringite and C4A 5.0C H12 (hemi-carbonate). Only a fewtenths of mass percent CO2 on cement mass is already sufficient to prevent the formationof mono-sulphate. When sufficient CO2 is present, hemi-carbonate can be replaced by the

CO2 richer mono-carbonate (C4AC H11). For the present analysis is not important whetherhemi-carbonate or mono-carbonate are formed, as the water retention by both substances is

almost identical, so that here attention is restricted to C4A 5.0C H12.In this paper, only the reaction (products) in saturated state is discussed, in P-dried

state can be find elsewhere (Brouwers (2003)). The major conclusion of the analysis of P-

dried products is that the sulphate in the cements used by Powers and Brownyard mostlikely has been present as anhydrite and/or in the clinker.

Saturated state

In saturated state, the amount of retained water in relation to the moles of C3A and C S isprovided by eq. (8) as (approximately)

S C AC d H nnn 2213, += (15)

In the absence of C S )0( =S C

n and in the presence of CH, only the aluminate hydrate and

hemi-carbonate can be formed. The high water retention renders the formation of hemi-carbonate unlikely, so from eq. (15) it readily follows that in such case

C3A + CH + 21 H → C4AH22 (16)

This high water retention is higher than given by Taylor (1997), but in line with theC4AH21 found by Le Chatelier in the 19th century (quoted by Schwiete and Ludwig(1969)). Fischer and Kuzel (1982) synthesised aluminate hydrates and with XRD, IR andDTA, also measured the presence of C4AH19 and an indication of hydrates with even more(interlayer) water. At a relative humidity lower than 65%-88%, the hydrate is readilydehydrated to C4AH13 (Schwiete and Ludwig (1969), Fischer and Kuzel (1982)).

In case sulphate is present too, ettringite and mono-sulphate will be formed as well(Taylor (1997)). In case of carbon dioxide present, carbonation will occur, whereby mono-

sulphate is the most unstable and will first react to hemi-carbonate (C4A 5.0C H12), and

possibly also to mono-carbonate (C4A C H11) in case sufficient C is present. Sources of C

are mixing water, ambient air and C C in the cement (Kuzel (1996), Taylor (1997)). The P-dried analysis has revealed that the sulphate phase does not carry water (Brouwers (2003)).



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Furthermore, in saturated conditions, most likely C4AC H11, C4A S H14 and C6A 3S H36 areformed (Dosch et al. (1969), Pöllmann et al. (1989), Kuzel (1996), Pöllmann (2003)). Thisinformation is used to formulate the following additional reactions:

C3A + ½ CH + ½ C C + 11.5 H → C4A 5.0C H12 (17)

C3A + C S + 14 H → C4A S H14 (18)

C3A + 3 C S + 36 H → C6A 3S H36 (19)

The formation of hemi-carbonate has been imposed, but with the formation of mono-carbonate almost the same amount of water is involved (11 moles of H instead of 11.5).Implicitly we assume that the initial mono-sulphate is able to consume all CO2 and hence,that the ettringite and aluminate hydrate (and portlandite) do not carbonate.

Now it is possible and interesting to compute which part of the C3A is converted

into C4AH22, C4A 5.0C H12, C4A S H14 and into C6A 3S H36. Mole balances of C3A, S and Hyield

AC H S AC H S AC H C AC AH C nnnnn33636144125.04224

=+++ (20)

S C H S AC H S AC nnn =+

36361443 (21)

d H H S AC H S AC H C AC AH C nnnnn ,3636144125.04224

36145.1121 =+++S C AC nn 221

3+= (22)

respectively, whereby eq. (15) has been inserted. This set of three equations contains 4unknowns. Most likely, as discussed in the foregoing and as in most real pastes, fullcarbonation of mono-sulphate has occurred. Accordingly, henceforth it is assumed that thedata of Powers and Brownyard (1948) are based on carbonated pastes. According to Kuzel

(1996) this situation is most likely in practice as CO2-free conditions are very difficult toachieve and only a little CO2 is needed to prevent the formation of mono-sulphate.The amounts formed products in that case follow from eqs. (20)-(22) as

S C AC S C AC AH C nnnnn3

2

57

3733224−≈−= (23)

0144=

H S AC n (24)

S C H S AC nn

3

13636= (25)

S C S C H C AC nnn

3

1

57

18125.04

≈= (26)

Note that the sum of all these hydration products formed from C 3A, times theirwater retention (eqs. (16)-(19)), indeed complies with eq. (15).

