Effects of Different Nanomaterials on Hardening and Performance 1
of Ultra-High Strength Concrete (UHSC) 2
3
Zemei Wu
a,b
, Caijun Shi
a,*
, K.H. Khayat
b
, Shu Wan
a
4
a
College of Civil Engineering, Hunan University, Changsha 410082, PR China, 5
b
Department of Civil, Architectural and Environmental Engineering, Missouri University of Science and 6
Technology, Rolla, Missouri, USA 7
8
Abstract: Nanomaterials have attracted much interest in cement-based materials during the past 9
decade. In this study, the effects of different nano-CaCO
3
and nano-SiO
2
contents on flowability, heat 10
of hydration, mechanical properties, phase change, and pore structure of ultra-high strength concrete 11
(UHSC) were investigated. The dosages of nano-CaCO
3
were 0, 1.6%, 3.2%, 4.8%, and 6.4%, by the 12
mass of cementitious materials, while the dosages of nano-SiO
2
were 0, 0.5%, 1.0%, 1.5%, and 2%. 13
The results indicated that both nano-CaCO
3
and nano-SiO
2
decreased the flowability and increased 14
the heat of hydration with the increase of their contents. The optimal dosages to enhance 15
compressive and flexural strengths were 1.6% to 4.8% for the nano-CaCO
3
and 0.5% to 1.5% for the 16
nano-SiO
2
. Although compressive and flexural strengths were comparable for the two nanomaterials 17
after 28 d, their strength development tendencies with age were different. UHSC mixtures with 18
nano-SiO
2
showed continuous and sharp increase in strength with age up to 7 d, while those with 19
nano-CaCO
3
showed almost constant strength between 3 and 7 d, but sharp increase thereafter. 20
Thermal gravimetry (TG) analysis demonstrated that the calcium hydroxide (CH) content in UHSC 21
samples decreased significantly with the increase of nano-SiO
2
content, but remained almost 22
constant for those with nano-CaCO
3
. Mercury intrusion porosimetry (MIP) results showed that both 23
porosity and critical pore size decreased with the increase of hydration time as well as the increase of 24
nanoparticles content to an optimal threshold, beyond which porosity decreased. The difference 25
between them was that nano-CaCO
3
mainly reacted with C
3
A to form carboaluminates, while 26
*Corresponding author. Tel./fax: +86 731 8882 3937.
E-mail address: cshi@hnu.edu.cn (C. Shi)
© 2016. This manuscript version is made available under the Elsevier user license
http://www.elsevier.com/open-access/userlicense/1.0/
2
nano-SiO
2
reacted with Ca(OH)
2
to form C-S-H. Both nano-CaCO
3
and nano-SiO
2
demonstrated 27
nucleation and filling effects and resulted in less porous and more homogeneous structure. 28
29
Keywords: UHSC; Nano-CaCO
3
; Nano-SiO
2
; Hydration; Strength; Calcium hydroxide; Pore 30
structure 31
32
1. Introduction 33
Ultra-high strength cement based-material (UHSC) is a novel type of composite materials with 34
superior static and dynamic mechanical properties, and excellent durability. Such material can be 35
used in marine structures, defense and military engineering applications, and high building 36
construction [ 1 - 3 ]. However, as an intrinsically heterogeneous material, the structure of 37
cement-based materials can be generally discretized into four multi-scale phases: nano, micro, meso, 38
and macro [4]. The macro-properties of cement-based materials are dominated by the structure at the 39
nano-scale level. The main hydration product, C-S-H, occupies at least 60-70% by volume of the 40
hardened cement paste. It is a nano-scale material with average diameter around 10 nm [5]. It is 41
suggested that C-S-H has low, high, and ultra-high density forms with different hardness and elastic 42
modulus values and volume fractions [4,6]. High density C-S-H degrades much slower than low 43
density C-S-H under external environmental condition [6]. Furthermore, water loss from pores in the 44
C-S-H gel can lead to considerable autogeneous shrinkage, which can cause cracking and loss in 45
strength and durability of UHSC [7]. Therefore, it is vital to optimize the microstructure of 46
cement-based materials from the nano-scale to ensure high performance. 47
Nanotechnology has attracted much interest over the past decade. Since the introduction of 48
nanomaterials, extensive research has been conducted to promote their use in cement-based material. 49
3
It is well known that nanomaterials can provide significant enhancement in performance of 50
cement-based material given their physical effect (filling and nucleation effects) as well as the 51
chemical reactivity [8]. Nano-silica (nano-SiO
2
) [9], nano-alumina (nano-Al
2
O
3
