A gap-graded particle size distribution for blended cements: Analytical approach and experimental validation

Powder Technology 214 (2011) 259–268

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A gap-graded particle size distribution for blended cements: Analytical approach and experimental validation Tongsheng Zhang, Qijun Yu ⁎, Jiangxiong Wei, Pingping Zhang, Peixin Chen Key Laboratory of Specially Functional Materials of the Ministry of Education, South China University of Technology, 510640 Guangzhou, China

a r t i c l e

i n f o

Article history: Received 28 June 2011 Received in revised form 10 August 2011 Accepted 19 August 2011 Available online 25 August 2011 Keywords: Blended cement Particle size distribution Packing density Microstructure Mechanical properties

a b s t r a c t To increase the packing density of blended cement paste, a gap-graded particle size distribution (PSD) was theoretically deduced and modified according to the wet density of actual paste. Then experiments were conducted to validate the hypothesis of improvement of the properties of blended cements by the gap-graded PSDs proposed. The experimental results show that the gap-graded PSD resulted in a decreased water requirement and an increased packing density of blended cement paste, and modified gap-graded PSDs gave further effects. The heat of hydration of gap-graded blended cement pastes released slowly in the first 24 h and increased rapidly afterward. The microstructure of gap-graded blended cements was much more homogeneous and denser than that of reference blended cement, therefore both early and late mechanical properties of low clinker gap-graded blended cements were improved significantly and even higher than those of Portland cement. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

1. Introduction As cement industry adapts to growing environmental regulations, the efficient utilization of materials and energy and reducing CO2 emissions in cement manufacture become even more important. Utilization of fly ash, granulated blast furnace slag (GBFS), steel slag, limestone and natural pozzolans as supplementary cementitious materials (SCMs) in blended Portland cements produced has been noted for engineering, ecological and economic benefits [1]. Some properties of blended cements are superior to that of Portland cements [2]. However, one shortcoming of blended cements that has not yet been solved perfectly is their relatively low early strengths due to low pozzolanic activity of SCMs and pozzolanic reaction mainly takes place in late ages. Many attempts, such as chemical activation [3] and ultra-fine grinding [4], have been made to improve the early properties of blended cements. Most of these methods are focused on enhancement of the pozzolanic activity of SCMs, which usually lead to increased costs or difficulties in engineering practices. Inventions of ultra-high strength cement-based materials, such as hot pressing cement [5], macro-defect-free cement (MDF) [6] and densified systems containing homogeneously arranged ultra-fine particles (DSP) [7], prove that a large amount of hydration products or a high hydration degree does not mean high compressive strengths, the key lies in the porosity and the pore size distribution of the hardened

⁎ Corresponding author. Tel./fax: + 86 20 87114233. E-mail address: [email protected] (Q. Yu).

paste, which are largely depended on the initial packing density of cement paste [8]. An appropriate particle size distribution (PSD) may lead to high initial packing density of cement paste, which means smaller amount of hydration products is needed to achieve dense microstructure, thus both early and late properties of blended cement can be improved significantly. To that purpose, characteristics of typical PSDs used in cement-based materials were analyzed. It is found that few existed PSDs are suitable to be applied to blended cements, which consist of three or more components with different hydraulic activities. Consequently, a gap-graded PSD was theoretically deduced based on close packing theory, and modified according to the wet density of actual paste. 2. Analytical approach of a new gap-graded PSD 2.1. Classical PSDs used in cement-based materials Some researchers have confirmed that high packing density needs wide PSDs [9], in which a large amount of fine particles is needed to fill voids. However, it may result in high water requirement, fast setting, deterioration in rheological properties and strength retrogression [10,11]. To avoid those shortcomings, narrow PSDs are suggested for cementitious materials (such as Portland cements) [12]. Some theoretical models (such as Horsfield model and Hudson model) and empirical PSDs (i.e. Aim and Goff distribution [13], Stovall distribution [14,15], S. Tsivilis distribution, Fuller distribution, Andersen distribution and Rosin–Rammler–Bennet distribution [16]) have been proposed for cement-based materials.

