Effect of rice husk ash fineness on porosity and hydration reaction of blended cement paste

Effect of rice husk ash fineness on porosity and hydration reaction of blended cement paste

Construction and Building Materials 89 (2015) 90–101 Contents lists available at ScienceDirect Construction and Building Materials journal homepage:...

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Construction and Building Materials 89 (2015) 90–101

Contents lists available at ScienceDirect

Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Effect of rice husk ash fineness on porosity and hydration reaction of blended cement paste Weiting Xu a,⇑, Yiu Tommy Lo b, Dong Ouyang c, Sharzim Ali Memon b, Feng Xing a, Weilun Wang a, Xiongzhou Yuan a a b c

Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, College of Civil Engineering, Shenzhen University, Shenzhen, China Department of Architectural and Civil Engineering, City University of Hong Kong, Hong Kong, China Department of Mechanics and Civil Engineering, College of Science and Engineering, Jinan University, Guangzhou, China

h i g h l i g h t s  We investigate the grinding effect on the pozzolanic reactivity of RHA.  We study the mechanical, hydration and porosity properties of RHA paste.  The strength, hydration reaction and porosity of RHA paste were studied.  Paste with 30-min grinding RHA performs best in strength, hydration and porosity.

a r t i c l e

i n f o

Article history: Received 10 June 2014 Received in revised form 11 March 2015 Accepted 21 April 2015

Keywords: Rice husk ash Grinding fineness Hydration reaction Porosity

a b s t r a c t An experimental investigation on the effect of grinding fineness of rice husk ash (RHA) on the mechanical, hydration and porosity properties of RHA blended paste was launched. Locally produced RHA with six different grinding durations of 5, 10, 30, 60, 90 and 120 min corresponding to mean particle size of 8.61, 8.15, 5.45, 6.25, 6.38 and 6.95 lm, respectively, were used in this study. The physical and chemical properties of the ground RHAs were determined and results show that variations in chemical composition of RHAs with different grinding fineness are small. The pastes with water binder ration of 0.4 and cement substitution of 10% by ground RHAs were tested. Comparisons between pastes with SF, and control paste (only with ordinary Portland cement) were also presented. Mechanical properties, hydration reaction and porosity of paste were investigated by compressive strength, thermogravimetric analysis (TGA) and environment scanning electron microscope (ESEM) plus Image Pro Plus software analysis, respectively. Results show that incorporation of 30-min grinding RHA exhibits a strong, high hydration and low porosity paste due to the better dispersion, filling effect and pozzolanic activity. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Rice husk (RH) is one of the main agricultural residues obtained from the outer covering of rice grains during the milling process. It constitutes 20% of the 500 million tonnes of paddy produced annually in the world [1]. In the past, RH has no useful application, although some of it is used as a low-grade fuel in brick kilns and low-pressure steam generation, etc. Most often it is dumped into water streams, which causes pollution and contamination of springs. ⇑ Corresponding author. Tel.: +86 755 2601 3871; fax: +86 755 2653 4021. E-mail addresses: [email protected] (W. Xu), [email protected] (Y.T. Lo), [email protected] (D. Ouyang), [email protected] (F. Xing), [email protected] hotmail.com (W. Wang). http://dx.doi.org/10.1016/j.conbuildmat.2015.04.030 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.

Studies have found that rice plants ingest orthosilicic acid from ground water, whereupon it is polymerized to form amorphous silica in the husk [2,3]. The amorphous silica is the most essential asset of pozzolanic activity of RHA. Approximately 85–95% of amorphous silica by weight of RHA can be produced by controlled burning [4,5]. Due to the high content of silica, RHA is an extremely reactive pozzolanic substance appropriate for use in lime-pozzolan mixes and for Portland cement substitution [6,7]. The reactivity of RHA is also attributed to its large surface area governed by the porous structure of the particles [8]. Cement replacement by RHA accelerates the early hydration of C3S. The increase in the early hydration rate of C3S is attributed to the high specific surface area of the rice husk ash [9]. This phenomenon specially takes place with fine particles of RHA. Although the small particles of pozzolans are less reactive than Portland cements [10], they produce

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a large number of nucleation cites for the precipitation of the hydration products by dispersing in cement pastes. Consequently, this mechanism creates the more homogenous and denser paste as for the distribution of the finer pores due to the pozzolanic reactions between the amorphous silica of the mineral addition and the CH [11]. It is reported that the finer particles of RHA by mechanical grinding speed up the reactions and form smaller CH crystals [4]. Furthermore, the ground ultrafine RHA could even minimize the adverse effect of the residual carbon and presence of the crystalline compounds in RHA produced in unsatisfactory burning conditions, which could compromise its pozzolanic activity [12]. Those not completely reacted pozzolanic particles in the cement paste may fill up the voids and enhance density of the paste at later age [13]. According to available literature, works have been carried out on the effect of grinding fineness by use of various sorts of grinding machine (attrition, vibration or ball mills) on the pozzolanic activity RHA [12,14–16]. Methods employed to evaluate the pozzolanic activity of RHA are compressive strength tests, variation in electrical conductivity of saturated portlandite (CH) solution with addition of RHA, and amounts of reaction products in CH-RHA suspension determined by (DTA/TG) [17–22]. Regardless of which experimental evaluation method uses, previous investigations present a positive linear relation between grinding duration and fineness of RHA, e.g., the more the grinding duration is, the higher the fineness or pozzolanic activity of RHA is. However, similar studies on improving the reactivity of silicates through mechanical activation manifest that prolonged grinding duration does not always lead to a finer particle size distribution or a greater surface area of powders, which may be attributed to particle agglomeration [23]. To the best of our knowledge, there are no detailed data regarding the effect of RHA grinding fineness on hydration reaction of RHA blended cement matrix at early and later age. The objective of this paper is to expand the existing research and systematically study the grinding fineness of RHA on hydration reaction at early and later hydration stage and porosity of cement matrix. In this study, RHA sample from controlled burning in electronic furnace was subjected to mechanical grinding in the laboratory ball mill. The effect of RHA grinding fineness on the hydration reaction and porosity of RHA paste is investigated. The pastes were made from raw and ground RHA of different fineness. Cement hydration degree in RHA pastes was tested at early and late age by calculating the weight loss of hydration products through TGA. The weight loss of hydration products corresponds to the amount of hydrated calcium silicates (CSH) and calcium hydroxide (Ca(OH)2), which can be evaluated through thermogravimetric analysis (TGA). The increasement of the content of CSH and the reduction of the concentration of lime in cement paste contribute to evaluating the role of grinding fineness on the pozzolanic action of RHA in the cement matrix. The porosity of paste incorporating RHA of different grinding fineness was also determined at selected curing ages using an environmental scanning electronic microscope (ESEM) and calculated using image analysis software. The pore size distribution helped to explain the difference in strength between various RHA pastes, because a denser matrix tends to produce high strength performance and good durability.

