International RILEM Conference on Material Science – MATSCI, Aachen 2010 – Vol. III, AdIPoC 265

INTERNAL CURING OF ULTRA HIGH PERFORMANCE CONCRETE BY USING RICE HUSK ASH

Nguyen Van Tuan, Guang Ye, Klaas van Breugel, Delft University of Technology, The Netherlands

ABSTRACT: Rice Husk Ash (RHA) is a treated agriculture waste with a porous structure. This may give the benefit of internal curing for concrete with low water to cement ratio. The objective of this paper is to study internal curing of Ultra High Performance Concrete (UHPC) provided by the addition of RHA. The result was compared to UHPC sample made with Silica Fume (SF). The experimental studies on microstructure, thermogravimetric analysis, autogenous shrinkage and the development of compressive strength were performed. The results show that, compared to the SF sample, the addition of RHA attributes the increased hydration, the absence of large empty pores (hollow shell or partly hollow shell pores), the less pozzolanic reaction, the mitigation of autogenous shrinkage and the enhancement of later age compressive strength of UHPC. This is suggested to be provided by internal curing of RHA in UHPC.

1 INTRODUCTION

Ultra High Performance Concrete (UHPC) refers to concrete with superior mechanical properties and very high durability due to its dense microstructure. Normally, the UHPC is made of coarse, fine and micro fine aggregates, silica fume, very low amounts of water, and high amounts of cement. However, the high cement content (900-1000 kg/m3) [Ric94, Ric95] and low water to binder ratio (0.14-0.20) [Hab08, Sch05] in UHPC will cause a high autogenous shrinkage, which is the consequence of chemical volume contraction during cement hydration and self-desiccation in concrete. Autogenous shrinkage will induce micro cracks at early ages. This may lead to reduced strength, decreased durability, loss of prestress in prestressing application and problems with aesthetics and cleanliness [Ben04].

In UHPC, silica fume (SF) is an essential ingredient with extreme fineness and high amorphous silica content. It plays a very important role with physical (filler, lubrication) and pozzolanic effects. The typical SF to cement ratio used in UHPC is 0.25 [Ric94] to achieve both these effects. However, the increase of SF content would also lead to a finer pore structure and increase self-desiccation deformation [Ben04]. Therefore the high amount of SF used to make UHPC will result in a significant autogenous shrinkage of this concrete.

Recently, a new technique of internal curing has been proposed to mitigate the autogenous shrinkage of UHPC, so-called water entrainment or using fine superabsorbent polymer (SAPs) particles as a concrete admixture [Jen01, Jen02]. It has been shown that the technology also works in practice [Dud08]. However, the average diameter size of SAPs is about 200 μm, which is relatively big compared to the size of fine sand. During cement hydration, the drop of internal relative humidity will be compensated by the water from SAPs. This means that SAPs gradually leave big voids in concrete. Moreover, due to a small amount of SAPs used in UHPC, 0.4% by mass of cement, if not dispersed well in concrete, it can easily make the concrete heterogeneous. This will have a negative effect on the properties of UHPC.

266 NGUYEN, YE, VAN BREUGEL: Internal Curing of UHPC by Using Rice Husk Ash

The disadvantages of both SF and SAPs in producing UHPC gives the motivation for searching for other materials with similar functions, especially in the developing countries because of the limited available resource and the high cost of these materials. One possibility is to use rice husk ash (RHA), which is an agricultural waste. RHA obtained after complete combustion of the husk in controlled conditions contains 90-96% silica in amorphous forms. SF and RHA have been classified as “highly active pozzolans” [Meh83]. The average particle size of RHA ranges in general from 5 μm to 10 μm with a very high specific surface area (even more than 250 m2/g) [Bui01]. This surface comes from the porous structure of RHA [Coo96,Meh92]. This allows RHA to absorb a large amount of water inward to reduce bleeding water [Hwa89]. With respect to autogenous shrinkage, this water may be released during the cement hydration process to mitigate the self-desiccation phenomenon. The effect of RHA on reducing autogenous shrinkage of cement paste is described in [Sen08].

