b o l e t í n d e l a s o c i e d a d e s p a ñ o l a d e c e r á m i c a y v i d r i o 5 8 (2 0 1 9) 103–114

www.elsevier.es/bsecv

Sulfate resistance of RFCC spent catalyst-blended

Portland cement

Ali Allahverdi a,b,∗ , Mahdi Nemat Shahrbabaki a , Mohammad Ghezelasheghi c ,

Mostafa Mahinroosta a
a Research Laboratory of Inorganic Chemical Process Technologies, School of Chemical Engineering, Iran University of Science and
Technology, Narmak 1684613114, Tehran, Iran
b Cement Research Center, Iran University of Science and Technology, Narmak 1684613114, Tehran, Iran
c Civil Engineering Department, Islamic Azad University of Arak (IAU), Arak, Iran

a r t i c l e i n f o a b s t r a c t

Article history: The reuse of spent catalysts from residue fluid catalytic cracking (RFCC) units as pozzolanic
Received 5 April 2018 materials in cement and concrete production offers a number of important benefits. In
Accepted 26 September 2018 spite of all these benefits, the durability performance of the produced blended cement is an
Available online 15 October 2018 important issue to be considered. This study investigates the effects of RFCC spent catalyst
on durability performance of hardened Portland cement paste in a highly aggressive sulfate
Keywords: environment. The 28-day cured paste specimens prepared from binary cement mixtures
Spent catalyst incorporating different replacement levels of 0, 10, 20, and 30% (by mass) RFCC spent catalyst
Pozzolanic activity at a constant water-to-cement ratio of 0.30 were exposed to 10 mass% solution of magnesium
Sulfate attack sulfate. The accelerated sulfate attack under alternative cycles of wetting and drying was
Blended cement studied by monitoring the changes in compressive strength, length, and mass of specimens
and also by the application of XRD, SEM and EDX techniques. Based on the results and a
comparison with plain Portland cement, binary cement mixtures exhibit a higher rate of
deterioration in spite of their significantly improved compressive strengths resulted from
pozzolanic reaction.
© 2018 SECV. Published by Elsevier España, S.L.U. This is an open access article under the
CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Resistencia al sulfato de cementos Portland con adiciones de catalizador

consumido RFCC

r e s u m e n

Palabras clave: La reutilización de catalizadores consumidos de unidades de craqueo catalítico de fluidos

Catalizador consumido residuales (RFCC) como materiales puzolánicos en la producción de cemento y hormigón
Actividad puzolánica ofrece una serie de beneficios importantes. A pesar de todos estos beneficios, el rendimiento
Ataque de sulfato de durabilidad del cemento con adiciones producido es un tema importante a consid-
Cemento con adiciones erar. Este estudio investiga los efectos del catalizador consumido RFCC en el rendimiento
de durabilidad de la pasta de cemento Portland endurecido en un entorno de sulfato

∗
Corresponding author.
E-mail address: ali.allahverdi@iust.ac.ir (A. Allahverdi).
https://doi.org/10.1016/j.bsecv.2018.09.001
0366-3175/© 2018 SECV. Published by Elsevier España, S.L.U. This is an open access article under the CC BY-NC-ND license (http://
creativecommons.org/licenses/by-nc-nd/4.0/).
104 b o l e t í n d e l a s o c i e d a d e s p a ñ o l a d e c e r á m i c a y v i d r i o 5 8 (2 0 1 9) 103–114

altamente agresivo. Los especímenes de pasta curada de 28 días preparadas a partir de

mezclas de cemento binarias que incorporan diferentes niveles de 0, 10, 20 y 30% (en masa)
de catalizador consumido RFCC a una proporción constante de agua a cemento de 0,30
se expusieron al 10% en masa de solución de magnesio sulfato. El ataque acelerado de
sulfato en ciclos alternativos de humectación y secado se estudió mediante el monitoreo
de los cambios en la resistencia a la compresión, la longitud y la masa de las muestras y
también mediante la aplicación de técnicas XRD, SEM y EDX. En función de los resultados y
en comparación con el cemento Portland sin adición, las mezclas binarias de cemento que
incorporan catalizador consumido muestran una mayor tasa de deterioro a pesar de sus
resistencias a la compresión significativamente mejoradas como resultado de la reacción
puzolánica.
© 2018 SECV. Publicado por Elsevier España, S.L.U. Este es un artı́culo Open Access bajo
la licencia CC BY-NC-ND (http://creativecommons.org/licenses/by-nc-nd/4.0/).