Table 1 Chemical composition and physical properties of cement, silica fume. Cement

Silica fume

Chemical composition (%) SiO2 Al2O3 Fe2O3 SO3 CaO MgO Na2O K2O LOI

22.52 5.80 3.52 2.54 62.08 1.55 0.05 0.56 0.94

94 0.21 0.09 – 0.12 0.33 – 0.38 1.5

Physical properties Specific gravity Retained on sieve No. 325 (%) Specific surface are (m2/kg) Median particle size, d50 (lm)

3.12 4.7 339 (Blain) 12

2.8 1.0 21,080 (BET) 0.3

Fig. 1. ESEM image of cement powders.

and the fineness of 339 m2/kg. Its physical and chemical properties are listed in Table 1. The particle shapes are shown in Fig. 1. 2.1.2. Silica fume A commercial silica fume (SF) was used as reference material in this study, and the physical and chemical properties are given in Table 1. The ESEM image of silica fume powders is seen in Fig. 2.

2. Experimental programs 2.1. Materials 2.1.1. Cement An ordinary Portland cement provided from Green Island Cement Co Ltd (Hong Kong) CEM II 52.5 complying with BS12 [24] standard was used. The cement had a specific gravity of 3.12

Fig. 2. ESEM image of silica fume powders.

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2.1.3. Rice husk ash RHA sample was obtained by 2-h calcination at specified temperature of 600 °C at a heating rate of 20 °C/min in an electronic furnace. In order to yield as much amorphous ash as possible for each batch burning, the rice husks were piled up evenly for a height of 10 cm in the stainless container with size of 70  40  10 cm in the furnace. For the grinding process, a laboratory ball mill manufactured by Nanjing University, China was used to grind the raw ash to smaller particles. The grinding process was achieved in four stainless tanks by means of rotating stainless balls as the grinding media, which agitate the samples into a random

state. The ball mill grinds material by rotating a cylinder (U500  500 mm) with stainless grinding balls, causing the balls to fall back into the cylinder and onto the material to be ground. As a result, the grinding ball broke the RHA sample into small particles. The controlled parameters of the ball milling, which were held constant, were milling rotational speed (1000 RPM), mass ratio of ball to powder (20:1). The grinding duration was set as 5, 10, 30, 60, 90 and 120 min, respectively. After each grinding intermission the ground sample was taken out to keep in a dry sealed container for subsequent tests. The ESEM image of ground RHA powders is seen in Fig. 3.

(a) 5-min grinding RHA (300×)

(b) 10-min grinding RHA (300×)

(c) 30-min grinding RHA (300×)

(d) 60-min grinding RHA (300×)

(e) 90-min grinding RHA (300×)

(f) 120-min grinding RHA (300×)

Fig. 3. ESEM image of ground RHAs.

W. Xu et al. / Construction and Building Materials 89 (2015) 90–101

2.2. Testing programs 2.2.1. Chemical and physical properties of raw and ground RHAs The chemical compositions of raw and ground RHAs were monitored by X-ray fluorescence (XRF). Mineralogical analysis on RHA samples was performed by X-ray diffractometry (XRD), which is the Phillips PW 1050 diffractometer with a copper tube and a nickel filter. The X-ray diffraction system was operated with a 50 kV, 50 mA Cu radiation source. The particle morphology of ground RHAs was taken by environmental scanning electron microscope (ESEM) (FEI XL30 S-FEG) with 20 kV accelerating voltage. The mean particle size of RHA with different grinding durations was determined by a laser particle size analyzer (Microtrac SR150). The total pore volume, volume pore size distribution and specific surface area of the ground RHAs were measured in triplicate by using nitrogen adsorption (NOVA 1200e). 2.2.2. Strength of paste The blended cement pastes were prepared with water to blender ratio of 0.3, 0.4 and 0.5 at room temperature around 20 ± 2 °C. The dosages of ground RHA are 10% of Portland cement by weight. Because of harsh mixing and bleeding effect, the cement paste blended with 10% of RHA with w/b ratio of 0.3 and 0.5 was excluded. The compressive strength of 20 mm paste cube at 1, 3, 7, 28 and 90 days of moist curing was determined in accordance with ASTM C109 standard [25]. The test yielded RHA sample, which showed highest pozzolanic activity. 2.2.3. Effect of cement, raw RHA and ground RHA on hydration process This test is aimed to determine the hydration reaction degree of CSH during the hydration process of RHA blended paste. The hydration processes of OPC paste and paste incorporating SF were

Table 2 TGA heating temperature and the corresponding decomposition materials [21]. TGA temperature (°C)

Weight loss corresponding to decomposition materials

110–170 °C

Decomposition of gypsum (with a double endothermal reaction), decomposition of ettringite and the loss of water from part of the carboaluminate hydrates Loss of bond water from the decomposition of C–S–H and carboaluminate hydrates undergoes Dehydroxylation of the portlandite (calcium hydroxide) Decarbonation of calcium carbonate