0032-5910/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2011.08.018

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2.1.1. Horsfield model Horsfield model is only applied to ideal non-continuous particle systems, in which all physical and chemical forces are neglected. The diameter of particles of each filling grade is given in Table 1 [17]. 2.1.2. S. Tsivilis distribution S. Tsivilis distribution is widely applied to Portland cements and can be expressed as Eq. (1) [10,18]: yð30Þ−yð3Þ≥65% & yð3Þ≤10%

Table 2 Target parameters of the gap-graded PSD. Filling grade

Fifth

Fourth

Third

Second

First

Mean size (μm) Size range (μm) Incremental volume (%) Cumulative volume (%)

3 b4 30.2 30.2

6 4–8 9.6 39.8

16 8–24 22.0 61.8

32 24–45 17.7 79.5

63 45–80 20.5 100

ð1Þ

where, y(x) is cumulative volume of particles smaller than x μm (%). S. Tsivilis distribution is a typical narrow PSD and mainly concentrates on initial hydration rate and hydration degree of cements, which link to workability and ultimate strengths, respectively. While blended cements with S. Tsivilis distribution are characterized of low early strengths due to a low packing density of cement paste and a small amount of hydration products. 2.1.3. Fuller distribution Fuller distribution is a classical wide PSD and is originally applied to concrete aggregates [19]. It can also be applied to inert or low hydraulic activity powders after modification by Kuhlmanm [20,21]. For cement particles, Fuller distribution can be expressed by Eq. (2).
x 0:4 U ðxÞ ¼ 100 D

ð2Þ

where, U(x) is cumulative volume of particles under x μm (%); D is the maximum diameter of particles (μm). The purpose of Fuller distribution is to achieve maximum packing density regardless of undesirable workability or fluidity caused by rapid hydration of fine cement clinker particles. Thus, cements with Fuller distribution are characterized of high water requirement and high early strength, while the late strengths of these cements are relatively low due to high water requirement and large amount of unhydrated cement particles (more than 40% by mass). 2.2. A new gap-graded PSD for blended cements It can be inferred from the above analyses that S. Tsivilis distribution and Fuller distribution are applied to high activity powders (such as Portland cements) and inert (or low activity) powders, respectively [11,22]. Different from high activity and inert powders, blended cements consist of cement clinker and SCMs with different hydraulic activities (perhaps even inert particles). Thus few above-mentioned PSDs are suitable to be applied to blended cements. A gap-graded PSD will be proposed as follows: according to Horsfield model [14], if the diameter of first grade particles is 80 μm (the typical maximum diameter of cement particles), the diameters of particles of each filling grade are 33.12 μm, 18.00 μm, 14.16 μm, 9.28 μm, 5.80 μm and 2.40 μm respectively, and are simplified as being 63 μm, 32 μm, 16 μm, 6 μm, 3 μm for easy operation. Then cement particles are divided into five fractions as being b4 μm, 4–8 μm, 8–24 μm, 24–45 μm and 45– 80 μm, respectively. The PSD of each fraction should be as narrow as possible and concentrates to the diameter of corresponding filling grade (see Table 2). In that case, both a relaxation effect caused by large fillers

and an empty void effect due to small fillers can be avoided, therefore a high packing density can be easily achieved as voids are filled in, grade by grade. To achieve maximum packing density without Blaine fineness increase, the overall PSD of the blended cements should be as close as possible to Fuller distribution (see Fig. 1). The target parameters of the gap-graded PSD proposed are listed in Table 2, in which volume percentage of particles in each fraction is calculated according to Fuller distribution. 2.3. Modification of the gap-graded PSD It is known that a water film is absorbed on the surface of cement particles immediately after mixing with water, and each solid particle is not in contact with neighboring ones in actual cement paste. Thus the volume percentage of solid particles in each fraction cannot be calculated according to Fuller distribution, which has an assumption that particles are in contact with each other (dry particle systems). Each detached solid particle together with space coated around (water film is included) is viewed as compound particle, which is contact with neighboring compound particles [23]. The diameter of compound particle (dCP) can be calculated by Eq. (3): dCP ¼ dP þ dHCP

ð3Þ

where, dP is the diameter of solid particle; and dHCP is distance between solid particles in paste (see Fig. 2). For the sake of convenience, some hypotheses are taken as follows: (I) both solid particles and compound particles are spherical particles. (II) The value of dHCP only depends on surface properties (governed by the category of cementitious materials) and the size of solid particle. Then PSD of solid particles is attainable by establishing the relationship between PSDs of solid particles in actual paste and compound particles in imaginary dry particles system.