disposed as industrial waste materials. Investigations have

Introduction confirmed that spent catalysts of both FCC and RFCC types
consisting mainly of silica and alumina and sometimes some
Sulfate attack on Portland cement (PC)-based products was amount of faujasite crystals are usually polluted with heavy
first reported by United States Bureau of Reclamation in 1908 metals and therefore must be disposed according to some spe-
and from that time research works for improving the chem- cific regulations [25]. On the other hand, it is well known that
ical resistance of cement-based materials and products are encapsulation or immobilization of heavy metals-polluted
still being carried out [1]. The improvements achieved in these materials in cement matrices is an effective way to pre-
extensive and long-time research works were mostly based on vent their environmental pollutions. Such a disposal method
the following techniques [2,3]: for these spent catalysts, not only results in considerable
improvements in physical properties of the cement, but also
1. Improving the physicochemical properties of PC effectively prevents any type of environmental pollution from
2. Utilizing suitable blended cements such polluted materials [19,20,27–30]. The durability and long-
term performance of RFCC spent catalyst- blended PC is
Many researchers have studied the performance and dura- therefore important and it is necessary to investigate the
bility of blended cements exposed to sulfate environments effects of aggressive environments on this cement and to eval-
and claimed that supplementary cementing materials (SCMs) uate its chemical resistance.
of either natural or artificial origins with relatively high con- The aim of this study is to evaluate the effects of RFCC
tents of silica and alumina not only improve the long-time spent catalyst on durability performance of hardened PC paste
physical properties of PC, but also increase the chemical resis- against magnesium sulfate as a very aggressive salt for cement
tance of PC against sulfate attack [4–6]. In the experimental matrices. Within the scope of this study, the changes in com-
results, such an improvement in sulfate resistance of blended pressive strength, length and mass of hardened cement paste
cements brought about by the effect of SCMs utilized as a par- specimens exposed to magnesium sulfate are measured at
tial cement substitute is quite evident [1,7–12]. In some cases, different time intervals and considered as measures for eval-
the utilized SCMs were fly ash [13], rice husk ash [14], the ash uating the extent of deterioration by salt attack.
produced from combustion of agricultural wastes [15], ground
granulated blast furnace slag [16,17], phosphorus slag [18], and
a blend of various additives [19–23]. In such cases, in addition Experimental program
to the improvements happened in mechanical strength and
chemical resistance of the cement, considerable environmen-
Materials
tal polluting materials could also be consumed [15].
According to previous research activities [24–29], spent cat- The materials used in this study include RFCC spent catalyst
alysts from fluidized catalytic cracking (FCC) units or residue (SC), ASTM standard type II PC of grade 42.5, silica fume and
fluid catalytic cracking (RFCC) units exhibit strong pozzolanic natural pozzolan. This type of PC contains less than 8% by
activity. These materials have a highly porous microstructure mass calcium aluminate phase and therefore has a medium
causing a relatively high water absorption capacity which in sulfate resistance [31]. Knowing that particle size distribu-
turn considerably lowers the workability of cement paste, mor- tion of SC could strongly affect both wet and dry properties
tar, or concrete. The important research result to consider is of PC, the SC was therefore ground in a laboratory ball mill
that an optimal replacement of cement with FCC spent cat- for 100 min to obtain a relatively fine powder comparable to
alyst shows an opposite effect on carbonation resistance of PC. The particle size distribution of the SC powder was deter-
concrete [28]. mined by a laser particle size analyzer. The values of specific
These catalysts, composed of aluminosilicates, are deacti- surface area and bulk density of ground SC were measured
vated after some limited life time of service under operating in accordance with ASTM C204 [32] and ASTM C188 [33] stan-
conditions in the catalytic cracking units and therefore are dards, respectively. Fig. 1 shows the particle size distributions
 b o l e t í n d e l a s o c i e d a d e s p a ñ o l a d e c e r á m i c a y v i d r i o 5 8 (2 0 1 9) 103–114 105