180–300 °C 450–550 °C 700–900 °C

also studied for comparison test. Pastes with water binder ratio of 0.40 and cement replacement ration of 10% (by ground RHAs or SF) were cured in plastic containers at a temperature of 20 °C and vacuum environment for different period of time (2, 4, 8 and 12 h; 1, 3, 7 and 28 days). Then, the samples were ground to obtain homogeneity of the grain size below 50 microns for the quantitative thermogravimetric analysis (TGA). The TGA heating temperature range was run at 50–900 °C and with a heating rate of 10 °C per minute. The TGA curves were plotted and the weight loss of the paste specimens was calculated. As the different chemical compound in cement corresponds to certain decomposition weight loss of TGA curves [26], the qualitative evaluation of the hydration products in paste can be assessed. The TGA temperature and the corresponding decomposition materials are listed in Table 2. 2.2.4. Morphology and pore size distribution of paste This test is aimed to determine the pore size distribution of each paste by image analysis. The backscatter electron image micrographs of cement paste and pastes incorporating raw RHA, ground RHAs and SF at the age of 3, 7 and 28 days were captured using the ESEM. Pores counting for paste are conducted by using Image-ProPlus software. A small piece of each paste will be examined under two magnifications at 250 and 1000. An example of typical backscatter electron image micrographs obtained at 250 magnifications for a water cement ratio 0.40 paste is shown in Fig. 4a. The structure was composed of solid grains appearing in white and large pores and interconnected pores appearing in dark gray. Some large pores appeared to be the space between hydrated grains. After processing with image analysis program, the pores were coded with color depending on their size as shown in Fig. 4b. The program calculates the pore size as the diameter of a circular area equal to the area of each pore in the micrograph. In this study the pore size distribution is measured. Due to the limitation in distinguishing the pixel, the minimum size that can be measured by this procedure is 0.8 microns. The liquid displacement technique [27] is used to determine the porosity in this study. Paste specimen with size of 20  20  20 mm is vacuum saturated in water for 1 day and weigh in the saturated, surface dry condition (Ws). The buoyant weight of the sample (Wb) is then measured by suspending it in the water. The water is then removed by oven drying the sample at 110 °C to constant weight (Wd). The mass of water lost from saturation for drying is an estimate of the evaporable water content in the specimen. The percent porosity can then be calculated as equation:

Porosity ð%Þ ¼ ðWs  WdÞ=ðWs  WbÞ100:

(a) Before Processing

93

(b) After Processing

Fig. 4. Representative backscatter electron images of a 28-day RHA Paste (250).

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The pore size distribution is calculated as follows:

Table 4 Mean particle size, surface area, and pore characteristics of raw and ground RHAs.

Total Pore Size Distribution ð%Þ ¼ Porosity  Pore size distribution from image analysis of ESEM:

3. Results and discussions 3.1. Chemical and physical properties of RHA The XRF chemical composition analysis of the typical RHA samples with and without grinding is listed in Table 3. According to the chemical composition results, there are no significant variations in chemical composition of the RHAs with different grinding time. It is observed in XRD diagrams of Fig. 5 that all the RHA samples are X-ray amorphous, since there is no sharp peak observed at 2h = 22°. There is not any obvious change in mineralogy of the different ground ash samples. It indicates that grinding does not affect the crystalline mineralogy of RHA samples. The mean particle size, surface area, and pore characteristics of raw and ground RHAs are shown in Table 4. It can be observed that the mean particle size are decreased with increase in grinding duration from 5 to 30 min, which is attributed to the mechanical grinding effect on reduction of particle size (or increasement of surface area). The lowest mean particle size (5.45 lm) and the highest surface area (19.558 m2/g) are present in the 30-min grinding RHA. When the grinding duration rises over 30 min, some larger particles show up which manifested by the increasing mean particle

Table 3 Chemical compositions of raw and ground RHAs. Chemical composition (%)

Raw RHA

RHA5 min

RHA30 min

RHA120 min

SiO2 Al2O3 Fe2O3 SO3 CaO MgO Na2O K2O LOI

81.4 0.26 0.93 1.47 2.42 1.02 0.18 6.79 3.13

82.9 0.28 1.69 1.56 2.35 0.98 0.11 6.83 3.14

82.9 0.29 1.83 1.34 2.29 0.93 0.06 6.82 3.14

83 0.34 1.83 1.58 2.16 1.07 0.01 6.83 3.13

RHA sample

Raw RHA RHA5 min RHA10 min RHA30 min RHA60 min RHA90 min RHA120 min

Mean particle size/(lm) 8.61 8.15 5.45 6.25 6.38 6.95

BET surface area/(m2/g)

Total pore volume/(mL/ g)

Average pore diameter/(nm)

2.765 6.388 10.819 19.558 11.911 9.587 7.546

0.073 0.023 0.029 0.053 0.049 0.035 0.031

3.7038 4.5000 3.9312 3.0728 4.2458 4.2496 4.5806

size of 6.25 lm, 6.38 lm and 6.95 lm corresponding to 60-, 90and 120-min grinding, respectively. This is due to the particle agglomeration associated with excessive grinding duration [23]. This trend can be confirmed by the ESEM image shown in Fig. 3. It can be seen that the finely grinding effect took place when raw RHA sample was ground at a certain range of grinding duration. However, some larger particles show up with the excessive grinding. Thus, the excessively long grinding duration should be avoided. 3.2. Strength of RHA paste The compressive strength testing results of cement paste and paste with incorporation of 10% cement replaced by RHA and SF is shown in Table 5. It can be seen that the paste incorporating raw RHA without grinding shows the lowest compressive strength of all the testing curing ages, which is due to the dilution effect [28]. Strength of paste incorporating RHA is increased with increase in RHA grinding duration of 5–30 min. For the ground RHA blended paste, maximum strength of paste (79.57 MPa) occurs at the paste incorporating RHA grinding for 30 min at the age of 90 days, which is 12.8% higher than that of control paste. The paste with 30-min grinding RHA shows the second highest strength value after SF blended paste (80.70 MPa) among all the paste at the age of 90 days. This is due to the high fineness of RHA that exhibits good pozzolanic properties and packing effect. However, when the RHA ground for above 30 min, the corresponding strength of paste is gradually reduced. This indicates that long grinding duration does not always lead to high pozzolanic activity, due to the particle aggregation by excessive grinding. The 30-min grinding RHA shows the best pozzolanic activity among all the ground ash samples.