For the full carbonation of mono-sulphate,C C

n / S C

n apparently needs to be 1/6 (see

eq. (26)), orC C

m / S C

m = 12%. So, related to the mass of calcium sulpate, only 12% of

calcite is required for full carbonation. As the sulphate constitutes only a few percent of the



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cement mass, a few tenths of a percent of calcite in relation to cement mass is required,which is in line with findings in literature (Kuzel (1996), Taylor (1997)). Ettringite andhemi-carbonate are stable (in contrast to mono-sulphate) and delayed ettringite formationis not likely (Kuzel (1996)) when mono-sulphate is absent.

From eqs. (23)-(26) the following quantitative conclusions follow. The moles of

hemi-carbonate (plus possible mono-carbonate) and ettringite formed only depend on S C n ,but their ratio being constant

119

18

3636

125.04 ≈= H S AC

H C AC

n

n(27)

For full carbonation of the mono-sulphate, as assumed to prevail during the experiments of Powers and Brownyard (1948) and in most practical cases, the molar ratio of hemi-carbonate and ettringite is about unity.

The hemi-carbonate formed can be expressed in amount of ettringite formed, as this

latter product is always present (in case sulphate is present). The ratios appear not todepend on AC n

3nor

S C n . It is interesting now to determine the ratio of tetra calcium

aluminate hydrate formed to ettringite formed. Combining this eq. (23) with eq. (25)yields:

−≈

−=

3

23

57

373 33

3636

224

S C

AC

S C

AC

H S AC

AH C

n

n

n

n

n

n(28)

Apparently, to bind all sulphate (i.e. so that no aluminate hydrate is formed), the number of

moles C3A ( AC n 3 ) needs to be 2/3 the number of moles of C S ( S C n ). This minimum valuefor AC n

3 /

S C n (= 2/3) is larger than the value that one would expect if all C 3A would be

converted into ettringite, namely 1/3. This is due to the fact that mono-carbonate is formedsimultaneously in a carbonating environment, which is also binding C3A.

If no (or partial) carbonation takes place in the cement paste, then another line of reasoning needs to be followed and alternative expressions are obtained for the quantitiesof the four products formed. The quantities then depend on the degree of carbonation(Brouwers (2003)).

Reaction of ferrite phase

The clinker phases C3S, C2S and C3A were already known to Le Chatelier in 1887. Theferrite phase, on the other hand, was discovered much later, in 1928 (Steinour (1961)). Thereaction of this phase, which is the most impure one, in a cement-water system still raisesquestions. First of all, distinction needs to be made between the reaction of the pure phaseat one hand, and the reaction of the impure phase as found in cement clinker, reacting inthe presence of other clinker phases (Taylor (1997)). The first situation is prevailing in a



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laboratory setting and the results reported (Collepardi et al. (1979), Fukuhura et al. (1981)),but the latter situation is obviously of more relevance to real cement pastes.

As said, in many publications it is reported that similar products can be formed as

formed by the C3A reactions, such as C4AH13-19, C4A S H12-14 and C6A 3S H32-36, wherebyA can be partly substituted by F (Schwiete and Ludwig (1969), Taylor (1997)). The –

partial- replacement of A by F in ettringite, yielding approximately C6A0.75F0.25 3S H31,

however, has only been found in pure ferrite and C S hydration experiments (e.g. Fukuharaet al. (1981)). The replacement has not been found in mono-sulphate yet. The formation of FH3 has not been detected either in real cement pastes, though it could be amorphous andtherefore difficult to detect. These findings render the similarity between C3A and C4AFreactions questionable (Taylor (1997)).