The objective of this paper is to study the internal curing of UHPC provided by the addition of RHA. Experimental investigations on the microstructure, thermogravimetric analysis, autogenous shrinkage and the development of compressive strength were evaluated.

2 MATERIALS AND METHODS

2.1 Materials

The materials used in this study were silica sand with a mean size of 225 μm, Portland cement CEM I 52.5 N, condensed silica fume, rice husk ash, and polycarboxylate based superplasticizer with 30% solid content by weight. The SF possesses an amorphous SiO2 content of 97.2% and its mean particle size of about 0.1 - 0.15 μm.

Rice husk, an agricultural waste material from Vietnam, was burnt in a drum incinerator developed by Pakistan Council of Scientific & Industrial Research [UNI17] under uncontrolled combustion conditions. The obtained ash was ground in a vibrating ball mill for 90 minutes. The ash contains 88.0% amorphous SiO2. The loss on ignition is 3.8% and its mean particle size (dRHAmean) is 5.6 µm, determined by laser diffraction.

Fig. 1 shows SEM images of RHA and SF. It can be seen that RHA is porous (Fig. 1a-c). Around the coarse particles, RHA also contains ultra fine particles with the same size as SF particles (Fig. 1c).

International RILEM Conference on Material Science – MATSCI, Aachen 2010 – Vol. III, AdIPoC 267

Fig. 2.1. SEM images of (a) RHA before grinding, (b-c) RHA after grinding and (d) SF

2.2 Mix compositions

The UHPC mix compositions used in this study were prepared as shown in Table 2.1. The binder herein is the sum of the cement and the SF and/or the RHA. The amounts of superplasticizer were used to keep the workability of UHPC mixtures between 210 and 230 mm, measured by means of flow table test. For each cement replacement percentage, RHA samples were compared to SF samples.

Table 2.1. UHPC compositions used in this study

Mix No. Water to binder ratio, by weight

Sand to binder ratio, by weight

RHA (% by weight)

SF (% by weight)

Superplasticizer (solid % by weight of binder)

RHA10 0.18 1 10 0 1.00 RHA20 0.18 1 20 0 1.20 SF10 0.18 1 0 10 0.63 SF20 0.18 1 0 20 0.76

2.3 Experimental methods

The experimental program designed to study the effect of RHA on the autogenous shrinkage of UHPC focuses on the autogenous deformation, degree of hydration, compressive strength and the microstructure. The autogenous shrinkage was measured following the ASTM standard [AST09], in which three sealed corrugated moulds of 440 mm Ø28.5 mm were determined for each mix composition. The average results were used to present in this paper.

5 µm

Clusters2 µm

(c) (d)Ultra fine particles

20 µm 20 µm

(a) (b)

268 NGUYEN, YE, VAN BREUGEL: Internal Curing of UHPC by Using Rice Husk Ash

All materials were prepared in a 20-liter Hobart mixer. The volume of each batch was 3.5 liters. After mixing, each mixture was firstly carefully filled into three sealed corrugated tubes to determine the autogenous shrinkage.

Secondly, mixtures were cast into the 40×40×40 mm cubes for the compression tests. All mixtures were vibrated for 1 minute using a vibrating table with a frequency of 2500 cycles/min. After casting, samples were cured in the moulds in a fog room (20±2oC, R.H. > 95%) for one day. After de-moulding the samples were stored in the fog room until the day of testing.

For microstructural studies and thermal analysis, mixtures were poured into 500 ml plastic bottles and then sealed with a plastic lid. The samples were cured at 20oC until the age of testing. At the end of each curing period, the plastic bottle was removed. Samples were cut into small pieces of about 1 cm3 by saw with cooling water. The hydration of all samples was stopped by liquid nitrogen at temperature of -195oC freeze drying method. After drying, samples were separated into three groups for microstructural investigations including Mercury Intrusion Porosimetry (MIP), Environmental Scanning Electron Microscopy (ESEM), and determination of calcium hydroxide content by Thermogravimetric analysis (TGA)

2.3.1 Microstructural investigation

MIP The porosity and pore size distribution of samples was evaluated by means of MIP. For each test, 2 or 3 samples with a total weight of approximately 4-6 g were used. The contact angle of mercury and surface tension used were 139o and 480 mN/m. Details of method are given elsewhere [Ye03]. Samples were tested at 7 days and 28 days.