Table 1 Parameters of Horsfield model. Particle grade

Diameter (ratio by first grade particle)

Number of particles

Porosity (%)

First Second Third Fourth Fifth

1.000 0.414 0.225 0.177 0.116

– 1 2 8 8

25.9 20.7 19.0 15.8 14.9

Fig. 1. Schematic outline of the gap-graded PSD.

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The number of solid particles (Ni) in per cube (such as 1 cm 3 in this study) can be calculated by Eq. (7): Ni ¼

1⁎φi Vi

ð7Þ

The number of solid particles (ni) on a line paralleled with any side of the cube can be obtained by Eq. (8): 1

ni ¼ Ni 3 :

ð8Þ

It is generally considered that the arrangement of solid particles in actual paste should be between cubic close packing and hexagonal close packing, and inclines to hexagonal close packing. According to hexagonal close packing (Fig. 3), if the diameter of compound particles of the fraction i is dCPi, the relationship between dCPi and the length of any side of the cube (1 cm in this study) obeys Eq. (9): dCPi þ

ðni −1Þ pffiffiffi 3dCPi ¼ 1 cm: 2

ð9Þ

Distance between solid particles (dHCPi) of fraction i in actual cement paste can be calculated by Eq. (10):

Fig. 2. Sketch map of compound particles.

dHCPi ¼ dCPi −dPi :

Fig. 3. Sketch map of hexagonal close packing model.

2.3.1. Deduction of distance between solid particles in actual paste The maximum volume concentration of solids (φ) was used to characterize the packing density of cementitious material pastes [24], which obeys the following formulas: ρc ⋅ φ þ ρw ⋅ ð1−φÞ ¼ ρwet φ¼

ð4Þ

ρwet −ρw ρc −ρw

ð5Þ

where, ρw and ρc are, respectively, the densities of water and the cementitious material fraction. ρwet is the maximum density of cementitious material pastes. Since PSDs of cementitious material fractions prepared are fairly narrow, particles in a given fraction can be seen as spherical particles that have equal diameter dP (dP = D50). The volume of a particle (Vi) of fraction i can be calculated by Eq. (6):

Vi ¼

1 3 πd 6 Pi

ð6Þ

ð10Þ

2.3.2. Measurement of the wet density of cementitious material pastes A GBFS, low calcium fly ash and Portland cement clinker were ground and then classified by a laboratory air classifier. The chemical compositions of the raw materials are given in Table 3. The PSD of each cementitious material fraction was determined by laser diffraction (Malvern Mastersizer 2000, refractive index of dispersant (ethyl alcohol): 1.32 and obscuration: 12.4%) as shown in Fig. 4. Each cementitious material fraction was mixed with water homogeneously at different water to cementitious material ratios, and then maximum density of wet pastes was determined. Meanwhile specific density of each fraction was also measured. Distance between solid particles calculated by Eq. (10) is listed in Table 4. It can be seen that dHCP is decreased linearly with the decrease of mean size of solid particles for given cementitious material. For instance, dHCP varied from 8.69 μm to 0.53 μm when mean size declines from 56.61 μm to 1.50 μm for cement clinker fractions. That is to say, the coarser particles of the cementitious material fractions, the longer distance between particles is. It is accepted that the thickness of water film adsorbed on the surface of cement particles is in the range of 0.11–0.36 μm [19]. The experimental results reveal that there exists a certain distance between solid particles in cement paste apart from water film. 2.3.3. Modification of the gap-graded PSD according to the wet density of paste To achieve high packing density, the optimal PSD of compound particles should obey Fuller distribution as Eq. (11):  0:4 d U ðdCP Þ ¼ 100 CP DCP

ð11Þ

Table 3 Chemical compositions of Portland cement clinker, GBFS and low calcium fly ash used in the experiments. Material

Chemical composition (%) SiO2

Al2O3

Fe2O3

CaO

MgO

K2O

Na2O

SO3

TiO2

LOI

Sum

Cement clinker GBFS Low calcium fly ash

21.86 35.22 45.43

4.45 12.15 24.36

2.35 0.25 9.70

63.51 37.08 5.23

1.67 11.25 1.46

0.55 0.49 0.23

0.26 0.25 0.36

2.91 1.19 1.03

0.11 0.73 0.15

1.89 − 0.36 11.88

99.56 98.61 99.68

Note: LOI, loss on ignition.