100 the sample by reducing the iron to the ferrous state with SnCl2
90 and titrating with a standard solution of K2 Cr2 O7 . The Al2 O3
80
content is obtained from the ammonium hydroxide group by
70
subtracting the separately determined constituents that usu-
Weight (%)

60
50 ally are present in significant amounts in the ammonium
40 hydroxide precipitate. Magnesium is precipitated as magne-
30 sium ammonium phosphate from the filtrate after removal of
20 (c) (b) (a)
calcium. The precipitate is ignited and weighed as Mg2 P2 O7 .
10
The MgO equivalent is then calculated. To determine the sul-
0
1 10 100 1000 fur content, sulfate is precipitated from an acid solution of
Particle size (µm) the sample with BaCl2 . The precipitate is ignited and weighed
as BaSO4 and the SO3 equivalent is calculated. The sample
Fig. 1 – Particle size distributions of (a) RFCC spent catalyst
is ignited in a muffle furnace at a controlled temperature.
before grinding, (b) RFCC spent catalyst after grinding, and
The loss is supposed to represent CO2 and the total mois-
(c) type II Portland cement.
ture in the sample. This test method covers the determination
of Na2 O and K2 O by atomic absorption or flame photometry.
For PC, the relative errors related to CaO and SiO2 contents
of PC and SC before and after grinding. A comparison of the were less than 3% and the relative errors of the remaining
two particle size distribution curves (Fig. 1a and b) clearly components were less than 5%. For SC and natural pozzolan,
shows that after grinding, the mass fraction of particles less the relative errors of the Al2 O3 and SiO2 contents were less
than 50 ␮m has significantly increased. As seen, in the ground than 4% and the relative errors of the remaining compo-
SC almost 95% of the grains are in the range between 5 and nents were less than 6%. For silica fume, the relative errors
90 ␮m, whereas such a percentage is related to the grain size of the SiO2 the remaining constituents were less than 2 and
of 2–35 ␮m in PC. 5%, respectively. The obtained results revealed that the cata-
The chemical composition and physical properties of SC, lyst was mainly comprised of SiO2 and Al2 O3 . As seen, these
PC, silica fume, and natural pozzolan are presented in Table 1. two components account for over 95% of the total mass of
To determine the chemical composition of the SC, wet ana- the material. This SC is therefore a relatively high siliceous
lytical methods were applied in accordance with ASTM C114 material and according to ASTM C618 [35], it could chemi-
[34] and for the results to be enough accurate, two individ- cally be considered as a relatively good artificial pozzolana.
ual samples were analyzed separately and the average values Silica fume and natural pozzolan were used in pozzolanic
were reported as the result. In these test methods, silicon diox- activity measurements as reference materials for comparison
ide (SiO2 ) is determined gravimetrically after the dissolution purposes.
of sample in HCl. The ammonium hydroxide group, namely The mineralogical phase composition of the SC was deter-
aluminum, iron, titanium, and phosphorus are precipitated mined using powder X-ray diffractometry (Cu, K␣ radiation).
from the filtrate, after SiO2 removal, by means of NH4 OH. The Fig. 2 depicts the X-ray diffraction pattern of the material.
precipitate is ignited and weighed as the oxides. The Fe2 O3 As seen, the shape of the pattern and the broad diffuse halo
content of the sample is determined on a separate portion of at 2 = 23◦ clearly show that the SC is mainly an amorphous

Table 1 – Chemical composition and physical properties of the used materials.

