Characteristics of cement pastes containing sulphoaluminate and belite prepared from nano-materials

Construction and Building Materials 38 (2013) 14–21

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Characteristics of cement pastes containing sulphoaluminate and belite prepared from nano-materials H. El-Didamony a, Mohamed Heikal a,b,⇑, Kh.A. Khalil a a b

Chemistry Department, Faculty of Science, Zagazig University, Zagazig, Egypt Chemistry Department, Faculty of Science, Benha University, Benha, Egypt

h i g h l i g h t s " Substitution of OPC with C4 A3 S or C4 AS + b-C2S tends to shorten the setting times. " The fast setting of cement pastes is due to the formation of Aft and AFm. " Substitution of OPC with C4 A3 S or C4 AS increases the chemically combined water. " The addition of C4 A3 S or C4 AS with belite decreases the free portlandite. " Pastes containing C4 A3 S or C4 AS give higher compressive strength.

a r t i c l e

i n f o

Article history: Received 19 April 2012 Received in revised form 10 June 2012 Accepted 21 July 2012 Available online 13 September 2012 Keywords: Sulphoaluminate Belite cement Hydration characteristics and compressive strength DSC and XRD

a b s t r a c t The sulphoaluminate phase C4 A3 S as well as b-C2S represent an important factor for the preparation of rapid hardened sulphoaluminate–belite cement. The hydration of C4 A3 S forms ettringite and monosulphate hydrates in addition to the liberation of aluminium hydroxide. The hydration rate of belite is much slower than that of C4 A3 S, but becomes significant at later ages. Two mixes containing sulphoaluminate (C4 A3 S) and monosulphate mix (C4 A3 S, with 8CS þ 6C) in addition to belite (b-C2S) were synthesized from nano-materials after firing at 1250 °C. The hydration characteristics of 10% C4 A3 S or 10% C4 AS in addition to 10% b-C2S on the expanse of OPC were studied. The results revealed that the substitution of OPC with sulphoaluminate mixes as well as belite shortens the setting times. The rate of hydration of OPC with sulphoaluminate–belite phases is increased from the formation of more sulphoaluminate hydrates and the consumption of protlandite. Also, these mixes give higher chemically combined water contents. The monosulphate hydrate is deposited in some of open pores in the dormant period of hydration that tends to increase the compressive strength of sulphoaluminate–belite cement pastes. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Calcium sulphoaluminate, C4 A3 S, is an important phase for the rapid hardened sulphoaluminate cement. Its formation from molar mixes of the oxides of calcium, aluminium and sulphur derived from synthetic pure chemicals and commercial grade natural minerals, such as limestone, bauxite and gypsum have been studied. The degree of formation of C4 A3 S phase in the temperature range of 1100–1325 °C is comparatively higher from pure chemicals. This is due to increased solid state reaction between these oxides [1]. Calcium sulphoaluminate may be also, produced from a starting blend of an appropriate oxide composition by burning at a temperature of about 1250–1300 °C [2,3]: ⇑ Corresponding author at: Chemistry Department, Faculty of Science, Benha University, Benha, Egypt. Tel.: +20 10 3598184; fax: +20 13 3222578. E-mail address: [email protected] (M. Heikal). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.07.098

3CaCO3 þ 3Al2 O3 þ CaSO4  2H2 O ! 4CaO  3Al2 O3  SO3 þ 3CO2 þ 2H2 O The sulphoaluminate phase is stable up to about 1350–1400 °C. Its crystalline structure consists of a three-dimensional framework of AlO4 tetrahedra sharing corners, with Ca2+ and SO2 4 ions located in the existing cavities. It belongs to the tetragonal system. Above about 1350 °C this phase becomes unstable and starts to decompose [4]. The Al3+ within the structure of the C4 A3 S phase may be partially substituted by Fe3+ ions. The reactivity of this phase will decline with iron content in the crystalline lattice. The hydration of C4 A3 S depends on whether calcium sulphate and calcium hydroxide are also present, and progresses at temperatures up to 75 °C [2,5,6]. In pure water C4 A3 S yields C4 ASH12 and AH3 as products of hydration. Two mixes of C4 A3 S and CSH2 yield ettringite alone, if mixed at a molar ratio of at least 1:2, and a combination of ettringite and monosulfate if the amount of gypsum is