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where, U(dCP) is cumulative volume of particles under dCP μm (%); DCP is the maximum diameter of compound particles (μm). The incremental PSD of compound particles can be obtained as Eq. (12) F ðdCP Þ ¼ d½U ðdCP Þ ¼

  40 dCP −0:6 : DCP DCP

ð12Þ

Relationship between PSDs of solid particles and compound particles can be expressed by Eq. (13): Incremental volume of solid particles F ðdP ÞdðdP Þ ¼ Incremental volume of compound particles F ðdCP ÞdðdCP Þ  3 π d 3 dP ¼ π6 P 3 ¼ d d CP CP 6

ð13Þ

where, F(dP) is incremental PSD of solid particles. According to relationship between diameters of solid particles and compound particles in Fig. 5, dCP can be expressed by Eq. (14): dCP ¼ λdP þ C

ð14Þ

where, λ and C are constants (can be obtained from Fig. 5) and only depend on surface properties (category of cementitious materials). Eq. (15) can be obtained by combination of Eqs. (12), (13) and (14):  F ðdP Þ ¼ λ

dP λdP þ C

3

F ðλdP þ C Þ:

ð15Þ

The cumulative PSD of solid particles (U(dP)) can be expressed by Eq. (16): DP

U ðdP Þ ¼ ∫ F ðdP ÞdðdP Þ

ð16Þ

0

where, DP is the maximum diameter of the solid particle. After normalization of Eq. (16), the optimal cumulative PSD of solid particles can be obtained as Eq. (17): 3      λDP þ C 3 dP λdP þ C 0:4 ⋅ ⋅ ΦðdP Þ ¼ 100⋅ DP λDP þ C λdP þ C

ð17Þ

where, Ф(dP) is cumulative volume of solid particles under dP μm (%). Table 4 Particle size distribution of each cementitious materials fraction and distance between particles in wet pastes. Fraction D10

C1 C2 C3 C4 C5 C6 C7 C8 G1 G2 G3 G4 G5 G6 G7 G8 F1 F2 F3 F4 F5 F6 F7 Fig. 4. PSDs of different cementitious material fractions. (a) Cement clinker, (b) GBFS, and (c) low calcium fly ash.

Maximum Maximum volume dHCP wet density concentration of solids

D50

D90

Specific density

(μm)

(μm)

(μm)

(g/cm3) (g/cm3)

(%)

(μm)

50.81 29.45 22.07 16.81 10.29 6.17 1.90 0.81 35.74 18.53 14.39 12.57 7.42 5.12 2.80 0.68 45.26 20.01 11.48 7.58 5.85 3.61 0.29

56.61 36.37 28.27 21.48 13.19 8.24 3.98 1.50 44.84 22.27 18.95 16.08 9.52 6.81 4.72 1.67 58.85 26.38 15.80 10.41 8.32 5.06 0.45

70.83 46.55 35.05 26.84 16.53 11.23 4.72 2.28 51.09 29.57 25.03 21.74 13.09 9.17 7.55 3.46 61.56 32.45 19.98 13.25 11.24 7.58 0.74

3.259 3.252 3.241 3.209 3.159 3.112 3.06 3.021 2.907 2.903 2.901 2.892 2.886 2.884 2.846 2.789 2.562 2.562 2.574 2.582 2.590 2.612 2.614

52.39 52.41 51.27 48.08 46.96 44.68 41.90 32.52 55.23 54.28 53.94 53.42 51.39 49.53 46.64 33.86 48.50 46.45 44.47 42.05 40.32 35.51 22.16

8.69 5.59 4.59 4.03 2.60 1.79 0.97 0.53 5.99 3.13 2.71 2.36 1.54 1.20 0.94 0.56 10.8 5.31 3.46 2.52 2.16 1.59 0.78

2.183 2.180 2.149 2.063 2.014 1.944 1.863 1.657 2.053 2.033 2.025 2.010 1.969 1.933 1.861 1.606 1.758 1.726 1.700 1.665 1.641 1.572 1.358

Note: C, G and F are cement clinker, GBFS and low calcium fly ash, respectively. D10, D50 and D90 are the particle diameters at which the cumulative volume reaches 10%, 50%, and 90%, respectively.

