Discussions About Incorporated Graphite On
Unitary/Binaries Matrices Geopolymeric
Systems: Part A
Madeleing Taborda Barraza*, Juliano Rodrigues Spínola, Laura Silvestro and Philippe Jean Paul Gleize
Department of Civil Engineering, Federal University of Santa Catarina, Brasil
Submission: October 14, 2022; Published: November 03, 2022
*Corresponding author: Madeleing Taborda Barraza, Department of Civil Engineering, Federal University of Santa Catarina, Brasil
How to cite this article: Madeleing T B, Juliano R S, Laura S, Philippe J P G. Discussions About Incorporated Graphite On Unitary/Binaries Matrices
Geopolymeric Systems: Part A . Trends Tech Sci Res. 2022; 5(4): 555669. DOI: 10.19080/TTSR.2022.05.555669
This work assessed the graphite incorporation on binary geopolymeric matrices, aiming to improve their mechanical performance. The graphite was previously added to the alkaline solution and dispersed via sonication. The geopolymeric pastes were cured for 8 days at 40°C and were evaluated through isothermal conduction calorimetry, compressive strength, flexural strength, open porosity, and X-ray diffraction. Comparatively, when graphite was added into a metakaolin-based system, the presence of the nanomaterial caused reductions of 34% and 55% in compressive and flexural strengths, respectively. Conversely, incorporating a graphite content of 0.1 wt.% on a system containing 30% fly ash and 70% metakaolin increased the compressive and flexural strengths by 166.0% and 184.0%, respectively. The results suggested that the effect of graphite addition is conditioned to the type of matrix where it is incorporated. The graphite incorporation possibly caused discontinuities in the systems with better performance (i.e., metakaolin-based), reducing their mechanical properties.
Geopolymer systems continue to be searched because they constitute an alternative binder for civil construction elements [1-3]. In research, it is common characterized unitary systems to understand them physical and chemical properties, manufacturing techniques and his durability [3-4]. However, it has been identified that unitary systems may be easier to register chemical and physical instability [5-6] and the incorporation of another phase may improve the system with alternative compounds and tourns the samples manufacture more ecological. For this reason, there is a greater appeal for the execution of binary geopolymeric systems [7-10]. Materials such as fly ash, blast furnace slag, rice husk ash, and waste from biomass burning or from industries are commonly used [11-12]. However, a replacement of precursors does not always mean improved mechanical performance. Specifically, the performance of each material depends on its reactivity, which varies from case to case. What determines the feasibility of use is the purpose of its application. Thus, it is possible to make use of binary systems with a regular mechanical performance that can guarantee reasonable durability.
Specifically, about geopolymers, even defined as polymer inorganic can register a ceramic response and from there, a series of typical disadvantages arise, such as fragility and crack propagation. In this sense, the incorporation of carbon-based nanomaterials proves to be a great complementary alternative to the system, mainly due to the benefits of compatibilization between nanomaterials and cementitious materials [13-15]. Many authors indicate that materials such as carbon nanotubes, silicon carbide whiskers and graphene oxide favor the mechanical performance of systems due to the effects they are capable of promoting [9,15], such as densification capacity (by the action of points of nucleation -seed effect)  and the contribution of mechanical properties inherent to the incorporation itself, either by increasing the mechanical resistance, thermal resistance, to chemical attack or by the excellent capacity of electric conductivity [14,17].
Despite the benefits, nanomaterials, when participating in
nanocomposites, need special treatment to unleash the potential
nature of each of them. The application of physical or chemical
techniques that guarantee good dispersion of these in the matrix
is necessary [18,19]. Thus, with the intention of developing
different unitary and binary geopolymer systems, the effects
that the introduction of graphite nanoparticles, dispersed by
ultrasonication, can cause on the mechanical performance of
nanocomposites based on fly ash and metakaolin in different
proportions are evaluated.
As material precursors, commercial metakaolin (MK) and fly
ash (FA) were used. MK was obtained from Metacaulim do Brasil,
São Paulo, Brazil, and FA from Candiota thermoelectric plant. The
chemical composition of each one is shown in table 1. MK and FA
mineralogical phases are illustrated on figure 1a, and the particle
size distribution is showed on figure 1b. For MK, mean particle size
was was 20,6μm whereas the mean particle size for FA was 4,62
μm. The X-ray spectrum of both materials defined minerals like
kaolinite (K), quartz (Q), hematite (H), Magnetite (G), Sulfate (P)
and of illite (I), both registered anamorphous phases associated
to its nature. The geopolimeric system was activated by sodium
silicate solution acquired from MQB S.A with silica modulus (Ms)
equal to 2,33 and NaOH, microbeads were get it from Neon LTDA.,
a product with 98% purity. At last, graphite powder (GRAP) was
purchased from XFnano Materials Company, China, with 3-9
nm thickness and 98% carbon content. XRD spectra and FTIR
chemical bonds are registered on figure 2.
Sample preparation began with the preparation of the activator
solution (A.S): dissolution of sodium hydroxide (HS) into solution
of sodium silicate (SS). The proportion was a modification from
[18,22]. The new mixed solution was cooled for 24 hours and then,
graphite at 0,1% was added to it, which received ultrasonication
energy to disperse the particles using a tip sonicator (model
VCX 750 W, Sonics & Materials Inc.) for 10 minutes. Plus, a wave
amplitude of 80% with 20s-cycles was applied. The aspect of
solution containing (nano)graphite is illustrated on figure 3a. To
prepare the paste, the activator (containing or not graphite) was
added on the precursor and mixed by 5 minutes using a highpower
mixer at 10,000 rpm. Then, the samples were cured at 40 °C
for 8 days. After demolding, samples presented the aspect showed
on figure 3b. Table 2 organizes the specification of unitary/binary
systems and the investigated proportions.