ESEM Microstructural analysis, i.e. phases identification, the volume of each phase and morphology, were conducted on Philips-XL30-ESEM in a gaseous (water vapor) environment. The acceleration voltage of 15-20 kV was applied. The magnification of 540× was chosen for analysis, the backscattered electron images (BSE) size was 1424×968 pixels. For each sample, 12 images were captured in order to achieve the acceptable confidence of results [Ye03]. Samples were tested at the ages of 1, 3, 7, 28 and 91 days.

According to stereology analysis [Dia95], with enough sampling, the volume fraction can be assumed to be equal to the area fraction in BSE images. With the volume fraction of unhydrated Portland cement, the degree of cement hydration can be calculated.

2.3.2 Calcium hydroxide content Calcium hydroxide (CH) content of UHPC samples was determined by TGA. The weight of each sample after grinding was in the range of 70 and 80 g. The TG runs were dynamic at 10oC/min from 35oC to 1100oC in a flowing nitrogen atmosphere. The CH content from weight loss curve during thermal analysis was determined by a graphical technique used previously [Mar88,Hai02]. Samples were tested at the ages of 0.25, 1, 3, 7, 28 and 91 days.

International RILEM Conference on Material Science – MATSCI, Aachen 2010 – Vol. III, AdIPoC 269

3 RESULTS AND DISCUSSIONS

3.1 Microstructural investigation

3.1.1 Degree of cement hydration of UHPC by image analysis

Fig. 3.1-3.2 shows the BSE images of the samples incorporating RHA and SF.

It is observed that the hollow-shell pores (Hadley grains) [Kje97, Had00] only appear in SF sample, not in the RHA sample. The cement grains form hollow shells with remnant anhydrous cores, which are marked with arrows (Fig. 3.2).

The percentage of unhydrated Portland cement in pastes of UHPC are depicted in Fig. 3.3, and the degree of cement hydration is shown in Fig. 3.4. Generally, the degree of cement hydration in UHPC samples is relative low, around 0.40 after 91 days. The degree of cement hydration in RHA sample is significant at later ages, i.e. after 7 days, even higher than that in the SF sample at 91 days (Fig. 3.4).

Fig. 3.1. BSE image of the 0.18 w/b ratio paste

in RHA sample at 28 days; black crosses indicate unhydrated cement; white circles indicate RHA particles, dRHAmean = 5.6 μm

Fig. 3.2. BSE image of the 0.18 w/b ratio paste

in SF sample at 28 days; black crosses indicate unhydrated cement; white arrows indicate hollow-shell pores

35.5

1

37.7

0

31.1

2

31.9

3

29.1

6

28.4

0

30.4

4

30.1

2

0

5

10

15

20

25

30

35

40

45

SF20 RHA20Samples

% A

rea

of u

nhyd

rate

d ce

men

t in

past

e of

UH

PC

1 day 7 days

28 days 91 days

Fig. 3.3. The percentage of unhydrated cement area in paste of UHPC at 1, 7, 28 and 91 days, dRHAmean = 5.6 μm

912870.20

0.24

0.28

0.32

0.36

0.40

1 10 100Time [days]

Deg

ree

of c

emen

t hyd

ratio

n

Fig. 3.4. The degree of cement hydration in UHPC vs. time, dRHAmean = 5.6 μm

SF20

RHA20

SA

ND

X X

X

X

X

o

o o

o

X

X

X

SA

ND

X

X

X

X

X

X

X

X

X

270 NGUYEN, YE, VAN BREUGEL: Internal Curing of UHPC by Using Rice Husk Ash

3.1.2 Pore structure Pore size distribution and total porosity of UHPC samples at 7 and 28 days is presented in Fig. 3.5. It is observed that the effect of RHA on pore refinement of UHPC is less than that of SF. The maximum pore diameter of the pore size distribution of samples is below 0.03 μm at both ages. The total porosity of both RHA and SF samples is about 5% at 28 days.