PC SC Silica fume Natural pozzolan

Physical properties
Blaine fineness (m2 /kg) 320 ± 5 315 ± 5 18,000 ± 70 309 ± 4
Bulk density (kg/m3 ) 3130 ± 20 2470 ± 15 2130 ± 10 2650 ± 15

Chemical composition (mass%)

CaO 64.96 – – 6.69
SiO2 20.26 58.43 96.12 61.57
Al2 O3 5.43 37.32 0.82 18.00
Fe2 O3 3.87 1.19 – 4.93
MgO 0.48 0.65 – 2.63
SO3 2.09 0.20 – 0.10
K2 O 0.60 0.11 0.40 1.95
Na2 O 0.27 0.82 – 1.65
LOI 1.95 1.25 0.63 2.15
Free lime 0.45 – – –

Bogue’s potential phase composition (mass%)

C3 S 66.58 – – –
C2 S 7.95 – – –
C3 A 7.84 – – –
C4 AF 11.78 – – –
106 b o l e t í n d e l a s o c i e d a d e s p a ñ o l a d e c e r á m i c a y v i d r i o 5 8 (2 0 1 9) 103–114

F: Faujasite
F
Q F K: Kaolinite

Intensity (a.u.)
F Q: Quartz
F
F F
K

20 30 40 50 60
2Theta angle (degree)

Fig. 2 – X-ray diffraction pattern of RFCC spent catalyst.

Paste specimen
cm
2

2 cm

2 cm
Polyamide mold Hardened paste Tap water Sulfate solution

(1) (2) (3) (4)

Fig. 3 – Sulfate exposure test: (1) molding the paste specimens, (2) demolding the specimens after 24 h, (3) curing of
specimens in tap water for 28 days, and (4) immersing the cured specimens in sulfate solution.

material. The few minor crystalline mineral phases present in specimens were cured in tap water at 25 ◦ C until the age of 28
the material are faujasite, quartz, and kaolinite. days.

Procedures Sulfate exposure test

In order to perform a sulfate exposure test, the steps shown
Measurement of pozzolanic activity in Fig. 3 were followed. After paste specimens were stored in a
In this study, for measuring the pozzolanic activity of the SC, humid chamber for 24 h, they were demolded and cured in tap
silica fume and natural pozzolan, thermogravimetric analysis water for 28 days. After 28 days of curing, the specimens were
was used. For this purpose, a homogenized mixture of from exposed to sulfate solution in accordance with ASTM C1012
each material and calcium hydroxide with a mass ratio of 1:1 [36]. Among the sulfate salts, magnesium sulfate that is the
was used to prepare a paste by adding equal amount of water. most aggressive one was selected. The concentration of the
The obtained paste was left in a nitrogen gas atmosphere at salt solution was considered at a relatively high level of 10
a constant temperature of 60 ◦ C. Then at given time intervals mass% and alternative wetting-drying cycles were applied to
of 3, 7, 14, and 28 days, samples taken from the paste were accelerate the process of deterioration. After each two days
dried with acetone and nitrogen gas. Finally, the dried samples of full immersion, the specimens were exposed to open air
were used for thermogravimetric analysis to determine the atmosphere and allow to dry for 24 h. The ratio of the salt solu-
amount of reacted calcium hydroxide. For each time interval, tion volume to the total exposure surface was kept constant
the described experiment was repeated twice and the average at 10 cm3 /cm2 . The solution temperature was kept constant
value plus error percentage was reported as the result. at 25 ◦ C throughout exposure time. The sulfate solution was
renewed once per month. The sulfate exposure test was con-
Preparation of specimens tinued for about 4 months. A similar trend was applied for
Binary cement mixtures were prepared by replacing PC with paste bars.
ground SC at different replacement levels of 0, 10, 20, and
30% by mass. Each dry binary mixture was thoroughly homog- Measurement methods and complementary techniques
enized using a laboratory ball mill containing a few balls Changes in compressive strength, length and mass of the
for 20 min. Water-to-cement ratio was taken constant at 0.30 specimens were measured and considered for determining
for all the mixtures and pastes were cast into cubic speci- the extent of deterioration. For each measurement, three spec-
mens of the size 2 cm × 2 cm × 2 cm. Also, paste bars of the imens were used and the average value was reported as the
size 2 cm × 2 cm × 10 cm were prepared to monitor the length result. For measurement of mass, the specimens were gently
change. The molds were stored at an atmosphere of more than weighed as soon as removing their surface free salt solution
95% relative humidity at 25 ◦ C for the first 24 h and then the with a towel. The mass measurements were done with 0.01 g
 b o l e t í n d e l a s o c i e d a d e s p a ñ o l a d e c e r á m i c a y v i d r i o 5 8 (2 0 1 9) 103–114 107

