Evaluation on the Surface Modification of Recycled Fine Aggregates in Aqueous H2SiF6 Solution
International Journal of Concrete Structures and Materials
Evaluation on the Surface Modification of Recycled Fine Aggregates in Aqueous H2SiF6 Solution
Recycled aggregates (RAs) production techniques are essential for the material circulation society because RAs from demolished concrete waste can sustainably be reused as a concrete material. However, RAs can bring about several performance decreases when they are used for recycled aggregate concrete (RAC) because of the low qualities (i.e., high water-absorption rate and low density) caused by the attached hydrated cement paste on the RA surface. Therefore, both the production of high-quality RAs and the surface modification of RAs are significantly important for the extension of RAC utilization. This paper focuses on the surface modification of RFA to reduce the water absorption rate and increase density. Hydrofluorosilicic acid (H2SiF6), which is one of the by-products in phosphoric acid manufacture, is used herein for the surface modification of the RFA. The physical properties and mechanical performance of mortar using RFA were evaluated after RFA modification. Consequently, the proposed method is effective in reducing water absorption rate and increasing density of RFA. The density of RFAs was slightly increased by 0.5-2.6% after modification. On the other hand, the water absorption rate decreased by 4-18% after modification. The compressive strengths of mortar at 28 days ages showed 18.1 MPa with modified RFA and 16.2 MPa with RFA.
recycled aggregate; recycled fine aggregate; surface modification; hydrofluorosilicic acid; mortar
The current annual industrial waste in South Korea
amounts to approximately 130 million tons, with more than
50% of that being construction waste such as excess
concrete. Specifically, the amount of concrete waste comprises
70% of construction waste, which is approximately 40% of
all industrial waste (46 million tons). More than 90% of
concrete waste, or 40 million tons each year, is produced as
recycled aggregates (RAs) after going through recycling
processes. The reproduced RAs are mainly reused as
subbase materials for roads, roadbed materials, and RAs for
(Noguchi et al. 2015; Shi et al. 2015)
. The quality
of RAs produced from waste concrete via processes such as
crushing, rubbing, smashing, and heating varies, and
standards dependent on the physicochemical properties of the
aggregates have been established for various countries as
shown in Table 1
(AS 1996; DIN 4226–100 2002;
Construction standard 2013; BS 812: Part 2 1994; JIS A
5021 2005; JIS A 5022 2007; JIS A 5023 2007; KS F 2573
1999; RILEM TC 121-DRG 1994; Spanish Minister of
Public Works 2008)
The current standardized quality grades of RAs are
divided according to aggregate absorption rate and specific
gravity, and they are classified into structural concrete
aggregates, non-structural concrete aggregates, roadbed
materials, and so on, based on these grades as shown in
(Shi et al. 2015; Behera et al. 2014)
. In general, the
reason that RAs are lower in quality than natural aggregates
is because mortar adheres to the surface of RA (cement
matrix). Low-quality RAs having a high absorption rate and
low weight have limitations in terms of being reused for
(Behera et al. 2014; Choi et al. 2014; Ismail and
. Accordingly, most RAs are not used as
concrete aggregates, but as roadbed and sub-base materials;
however, this demand is gradually decreasing, while the
demand for their use in concrete is continuously required. In
addition, regarding RAs for roads and landfills, there are
concerns about alkali extraction due to Ca(OH)2, which is
contained in adhered mortar (cement matrix) on the surface
of aggregates, and also other long-term problems of
(Shi et al. 2015; Chen et al. 2017; Tam
et al. 2007)
For the sustainable recycling of resources, in order to
recycle the waste concrete that is produced in large
quantities, not only an active use, but also an eco-friendly use of
RAs is important
(Choi et al. 2014; Ismail and Ramli 2014;
Chen et al. 2017; Tam et al. 2007; Kong et al. 2010)
requires taking measures to simultaneously address the
problems of low weight, high absorption rate, and alkali
extraction that RAs currently have, and also requires the
quality improvement of aggregates for use in concrete
et al. 2010; Al-Bayati et al. 2016; Zhan et al. 2014)
There have been numerous studies reporting on surface
modifications of RAs using carbonation and acid treatment,
focusing on the fact that mortar adhered to the surface of
RAs (cement matrix) contains profuse Ca(OH)2
(Chen et al.
2017; Tam et al. 2007; Kong et al. 2010; Al-Bayati et al.