) [10], nano-titanium 52
oxide (nano-TiO
2
) [11], nano-CaCO
3
[12], nano iron (Fe
2
O
3
), and nanotubes [13] have been studied 53
for use in cement-based materials. Among those, nano-CaCO
3
and nano-SiO
2
are commonly used. 54
This is because nano-CaCO
3
is relatively cheap due to abundant supplying of CaCO
3
in limestone, 55
chalk, and marble, and nano-SiO
2
can present superior performance given its high specific area and 56
pozzolanic activity [ 14 ]. Prototypes of limestone and silica fume have been employed in 57
cement-based materials for many years [15-17]. 58
Camiletti et al. [14] investigated the effects of nano- and micro-limestone on early age 59
properties of ultra-high performance concrete (UHPC) and found that the addition of nano- and 60
micro- limestone reduced its setting time. Moreover, the addition of 2.5% to 5% nano-limestone
61
could
lead to 32% to 75% improvement in 24 h compressive strength in comparison to that without 62
any nano-limestone. Shaikh et al. [12] found that samples incorporating 1% nano-CaCO
3
particles 63
showed the highest compressive strength for high volume fly ash concrete. Rong et al. [18] found 64
that nano-SiO
2
accelerated the hardening and enhanced mechanical properties of UHPC when 3% 65
nano-SiO
2
,
by mass of cementations materials, was incorporated. Ghafari et al. [19] reported that 66
nano-SiO
2
reduced the workability of UHPC and increased compressive strength, especially at early 67
age. Although both the nano-CaCO
3
and nano-SiO
2
could improve mechanical properties, their 68
hydration mechanisms, hardening processes, and age dependencies are different [20]. This could lead 69
to different hydration products and thereby change in mechanical properties [21-23]. Furthermore, 70
mechanical stirring and ultrasonic dispersion techniques are often adopted to avoid agglomeration of 71
nanomaterials [24]. However, the dispersion time and speed would contribute to the experimental 72
4
results, which are often neglected. If nanomaterials can be efficiently dispersed under normal mixing 73
procedure, this would not only facilitate their applications in cement-based materials but also reduce 74
energy consumption. 75
In order to understand the hydration mechanism and hardening process of UHSC made with 76
either nano-CaCO
3
or nano-SiO
2
, the flowability, heat of hydration as well as compressive and 77
flexural strengths of UHSC mixtures with five different contents of each nanomaterial were 78
investigated. The crystalline phases and pore structure of the samples were investigated by thermal 79
gravimetry (TG) and mercury intrusion porosimetry (MIP), respectively. The study seeks to 80
understand the hydration mechanisms of these types and contents of nanomaterials. 81
82
2. Experimental program 83
2.1. Materials 84
Portland cement complying with the Chinese Standard GB175-2007 was used [25]. The 3-d 85
compressive and flexural strengths of standard mortar sample are 28.3 and 5.6 MPa, respectively. 86
Silica fume with particle size rangeing between 0.02 and 0.28 μm was used. Nano-CaCO
3
and 87
nano-SiO
2
were used, as shown in Fig. 1. The nano-CaCO
3
has a size of about 15 to 105 nm with 88
97.8% calcite content. The nano-SiO
2
has a size of 5 to 35 nm with 99.8% SiO
2
content. Table 1 89
summarizes the chemical composition and physical properties of the cementitious materials. 90
5
91
(a) Nano-CaCO
3
particles [26] (b) Nano-SiO
2
particles [18] 92
Fig. 1 Morphogy of nano-particles 93
94
Natural river sand with a fineness modulus of 3.0 was used. Particles with size greater than 2.36 95
mm were removed by sieving in order to enhance mechanical properties of the UHSC. 96
A polycarboxylate-based superplasticizer (SP) with a solid content of 20% was incorporated. Its 97
water-reducing capacity is greater than 30%. The dosage of SP in the all mixture was set to 2%, by 98
mass of cementitious materials. This dosage corresponds to a well-dispersed system for the reference 99
mixture without any nano-material [27,28]. 100
101
Table 1 Chemical composition and physical properties of cementitious materials 102
Materials
Cement
Silica fume
Nano-CaCO
3
Nano-SiO
2
SiO
2
(%)
21.18
93.90
-
99.80
Al
2
O
3
(%)
4.73
-
-
-
Fe
2
O
3
(%)
3.41
0.59
0.02
-
SO
3
(%)
2.83
-
-
-
CaO (%)
62.49
1.85
97.90
-
MgO (%)
2.53
0.27
0.50
-
Na
2
O (%)
-
0.17
-
-
K
2
O (%)
-
0.86
-
-
Loss on ignition (%)
1.20
0.30
-
-
Surface area (m
2
/kg)
350
18,500
42,000
160,000
Specific gravity (kg/m
3
)
3140
2200
-
-
Particle size (nm)
36,700
20-280
15-105
5-35
Setting time (min)
Initial
172
-
-
-