Table 2 – Pozzolanic activity of spent catalyst and reference materials.

Material Pozzolanic activity (mass% of reacted calcium hydroxide)

3 days 7 days 14 days 28 days

Spent catalyst 66.8 ± 2.4 67.7 ± 4.1 69.2 ± 3.3 70.2 ± 2.2
Silica fume 97.6 ± 1.7 100.0 ± 1.5 100.0 ± 1.8 100.0 ± 1.3
Natural Pozzolan 42.3 ± 3.2 58.8 ± 3.9 60.4 ± 4.4 64.9 ± 3.1

accurate digital balance. Enough number of 28-day cured paste

specimens were also stored in tap water at 25 ◦ C for compar-
ison purposes and for an accurate evaluation of the extent
of deterioration. The length of specimens was measured at
(b)
different time intervals using an ordinary 150-mm digital
calipers with a rated accuracy of 0.02 mm and length changes
were determined using the following equation [36]:

L − L  970
x i 1366-1417
L= × 100 (1)
Li 3640 1100 (a)

420
where L (%) is length change at the age of x; Lx (mm) is average
length of three bars at the age of x; Li (mm) is average initial 4000 3600 3200 2800 2400 2000 1600 1200 800 400

length of the same three bars after 28 days of curing. Wavenumber (cm-1)
The mass changes were calculated by the following equa-
Fig. 4 – FTIR spectra of spent catalyst/lime (1:1) after (a) 7
tion:
and (b) 28 days of reaction.
M − M 
x i
M= × 100 (2)
Mi
140
in which M (%) is the mass change; Mi (g) is average mass of
Compressive strength (MPa)

130
three specimens after 28 days of curing; and Mx (g) is aver-
120
age mass change of the same three specimens at the age of
x. 110
Laser particle size analyzer (Sympatec, GmbH, HDD) was 100
used for determination of particle size distribution of SC
90
before and after grinding. The pozzolanic activity of the SC
OPC
was measured using a thermogravimetry equipment (Netzsch 80
0.9OPC+0.1RFCC
model 429). The mineralogical phases were determined with 0.8OPC+0.2RFCC
70
a JEOL JDX-8030 X-ray diffractometer using Cu-K␣ radiation 0.7OPC+0.3RFCC