2016; Zhan et al. 2014)
. The surface modification with
carbonation is a method that uses accelerated carbonation,
which reacts highly concentrated carbon dioxide with
Ca(OH)2 of the adhered mortar on the aggregate surface
under dry or wet environments
(Zhan et al. 2014; Zhang
et al. 2015)
. The mechanism of surface modification used for
RAs by accelerated carbonation is described below.
Ca(OH)2 þ CO2 ! CaCO3 þ H2O
C S H þ CO2 ! CaCO3 þ SiO2 nH2O
On the other hands, the hydration products of cement in
hardened paste can be dissolved in acid solution
(Shi et al.
2015; Tam et al. 2007; Al-Bayati et al. 2016)
. Thus, acidic
solution can be used to remove the adhered mortar
effectively and enhance the quality of RA. Phosphoric acid,
Sulfuric acid and hydrochloric acid are used in a typical acid
treatment. The mechanisms of surface modification used for
RAs by acid treatments are described below.
Reactions under HCl:
CaO þ 2HCl ! CaCl2 H2O
Fe2O3 þ 6HCl ! 2FeCl3 3H2O
Al2O3 þ 6HCl ! 2AlCl3 3H2O
Reactions under H2SO4:
CaO þ H2SO4 ! CaSO4 H2O
Al2O3 þ 3H2SO4 ! Al2ðSO4Þ3 3H2O
Fe2O3 þ 3H2SO4 ! Fe2ðSO4Þ3 3H2O
Reactions under H3PO4:
2CaO þ H3PO4 ! 2Ca2þ þ Hþ þ PO34 þ 2OH
Al2O3 þ 2H3PO4 ! 2Al3þ þ 3Hþ þ 2PO34 þ 3OH
Fe2O3 þ 2H3PO4 ! 2Fe3þ þ 3Hþ þ 2PO34 þ 3OH
In the ‘‘mechanical’’ first processing stage, RAs are
produced and then subjected to the ‘‘chemical’’ second
processing stage, in which carbonation and acid treatment are
applied. Ultimately, both methods improve the physical
properties of RAs. Ca(OH)2 within the adhered mortar is
converted to CaCO3 by carbonation, and the porosity of the
adhered mortar decreases, leading to an increase in the
specific gravity of aggregates and a decrease in its absorption
(Ryou and Lee 2014; Ondova and Sicakova 2016)
acid treatment results in an increased specific gravity of the
aggregates and a decreased absorption rate through partially
removing the adhered mortar and neutralizing the hydroxide
(OH-) ions simultaneously. However, carbonation is
associated with problems such as high costs of equipment,
limited processed amount of aggregates per unit time, and
difficult retrieval of unreacted carbon dioxide
et al. 2016; Zhan et al. 2014; Zhang et al. 2015)
Furthermore, for acid treatment, there are issues with high
processing costs, secondary contaminated water, and
(Shi et al. 2015; Chen et al. 2017; Tam
et al. 2007; Kong et al. 2010; Al-Bayati et al. 2016)
. Thus, a
new processing method that can both achieve surface
modification of RAs and improve the issues associated with
existing methods is required.
This study focused on achieving efficient surface
modification of RAs through acid treatment. In order to overcome
the problems of high costs and secondary contaminated
water generation, we applied strongly acidic
hydrofluorosilicic acid (H2SiF6) as an industrial by-product that is
usually generated in processing and production stages of
phosphogypsum or phosphatic fertilizer
(Yang et al. 2005;
Kim et al. 2004)
. The uses of H2SiF6 are various such as
metal refinement, soil hardening agent, surface treatment
agent, sterilizer and water system purification. In this
research, for RFAs, we evaluated the decrease in absorption
rate, increase in specific gravity, and reduction in pH, in
order to develop a method to achieve surface modification of
RAs using H2SiF6. In addition, we evaluated the physical
properties of mortar with modified RFAs using a
physicochemical analysis, through which we were able to verify the
usefulness of the RFAs, surface-modified by the method
proposed in this study, as concrete aggregates.
2. Experimental Procedure
In the experiments, H2SiF6 of pH 1.0 (10% solution) was
used as acid treatment for surface modification of RFAs.
And both natural fine aggregate (NFA) and RFAs were used,
and the basic physical properties of the fine aggregates are
shown in Table 3. Three types of recycled aggregates (RFA
1, 2 and 3) were used in this study. As shown in Tables 1 and
2, RFA-2 and 3 are considered low-quality recycled
aggregates used as roadbed. RFA-1 can be used as aggregate for
mortar or concrete because it has higher physical quality
than relatively different recycled aggregate. The chemical
composition of cement is shown in Table 4.