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Table 5 Parameters of modified gap-graded PSDs. Filling grade

Fifth

Fourth

Third

Second

First

Size range (μm) Incremental volume (%)

b4 30.2 25.1 23.4 21.4

4–8 9.6 11.3 11.8 12.3

8–24 22.0 24.0 24.6 25.4

24–45 17.7 18.5 18.8 19.2

45–80 20.5 21.1 21.4 21.7

F0 FG FC FF

Note: F0 is the original gap-graded PSD calculated according to Fuller distribution shown in Table 2. FG, FC and FF are gap-graded PSDs calculated according to MC Fuller, MG Fuller and MF Fuller distributions in Fig. 6, respectively.

Fig. 5. Relationship between diameters of solid particles and compound particles.

Modified Fuller distributions calculated from Eq. (17) are shown in Fig. 6, MG Fuller, MC Fuller and MF Fuller are modified Fuller distributions according to the wet densities of GBFS, cement clinker and fly

ash pastes, respectively. The contents of solid particles in each fraction of gap-graded PSDs before and after modification are summarized in Table 5, in which F0 is the original gap-graded PSD calculated according to Fuller distribution (Table 2), and FG, FC and FF are modified gapgraded PSDs calculated according to MC Fuller, MG Fuller and MF Fuller distributions (Fig. 6), respectively. Clearly, the content of fine particles (b4 μm) reduced sharply, especially for particles finer than 1 μm, while the amount of coarse particles increased slightly after modification. 3. Improvement of the properties of blended cements through the gap-graded PSDs To directly validate the hypothesis of improvement of the properties of blended cements by the gap-graded PSDs proposed, fundamental properties of powder, paste and mortar of GBFS-incorporated blended cements with the gap-graded PSD or modified gap-graded PSDs were investigated. 3.1. Preparation of gap-graded blended cements Portland cement clinker and GBFS were ground and then classified. By changing operational parameters of the classifier, cement clinker and GBFS fractions, which roughly meet the requirements of the gapgraded PSD, were obtained. Fig. 7 shows that cement clinker and GBFS fractions prepared gave a narrow PSD, and their mean sizes were mainly consistent with the diameters of particles of corresponding filling grades (Table 2). Gap-graded blended cements were prepared by mixing GBFS fractions, cement clinker fraction and gypsum dihydrate homogeneously, in which cement clinker fraction was arranged in 8–24 μm fraction (the cement clinker fraction which contributes most to strength development of Portland or blended cement [25]), other fractions were filled by corresponding GBFS fractions. GP, GPC, GPG and GPF in Table 6 are

Fig. 6. Comparison of the gap-graded PSDs before and after modification: MC Fuller, MG Fuller and MF Fuller are modified Fuller distributions according to distance between cement clinker, GBFS and fly ash particles, respectively. (a) Gap-graded PSD in the range of 0–80 μm, and (b) gap-graded PSD in the range of 0–4 μm.

Fig. 7. PSDs of five cementitious material fractions (the fraction with size 8–24 μm is cement clinker, other fractions are GBFS).

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Table 6 Target mixture proportions (volume percentage) of gap-graded blended cements. Filling grade

Fifth

Fourth

Third

Second

First

Fraction (μm)

b4

4–8

8–24

24–45

45–80

Cementitious material

GBFS

GBFS

Cement clinker

GP GPC GPG GPF Reference cement Portland cement

30.2 9.6 22.0 25.1 11.3 24.0 23.4 11.8 24.6 21.4 12.3 25.4 25% Cement clinker + 75% GBFS 100% Cement clinker

GBFS

GBFS

17.7 18.5 18.8 19.2

20.5 21.1 21.4 21.7

Note: GP, GPC, GPG and GPF are gap-graded blended cements with F0, FC, FG and FF distributions (Table 5), respectively. 5% of gypsum dihydrate by mass percentage of cementitious material was added for all cements in Table 6.

gap-graded blended cements with F0 and FC, FG and FF distributions (see Table 5), respectively. A Portland cement and a reference cement were also prepared by inter-grinding the mixture of cement clinker and GBFS as industrial practices, and the specific surface areas of these two cements were controlled to be in the range of 350–360 m2/kg, which is seen to be equal to that of the gap-graded blended cements. 3.2. PSDs of gap-graded blended cements