0.00

0.02

0.04

0.06

0.08

0.10

0.001 0.01 0.1 1Pore diameter (µm)

dV/d

logD

(ml/m

l)

0.00

0.02

0.04

0.06

0.08

0.10

0.001 0.01 0.1 1Pore diameter (µm)

dV/d

logD

(ml/m

l)

Fig. 3.5. Pore size distribution and total porosity of UHPC samples at (a) 7 days, (b) 28 days, dRHAmean = 5.6 μm

3.2 The calcium hydroxide content

Fig. 3.6 shows the calcium hydroxide (CH) content in UHPC samples with time. It can be seen that both RHA and SF strongly reduce the CH content. Combined with the result of cement hydration by BSE image analysis, the low CH content in the RHA sample at the beginning (1-3 days) is caused by the slow cement hydration in RHA sample. The effect of RHA on reducing CH content is clear after 7 days. Meanwhile, with SF sample, after reaching a maximum (between 1 and 3 days) the CH content strongly decreases. Apparently, the RHA consumes less CH than SF beyond 7 days. This implies that the pozzolanic reaction of RHA-CH is less pronounced than that of SF-CH.

3 7 28 910.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

1 10 100Time (days)

Ca(

OH

)2 c

onte

nt (g

/g c

emen

t)

Fig. 3.6. The calcium hydroxide content of UHPC samples with time

SF20 (6.49%)

RHA20 (7.63%)

(a)

SF20 (4.55%)

RHA20 (5.76%)

(b)

SF20

RHA20

International RILEM Conference on Material Science – MATSCI, Aachen 2010 – Vol. III, AdIPoC 271

3.3 Early age autogenous shrinkages

Fig. 3.7 shows the curves for the development of autogenous shrinkage of UHPC incorporating different amounts of RHA (or SF) over time obtained during the first 28 days following the final set of UHPC mixtures. Generally, all samples show dramatic autogenous shrinkage at a very early age, indeed in the first twelve hours from final set, and then remain more or less stable afterward.

It can be observed that when more RHA is added, the autogenous shrinkage of UHPC is significantly reduced. After the first twelve hours from final set, both RHA10 and RHA20 samples show only a very minor increase in autogenous shrinkage. However, after reaching the age of 10 days, the autogenous shrinkage of RHA samples was further mitigated. Especially, the autogenous shrinkage of 20% RHA sample was even eliminated after 15 days. Meanwhile, it was not surprising that the autogenous shrinkage of SF samples increases with an increase of SF amount, also suggested by [Ben04, Sel92, Lur03].

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28Time (days)

Aut

ogen

ous

shrin

gkag

e [m

m/m

]

Fig. 3.7. Autogenous shrinkage of UHPC incorporating different amounts of RHA (or SF) measured from final set, dRHAmean = 5.6 μm

3 7 28 9150

70

90

110

130

150

170

190

210

1 10 100Time (days)

Com

pres

sive

stre

ngth

(MPa

)

Fig. 3.8. Strength development of UHPC samples vs. time, dRHAmean = 5.6 μm

3.4 Compressive strength

To investigate the effect of RHA on compressive strength and microstructure of UHPC, only RHA with a mean particle size of 5.6 μm was chosen.

Fig. 3.8 shows the result on compressive strength of UHPC versus time. It is observed that the development of compressive strength of the RHA sample is more pronounced than that of the control sample, even the SF sample at the later ages, i.e. after 7 days. It should be noted that the compressive strengths of both samples were about 170 MPa and 185 MPa at the ages of 28 and 91 days, respectively.

3.5 Discussion

Compared to the SF sample, with the same amount of cement, the evolution of autogenous shrinkage of RHA sample is different. The addition of SF increases the autogenous shrinkage but vice versa in case of the RHA. This may be attributed to the pozzolanic reaction, the pore refinement, and the internal curing, which will be discussed in the following.