RHA120min 3.3. Hydration process of cement, RHA and SF paste at different ages

RHA90min RHA60min RHA30min RHA10min RHA5min Raw RHA 10

20

30

40 o

2θ ( ) Fig. 5. XRD analysis of RHA samples.

50

60

3.3.1. Hydration process of paste at the age of 28 days The results of the thermogravimetric testing on control, RHA and SF paste at the age of 28 days obtained at heating temperature rate of 10 °C/min are plotted in Fig. 6. It can be observed that the curve of all the paste shows three rapid weight losses. Referring to Table 2, the first weight loss, located between 110 and 300 °C, is the result of dehydration reactions of several hydrates (C–S–H, carboaluminates, ettringite, etc.) and is mainly due to the dehydration of the C–S–H. The second major weight loss, observed at 450– 550 °C, corresponds to the dehydroxylation of portlandite, another hydration product. The third weight loss appears at 750–900 °C and corresponds to the decarbonation of calcium carbonate coming from the cement clinker. The weight losses for paste specimens at the age of 28 days are calculated in Table 6. For the mixtures at the age of 28 days, since the free water in the matrix is consumed, the weight loss at the first stage corresponding to the dehydration reaction of C–S–H can be used as a direct indicator for the hydration reaction degree

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W. Xu et al. / Construction and Building Materials 89 (2015) 90–101 Table 5 Compressive strength of RHA and SF paste (W/C = 0.4, cement replacement ratio: 10%). Paste specimen

Compressive strength (MPa)

Control Raw RHA RHA5 min RHA10 min RHA30 min RHA60 min RHA90 min RHA120 min SF

1 day(MPa)

3 day(MPa)

7 day(MPa)

28 day(MPa)

90 day(MPa)

28.15 14.82 27 27.8 33.96 32.8 32.79 32 34.93

41.62 22.26 41.66 41.8 44.23 36.03 36.02 35.22 39.43

48.42 25.58 43.96 49.47 50.58 46.78 46.7 46.15 59.5

66.83 36.87 65.03 66.93 71.02 65.16 65.39 64.93 75.59

70.51 43.39 67.34 67.88 79.57 71.52 70.79 68.62 80.70

102 Control paste at 28d Paste incorporating raw RHA at 28d Paste incorporating 5 min-grinding RHA at 28d Paste incorporating 10 min-grinding RHA at 28d Paste incorporating 30 min-grinding RHA at 28d Paste incorporating 60 min-grinding RHA at 28d Paste incorporating 90 min-grinding RHA at 28d Paste incorporating 120 min-grinding RHA at 28d Paste incorporating SF at 28d

99

TGA (%)

96 93 90 87 84 81 0

100

200

300

400 500 600 o Temperature ( C)

700

800

900

Fig. 6. TGA curve of control paste and paste incorporating RHA and SF at the age of 28 days (10% cement replacement by grinding RHAs and SF).

Table 6 Weight losses of pastes at the age of 28 days. Paste specimen

Control Raw RHA RHA5 min RHA10 min RHA30 min RHA60 min RHA90 min RHA120 min SF

Weight loss (%)

Weight loss with respect to total weight loss (%)

Stage 1

Stage 2

Stage 3

Stage 1

Stage 2

Stage 3

5.21 4.65 4.73 5.41 5.51 5.38 5.25 5.07 5.33

3.21 3.72 3.74 3.40 2.89 3.15 3.14 3.28 2.56

2.71 3.19 2.97 2.67 2.43 2.62 2.62 2.74 2.35

46.80 40.20 41.34 47.11 50.90 48.25 47.71 45.72 52.04

28.85 32.20 32.66 29.60 26.6 28.23 28.51 29.60 25.04

24.35 27.60 26.00 23.29 22.43 23.52 23.78 24.68 22.92

Note: Stage 1: dehydration reactions mainly due to the loss of water from C–S–H; Stage 2: dehydroxylation of portlandite; Stage 3: decarbonation of calcium carbonate.

of mixture. It can be seen in Table 6 that the paste with incorporation of SF shows the highest weight loss at the first stage, which is 11.2% higher than control paste. The paste incorporating 30-min grinding RHA ranks a weight loss percentage second with 50.9% among all the paste samples, which is 8.8% higher than control paste in the stage of dehydration. It indicates that the pozzolanic activity of RHA with grinding for 30 min can be comparable with that of silica fume at the age of 28 days. The raw RHA shows the lowest percentage in weight loss among all the paste specimens in the dehydration stage due to the dilution effect. With grinding duration of RHA increases from 5 to 30 min, the weight loss of corresponding blended paste is increased at the first stage. The C–S–H

becomes more with increase in fineness of RHA particles. The reason is that the fine RHA has a larger surface area to provide the silica for pozzolanic activity. While the weight loss of paste was reduced with incorporation of RHA grinding over 30 min. This is due to the particle agglomeration caused by the excessive grinding. The second and third thermogravimetric heating stage mainly corresponds to dehydroxylation of portlandite (Ca(OH)2) and decarbonation, respectively. The hydration products and calcium carbonate content are correlated. During the hydration reaction of cement, the increase proportion of hydration products (C–S–H and Ca(OH)2) is correlated to the decrease proportion of the decarbonation. For the second stage, it can be seen in Table 6 that paste with incorporation of 30-min grinding RHA or SF, shows lower value in the weight loss compared with control paste. It indicates that the addition of finely ground mineral admixture can enhance the activity of cement matrix and accelerate secondary hydration reaction of Ca(OH)2 in the paste. Paste incorporating 30-min grinding RHA shows the lowest weight loss value of Ca(OH)2 among all the RHA blended paste. The 30-min grinding RHA exhibits a good pozzolanic activity and can be comparable to SF.