In real cement pastes, on the other hand, F has been found in hydrogarnets. UsingXRD, Kantro et al. (1960) and Copeland et al. (1960) suspected a phase with acomposition in the vicinity C6AFS2H8 in hydrated cement. In a pure system of C3S andC4AF, Schwiete and Iwai (1964) found that with increasing C3S/C4AF, the S/F ratio (x) in

the formed C6AFSxH12-2x also increased, but x not exceeding a value of 2 (at roomtemperature). For x > 1.5 the hydrogarnet was furthermore stable to sulphate attack(practically no transition to ettringite). Using XRD and EMPA, Taylor and Newbury(1984) confirmed the presence of a hydrogarnet of composition near C6A1.2F0.8S2H18 inhydrated OPC. Based on SEM and TEM, Rodger and Groves (1989) suggested acomposition of C6A0.6F0.6S2Hx in OPC and OPC-fly ash blends. Paul and Glasser (2000)investigated OPC pastes that were prolonged cured (8.4 years) at 85 oC, which has limitedpredictability towards cement hydration at ambient temperature, of course. But using XRDand DTA/TGA, they estimated that the observed hydrogarnet in their paste is having a

composition near C6A1.26F0.51M0.46S2.8 S 0.58H5.59.By Brouwers (2003) it is demonstrated that the water retention results of Powers

and Brownyard (1948), both in P-dried and saturated state, render the occurrence of C 3Aanalogue reactions (to form F substituted calcium aluminate/ferrite hydrate, mono-sulphateand ettringite) not very likely, being in line with the literature summarized above.Consequently, here the possible formation of the F and S containing hydrogarnets will bediscussed only.

P-dried state

The formation of hydrogarnet can be modelled as follows

C4AF + x C2S + (2x + y – 2) H → C6AFSxHy + (2x – 2) CH (29)C4AF + x C3S + (3x + y – 2) H → C6AFSxHy + (3x – 2) CH (30)

In dried state, for the hydrogarnet furthermore holds

2x + y = 12 (31)

And from eq. (4) follows



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AF C S C S C n H nnnn432

74.537.251.1, ++= (32)

Applying eq. (31) to eq. (29) yields 10, =n H n . Furthermore, xn S C =2

, 03=S C n and

14

= AF C n . Substitution of these values and 10, =n H n into eq. (32) yields x = 2.8, and eq.

(31) then yields y = 6.4. Applying eq. (31) to eq. (30) yields xn n H +=10, , combining thiswith 0

2=S C n , xn S C =

3and 1

4

= AF C n , and substitution into eq. (32), now yields x = 3.1.

With eq. (31) then y = 5.8 is obtained.The computed compositions of the hydrogarnets are very close (an S/F ratio of

about 3), irrespective if they are formed from C2S or C3S, and their composition comesclose to composition that has been measured by the authors mentioned above. The analysishere supports the idea that C4AF reacts with C2S and/or C3S to form a hydrogarnet. The

absence of F and S containing phases could be due to the fact that C3A reacts morerapidly than C4AF, and might consume all sulphate in the cement system.

As previous authors found an S/F ratio of 2 at ambient temperatures, the C4AF and

calcium silicates most likely react to C6AFS2H8. Note that if the coefficient 5.74 appearingin eqs. (4) and (32) would only have been 25 % larger (i.e. a water retention of about 7.14moles of water per mole C4AF instead of 5.74), x 2 would have been obtained from thepresent analysis indeed.

Saturated state Using the values for x (2 or 3) as obtained from the analysis of the P-dried state, thehydrogarnet reactions in saturated state can modelled by equations (29) and (30), wherebyy is yet unknown. Similarly as for the aluminate hydrate in the previous Section, whichappeared to retain extra interlayer water, the hydrogarnet will be allowed to retain morewater than prescribed by eq. (31).

From eq. (8) follows the following water retention is saturated state

AF C S C S C d H nnnn432

74.1223.458.3, ++= (33)

Applying x = 2 to eq. (29) yields yn d H += 2, , 22=S C n , 0

3=S C n and 1

4= AF C n .

Substitution of these values into eq. (33) yields 92.192 =+ y and hence, y = 17.92 (i.e.

