Lukuttsova N., Matveeva
E., Pustovgar A.
Bryansk State Academy of
Engineering and Technology, Bryansk, Russia
Nanomodified fine-graned concrete
The purpose
of the given work is the research of the additive "NANO-F" containing
stabilised nanosilica and its influence on structural and strengthening
characteristics of fine-grained concrete (FGC).
The term
“nanodispersed silica” combines diverse varieties of dispersed silica (sols,
gels, suspensions, pastes) found naturally (quartz, opal, chalcedony), or created
by man in the process of technological activity (Aerosil, hydrosols).
Nanodispersed silica is an important natural feature and the main component of
the oxide materials obtained by sol-gel method. The most interesting and
important representatives of nanodispersed silica are sols (dispersed systems
with liquid dispersive medium and solid
disperse phase), which particles are involved in Brownian motion.
This work
resulted in obtaining the additive "NANO-F" and the research on how
the age of additive influences the strengthening characteristics of FGC on
Portland cement M 500 D0, produced at JSC op. “Maltsovsky Portlandcement”,
quarts sand MK = 1,2 and cement-water ratio = 0,38 was carried out. Researches on dispersion processes and
aggregation of silica particles due to different age of the additive were
conducted. The nanostructure additive in the amount of 10% was input with a
solution of a stabilizer into a concrete mixture, where the content of
nanoparticles of silica makes 0.23 %.
The regulation of a concrete mixture mobility was performed with
supersoftener Ñ-3 in the
amount of 1 % from cement mass.
The size of
silica paticles in the developed additive was determined with foton-correlation
spectroscopy (PCS) of quasielastic scattered
light (QELS) by means of multiangular system for determining particles
sizes 90 Plus/ Bi-MAS. MAS-OPTION is an automatic system for fixing sizes
particles of used either for concentrated suspensions of small particles or for
the sizes of marcoparticles. Raster ionic-electronic microscope Quanta 200 3D
was used for studying microstructure. The X-ray-analysis was conducted on
diffractometer ARL X’TRA of the firm Thermo Scientific (Switzerland); the
thermal analysis with thermoanalyzer SETARAM LABSYS – by methods of TGA and DSC
at t=600ºÑ and heating
speed of 10˚/min; researches of samples porosity - by means of mercury
AutoPore IV 9500, which make it possible to measure pores in diameter in a
range from 0.0055 to 360 microns.
Additive
synthesizing was carried out by a chemical method of polycondensation [1]. Silicic acid sol is a noncrystalline
condensation nanodisperse structure from metastable solutions. Silicic sol is
characterised by aggregative instability at the change of temperature [2].
To find out
changes of nanosilica particles size in the developed additive due to their
aggregation in course of time they were examined at the of 1, 3, 5, 7,10 and 14
days.
Dependence
of ultimate compressions strength influence of FGC on nanosilica content in the
additive at the age from 1 till 14 days is given in Table 1.
Table 1 Ultimate
compression strength of fine-grained concrete depending on the nanosilica
particles content
|
¹ |
Composition of the FGC |
Ultimate
compression strength after 28 days of hardening, MPa |
The content of nanosilica particles size 20-100 nm, % |
|
1 |
Control
composition |
29 |
- |
|
2 |
FGC+ sol aged 1 day |
54 |
6 |
|
3 |
FGG + sol aged 3 days |
58 |
94 |
|
4 |
FGG + sol aged 5 days |
59 |
92 |
|
5 |
FGG + sol aged 7 days |
56 |
94 |
|
6 |
FGG + sol aged 10 days |
57 |
90 |
|
7 |
FGG + sol aged 14 days |
56 |
56 |
|
8 |
FGG+sol aged 4months |
- |
18 |
As we see
from the Table 1, the nanosilica content in the additive after 3 days increases
from 6 to 94 %, and after 14 days decreases to 56 %. The maximum value of FGC
compression strength is reached while using silicic sol at the age from 3 till
14 days, it is in 2 times exceeds the value of ultimate compression strength of
test samples.
Increase in
strength of FGC is also caused in our opinion due to the presence of
stabilizing agents- the acetates-ions preventing the aggregation of silica
particles. In the interaction of calcium hydroxide with sodium acetate calcium acetate is formed [5]. According to
[3], calcium acetates and others calcium salts refers to a group of the
additives, joining with the binder in addition reactions forming almost
insoluble mixed salts - hydrates. Calcium acetate, in its turn, interacts with
major cement clinker minerals to form almost insoluble mixed calcium salts.
At the initial stage of hydration
crystallization speed of mixed
insoluble calcium salts are above the speed of ettringite formation.
Crystals of these formations have microreinforcing effect on a cement stone
raising its density. Nanosilicaparticles in
combination with forming insoluble calcium salts directly participate in formation of cement
stone structure, being built in hydrates structure and filling a pores, and,
thereby, raising density of concrete [3]. It also leads to formation of a
primary skeleton, which provides the
process of increasing strength of a cement stone at early stages of hardening.