2.2 Experiment Outline
In order to evaluate the surface modification of RFAs by
the H2SiF6 proposed for use in this research, we measured
the change in pH, specific gravity, and absorption rate of the
fine aggregates. To evaluate the physical–mechanical
properties of the mortar before and after surface modification of
the fine aggregates, the setting time, flow, and compressive
and flexural strengths were measured. Additionally, we
performed Scanning Electron Microscope (SEM),
Thermogravimetric Differential Thermal Analysis (TG/DTA),
Mercury Intrusion Porosimetry (MIP), and X-ray diffractometer
(XRD) as chemical analyses. The mortar specimens were
created by using natural, recycled, and modified-RFAs.
Table 5 shows the test items and methods for RFAs
presoaking in water and acid. And Table 6 shows the test items
and method for mortar and their related standards.
2.2.1 Specific Gravity and Absorption Rate of Fine
As shown in Tables 3 and 5, we measured the specific
gravity and absorption rates of natural and RFAs that were
used in our experiments and also the air-dried specific
gravity and over-dry one for the specific gravity of
2.2.2 pH Measurement
We measured the respective changes in pH by Ca(OH)2
within the cement matrices adhered to the RFAs and by
H2SiF6 in the state of aqueous solutions under three different
conditions, as displayed in Table 5. First, RFAs and distilled
water were combined with a weight ratio of 1:6 and
measured the pH values for different stirring times of the
solution (0, 1, 3, and 5 min). Also, we conducted the first stirring
of distilled water and RFAs for 5 min, deposited 8, 10, and
12 g of H2SiF6 into three different solutions, and measured
their pH values based on second stirring times of 0, 1, 3, and
5 min. Lastly, we mixed RFAs and distilled water with a
weight ratio of 1:6, and deposited 4, 6, 8, and 10 g of H2SiF6
into four different solutions, and measured their pH values
based on different stirring times if 0, 1, 3, and 5 min.
2.2.3 Evaluation of Material Properties of Mortar
Mortar specimens were produced using natural and RFAs.
For the mortar specimen with modified-RFAs, the method
having the best surface modification was used.
3. Results and Analysis
Table 7 displays the pH measurement results based on
different stirring times of the distilled water and RFAs
(Method-1 in Table 5). It was shown that the RFAs had a pH
of about 10.51 when soaked in distilled water and the pH
Number of tests
8 g mixed
10 g mixed
12 g mixed
tended to increase depending on the stirring time of the
solution. We also confirmed consistent alkali extraction due
to the Ca(OH)2 in the adhered cement matrices.
Table 8 shows the results of the pH measurements based
on the second stirring times after conducting the first stirring
of distilled water and RFAs for 5 min and depositing H2SiF6
into the solutions (Method-2 in Table 5). For the solutions
containing 8, 10, and 12 g of H2SiF6, the pH values
measured immediately after the addition of H2SiF6 were less
than 5.0, implying that the solutions were acidic. The
solutions that had less than 10 g of H2SiF6 added after 3 min of
stirring showed pH values of less than 10, implying
alkalescence. For a solution to have a pH range under 10 in up to
5 min of stirring, more than 10 g of H2SiF6 was required.
Meanwhile, the pH measurements after depositing RFAs
into distilled water and adding H2SiF6 are shown in Table 9
(Method-3 in Table 5). When the properties of the solutions
with 4, 6, 8, and 10 g of H2SiF6 were evaluated, for the
unextracted RFAs, the pH values of the solutions fell within
the ‘‘acidic’’ range under 3.0; when each specimen was
stirred, its pH tended to increase inversely proportional to
the amount of H2SiF6 added.
3.2 Specific Gravity and Absorption Rates
of Fine Aggregates
The results of testing the specific gravity and absorption
rates of NFA and RFAs are shown in Table 10. The test
results of the modified RFAs correspond to the case with
10 g of H2SiF6 added and 5 min of stirring in Table 9
(Method-3 in Table 5). The density of RFAs was slightly
increased by 0.5–2.6% after modification. On the other hand,
the water absorption rate decreased by 4–18% after
modification. The decrease of the absorption rate of the aggregate
due to the modification was found to be an increase of the
3.3 Mortar Setting Times and Flows
Three types of recycled aggregates (RFA 1, 2 and 3) were
used in this study. RFA-2 and 3 are considered low-quality
recycled aggregates used as roadbed materials because of
their high water uptake after modification. However, it has
confirmed the alkali leaching reduction effect and can be
used as a more environmentally stable roadbed material.