3.3.2. Hydration process of paste within 24 h Based on the above testing results, 30-min grinding RHA presents a good pozzolanic activity. In this test, the hydration properties of the control and the paste incorporating 30-min grinding RHA and SF within the early 24-hydration hours are further investigated. The results of the thermogravimetric testing on control, 30-min grinding RHA and SF paste within 24 h are plotted in Figs. 7a–c. The weight losses of the paste specimens are presented

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102

Control paste Control paste Control paste Control paste Control paste

99 96

at 2h at 4h at 8h at 12h at 24h

TGA (%)

93 90 87 84 81 78 75 0

100

200

300

400 500 600 o Temperature ( C)

700

800

900

Fig. 7a. TGA curve of control paste at 2, 4, 8, 12 and 24 h.

Paste incorporating Paste incorporating Paste incorporating Paste incorporating Paste incorporating

102 99 96

30 min-grinding RHA at 2h 30 min-grinding RHA at 4h 30 min-grinding RHA at 8h 30 min-grinding RHA at 12h 30 min-grinding RHA at 24h

90 87 84 81 78 75 72 0

100

200

300

400

500

600

700

800

900

T em p eratu re (% ) Fig. 7b. TGA curve of RHA paste at 2, 4, 8, 12 and 24 h (10% cement replacement by 30-min grinding RHA).

102

Paste incorporating SF at 2h Paste incorporating SF at 4h Paste incorporating SF at 8h Paste incorporating SF at 12h Paste incorporating SF at 24h

99 96 93

TGA (%)

TGA (%)

93

90 87 84 81 78 75 72 0

100

200

300

400

500

600

700

800

900

Temperature ( C) o

Fig. 7c. TGA curve of SF paste at 2, 4, 8, 12 and 24 h (10% cement replacement by SF).

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proportion, compared with that of control and SF paste for the initial 12 h hydration. This indicates that RHA does not retard the early hydration reaction rate of the cement matrix, which is consistent with some previous researchers’ study [9]. However, the relative percentage of decarbonation of all the paste samples is reduced when the hydration reaction proceeds to 24 h. This is because the free water in the cement matrix is gradually consumed and at the same time the hydration reaction is slowed down along with hydration reaction proceeding to longer hours.

Table 7 Weight losses of pastes at 2, 4, 8, 12 and 24 h. Paste specimen

Control-2 h Control-4 h Control-8 h Control-12 h Control-24 h RHA30 min-2 h RHA30 min-4 h RHA30 min-8 h RHA30 min12 h RHA30 min24 h SF-2 h SF-4 h SF-8 h SF-12 h SF-24 h

Weight loss (%)

Weight loss with respect to total weight loss (%)

Stage 1

Stage 2

Stage 3

Stage 1

Stage 2

Stage 3

11.03 3.24 3.42 3.19 3.16 16.36 7.01 6.71 5.98

0.33 0.36 0.56 0.69 2.16 0.24 0.58 1.08 1.86

1.08 1.47 1.66 2.16 2.47 0.94 1.34 2.67 3.57

88.71 63.94 60.62 52.81 40.57 93.24 78.47 64.20 52.39

2.63 7.08 9.88 11.36 27.73 1.38 6.53 10.30 16.30

8.66 28.98 29.51 35.83 31.70 5.38 15.00 25.50 31.31

4.42

2.74

2.84

44.21

27.43

28.36

17.79 8.8 6.38 5.16 3.02

0.23 0.58 0.45 1.35 1.67

0.78 1.13 2.61 2.81 1.79

94.60 83.78 67.61 55.39 46.59

1.23 5.50 4.78 14.48 25.73

4.17 10.72 27.62 30.13 27.68

Note: Stage 1: dehydration reactions mainly due to the loss of water from C–S–H; Stage 2: dehydroxylation of porlandite; Stage 3: decarbonation of calcium carbonate.

in the Table 7. It can be seen in Figs. 7a–c that there are sharp decent curve corresponding to the initial 2 and 4 hydration hours for all the testing paste samples, which may be due to the evaporation of the unconsumed free water and interlayer water with increase in the thermal gravimetric heating temperature. The unconsumed free water and interlayer water for the paste at the age of within 24 hydration reaction hours results in the inaccurate assessment on the weight loss of the first stage, i.e., dehydration. Since the original composition proportion of Portland clinker is stable, for the same cement replacement ration, the decarbonation in the third stage can be used as an indicator to assess the hydration reaction degree of paste samples for the initial 24 h. The lower relative proportion of decarbonation is, the higher the sum of CSH and CH i.e., hydration degree of paste is. It can be seen in Table 7 that the relative decarbonation proportion is increased (i.e., the sum of CSH and CH is reduced) with hydration reaction proceeding from 2 to 12 h for all the paste samples. It is because the hydration reaction proceeds rapidly for the initial hours and then gradually slowed down. The 30-min grinding RHA paste shows relative low weight loss of decarbonation