60
at 40 kV and 30 mA. For this purpose, PC and blended cement 0 20 40 60 80 100 120 140 160
paste specimens after 120 days of exposure to sulfate attack Curing time (day)
were used. The microstructural and elemental analyses were
carried out using a Cambridge Stereoscan 360 Scanning Elec- Fig. 5 – Compressive strength of plain Portland cement and
tron Microscope device at an accelerating voltage of 30 kV. mixtures containing RFCC spent catalyst stored in tap
Fourier transform infrared (FTIR) spectroscopy was performed water for continued curing.
using a Shimadzu FTIR 8400s Spectrophotometer in trans-
mittance mode from 400 to 4000 cm−1 using standard KBr
technique.
reactivity with hydrated lime and can be considered as a very
good pozzolanic material.
Results and discussion Pozzolanic activity of the SC was also confirmed by FTIR
spectroscopy studies. Fig. 4 shows the FTIR spectra obtained
Pozzolanic activity of spent catalyst from samples of SC/lime (1:1) after 7 and 28 days of curing.
The more relevant absorption bands include 3640 cm−1 for
The results of pozzolanic activity measurements for the SC –OH of calcium hydroxide, 1366–1417 cm−1 for carbonates (as
and the reference materials (silica fume and natural pozzolan) lime impurities), 1100 cm−1 for vibrations of valence Si-O(Al)-
are presented in Table 2. As can be seen and compared to silica O, 970 cm−1 for calcium silicate hydrates and 420 cm−1 for
fume as a material exhibiting very strong pozzolanic proper- calcium aluminate and calcium aluminosilicate hydrates. As
ties and also to a typical natural pozzolan that is currently can be seen, both bands at 3640 and 1100 cm−1 belonging to
being used by Iranian cement industry for the purpose of natu- lime and SC, respectively, noticeably decrease from 7 to 28 days
ral pozzolan-blended cements production, SC has a very good of reaction while the band at 970 cm−1 belonging to calcium
108 b o l e t í n d e l a s o c i e d a d e s p a ñ o l a d e c e r á m i c a y v i d r i o 5 8 (2 0 1 9) 103–114

120 during the first 15 days of the exposure time and after that
exhibited compressive strength losses at significantly much
100 higher rates compared to plain PC. All the mixtures containing
Compressive strength (MPa)