RFA-1 can be used as aggregate for mortar or concrete
because it has higher physical quality than relatively
different recycled aggregate. Therefore, RFA-1 and modified
RFA-1 were used in the preparation of mortar specimens for
the following experiments and analyzes.
Table 11 indicates the results of testing the flows and
setting times of mortar using NFA, RFA-1, and modified
RFA-1. In this case, the modified RFAs with 10 g of H2SiF6
added and 5 min of stirring in Table 9 was used for mortar.
The RFA had relatively smaller flows when compared to
the NFA, and the flows of the modified RFA were found to
be greater than those of the RFA (unmodified). Because of
the high absorption rates of RFA, the values of their flows
appeared to be relatively low, but on the other hand, there
was a positive effect on the flows of the modified RFA. Even
though there were relative differences among the mortar
samples with different setting times, it was found that the
initiation and termination times satisfied their prescribed
ranges in all levels.
3.4 Mechanical Properties of Mortar
The results of the compressive strength test and flexural
strength test of the mortar using each type of aggregates are
shown in Figs. 1 and 2. The results show that the
compressive strength of mortar with RFAs was lower than that of
mortar with NFA, and the compressive strength of mortar
with modified RFA was smaller than that of mortar with
NFA, but greater than that of mortar with RFA. The
improvement in compressive strength of the modified RFA
appears to result from the enhancement of its specific gravity
and reduction in its absorption rates. Because of the
characteristics of fine aggregates, it is very difficult to control
their moisture content in a surface-dry state, but even when
taken into consideration, it can be seen that there is an
evident relationship between the enhancement of material
properties of RFA through surface modification and the
compressive strength of mortar. In addition, after testing the
flexural strength of mortar, the flexural strengths of mortar
were in the same descending order as the compressive
strengths (28 days ages): NFA (22.7 MPa), modified RFA
(18.1 MPa), and RFA (16.2 MPa).
3.5 Chemical Analysis
Figure 3 shows the SEM results based on different mortar
curing periods. CSH and ettringite were clearly observed in
the NFA and RFA in the early age. CH, CSH, and ettringite
were observed on the 28th day regardless of the fine
aggregate type. When modified RFA was used, the hydration
reaction proceeded relatively late, but the effect of porosity
and compressive strength was better than that of RFA. The
effect of CaF2, SiO2 and residual H2SiF6 formed on the
modified aggregate surface were affected. However, the
decrease of the water absorption rate and the density of the
aggregate increased the compressive strength of mortar. TG–
DTA regarding three different types of mortar at the ages of
3 and 28 days, and the contents of Ca(OH)2 and CaCO3 are
shown in Table 12.
In addition, Fig. 4 represents the result of TG/DTA for the
mortar at the age of 28 days. There were changes in weight
for all specimens during the pyrolysis of Ca(OH)2 and
CaCO3, and based on this result, the existence of Ca(OH)2
and CaCO3 was verified within the analyzed specimens. At
up to 100 C on DTA, the physically bonded water within
the matrices evaporated, and there was physical dehydration
of C3A ettringite between 140 and 180 C, ettringite and
aluminate-based hydrates between 270 and 330 C, and
aluminate-based hydrates at around 570 C. In the case of
mortar with RFA and modified RFA, their Ca(OH)2 contents
were lower than that of mortar with NFA, and the CaCO3
contents of mortar with NFA and modified RFA,
respectively, were measured lower than that of mortar with RFAs.
As a result, the modification of RFA by H2SiF6 appeared to
The mortar porosity rate distribution dependent on mortar
age is displayed in Fig. 5. The porosity at 3 days of age
showed 20.025, 20.295, and 19.428% of NFA mortar, RFA
mortar and modified RFA mortar respectively. On the other
hand, the porosity at 28 days of age showed 16.950, 15.991,
and 15.903% of NFA mortar, RFA mortar and modified RFA
mortar respectively. The all mortar showed similar porosity.
The smaller the pore size, the higher the distribution in NFA
mortar. Modified RFA mortar showed a distribution of pores
similar to NFA mortar than RFA mortar.