3.3.3. Hydration process of control paste, 30-min grinding RHA paste and SF paste at the age of 1, 3, 7 and 28 days The thermogravimetric weight loss of control, 30-min grinding RHA paste and SF paste specimen was plotted in Figs. 8a–c. Since the final setting of paste is completed and the free water is mostly consumed after 24 h, the quantitative dehydration of paste can be used as the indication of hydration reaction degree at the age after 1 day. It is observed in Table 8 that the weight loss of the control paste is increased from 40.57% to 46.80% with increase in curing age from 1 to 28 days. As for the 30-min grinding RHA paste, the weight loss caused by the dehydration reaction in the first stage is increased from 44.21% to 50.90% with increase in testing age from 1 to 28 days. The weight loss of SF at the first stage is increased from 46.59 to 52.04% with testing age of 1–28 days. Comparing the weight loss of all the paste samples at the age of 28 days, the SF shows the highest value compared with control paste and 30-min grinding RHA paste. The 30-min grinding RHA shows higher dehydration percentage compared with control paste from 1 to 28 days. For the second weight loss stage at 28 days, the 30-min grinding RHA paste shows lower weight loss percentage in dehydroxylation reaction corresponding to Ca(OH)2 compared with that of control paste, which also indicates the excellent pozzolanic activity of ground RHA. For the decarbonation reaction at the third stage, the results show the weight loss of all the samples is decreased with increase in curing age from 1 to 28 days. 3.4. Porosity and pore size distribution of cement, RHA and SF paste at 28 days 3.4.1. Porosity of cement, RHA and SF paste at 28 days The porosity of paste samples at 28 days was obtained by the water displacement method and the results are shown in Table 9. It can be observed that the SF paste shows 33.30% in porosity and is the lowest value of all the hardened pastes at 28 days.

102 Control Control Control Control

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paste paste paste paste

at at at at

1d 3d 7d 28d

TGA (%)

96 93 90 87 84 81 0

100

200

300

400 500 600 o Temperature ( C)

700

Fig. 8a. TGA curve of control paste at 1, 3, 7 and 28 days.

800

900

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102 Paste incorporating 30 Paste incorporating 30 Paste incorporating 30 Paste incorporating 30

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min-grinding RHA min-grinding RHA min-grinding RHA min-grinding RHA

at 1d at 3d at 7d at 28d

TGA (%)

96 93 90 87 84 81 0

100

200

300

400 500 600 o Tempreature ( C)

700

800

900

Fig. 8b. TGA curve of RHA paste at 1, 3, 7 and 28 days (10% cement replacement by 30-min grinding RHA).

102 Paste Paste Paste Paste

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incorporating incorporating incorporating incorporating

SF SF SF SF

at at at at

1d 3d 7d 28d

TGA (%)

96 93 90 87 84 0

100

200

300

400 500 600 o Temperature ( C)

700

800

900

Fig. 8c. TGA curve of SF paste at 1, 3, 7 and 28 days (10% cement replacement by SF).

Table 8 Weight losses of pastes at the age of 1, 3, 7 and 28 days. Paste specimen

Control-1 day Control-3 days Control-7 days Control-28 days RHA30 min-1 day RHA30 min-3 days RHA30 min-7 days RHA30 min28 days SF-1 day SF-3 days SF-7 days SF-28 days

Weight loss (%)

Table 9 Porosity of cement, RHA and SF paste at the age of 28 days.

Weight loss with respect to total weight loss (%)

Stage 1

Stage 2

Stage 3

Stage 1

Stage 2

Stage 3

3.16 4.15 4.96 5.21 4.42 5.55 5.33 5.51

2.16 2.87 3.88 3.21 2.74 3.46 3.05 2.94

2.47 2.39 2.26 2.71 2.84 2.58 2.32 2.59

40.57 44.10 44.70 46.80 44.21 47.89 49.85 50.90

27.73 30.53 29.95 28.85 27.43 27.89 27.49 26.67

31.70 25.37 25.35 24.35 28.36 24.22 22.66 22.43

3.02 4.62 5.13 5.33

1.67 2.88 2.72 2.61

1.79 2.02 2.33 2.50

46.59 48.51 50.40 52.04

25.73 26.24 25.73 25.04

27.68 25.25 23.87 22.92

Note: Stage 1: dehydration reactions mainly due to the loss of water from C–S–H; Stage 2: dehydroxylation of porlandite; Stage 3: decarbonation of calcium carbonate.

Paste specimen

Porosity at the age of 28 days (%)

Control Raw RHA RHA5 min RHA10 min RHA20 min RHA30 min RHA40 min RHA50 min RHA60 min RHA90 min RHA120 min SF

45.00 67.50 49.05 43.20 40.50 36.10 37.80 44.55 45.09 46.80 48.60 33.30

The porosity of 30-min grinding RHA paste is 36.10%, which is 8.1% lower than that of control paste. Thus, the 30-min grinding RHA, like SF, presents good pozzolanic activity. The porosity of pastes with incorporation of RHA with grinding duration from 10 to 30 min was lower than that of control paste. The raw RHA paste

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Fig. 9. Pore size distributions of control paste and paste with 10% cement replacement by SF and RHAs at the age of 28 days.

presents the highest porosity value of all the paste samples. This is attributed to the large particle size and low surface area of unground RHA. The porosity of RHA with grinding for 5 min is 4.05% higher than that of control paste. The porosity of RHA 60 min, RHA90 min and RHA120 min paste are also higher than that of control paste. This is because grinding for 5 min is not enough to reach the required fineness for performing the pozzolanic activity. Excessive grinding for above 60 min causes the particles agglomeration and hence increasing the larger size particles and reducing the mean surface area of RHA particles. 3.4.2. Pore size distributions of cement, RHA and SF paste at 28 days Pore size distribution of cement, RHA and SF paste at 28 days is shown in Fig. 9. It can be seen that for all the pastes, the volume percentage of small pores is decreased more than that of large pores. This result is due to the formation of hydration products. The hydration products begin to fill the small pores at this time and hence reducing the pore size of paste. Comparing the pore size distributions among pastes with RHA of different grinding durations, it can be seen that the pore size reduction at 28 days in raw RHA pastes shows the largest values among all the paste samples, which is the result of its very large particle size. The large particles have smaller surface area compared with small particles and hence diminishing the nucleation process and reducing the strength of the matrix. It may cause larger gap among cement particles and thus impact the cement particles to complete the hydration reaction. The pore size distribution decreased as the fineness of RHA increased (with grinding duration of 5–30 min). The decrease in pore size in finer RHA paste is because it has more numbers of particles acting as a nucleation centers for hydration products. The RHA30 min paste showing second lowest pore size distribution in their pastes (followed SF paste), which is consistent with the compressive strength results. This tendency demonstrates the theory that mechanical properties of these pastes are usually controlled by pore structure. The paste with finer pore size distribution would have higher compressive strength. 3.5. Porosity and pore size distribution of cement, 30-min grinding RHA and SF paste at the age of 3, 7 and 28 days 3.5.1. Porosity of cement, 30-min grinding RHA and SF paste at the age of 3, 7 and 28 days The porosity at 3, 7, and 28 days of hardened cement, 30-min grinding RHA and SF pastes are shown in Table 10. It is observed