RHA10

RHA20

SF10 SF20 SF20

RHA20

272 NGUYEN, YE, VAN BREUGEL: Internal Curing of UHPC by Using Rice Husk Ash

Firstly, the experimental results above show that the pozzolanic reaction RHA-CH occurs in the later stage and the effect of RHA is less pronounced than that of SF on reducing CH content (Fig. 3.6). In fact, not only hydration of Portland cement but also the secondary reactions are accompanied by a chemical shrinkage. In flowable status, the chemical shrinkage of cement paste is totally converted into an external volume change. However, when cement paste starts hardening, the chemical shrinkage gives rise to the formation of water-air menisci. This causes a decrease of the relative humidity, and self-desiccation occurs in the cement paste [Ben04, Lur03]. It should be kept in mind that the pozzolanic reaction of SF may be much less sensitive to a lowering of the RH than the cement clinker mineral reactions [Jus92]. Besides, the pozzolanic reaction of SF and CH involves a chemical shrinkage of about 22 ml/100 g SF reacted [Jen01], whereas the chemical shrinkage of cement is 6-7 ml/100 g (cited in [Lur03]). Therefore, with the similar chemical composition of SF and RHA, the pozzolanic reaction of RHA will give a positive mitigation on autogenous shrinkage of UHPC compared to that of SF sample.

Secondly, the addition of RHA refines the pore structure of UHPC less than that of SF (Fig. 3.5). This can be understood because SF particles (0.1-0.15 μm) are much finer than cement (10-15 μm) and RHA (5.6 μm). As discussed, these SF particles create a very fine pore structure within the hydrating cement paste, which leads to an increase of self-desiccation shrinkage, especially for UHPC with very low w/c ratios. This means that the addition of RHA with a bigger particle size will give a smaller autogenous shrinkage of UHPC than that of SF.

The result of the cement hydration by image analysis (Fig. 3.4) shows that the RHA stimulates cement hydration at the later ages. The isothermal calorimetry results in the previous work [Ngu10] also show this tendency. The addition of RHA gives a smaller relative humidity drop in the paste over time [Sen08]. These can be explained by the porous structure of RHA. The pores in RHA particles (Fig. 2.1b-c) may act as "water wells". RHA allows water to penetrate and store in its pores during mixing [Hwa89]. Later the water is released from these pores when the relative humidity in the paste decreases with progress of cement hydration process. This not only compensates the internal relative humidity drop of concrete but also increases the hydration degree of blended cement. Additionally, the water entrainment of RHA will mitigate the self desiccation shrinkage, and hence reduces the autogenous shrinkage of concrete and the large hollow shell pores. This mechanism is similar to that proposed by Van Breugel [Bre99] when using saturated lightweight aggregates or proposed by Jensen and Hansen [Jen01,Jen02] when using SAP particles for internal curing of concrete. Besides, the increased hydration (Fig. 3.4) and absence of hollow shell pores in RHA samples (Fig. 3.1) may both contribute to the increase of the later age strength (Fig. 3.8). This is also attributed to the internal curing of RHA particles. The similar effect can also be seen in [Ben08] when pre-wetted lightweight fine aggregates were used in high performance mortars incorporating SF, slag, and Class F fly ash.

These analyses shows that the RHA not only improves the compressive strength, but also mitigates the autogenous shrinkage of UHPC compared to the SF due to the internal curing, the lower pozzolanic reaction, and less pore refinement.

More research is still needed to understand the effect of RHA on autogenous shrinkage of cement-RHA blends, especially to investigate whether the decrease in autogenous shrinkage is due to a swelling behavior or to a sudden increase in the paste rigidity, both of which are related to the pozzolanic reaction.

International RILEM Conference on Material Science – MATSCI, Aachen 2010 – Vol. III, AdIPoC 273

4 CONCLUSIONS

The following conclusions can be drawn from the results of this investigation with regard to the internal curing of UHPC provided by the addition of RHA:

• The addition of RHA contributes to the increase of hydration, the absence of large empty pores, the mitigation of autogenous shrinkage and the enhancement of later age compressive strength of UHPC. This is suggested to be due to internal curing of RHA in UHPC.

• From this study, RHA can compensate the disadvantages of both SF and SAPs in producing UHPC. As an agriculture waste, RHA can gain benefits from both technical and environmental points of view.

ACKNOWLEDGMENT

The principal author would like to express gratefulness for the PhD scholarship sponsored by The Vietnamese Government. The supply of materials from ELKEM, ENCI, FILCOM and BASF companies for this research is highly appreciated. The authors would like to thank G. Nagtegaal for helping to take the measurements.

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