C6AFS2H17.92 is formed). Applying x = 2 to eq. (30) yields yn d H += 4, , 02=S C n ,

23=S C n and 1

4= AF C n . Substitution of these values into eq. (33) yields 2.214 =+ y and

hence, y = 17.2 (C6AFS3H17.2 is formed). In comparison to dried hydrogarnet (C6AFS2H8),saturated hydrogarnet per mole apparently retains 9 to 10 moles of water more. When thesame procedure is also carried out with x = 3, then C6AFS3H19.48 and C6AFS3H18.43 followsfrom the reactions with C2S or C3S, respectively. Also in these cases the saturatedhydrogarnet holds 12 to 13 more moles of water than in dried state (C6AFS3H6).

This high water retention could be caused by the formation of very small crystalsand/or imperfectly ordered structures (Taylor (2002)). Another explanation could be thatthe hydrogarnet is formed by the reaction of an unstable F-substituted gehlenite hydrate(C2A0.5F0.5SH8) and CH, resulting in aforesaid super-saturated hydrogarnets. The analogue



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reaction of C2ASH8 and CH is explained by Damidot and Glasser (1995) using a phasediagram, and it was observed by Locher (1960) as well. A conceivable reaction would be:

C4AFS2H16 + 2 CH → C6AFS2H18 (34)

In other words, the formation of the gehlenite hydrate would be an intermediate step inoverall reactions (29) and (30), and that the involved water is retained in a super saturatedhydrogarnet.

However, it should be noted that this water retention has never been measured, northe presence of F-substituted gehlenite hydrate, and therefore remains speculative. But thewater retention experiments by Powers and Brownyard (1948) on real cement pastes andthe analysis there from here, confirm the formation of the afore-mentioned hydrogarnetwith high water retention. That this water retention has not been measured yet might becaused by the prevailing experimental conditions: mostly the paste is dried to some extent,

so being not saturated anymore. For the same reason usually also C4AH13, C4A S H12 and

C6A 3S H32 are detected, and not C4AH19-22, C4A S H14 and C6A 3S H36, which can persist

only in saturated state (Dosch et al. (1969), Fischer and Kuzel (1982), Pöllmann (1989),Kuzel (1996), Pöllmann (2003)).

Note that C4AF consumes C2S and/or C3S to form the crystalline hydrogarnet, andconsequently, less C2S and/or C3S is available for reactions (9) and (10), and less C-S-Hgel is formed. On the other hand, with the formation of hydrogarnet both C 2S and C3Sproduce per mole more CH than with the formation of C-S-H (no C 4AF involved). Thiscan easily be verified by considering eqs. (29) and (30) with x = 2, and eqs. (9) and (10)with x = 1.7. In the case of C2S, per mole of this clinker phase, 1 mole of CH is producedinstead of 0.3, in the case of C3S, per mole of this clinker phase, 2 moles of CH areproduced instead of 1.3. Sufficient CH needs to be produced for enabling reaction (16).

Summary

In the present article the water retention results as measured by Powers and Brownyard(1948) have been used to investigate the reactions of the clinker phases alite, belite,aluminate, calcium sulphate and ferrite in real cement pastes. Most likely, the ferrite reactswith the alite and belite to form a hydrogarnet. The remaining calcium silicates react toC1.7SH3.2 (when saturated) and CH. The sulphate seems to react solely with the aluminate.In case of carbonation, which is mostly the case in real cement paste, hemi-carbonate,ettringite and tetra calcium aluminate hydrate is formed. This insight leads to the followingmodified retention

nH,d = 4.5 S C n3

+ 3.5 S C n2

+ 21 AC n3

+ 13 AF C n4

+ 2S C

n (35)

This equation provides the moles of retained water in saturated state as a function of themoles of the individual clinker phases present in the cement. Using eqs. (2) and (3) thisequation can be written as



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wd /c = 0.355 SC3x + 0.366 SC2

x + 1.401 AC3x + 0.482 AFC4

x + 0.265S C

x (36)

Eq. (35) supports the formation of C1.7SH3.2 by both C3S and C2S, insofar these clinkerminerals are not involved with C4AF in the formation of hydrogarnet of compositionC6AFS2H18, and this is matched by the factor 13 in the equation.