Table 10 Porosity of cement, 30-min grinding RHA and SF paste at the age of 3, 7 and 28 days. Paste specimen

Control 30-min grinding RHA SF

Porosity (%) 3 day

7 day

28 day

61.20 60.12 50.62

58.50 55.80 46.95

45.00 36.10 33.30

that the porosity of all hardened pastes decreased with increase in curing ages. At all the testing ages, the porosity of SF presents the lowest value in porosity of all the testing samples. At the age of 3 days, the RHA paste shows similar porosity compared with the control paste. At 7 and 28 days, the porosity of RHA paste is reduced significantly and shows 4.6% and 19.8% lower than that of control paste, respectively. The pozzolanic properties of active amorphous silica in the ground RHA reduce the porosity of the paste. 3.5.2. Pore size distribution of cement, 30-min grinding RHA and SF paste at the age of 3, 7 and 28 days The pore size distribution data of cement, 30-min grinding RHA and SF pastes at 3, 7, and 28 days are plotted in Figs. 10a–c, respectively. It is observed that the pore size distribution of all the paste at 3 days was almost the same with those at 7 days. It suggests that there was small change in pore size between 3 and 7 days. The hydration products formed during this period may fill the larger void. After 7 days the spaces in the paste were filled further by hydration during that later period. It is evident that the pores of all RHA pastes were reduced with time. From 3 to 7 days, both 30-min grinding RHA and SF pastes reduce their pore sizes of >3 microns, while the pore sizes of <3 microns almost had little change. These large pores of >3 microns are believed to be the gap between particles. These gaps are becoming smaller because of the growth of hydration products on the particles at the age of 3–7 days. Whereas, the pores of <3 microns are not yet filled up yet. At 28 days, the volume percentage of small pores decreased more than that of large pores. This result is due to the formation of both hydration and pozzolanic products. The hydration and pozzolanic products begin to fill the small pores at this time, reducing the pore size. The pore size reduction in 30-min grinding RHA pastes at early age is the result of the nucleation process. This stage began immediately after mixing and continued until all the cement grains were

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Fig. 10a. Pore size distributions of control paste at 3, 7 and 28 days.

Fig. 10b. Pore size distributions of paste with 10% cement replacement by 30 min-grinding RHA at 3, 7 and 28 days.

Fig. 10c. Pore size distributions of paste with 10% cement replacement by SF at 3, 7 and 28 days.

fully hydrated. The hydrates such as C–S–H gel and Ca(OH)2 dissolving from the cement grains precipitated on to the RHA surface and cement surfaces. It is expected that the hydration product growing from nearby RHA particles or cement grains became connected and this bond increases the strength of the matrix. The

nucleation effect is present predominantly at the early age because there are many of cement grains ready for hydration. It may diminish at later ages after all cement grains have been hydrated. Hence, the pore size reduction at later age is probably the result of other mechanisms such as the pozzolanic action of RHA.

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4. Conclusions (1) Mechanical grinding cannot significantly affect the chemical composition of the RHAs with different grinding durations. The fineness of RHA is increased with increase in grinding duration from 5 to 30 min, which is attributed to the grinding effect on reduction of particle size (or increasement of surface area). The lowest mean particle size (5.45 lm) and the highest surface area (19.558 m2/g) are present in the 30-min grinding RHA. Grinding for above 30 min does not always lead to high fineness due to the particle aggregation. Therefore, excessively long grinding duration should be avoided. (2) The results of compressive strength of paste also show that the optimum pozzolanic active RHA can be obtained by grinding for 30 min, which can be comparable with that of SF. This finely ground RHA particles contributes better dispersion and filler effect to the cement matrix. (3) The weight loss of C–S–H at the thermodynamic stage by TGA testing can be used as an indicator for assessing the degree of hydration of paste at the later curing ages. While for the initial hydration age within 24 h, weight loss corresponding to the decarbonation can be used as an indicator to assess the hydration reaction degree of cement matrix, due to the unconsumed free water and interlayer water in the cement matrix. The TGA testing results indicate that the hydration degree of 30-min grinding RHA can be comparable with that of silica fume at the age of 28 days. However, paste with incorporating RHA grinding above 30 min gives reduced weight loss value corresponding to C–S–H, due to the particle agglomeration caused by the excessive grinding. As for the initial 12 hydration reaction hours, the 30-min grinding RHA paste shows relative low weight loss of decarbonation proportion compared with that of control paste. It indicates that RHA does not retard the hydration reaction of the cement matrix in the early age. (4) The pore size of all ground RHA pastes continues to decrease with increase in curing time by the nucleation and pozzolanic action, which fill the voids between grains with their products. The interconnection of these layers between particles is the major effect that strengthens the matrix bond at early ages. The finely ground RHA enhances dispersion, nucleation, and pozzolanic activity. The pore size of finely ground RHA paste is decreased with age prolonging. It is finer than that of cement paste at the age of 7 and 28 days. The pore reduction was a result of deflocculation, nucleation effect, and pozzolanic action. The fineness has positive contribution to all three of these processes. It is found that the paste with finer RHA had finer pore size distribution. The reduction of pore size helps to explain the difference in strength among various RHA pastes, because the denser matrix tends to produce high performance and good durability. The pore size distribution of SF paste was finer than that of 30-min grinding RHA and cement paste. This result holds true at all ages suggesting that the fineness of pure amorphous silica is an important factor contributing to deflocculation, nucleation and pozzolanic action.