Nanosilica directly participate in the process of structure formation of a
cement stone.
Gradual
formation of ettringite additionally microreinforces the structure of a cement
stone. Calcium hydrosilicates differ in uniform submicrocrystalline structure,
that also helps to increase strength [4]. At the age of 14 days in samples with
nanosilica recrystallization of ettringite in monohydrosulfaluminate is
excluded, because of Ñ3À deficiency
in the system, caused by its linkage into insoluble calcium salts.
To study the
character of influence of developed additive "NANO-F" on the
formation feature of FGC structure X-ray and the thermal analysis of samples
were carried out (Figures 1,2).
As a result
of the conducted qualitative and quantitative phase analysis according to
Ritveld’s method 14% decrease in intensity of the portlantide reflexion in
modified samples (4.91 ; 2.63; 2.75; 2.70 Å), 8% intensity increase of
ettringite reflexion (9.81; 3.86; 2.57;
5.62 Å) 18% increase of CSH reflexion (12.6; 11.84; 10.2; 3.07 Å),
in comparison with the control sample in which as opposed to modified one the
reflexion of monohydrosulfaluminate (8.93 Å) is fixed, it indicates the partial recrystallization of ettringite.
It is known,
that for Ñà (OH) 2
the friable structure which is a slaty hexagonal lattice constructed of
three-layer packages is typical. It is typical for Ca (OH)2 to have
large tetrahedronic cavities, into which not only atoms Si can get (to 25 % of
lump SiO2), but also larger ions Àl, Fe. At the same time firm
compounds in which the atom of one element (Si) does not replace completely
atoms Ñà are formed
and located in intervals between them in free cavities. Atoms Si introduced
into structure change the position of atoms of oxygen which results in
enlargements of peaks Ñà (OH)2 that
is observed in the XRD of the modified sample.
The thermal
analysis spent in common by methods DSC and ÒGA shows that for the received
curves general laws are typical at heating. (Figure 3). In the range of temperatures 475 - 500 °Ñ on the
curve ÒGA the
second step of weight loss is observed , on the curve DSK - corresponding to it
endothermal effect with a maximum is observed at temperature 486°Ñ. That
corresponds to the loss of crystal waters by compounds 3CaO·SiO2,
2CaO·SiO2 of a cement stone.
Figure 1. X-ray-analysis of FGC control simple
Figure 2. X-ray-analysis of modified sample
For all
samples there is a strongly pronounced endothermal effect with a maximum at
temperature 575°Ñ. At integration
of peaks, typical for Ñà (OH)2,
on curves TGA and DSC insignificant decrease in thermal effect on 14 %, and
also weight losses - on 25 % for the modified sample in comparison with control
one was revealed.
At the same
time it is necessary to notice, that at complex use of the additive and
supersoftener Ñ-3 the
formation of more dense structure of FGC, as well as the decrease in its
defectiveness (Figure 4) is observed.
With the
introduction of modifying additives in fine-grained concrete total pore volume
decreases from 0.0849 to 0,0687 mg/l, and average pore diameter - on 3.2
microns to 0.83 microns.
Thus, the
results of this research can draw the following conclusions.
1.
The sizes of nanosilica particles of
investigated additive depend on its age and vary from 30 nm to 540 nm, and the
content of particles 20-100 nm in size from 3 to 10 days is 90 ... 94%, so it is most effectively to
use it during this period
2.
Integrated use of modifying agent and
supersoftener Ñ-3 in FGC results not only in modification of the structure of cement stone by nanosilica,
but also in the formation of insoluble mixed salts which fill the pores of
fine-grained concrete. It helps to decrease the average diameter of pores from
3.2 to 0.83 microns and to redistribute pores according to their sizes towards
their reduction and also provide the
improvement of structural and strengthening parametres of FGC – obtaining
of samples with compression strength, in 2 times exceeding the value of ultimate compression strength
of control samples.
a b
Figure 3. TGA and DSC curves of: a- control sample,
and b- modified sample
.
a b
Figure 4. Microstuñture of FGC samples: a- control
samples, b- the modified samples.
References:
1.
N.Shabanova,
P.D.Sarkisov, Sol-gel technology of nanosilica, Moscow, 2004 (in
Russian).
2.
Yu. Frolov, Silicic acids: obtaining and
application of silica hydrosols, Moscow ,1979 (in Russian).
3.
V. Ratinov, T.Rosenberg. Additives in concrete, Moscow,
1989 (in Russian).
4.
P.Sadykov, Z. Estemesov, B. Dusipov, Features of hydration of the
cements containing dispersive polymeric powders. Journal: Technologies of
concrete, pp.68-69, Moscow, 2008 (in Russian).
5.
N. Lukuttsova, E. Matveeva, Nanomodified
fine-grained concrete. N 3 Scientific and technical Journal MSSU,
pp.84-102, Moscow, 2009 (in Russian).