H. El-Didamony et al. / Construction and Building Materials 38 (2013) 14–21

reduced. Simultaneously aluminium hydroxide is formed as a reaction product. Three mixes of C4 A3 S with calcium hydroxide, but without calcium sulfate, yield the hydrogarnet phase C3AH6 and an AFm phase of the approximate composition C3 A  1=2CS: 1=2CH:xH. Four in the presence of sufficient amounts of both gypsum and calcium hydroxide ettringite is formed as the sole reaction product. Calcium sulphoaluminate modified Portland cements may be produced by combining ordinary Portland cement clinker with limited amounts of calcium sulphoaluminate, produced separately, and calcium sulfate. Taczuk et al. [7] studied sulphoaluminatemodified cements with C4 A3 S contents between 5% and 20% and with a sulphoaluminate/calcium sulfate molar ratio of 1:8. Calcium sulfate was added as anhydrite or gypsum. The form of calcium sulfate present determined its dissolution rate (the dissolution rate of gypsum is much faster than that of anhydrite) and thus the properties of the produced cement. It was found that the presence of anhydrite tended to favour monosulfate precipitation at the expense of ettringite. As a consequence, such cements exhibited significantly shorter setting times and higher short-term strengths. Beretka et al. [8] produced a series of cements that contained only the phases C4 A3 S (15–50%), C5 S2 S (sulfospurrite) (25–77%), and CS (anhydrite) (8–25%), but no calcium silicates. Such cements exhibit a very fast initial ettringite formation and strength development, but no additional strength gain at later ages. Mixes with water contents too low for complete conversion of C4 A3 S to ettringite exhibit expansion due to delayed ettringite formation, associated with an uptake of water from the environment. While sulphoaluminate-modified Portland cements with limited amounts of C4 A3 S and calcium sulfate exhibit no expansion, those with higher amounts of these phases expand and are used as expansive cements. The hydration of the belite is much slower than that of C4 A3 S, and becomes significant only after longer hydration times. Very little is hydrated within the first 7 days, and between about 5% and 40% hydrates within 28 days [9–11]. The rate of hydration generally increases with SO3 content in cement [10]. The C–S–H phase formed in the hydration is mainly responsible for the ultimate strength of the hardened cement paste. Most or all portlandite produced in the hydration of C2S is consumed in the formation of ettringite, just like the free calcium oxide present in the original clinker. At later ages of hydration limited amounts of this phase may be also formed as larger needle-like crystals, to be precipitated in larger pore spaces still existing in the paste [10]. The gehlenite hydrate, C2ASH8, found in a different system, seems to be a result of the hydration of C3S or b-C2S in alumina saturated solution (alumina components), C4 A3 S [12]. In mature sulphobelite cement pastes the C–S–H phase and AFt (ettringite) are the two main hydration products. Variable amounts of monosulphate (AFm) and residual gypsum may be also present depending on the amount of calcium sulfate in the original cement. The material contains no, or very little, free calcium hydroxide, as it has been consumed in the formation of ettringite. The alkalinity of the pore liquid is relatively low, at around pH = 9.5–10 [9]. The aim of the present investigation is to study the main physico-mechanical properties of the sulphoaluminate–belite cement pastes containing C4 A3 S or monosulphate mix and belite prepared from nano-materials such as Ca(NO3)2, Al(OH)3, SiO2 and precipitated CaSO42H2O. 2. Experimental technique The materials used in this work were nano-Ca(NO3)2, nano-Al(OH)3, nano-silica and CaSO42H2O as obtained from Prolabo company. Nano-Al(OH)3 was prepared from Al-dross after leaching with commercial HCl then precipitated by ammonia solution at pH = 8 [13]. Nano-silica was also synthesized by acid hydrolysis of sodium silicate Na2SiO3 using 0.5 N HCl and stirred slowly at 60 °C at pH between 1