Acknowledgments The authors greatly acknowledge the financial supports from the National Natural Science Foundation of China (Grant No. 51408363, 51478207 and 51308344), the Project Funded by

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China Postdoctoral Science Foundation (Grant No. 2014M562210 and 2013M531875), the Natural Science Foundation of SZU (Grant No. 201421), and the Science Industry Trade and Information Technology Commission of Shenzhen Municipality (Grant No. GJHZ20120614144906248). We also thank the Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, College of Civil Engineering, Shenzhen University for providing facilities and equipments. References [1] Bronzeoak Ltd. Rice husk ash market study. Available online at: . [2] Rizwan SA. High-performance mortars and concretes using secondary raw materials [Ph.D. thesis]. Freiberg: Technischen Universistat Bergakademie; 2006. [3] Kamiya K, Oka A, Nasu H, Hashimoto T. Comparative study of structure of silica gels from different sources. J Sol-Gel Sci Tech 2000;19:495–9. [4] Mehta PK. Rice husk ash – a unique supplementary cementing material. In: Malhotra VM, editor. Proceeding international symposium on advances in concrete technology. Athens, Greece; 1992. P. 407–430. [5] Della VP, Kuhn I, Hotza D. Rice husk ash as an alternate source for active silica production. Mater Lett 2002;57:818–21. [6] Mehta PK. Role of pozzolanic and cementitious material in sustainable development of the concrete industry. ACI SP-178, vol. 1; 1998. p. 1–20. [7] Ganesan K, Rajagopal K, Thangavel K. Rice husk ash blended cement: assessment of optimal level of replacement for strength and permeability properties of concrete. Constr Build Mater 2008;22:1675–83. [8] Dakroury AE, Gasser MS. Rice husk ash (RHA) as cement admixture for immobilization of liquid radioactive waste at different temperatures. J Nucl Mater 2008;381:271–7. [9] Feng Q, Yamamichi H, Shoya M, Sugita S. Study on the pozzolanic properties of rice husk ash by hydrochloric acid pretreatment. Cem Concr Res 2004;34: 521–6. [10] Mehta PK, Aitcin PCC. Principles underlying production of high-performance concrete. J Cem Concr Aggregates 1990;12:70–8. [11] Isaia GC, Gastaldini ALG, Moraes R. Physical and pozzolanic action of mineral additions on the mechanical strength of high-performance concrete. Cem Concr Compos 2003;25:69–76. [12] Cordeiro GC, Filho RDT. Use of ultrafine rice husk ash with high-carbon content as pozzolan in high performance concrete. Mater Struct 2009;42:983–92. [13] Berry E, Hemmings RT, Zhang M, Cornelius BJ, Golden DM. Hydration in highvolume fly ash concrete binders. ACI Mater J 1994;91:382–9. [14] Van VTA, Rößler C, Bui DD, Ludwig HM. Mesoporous structure and pozzolanic reactivity of rice husk ash in cementitious system. Constr Build Mater 2013;43:208–16. [15] Cordeiro GC, Filho RDT, Tavares LM, Fairbairn EMR, Hempel S. Influence of particle size and specific surface area on the pozzolanic activity of residual rice husk ash. Cem Concr Compos 2011;33:529–34. [16] Zain MFM, Islam MN, Mahmud F, Jamil M. Production of rice husk ash for use in concrete as a supplementary cementitious material. Constr Build Mater 2011;25:798–805. [17] Boating AA, Skeete DH. Incineration of rice hull for use as a cementitious materials; the Guyana experience. Cem Concr Res 1990;20:795–802. [18] Zhang MH, Mohan MV. High-performance concrete incorporating rice husk ash as a supplementary cementing material. ACI Mater J 1996;93:629–36. [19] Cisse IK, Laquerbe M. Mechanical characterisation of filler sandcretes with rice husk ash additions: study applied to Senegal. Cem Concr Res 2000;30:13–8. [20] Chindaprasirt P, Rukzon S. Strength, porosity and corrosion resistance of ternary blend Portland cement, rice husk ash and fly ash mortar. Constr Build Mater 2008;22:1601–6. [21] Chindaprasirta P, Kanchandaa P, Sathonsaowaphaka A, Caob HT. Sulfate resistance of blended cements containing fly ash and rice husk ash. Constr Build Mater 2007;21:1356–61. [22] Habeeb GA, Fayyadh MM. Rice husk ash concrete: the effect of RHA average particle size on mechanical properties and drying shrinkage. Aust J Basic Appl Sci 2009;3:1616–22. [23] Opoczky L. Fine grinding and agglomeration of silicates. Powder Technol 1977;17:1–7. [24] British Standard Institute. BS12: specification for Portland cement. London, UK; 1989. [25] ASTM C109. Standard test method for compressive strength of hydraulic cement mortars. West Conshohocken, PA: ASTM International; 2013. [26] Lucia AR, Gerard P, Etienne M, Alain E. The use of thermal analysis in assessing the effect of temperature on a cement paste. Cem Concr Res 2005;35:609–13. [27] Bumrongjaroen W. Utilization of processed fly ash in mortar [Ph.D. thesis]. New Jersey Institute of Technology; 1999. [28] Lawrence P, Cyr M, Ringot E. Mineral admixtures in mortars effect of inert materials on short-term hydration. Cem Concr Res 2003;33:1939–47.