A fraction of the paste was put into a calorimeter by isothermal
Conduction (TA Instruments, New Castle) to register the kinetics
reactions. And to analyze the hardened state was molded into
prismatic shapes to measure the flexural strength and prismatic
compression strength (Instron 5569 machine), both performed
with a load application speed of 3000N/min and the open
porosity and apparent density was calculated using the ASTM
C20 -00(2022) procedure. Finally, the X-Ray diffraction (XRD)
was conducted on a X-Ray Diffractometer (Miniflex II Desktop,
Rigaku), operating at 30kV/15mA, 2θ from 5° to 65°, and 0.02°/s
In general, as shown in figure 4a, the intensity on heat
flow were proportional to the MK content and the addition of
graphite in the system. For instance, metakaolin-based system
with a graphite content of 0.1 wt%. (MK100+0.1%) exhibited a
higher heat flow values compared to the plain pastes (MK100).
Similarly, MK70/30FA +0.1% showed higher heat flow values
compared to MK70/30FA. Moreover, the hear flow curves of all
evaluated compositions are similar; most groups registered a
single peak associated with the dissolution of precursors in the
alkaline solution. Nevertheless, FA system could not develop any
peaks and a “stable” flow was maintained and recorded negative
cumulative heat (Fig. 4b). This phenomenon could indicate that
the heat curing (i.e., temperature) was not enough to promote the
reactions in this system and that it could need more time to react
or higher thermal energy of activation .
According to the literature, binary systems that incorporated
FA generally show a slow reaction kinetics on the first hours and
its showed by stability on heat flow at calorimetric analysis .
At the same time, systems that incorporate nanomaterials can
influence the intensity of heat flow peak and cumulative heat
due to the effect of nucleation that the nanoparticles provide or modified the setting time . This trend was observed for graphite incorporation on MK100 and MK30/70FA systems.
Nevertheless, the addition of graphite does not cause any effect on
cumulative heat of MK70/30FA system after 100 h.
Flexural and compressive strength were determined for 8
days. Three samples were tested by each group for bending and
four regarding prismatic compression. Figure 5a, 5b allows to
identify that the presence of 0.1% graphite generated a decrease
on both resistances regarding the MK100-based group, affirming
that this system could have consolidated a compact matrix that,
with the incorporation of the nanomaterial, led to a significant
increase in the fall of up to 42.30%. However, for systems
containing FA the presence of the nanomaterial increased the
flexural strength until to 223%. This can specify that do not exist
a unique optimal addition for materials groups due to this going
related to the type of matrix, and a matrix with low or medium
strength ends up registering more noticeable effects than exclusive
systems or with a predominance of more uniform precursors [26-
30]. Must be note that MK70/30+0,1% had opposite effects on the
flexural and compressive strengths. While decreased the flexural
strength by up to 37%, increased the compressive strength by
up to 63%, compared to plain geopolymer paste (MK70/30).
The inversion of effects on the two parameters allows us to
infer that the shape factor/orientation and its morphology can
improve almost one parameter and not the other. Author like 
registered this inversion in the values and attributed it to a poor
dispersion, however this justification would have caused decrease
in all systems but that not happen in this case.
It should be noted that although the accumulated heat
magnitudes have been obtained for the systems containing
graphite, the same did not follow for the mechanical behavior.
This finding could indicate that, even if exits an increase on
the parameter, due to the modifications on reaction’s kinetics
promote by the addition, this phenomenon is not enough to get
significant macrostructural modifications that will leave to an
increase in strength, such as  registered. An increase
in the percentage of addition could be considered in order to
manifest improvements in unitary systems [14, 16, 32]. Besides
that, different dispersion procedures and sonication parameters
can be evaluated to improve graphite dispersion and, thus, its
performance in MK-based systems.
The apparent porosity of the samples is shown in figure
6. Apparently, the graphite nanoparticles contributed to the
densification of the pastes once that similar or lower porosity
values compared to its reference systems were registered. In
addition, it can be highlighted that it was not possible to perform
the test in FA100-based group because the material shows
dimensional stability in contact with the water. The apparent
porosity values recorded in this study were higher for systems
containing exclusively FA  and higher for systems exclusively
based on MK . However, its downward trend agrees with the
literature [30,28]. Additionally, an increase in apparent density
can also be recorded [29,33].
In figure 7, the XRD spectra was recorded for all the samples,
and it can be observed that MK has typical amorphous halo in the
range 15 to 30° 2θ. After the activation, the displacement of this halo was registered in zones of greater angles near to the interval 20-40° 2θ [26-28] and the same way the FA activated develop a
bump broader than the FA anhydrous . These transformations
are characteristic of the geopolymeric gel. However, the peak
associated to carbon could be undercover by the intense peak
associated to graphite’s addition. It was not observed the
formation of any new phases or carbonates. This follows the
interaction between the precursors once that the intensity related
to one precursor started to disappear. At the same time the other
shows itself in a superior quantity within the system .
Geopolymers composed of FA and MK and with the addition of
a graphite content of 0.1 wt.% were produced. The nanomaterial
effect was conditioned to matrix characteristics. Even when
dispersion can be guaranteed in an alkaline solution (using highamplitude
energy sonication), they can act as discontinuous points in the high-performance matrix, so it can be specified that:
I. A definition of a unique optimal addition for materials
groups it’s not convenient for matrix with medium performance.
II. Significant changes in accumulated heat do not guarantee
an increase in mechanical behavior.
The authors are gratefully to the Brazilian government research
agencies, the National Council for Scientific and Technological
Development (CNPq) and the Santa Catarina Research Foundation
(FAPESC), for providing financial support for this research.