15

and 2 [14]. Nano-Ca(NO3)2 was freshly prepared by the addition of nitric acid (1:1) to CaCO3 to complete reaction then evaporated at 60 °C until solidification. The powder of Ca(NO3)2 was dried at 50 °C for 24 h. The materials were investigated for their chemical composition by using XRF, XRD and TEM. Table 1 summarizes the chemical composition of nano-SiO2 and nano-Al(OH)3. Fig. 1 shows XRD patterns of nano-silica and Al(OH)3. Fig. 2 shows TEM micrograph of nano-SiO2 and nanoAl(OH)3 with crystal sizes of 13 and 38 nm [14]. OPC cement was provided from Almasria Cement Company. The chemical analyses of investigated cements are given in Table 2. The mix composition of investigated mixes is given in Table 3. The calcium sulphoaluminate phase (C4 A3 S), anhydrous monosulphate mix (C4 AS) and active belite (b-C2S) were prepared from the stoichiometric amounts of above materials. Each mix was separately mixed in a ball mill for one hour to attain complete homogeneity; this was followed by firing at 1250 °C for 2 h, then cooled suddenly in air [14]. Table 3 shows the mix composition of investigated sulpoaluminate–belite cements. The water of consistency and setting times were measured according to ASTM Designation C191 [15], then pressed in 0.5  0.5  0.5 in.3 moulds. The moulds were cured in a humidity chamber at 100% RH at room temperature 23 ± 2 °C for 24 h, then demoulded and cured in the humidity chamber till the times of testing (3, 7, 28 and 90 days) were reached. At the end of each curing time, the hydration of the paste was stopped and the combined water and free portlandite contents were estimated on the ignited weight bases as described elsewhere [16,17]. The phase composition of hydration products was also followed by using XRD and DSC techniques.

3. Results and discussion 3.1. Sulphoaluminate–belite cement pastes This section deals with the characteristics of OPC (M0) and OPCsulphoaluminate (MS) as well as OPC-sulphoaluminate–belite (MSB) cement pastes. The hydration characteristics of these cement pastes were investigated by the determination of combined water and free lime contents as well as compressive strength at the different curing ages. The hydration products were identified by the aid of XRD and DSC techniques for some selected samples. 3.1.1. Water of consistency and setting time The water of consistency, initial and final setting times of M0, MS and MSB cement pastes are graphically represented as a function of sulphoaluminate and belite contents in Fig. 3. The results show that the water of consistency of sulphoaluminate cement pastes is higher than that of Portland cement. This is basically due to the formation of ettringite which has water content [18]. Moreover, the higher surface area of sulphoaluminate gives high surface area of blended cement compared with that of Portland cement due to the water demand increases. It is clear that the water of consistency of the sulphoalumminate–belite cement pastes (MSB) is nearly the same of the plain OPC, due to the low rate of hydration of belite at early ages. The results of Fig. 3 indicate also that the initial and final setting times are shortened with the addition of sulphoaluminate and belite phases. This is attributed to the very rapid hydration of C4 A3 S forming needle-like ettringite, which is responsible for the quick setting of sulphoaluminate cements [19]. Furthermore, with the addition of the nano-particles of these phases, the average particle size is getting smaller and the setting time is shortened. 3.1.2. Chemically combined water contents The chemically combined water contents of M0, MS and MSB cement pastes are graphically plotted as a function of curing time in Fig. 4. The combined water content generally increases with curing time due to the progress of hydration and formation of hydration products such as CSH, sulphoaluminate hydrates and C2ASH8 [12]. The chemically combined water contents of sulphoaluminate and sulphoaluminate–belite cement pastes are higher than those of OPC. This is related to the very fast hydration reaction rate of calcium sulphoaluminate phase which results in a rapid consumption of mixing water with the formation of ettringite which has a

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Table 1 Chemical analysis of the prepared nano-silica and nano-aluminium hydroxides, mass%. Material

SiO2

Al2O3

Fe2O3

CaO

MgO

SO3

Na2O

K2O

L.O.I.

Nano-silica Nano-Al(OH)3

87.59 2.34

0.52 55.73

0.09 1.98

0.53 0.89

0.13 0.06

0.02 2.31

1.44 0.07

0.08 0.03

9.60 36.40

Fig. 1. XRD patterns of (A) nano-SiO2 and (B) nano-Al(OH)3.