Table 7 Water requirement for normal consistency and maximum volume concentrations of solids of cement pastes. Cement

GP

GPC

GPG

GPF

Portland Reference cement cement

Specific density (g/cm3) 2.916 2.924 2.921 2.924 3.151 Water requirement for 0.325 0.317 0.313 0.322 0.303 normal consistency Maximum wet density 1.972 2.051 2.089 2.029 2.059 (g/cm3) 50.48 54.37 56.42 53.23 49.24 Maximum volume concentration of solids (%)

2.952 0.326 1.898 46.00

of gap-graded blended cement pastes were determined as shown in Table 7. Gap-graded blended cement pastes presented a lower water requirement for normal consistency and a higher maximum volume concentration of solids than the reference cement paste. Moreover, cement pastes with modified gap-graded PSDs (GPC, GPG and GPF) gave even higher maximum volume concentration of solids. For instance, maximum volume concentration of solids of GPG cement paste can be as high as 56.42%, being 10.42% higher than that of the reference cement paste (46.00%). The results prove that gapgraded cement pastes present high packing density.

PSDs of gap-graded blended cements and reference cement are given in Fig. 8. Reference cement and Portland cement presented similar PSDs due to slight difference in their grindability, therefore only PSD of reference cement is given in Fig. 8. Obviously, reference cement showed a high peak in its incremental PSD, indicating that cement prepared by co-grinding has a relatively narrow PSD. In contrast, gap-graded blended cements presented five peaks in their incremental PSDs, with the peaks corresponding to 2–3 μm, 5–7 μm, 15–17 μm, 30–32 μm and 60–65 μm, respectively. Although there was a minor overlap among fractions due to low efficiency of the classifier, the PSDs of gap-graded blended cements basically met the requirements of the gap-graded PSDs proposed. In addition, it should be noted that the content of fine particles (b4 μm) of gap-graded blended cement with modified gapgraded PSDs decreased significantly, which will have a critical influence on water requirement, packing density and hydration behavior of blended cement pastes. 3.3. Packing density of gap-graded blended cement pastes Water requirement for normal consistency (EN 196-3 [26]) and maximum volume concentrations of solids (specified in Section 2.3.1)

Fig. 8. PSDs of gap-graded blended cements and reference cement.

Fig. 9. Heat evolution curves of GPG cement, reference cement and Portland cement. (a) Rate of heat evolution, and (b) Cumulative heat of hydration.

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Fig. 10. SEM images of hardened cement pastes. (a) Reference cement paste cured for 24 h, (b) reference cement paste cured for 3 days, (c) reference cement paste cured for 28 days, (d) Portland cement paste cured for 24 h, (e) Portland cement paste cured for 3 days, (f) Portland cement paste cured for 28 days, (g) GPG cement paste cured for 24 h, (h) GPG cement paste cured for 3 days, and (i) GPG cement paste cured for 28 days.

3.4. Heat evaluation of gap-graded blended cement pastes Heat evaluation of cement pastes at water to cementitious material ratio of 0.5 was measured using a TAM-air isothermal calorimeter according to ASTM C 1702-09A [27], and the measurements were performed at constant temperature of 25 °C for 72 h. Compared with the calorimetric curves of Portland cement and the reference cement, the calorimetric curve of GPG cement paste presented an additional peak (occurring at 30–50 h) as shown in Fig. 9, which can be attributed to pozzolanic or hydraulic reactions of fine GBFS particles. Clearly, the pozzolanic reaction (or hydration) of the GBFS particles in GPG cement paste is much more significant than that in the reference cement paste. In addition, most of heat of hydration of the Portland cement paste is released in the first 24 h, while that of GPG cement paste is mainly released after 10 h and increased rapidly afterward. As a result, the cumulative heat of hydration of the GPG cement paste tested in the first 72 h was about 90% of that of the Portland cement. 3.5. Microstructure and hydration products of gap-graded blended cement pastes Cement pastes of normal consistency (see Section 3.3) were cast into plastic tubes and sealed before placing the pastes in a 20 ± 1 °C water bath. Small pieces taken from different parts of the hardened