In practical application, e.g. in concrete, the bulk of the cement paste is isolated andcan effectively be considered as sealed. Hydration stops at a relative humidity of about80% (Powers (1947)), and water retention under this sealed condition is interesting todetermine as well. As long as water is present the reactions proceed to form the saturatedreaction products. When this water is consumed, it is likely that the saturated hydrationproducts partly dehydrate with falling relative humidity. This enables the cement hydrationto proceed till a final relative humidity in the entire paste of about 80% is achieved. So, itis necessary to assess the water retention of all hydration products formed at this relativehumidity to assess the minimum required water for full hydration in a sealed system.

It can be expected that the saturated hydrogarnet (C6AFS2H18), aluminate hydrate

(C4AH22) and ettringite (C6A 3S H36) lose their most loosely bound water and are readily

dehydrated to C6AFS2H8, C4AH13 and C6A 3S H32, respectively. By Brouwers (2003) it isderived that at 80% relative humidity the C-S-H is dehydrated to C1.7SH2.5. The portlandite

(CH) and hemi-carbonate (C4A 5.0C H12) retain their water. To assess the quantity (moles)of aluminate hydrate, hemi-carbonate and ettringite formed, eqs. (23)-(26) can be used.

All information condenses in the following water retention relation at 80% relativehumidity:

nH,d80 = 3.8 S C n3

+ 2.8 S C n2

+ 12 AC n3

+ 4.4 AF C n4

+ 6.5S C

n (37)

Using eqs. (2) and (3) this equation can be written as

wd80 /c = 0.300 SC3x + 0.293 SC2

x + 0.800 AC3x + 0.163 AFC4

x + 0.860S C

x (38)

In eq. (37) the factor 4.4 pertaining to C4AF is introduced to match the factors pertaining toC3S/C2S, this former factor is required to stochiometrically permit the formation of

C6AFS2H8 and 4CH/2CH. The factors pertaining to C3A and C S follow from eqs. (23)-(26) whereby the proper water retention is used (eqs. (16)-(19)).

To compare the water required for full hydration in saturated and in sealedcondition, in eqs. (36) and (38) the composition of a typical CEM I (ASTM Type III) aresubstituted: SC3

x = 0.61, SC2x = 0.15, AC3

x = 0.06, AFC4x = 0.10 and

S C x = 0.04, yielding

wd /c = 0.40 and wd80 /c = 0.33. In other words, a relatively small drop in humidity (from100% to 80%) leads to a substantial reduction in water required for complete hydration.

The moles of formed hydration products in the paste have been quantified, and arerelated to the mass (moles) of the clinker phases originally present in the cement. As seen,this new reaction model enables relations between clinker composition and water retentionat different humidity. Furthermore, each product formed affects the macroscopic propertiessuch as permeability, strength, creep, chemical shrinkage and binding capacity of the paste.



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Quantification of these products in the paste facilitates the future engineering with OPCand application of OPC-based materials.

Acknowledgements

The author is grateful for the advice given by late Dr. H.F.W. Taylor, Emeritus Professorof Inorganic Chemistry, University of Aberdeen, U.K. and by Dr. Dr. H. Pöllmann,Professor of Mineralogy/Geochemistry, Martin-Luther University of Halle-Wittenberg,Germany. He furthermore wishes to thank the following persons and institutions forproviding copies of references: Mr. W.J. Burns from the Portland Cement Association(PCA), Skokie, Il, U.S., and Dr. M. Schneider and Mrs. B. Bäumer from the VereinDeutscher Zementwerke e.V. (VDZ), Düsseldorf, Germany.

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Brunauer, S., and Greenberg, L.E. (1960), The hydration of tricalcium silicate and β-

dicalcium silicate at room temperature, Bull. 152, Res. Lab. of Portland CementAssociation, Skokie, IL, U.S., reprinted from Proc. 4th ISCC, Vol. 1, p. 135-165.

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Dosch, W., Keller, H. and Zur Strassen, H. (1969), Written discussion, Proc. 5th ISCC,Vol. 2, p. 72-77.

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Author

Dr. ir. H.J.H. BrouwersDepartment of Construction Technology (Bt)Faculty of Engineering Technology (CTW)University of TwenteP.O. Box 217

NL – 7500 AE Enschede

conference 13

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