Fig. 2. TEM micrograph of (A) nano-SiO2 and (B) nano-Al(OH)3.

Table 2 Chemical oxide composition of investigated cements, mass%. Materials

SiO2

Al2O3

Fe2O3

CaO

MgO

SO3

Na2O

L.O.I.

OPC C4 A3 S

20.34 2.73

4.75 34.27

3.63 1.18

62.82 47.95

1.64 0.20

3.68 11.5

2.42 0.22

4.31 1.5

C4 AS b-C2S

1.60

22.12

0.40

54.02

0.20

17.5

0.31

4.05

34.70

0.45

0.07

62.70

0.32

—

0.68

1.92

Table 3 Mix composition of the investigated cements. Symbol

OPC

C4 A3 S

C4 AS

b-C2S

M0 MS MSB MM MMB

100 90 80 90 80

00 10 10 00 00

00 00 00 10 10

00 00 10 00 10

very high water content that increases the chemically combined water content. Moreover, the hydration products such as gehlenite or gismondine, that have a high water content, tend to increase the

values of chemically combined water content. However, the chemically combined water content decreases with calcium sulphoaluminate–belite cement pastes in comparable with sulphoaluminate cement pastes. This is mainly due to the decrease of OPC fraction and/or the slow rate of hydration of belite in comparison of OPC. 3.1.3. Free portlandite content The free portlandite, Ca(OH)2, contents of OPC as well as sulphoaluminate and sulphoaluminate–belite cement pastes (M0, MS and MSB) are graphically plotted as a function of curing time in Fig. 5. It can be noticed that the free lime contents of OPC and sulphoaluminate cement pastes increase up to 90 days. This is mainly related to the continuous hydration of cement clinker such as C3S and b-C2S producing CSH and CH. Also, the free portlandite contents of MS paste are slightly increased with curing time with lower values than those of OPC pastes. The decrease of free portlandite contents of MS cement paste is due to the reaction of CH with C4 A3 S to give ettringite as well as the decrease of OPC fraction which is the source of free CH. The results of Fig. 5 revealed also that the free portlandite contents of sulphoaluminate–belite cement pastes increase with curing time up to 7 days, then decrease

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H. El-Didamony et al. / Construction and Building Materials 38 (2013) 14–21 M0

250

28 200

150

26

100 24 Water of concistency

50

Initial set

M0

6

MS MSB

5

4

3

2 1

10

100

Curing time, Days

Final set

0

22 0

5

10

15

20

Sulphoaluminate and belite cement content, % Fig. 3. The water of consistency, initial and final setting times of M0, MS and MSB cement pastes.

up to 90 days. The increase of portlandite up to 7 days is due to the rapid hydration of Portland cement. As the curing time proceeds up to 90 days, the free portlandite contents decrease. This is ascribed to the consumption of portlandite in the formation of gismondine from CSH and amorphous AH3 associated with portlandite at later ages [20]. Generally, the sulphoaluminate–belite cement pastes give a lower free portlandite content in comparison with M0 and MS cement pastes. Moreover, the low rate of hydration of b-C2S at early ages tends to decrease the portlandite content.

3.1.4. X-ray diffraction analysis Fig. 6 depicted the XRD patterns of OPC as well as sulphoaluminate and sulphoaluminate–belite cement pastes (M0, MS and MSB) cured for 28 days. The results indicate that the intensity of ettringite increases with the addition calcium sulphoaluminate compared with that of OPC due to the formation of high content of ettringite produced from the hydration of C4 A3 S, and then decreases with the addition belite in MSB paste. The decrease of ettringite phase in MSB cement paste may be due to the decrease in the amount of liberated portlandite from b-C2S phase in comparison with OPC. Portlandite stabilizes the formation and crystallization of ettringite,

Chemically Combined water contents, %

Free portlandite contents, %

MSB

Initial and final setting time, (min)

MS

Water of consistency, %

7

300

30

Fig. 5. Free portlandite contents of M0 as well as MS and MSB cement pastes with time.