pastes were put into ethyl alcohol to stop their hydration after curing for 1, 3 and 28 d, then vacuum-oven-dried at 65 °C and 0.1 atm for at least 24 h. The microstructure of hardened cement pastes was observed using scanning electric microscope (SEM, Nano 430, 10 kV). Hydration products of cement pastes were characterized by differential thermal analysis–thermogravimetry (DTA–TG) with a heating rate of 10 °C/min in nitrogen atmosphere. Fig. 10a, b and c showed only a small amount of hydration products in the reference cement paste cured for 1, 3 and 28 d, while a large amount of hydration products with small capillary pores were observed in Portland cement paste at the age of 1, 3 and 28 d (Fig. 10d, e, and f). GPG cement paste showed significantly denser microstructure than reference cement paste and apparently even slightly denser than Portland cement paste at all ages (Fig. 10g, h and i). In addition, there was an obvious gap between GBFS particles and hydration products in the reference cement paste, the hydration products appeared to bond better with the un-hydrated particles in GPG cement paste. DTA curves in Fig. 11 indicated the presence of an endothermal peak at about 104 °C, attributed to C–S–H gel (or C–A–H gel); endothermal peaks at about 439 °C and 709 °C can be related to decomposition of Ca (OH)2 and CaCO3, respectively [28–30]. It can be inferred from TG curves that although with slightly lower amount of hydration products than the Portland cement paste, GPG cement paste showed a larger amount of hydration products and a smaller amount of Ca (OH)2 relative to reference cement paste.

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Fig. 11. DTA and TG curves of hardened cement pastes. (a) Cement paste cured for 3 days, and (b) cement paste cured for 28 days.

3.6. Compressive and flexural strengths of gap-graded blended cement mortars Mortar prisms of 40 × 40 × 160 mm were prepared at water:cementitious material:sand mass ratio of 0.5:1:3. After curing at 20 ± 1 °C and 90% relative humidity (RH) for 24 h, mortar specimens were demoulded and cured in lime-saturated water at 20± 1 °C. Compressive and flexural strengths were tested at 3 d and 28 d according to EN 196-1 [31]. Fig. 12a showed that both 3 d and 28 d compressive strengths of the gap-graded blended cements with about 25% cement clinker were much higher than those of the reference cement, and can be comparable with or even higher than that of the Portland cement. For instance, the 3 d and 28 d compressive strengths of the GPG cement were 25.4 MPa and 49.2 MPa respectively, being 75.2% and 44.7% higher than that of the reference cement. Fig. 12b demonstrated that the gap-graded blended cements also presented higher flexural strengths than the Portland cement.

Fig. 12. Compressive and flexural strengths of gap-graded blended cements and reference cement. (a) Compressive strength, and (b) flexural strength.

subsequently cured in lime-saturated water at 20 ± 1 °C for 2 d. Initial length of the mortar prisms was measured immediately after curing, then the specimens were exposed to a 20 ± 1 °C and 50% RH environmental chamber and the length change was measured at different ages [32]. Irreversible shrinkage was also determined by re-immersion of the mortars in lime-saturated water at 20± 1 °C for 14 d. A time of 0 d in Fig. 13b is equivalent to the final time in Fig. 13a. Total drying shrinkage and irreversible shrinkage in Table 8 are the equilibrium values in Fig. 13a and b respectively, and reversible shrinkage is the difference between the two. Portland and reference cement (both prepared by inter-grinding) mortars gave a high total drying shrinkage and a high reversible shrinkage. In contrast, the gap-graded blended cement mortars presented a lower total drying and reversible shrinkage. That is to say, most of shrinkage of the gap-graded blended cement mortars is irreversible shrinkage.

4. Discussions 4.1. Optimization of hydration process of gap-graded blended cements

3.7. Drying shrinkage of gap-graded blended cement mortars Mortar prisms of 25 × 25 × 280 mm were prepared at cement to sand mass ratio of 0.5, and the fluidity of the mortars was controlled in the range of 130–140 mm by adjusting water addition. After being cured at 20± 1 °C and 90% RH for 24 h, the specimens were demoulded and

The hydration process of gap-graded blended cements can be optimized by arranging cementitious materials in the gap-graded PSDs according to their hydraulic (or pozzolanic) activity. High activity SCMs (GBFS in the experiment) should preferably be arranged in the fine fractions (b4 μm and 4–8 μm), therefore not only high water