An XRD compound analysis was conducted on the test
bodies of NFA, RFA, and modified RFA, and the results
from day 3 and day 28 are shown in Fig. 6. Modified RFA
mortar showed different compound composition compared
Fig. 4 Mortar TG/DTA (28 days). a mortar using NFA
(28 days), b mortar using RFA (28 days), and c mortar
using Modified RFA (28 days).
to RFA mortar. For the mortar on day 3, the early stage
amount of Ca(OH)2 was relatively higher for mortar with
RFA than for mortar with modified RFA. This is considered
to result from the increase in the hardened cement paste
attached to the surface of the RFA resulting from Ca(OH)2.
Meanwhile, a relatively higher CSH generation was verified
for mortar with modified RFA. For the mortar on day 28
with modified RFA, the amount of CSH generation was
found to be increasing, which was considered to result from
the supply of Si ions in the H2SiF6.
To investigate the RA surface modification method
proposed in this paper, the absorption rates and specific
gravity of RFA before and after treating with H2SiF6 were
measured. Additionally, a physicochemical property
evaluation and chemical analysis of mortar with modified RFA
were conducted. The results verified the surface
modification effect and the presence of changes in the physical
properties of the RFA owing to the H2SiF6 in each
experiment. The values of various mechanical properties
of the modified RFA mortar were also found to be
relatively high. The RFA surface modification method
proposed in this study is based on the chemical reaction
mechanism between H2SiF6 and abundant Ca(OH)2
contained in the hardened cement paste in the aqueous
solution state as below.
Reactions under H2SiF6
H2SiF6 þ 3CaðOHÞ2 ! 2Hþ þ Si2þ þ 6F þ 3Ca2þ
The reaction between the Ca(OH)2 and H2SiF6 involves a
chemical reaction that produces inorganic fine powders of
CaF2 and SiO2. Wastewater is generated in recycled
aggregate processing using the proposed method. Removal of
fluoride from wastewater may be worse than acid
wastewater. However, the fluoride in the wastewater is precipitated
as an insoluble CaF2 form as described in the paper. During
the wet process, CaF2 precipitate can be collected and
disposed of separately.
On the other hand, it is thought that this reaction
progresses gradually from the surface of the hardened cement
paste attached to the RFA towards the inside, causing the
cement matrix to become denser. The decrease in the
absorption rate and the consequent increase in specific
gravity are considered to be the filling effect of the surface
porosity due to the densification of the matrix inside the
hardened cement paste. A decreased aggregate absorption
rate and an increased specific gravity were revealed as
increased mortar compression strength compared to before
the modification as the compressive strengths (28 days
ages): NFA (22.7 MPa), modified RFA (18.1 MPa), and
RFA (16.2 MPa). Therefore, it has been confirmed that the
acid treatment of the RFA and H2SiF6 in aqueous solution,
as proposed in this study, shows a surface modification effect
similar to that of conventional carbonation treatment and
In this study, RFAs with high absorption rates and low
specific gravities were wet-reacted with H2SiF6 having
relative strong acidity, and various physical properties of the
RFAs and mortar using the byproduct were confirmed. The
results of this study are summarized as follows.
(a-1) Mortar using NFA (3 days)
(a-2) Mortar using NFA (28 days)
(b-1) Mortar using RFA (3 days)
(b-2) Mortar using RFA (28 days)
(c-1) Mortar using modified RFA (3 days)
(c-2) Mortar using modified RFA (28 days)
as the result of the reaction with H2SiF6, an
improvement in aggregate specific gravity and absorption rate
was found to occur, and mortar test results confirmed
3. By surface modification, mortar by using modified RFA
showed improvements of mechanical properties in both
compressive strength and flexural strength relatively tin
comparison with mortar by using RFA.
4. Therefore, it was confirmed that the proposed RFA
surface treatment method using H2SiF6, which is an
industrial byproduct, is effective in improving the
physical properties (high water absorption rate and
low density) of RFA.
(a) 3 days
(b) 28 days
This research was supported by 2015 Research Grant from
Kangwon National University, a research grant from
Technology Advancement Research Program (TARP) funded by
Ministry of Land, Infrastructure and Transport of Korean
Government (15CTAP-C097331-01), and by the National
Research Foundation of Korea (NRF) grant funded by the
Korea government (No.NRF-2015R1D1A1A09059522).
This article is distributed under the terms of the Creative
Commons Attribution 4.0 International License (http://
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and the source, provide a link to the Creative Commons
license, and indicate if changes were made.
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