which is formed in high pH. Meanwhile, the intensity of portlandite decreases with calcium sulphoaluminate and sulphoaluminate–belite cement pastes related to the decrease of C3S phase of OPC fraction that generates high content of portlandite during hydration. On the other side, the intensity of portlandite and CSH decreases with the sulphoaluminate–belite cement pastes. This is ascribed to the reaction of C2S with amorphous Al(OH)3 associated with portlandite forming gismondine at 28 days [20]. 3.1.5. Thermal analysis The DSC patterns of OPC as well as sulphoaluminate and sulphoaluminate–belite cement pastes after 28 days of hydration are illustrated in Fig. 7. The thermogram of OPC (M0) paste shows four endothermic peaks located at 112,150, 460 and 750 °C. The endothermic peaks below 200 are mainly due to the dehydration of CSH as well as ettringite. The endothermic peaks at 460 and 750 °C are related to the decomposition of portlandite and CaCO3, respectivly. The thermogram of sulphoaluminate cement paste

24

20

16 M0 MS MSB

12 1

10

100

Curing time, Days Fig. 4. Chemically combined water contents of M0, MS and MSB cement pastes with curing time.

Fig. 6. The XRD patterns of M0 as well as MS and MSB cement pastes cured for 28 days.

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Compressive strength, kg/cm2

1000 M0 MS MSB

800

600

400 1

10

100

Curing time, Days Fig. 8. The Compressive strength of M0 as well as MS and MSB cement pastes with time.

32

Fig. 7. DSC thermograms of M0, MS and MSB cement pastes cured for 28 days.

M0

300 MMB

Water of consistency, %

250

(MS) illustrates a strong endothermic peak at 112 °C due to the dehydration of nearly amorphous CSH and ettringite; whereas, the endothermic peak located at about 150 °C may be attributed to decomposition of more crystalline CSH and ettringite. On the other hand, the thermogram of sulphoaluminate–belite cement paste shows the appearance of CSH, ettringite, and a small peak at 300 °C corresponding to the dehydration of gismondine. It can be concluded that the main hydration products are ettringite, CSH, gismondine and CH as well as CaCO3.

28

200

150

24

100 Water of concistency

50 Initial set

3.1.6. Compressive strength The compressive strength of OPC as well as sulphoaluminate and sulphoaluminate–belite cement pastes is graphically plotted as a function of curing time in Fig. 8. The results illustrate that the compressive strength of the cement pastes increases with the curing time, this is attributed to the progress of the hydration and accumulation of hydration products acting as cementing materials within the available pore spaces leading to higher strength values. Fig. 9 shows also that MS cement paste gives high strength values as compared with M0 as well as MSB cement pastes. This is mainly due to the rapid formation of ettringite at early ages which fills the pores and densifies the paste structure; consequently, the compressive strength increases. Ettringite has a larger specific volume which can precipitate in some pores leading to an increase in the strength values, especially as the early ages of hydration. Also, the formation of AH3, as an amorphous material during the hydration of C4 A3 S, can bind the cement grains of sulphoaluminate cement pastes. On the other side, the substitution of additional 10% of OPC with belite phase in addition to C4 A3 S affects the formation of hydration products of MSB paste and the consequent increase in compressive strength. It is also clear that, the MS cement paste shows higher rate of hydration than that MSB paste. Therefore, the compressive strength values of MS cement paste are higher than those of MSB cement paste. On the other side, sulphoaluminate–belite cement (MSB) paste shows higher strength values at early and later curing ages as compared with those of OPC cement (M0) paste at all ages of hydration. This is basically due to that the nano-particles of belite phase fill some spaces between cement grains and act as nuclei for cement

Initial and final setting time, (min)

MM

Final set

20

0 0

5

10

15

20

Monosulphate mix and belite content, % Fig. 9. Water of consistency, initial and final setting time of M0, MM and MMB cement pastes.

hydrates leading to acceleration of the hydration of OPC clinker phases with the formation of a more dense structure and accordingly, the compressive strength increases at early ages. Later on, the compressive strength increases due to the relatively high rate of hydration of belite at later ages. Furthermore, the formation of gismondine contributes in the later strength of MSB paste [21].