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4.2. Microstructure development of gap-graded blended cement pastes Gap-graded PSD leads to a decreased water requirement and an increased packing density of blended cement pastes, and modified gapgraded PSDs (especially for GPG) show further effects. That is to say, smaller amount of hydration products is needed to achieve dense microstructure. Certain amount of hydration products is generated in the gap-graded blended cement pastes in the first 24 h due to hydration of cement clinker particles, Ca (OH)2 reacts with fine GBFS particles to generate more hydration products after 24 h (see heat evolution results). Although with a slightly lower amount of hydration products, the gap-graded blended cement pastes present more homogeneous and denser microstructure than Portland cement paste due to grain size reinforcement and pore size reinforcement (pozzolanic effect). As a result, both early and late mechanical properties of the gap-graded blended cements with about 25% cement clinker can be comparable with or higher than that of 100% Portland cement. 4.3. Feasibility of producing gap-graded blended cements

Fig. 13. Total drying shrinkage and irreversible shrinkage of cement mortars at 50% RH and 20 ± 1 °C. (a) Total drying shrinkage, and (b) irreversible shrinkage.

requirement can be avoided as the pozzolanic reaction (or hydration) of SCMs can be neglected compared with rapid hydration of clinker particles in the first few minutes, but also the pozzolanic activity of fine SCM particles increases significantly due to their large specific surface areas. Cement clinker particles in the range of 8–24 μm have low water requirement and appropriate hydration rate in the first few minutes, and the hydration degree of these particles continues to increase rapidly afterward [25]. Thus cement clinker should be arranged in the middle fraction and is anticipated to play dominant contribution to the properties of gap-graded blended cements. Coarse particles mainly play “packing effect” and do not make much contribution to strength development [4]. Thus low activity SCMs or inert fillers can be placed in the coarse fractions (24–45 μm and 45–80 μm) for environmental and economical benefits.

Table 8 Drying shrinkage of cement mortars at 50% RH and 20 ± 1 °C (percentage change in length). Cement

GP

GPC

GPG

GPF

Portland Reference cement cement

Type of drying Total (%) 0.085 0.076 0.072 0.092 0.116 shrinkage Irreversible (%) 0.063 0.057 0.056 0.067 0.077 Reversible (%) 0.022 0.019 0.016 0.025 0.039

0.120 0.084 0.036

Portland cements prepared by ball mill in cement plant present narrow PSD like the reference cement (Section 3.2), and those prepared by vertical mill have even narrower PSD. Generally, about 70% Portland cement particles lie in the range of 3–30 μm. If a proper air classifier is applied to milling cycle of cement clinker, it is easy to control about 80% clinker particles in the range of 8–24 μm (about 80% of particles was controlled in the target size fraction in experimental validation). Ultra-fine SCM powders are now widely applied in cement manufacturing and concrete making in the world (such as ultra-fine GBFS powder), then fine SCM fractions (b4 μm and 4–8 μm) can be obtained by classification of ultra-fine SCM powders. Coarse SCM (maybe inert fillers) fractions (24–45 μm and 45–80 μm) are much easier to produce through ball mill. Since each cementitious materials fraction can be obtained, gap-graded blended cements can be prepared by blending fine SCM (mainly high activity SCMs) fractions, cement clinker fraction (in the range of 8–24 μm) and coarse SCM (or inert fillers) fractions homogeneously. It will be an effective countermeasure to reduce CO2 emissions and produce substantial energy and costs saving, due to a reduction in cement clinker content (about 25%) as it is replaced by SCMs to a larger extent. Meanwhile, both cement clinker and SCMs are utilized more efficiently. 5. Conclusions The main conclusions that can be drawn from this study are summarized as follows: (a) To achieve high packing density of blended cement pastes, a gap-graded PSD was proposed and modified according to the wet density of actual paste. (b) The gap-graded PSD leads to a decreased water requirement and an increased packing density of blended cement paste, and modified gap-graded PSDs show further effects. (c) The heat of hydration of gap-graded blended cement pastes released slowly in the first 24 h and increased rapidly afterward. In addition, the gap-graded blended cements also gave a low drying shrinkage. (d) Both early and late mechanical properties of low clinker gapgraded blended cements were improved significantly and even higher than those of Portland cement, due to more homogeneous, denser microstructure. Acknowledgments This work was funded by 973 National Foundational Research of China (No. 2009CB623104), National Natural Science Foundation of

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