3.2. Monosulphate mix–belite cement pastes 3.2.1. Water of consistency and setting time The water of consistency, initial and final setting times of M0, MM and MMB cement pastes are graphically represented as a function of monosulphate mix and belite contents in Fig. 9. The results show that the water of consistency increases markedly with the substitution of OPC with monosulphate mix (MM) and slightly with monosulphate mix and belite (MMB). This is mainly due to that monosulphate mix contains 8 mol of CaSO4 in addition to 6 mol of CaO in respect of one mole of C4 A3 S which form ettringite on hydration. The formation of ettringite together with the increase of surface area of the admixed materials increase the water

19

7

24

Free portlandite contents, %

Chemically combined water contents, %

H. El-Didamony et al. / Construction and Building Materials 38 (2013) 14–21

20

16 M0 MM

M0 MM

6

MMB

5

4

MMB

3

12 1

10

100

Curing time, Days Fig. 10. Combined water contents of M0, MM and MMB cement pastes with time.

1

10

100

Curing time, Days Fig. 11. The free lime contents of M0 as well as MM and MMB cement pastes with time.

of consistency of the paste made of mix MM. Evidently, the water of consistency decreases with monosulphate–belite cement (MMB) paste due to the decrease of the rate of hydration of belite phase. On the other hand, the initial and final setting times are shortened with the substitution of monosulphate (MM) or monosulphate–belite (MMB) phases as compared with those of OPC (M0). This is related to a rapid hydration of C4 A3 S leading to the formation of needle-like ettringite crystals, which is responsible for quick setting. The decrease of water of consistency of MMB cement pastes tends to elongate the setting of cement pastes in comparison with OPC and MM. 3.2.2. Chemically combined water content The chemically combined water contents of M0, MM and MMB cement pastes are graphically represented as a function of curing time in Fig. 10. The results indicate that the chemically-combined water contents increase with curing time for all of hydrated cement pastes; this is due to the progresses of hydration. It can be noticed that the chemically combined water contents are higher with the monosulphate and monosulphate–belite cement blends (MM and MMB) as compared with those of neat OPC paste. This is due to that monosulphate hydration gives ettringite with its higher water content. In addition, the ultra-fine particles of these phases act as accelerators for the hydration of cement clinker and accordingly, the combined water contents increase. However, monosulphate–belite cement paste (MMB) shows lower chemically combined water contents than those of monosulphate cement pastes (MM); this is related to the decrease of cement clinker content in addition to the relatively slow rate of hydration of belite phase. 3.2.3. Free portlandite content The free portlandite, Ca(OH)2, contents of M0, MM and MMB cement pastes are graphically plotted as a function of curing time in Fig. 11. Obviously, the free portlandite contents of all cement pastes increase up to 90 days due to the continuous hydration of C3S and C2S phases of OPC clinker with the formation of calcium silicate hydrates and calcium hydroxide. In addition, the hydration of the added belite phase in mix MMB forms calcium hydroxide. Although monosulphate contains certain amount of free portlandite, the results reveal that MM paste has lower free portlandite contents in comparable with OPC cement paste(M0) due to the consumption of free lime in the formation of ettringite at early hydration ages and the decrease in cement clinker phases as C3S and C2S. The addition of monosulphate–belite phases at the expense of OPC and the low rate hydration of belite at early time tends to decrease the free lime contents of MMB paste. Fig. 11

Fig. 12. The XRD patterns of MM mix as a function of curing time.

shows also the higher rate of hydration of belite at later ages of hydration of MMB paste. 3.2.4. X-ray diffraction analysis XRD patterns of MM pastes are illustrated as a function of curing time in Fig. 12. The results reveal that sample hydrated up to 3 days shows the presence of ettringite, portlandite, unhydrated C4 A3 S, belite, alite as well as CSH which is overlaped by calcite peak. It is obvious that the intensity of ettringite increases up to 28 days. This is basically due to the hydration of C4 A3 S with CH and CS producing ettringite. On the other side, the intensity of portlandite and CSH increases due to the hydration of C3S and C2S phases of clinker to form CSH and portlandite. It can be seen that unhydrated C4 A3 S is completely consumed after 3 days. Fig. 13 illustrates the XRD patterns of M0, MM and MMB cement pastes cured for 28 days. The patterns show that the intensity

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H. El-Didamony et al. / Construction and Building Materials 38 (2013) 14–21

Compressive strength, kg/cm2

1000 M0 MM MMB

800

600

400 1

10

100

Curing time, Days Fig. 15. Compressive strength of OPC as well as monosulphate and monosulphate – belite cement pastes with curing time.

Fig. 13. The XRD patterns of M0 as well as MM and MMB cement pastes cured for 28 days.

Fig. 14. DSC thermograms of M0, MM and MMB cement pastes cured for 28 days.

of ettringite peak increases with the addition of monosulphate mix (MM). This is due to the hydration of monosulphate mix forming ettringite. Meanwhile, the addition of monosulphate and belite at the expense of Portland cement (mix MMB) decreases the amount of ettringite. It is clear that the intensity of portlandite peaks also decrease with the addition of monosulphate mix (MM). The lines of CSH appears in all of the hydrated cement pastes with lower intensities for MM and MMB pastes due to the decrease of cement clinker phases (C3S and C2S). 3.2.5. Thermal analysis Fig. 14 illustrates the DSC thermograms of the hydrated M0, MM and MMB cement pastes cured for 28 days. The DSC thermograms show the appearance of some endothermic peaks at

90–120, 380, 460 and 650–700 °C. The endothermic peak located at 90–120 °C is mainly due to the dehydration of the bound water of ettringite and calcium silicate hydrates; this endotherm appeared with a higher intensity for MM and MMB cement pastes, which form excessive amounts of ettringite, as compared to the neat OPC paste (M0). The endotherm located at about 380 °C is mainly attributed to the decomposition of gehlenite-like hydrate and hydrogarnt. This peak appeared with MMB cement paste at 28 days which is related to the reaction of belite with AH3 produced from the hydration of C4 A3 S forming gehlenite-like hydrate. Meanwhile, the peaks at 460 and 650–700 °C are characteristic for the decomposition of portlandite as well as amorphous and crystalline CaCO3. It is clear that these two peaks appeared with lower intensities for MM and MMB cement pastes as compared to M0 paste due to the decrease of cement clinker content. These results are in a good agreement with the results of XRD. 3.2.6. Compressive strength Compressive strength results of M0, MM and MMB cement pastes are graphically illustrated as a function of curing time in Fig. 15. The compressive strength of all cement pastes increase with the increase of curing time. This is due to the formation of cementitious materials from the hydration of cement clinkers, in addition to the formation of ettringite from C4 A3 S hydration and CSH form the hydration of b-C2S. These hydration products precipitate in the pores forming more compact structure which increase the compressive strength. It is observed that MM cement paste shows high early strength values as compared with those of OPC paste (M0). This is related to the contribution of the ettringite formation at early ages, which has needle structure that decreases the total porosity or increases the compressive strength. On the other side, the substitution of OPC by 10% belite in addition to 10% MM in MMS paste shows reasonable higher compressive strength than that of M0 paste at 3 days. These results are in a good agreement with the free portlandite and combined water contents. 4. Conclusions From the above findings it can concluded that: 1. The substitution of OPC with 10% C4 A3 S or 10% C4 AS þ bC2 S tends to shorten the setting times of cement pastes and slightly increases the water of consistency. The fast setting time of cement pastes is mainly due to the formation of calcium sulphoaluminate hydrates, Aft and AFm.

H. El-Didamony et al. / Construction and Building Materials 38 (2013) 14–21

2. The substitution of OPC with 10% C4 A3 S or 10% C4 AS increases the chemically combined water contents of sulphoaluminate hydrates with higher values of chemically combined water contents. The substitution of more 10% belite on the expense of OPC in mixes MSB and MMB decreases the chemically combined water contents than those made with only C4 A3 S (MSB) or C4 AS (MMB) but results are still higher than those of OPC paste. 3. The addition of C4 A3 S or C4 AS with belite decreases the free portlandite due to its consumption in the formation of calcium sulpholuminate hydrates, Aft and AFm. 4. Cement pastes containing C4 A3 S or C4 AS have higher compressive strength values those of OPC pastes. 5. The results of the XRD are in a good agreement with those of DSC, free portlandite and compressive strength.

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