SC, therefore, showed a significantly higher extent of deterio-

80 ration compared to plain PC.
In fact, two different chemical phenomena in contrary
60 to each other are at work for the observed changes in
compressive strengths. From one side, the time progress of
40 hydration reactions of PC phases (in plain PC specimens)
OPC
along with pozzolanic reaction (in the case of SC-containing
0.9OPC+0.1RFCC
20 specimens) result in more matured microstructure and hence
0.8OPC+0.2RFCC
increased compressive strengths. From the other side, mag-
0.7OPC+0.3RFCC
0 nesium sulfate attack causes undesirable reactions leading
-20 0 20 40 60 80 100 120 140 to degradation of the microstructure and reduced com-
Exposure time (day) pressive strengths. The final governing phenomenon upon
continued exposure, however, is sulfate attack deteriorating
Fig. 6 – Compressive strength changes versus exposure
all the mixtures. When PC-based materials are exposed to
time for plain Portland cement and mixtures containing
a source of sulfate ions, reactions between alumina con-
RFCC spent catalyst.
taining compounds and calcium hydroxide (Portlandite) as
the PC hydration product and diffusing sulfate ions pro-
duce gypsum and ettringite [37–39]. Considering Table 1, the
silicate hydrates increases as a consequence of the pozzolanic SC contains around 37% alumina and this means that the
reaction. cements incorporating SC are susceptible to the formation
of gypsum and ettringite in exposure to sulfate solution.
Compressive strength changes This will be confirmed in later subsections by XRD and
SEM/EDX techniques. The interesting point is the significant
The results of compressive strength measurements are shown difference in the starting time and the extent of deteri-
in Figs. 5 and 6. Fig. 5 representing the results obtained from oration in mixtures containing SC compared to plain PC.
specimens stored in tap water not only shows the effect of SC Replacement of PC by SC causes a considerably sooner and
on compressive strength of PC, but also provides the possibility deeper deterioration due to sulfate attack. This is in con-
of comparing the results with those obtained for specimens trary to the general expectations and the experimental results
kept in sulfate solution (Fig. 6) and evaluating the extent of reported by the researchers for the other pozzolanic materials
deterioration. [1,7–12].
As seen in Fig. 5, the compressive strength continued to One of the most important parameters determining the
increase up to the age of 60 days with a higher growth rate starting time and the extent of deterioration due to sulfate
(steeper slope) especially for mixtures incorporating 10 and 20 attack is the permeability of the hardened cement paste, in
mass% SC. After 60 days, the compressive strength still kept addition to its chemical and mineralogical phase composi-
increasing, but at a slower rate. Mixtures with 10 and 20 mass% tion. If the permeability of the hardened cement paste is
replacements with the highest compressive strengths of 129 relatively low, the deteriorating effects of the sulfate attack,
and 136 MPa after 130 days of continued curing, respectively, which are limited to the regions close to the exposed sur-
exhibited higher values of compressive strengths at all the faces appear at a relatively slower rate. The reverse is also
curing ages after the first 28 days of curing. These noticeably well accepted. In relatively high permeable hardened cement
higher compressive strengths at replacement levels of 10 and pastes, the increased diffusion rate of sulfate ions into inter-
20 mass% are due to the pozzolanic reaction between SC and nal regions results in a faster and deeper deterioration process
calcium hydroxide produced in the hydration of the PC phases. [40–43]. The relatively high porous microstructure of the RFCC
Such a reaction generates additional calcium silicate hydrates. SC has been confirmed by other researchers [44,45]. Another
These secondary hydration products are the principal compo- point to be taken into account is that according to the bulk
nents responsible for the increase in the mechanical strength densities values in Table 1 and the particle size distributions
of the blended cement pastes [1,8]. presented in Fig. 1, it is realized that the difference between
Specimens of all four cement mixtures exposed to particle sizes of the SC and PC and also the difference between
magnesium sulfate attack under the condition of alterna- their bulk densities lead to a porous structure in the cement
tive wetting-drying cycles showed considerable compressive matrix. In a previous work [46], the same authors also per-
strength losses compared to those stored in tap water for con- formed capillary and gel porosity measurements on 28-day
tinued curing, as seen in Fig. 6. An important point, however, cured paste specimens of plain PC and the binary mixture
is the beginning time for compressive strength reduction and containing 30 mass% SC using mercury intrusion porosime-
the extent of deterioration. As can be seen in Fig. 6, the com- try technique. They reported a higher total porosity of about
pressive strength of the plain PC increases up to about 104 MPa 20.75% by volume for the binary mixture compared to the
after 75 days from the beginning of the exposure time and value of 17.80% by volume for plain PC. Partial replacement
starts decreasing gradually, thereafter. All the mixtures con- of PC by SC, therefore, increases the permeability of the hard-
taining SC showed a small increase in compressive strength ened cement paste. This increased permeability along with
 b o l e t í n d e l a s o c i e d a d e s p a ñ o l a d e c e r á m i c a y v i d r i o 5 8 (2 0 1 9) 103–114 109

0.6 3
OPC OPC
0.5 0.7OPC+0.3RFCC
2.5 0.9OPC+0.1RFCC
0.8OPC+0.2RFCC
Length change (%)

0.9OPC+0.1RFCC
0.8OPC+0.2RFCC
0.4
2 0.7OPC+0.3RFCC

Mass change (%)

0.3
1.5
0.2
1
0.1

0.5
0
15 20 25 30 35 40 45 50 55 60 65 70 75 80
Exposure time (day) 0
15 30 45 60 75
Fig. 7 – Length changes versus exposure time for plain -0.5
Portland cement and mixtures containing RFCC spent
catalyst. -1
Exposure time (day)

Fig. 8 – Mass changes versus exposure time for plain

capillary suction forces exerted by alternative wetting-drying
Portland cement and mixtures containing RFCC spent
cycles can significantly reduce the sulfate resistance of the
catalyst.
cement paste. This means that RFCC SC reduces the sulfate
resistance of PC in spite of its considerably effective pozzolanic
property.
of the pozzolanic reaction can be effective in reducing the
Length changes permeability of the hardened cement paste and hence result-
ing in much less severe degradation. As can be seen in Fig. 7
The results of measurements of length changes are presented and compared to binary mixtures, the plain PC bar specimens
in Fig. 7. As can be seen, all specimens exposed to the sulfate show the lowest expansion, which more likely is due to
solution experienced significant length changes. limited formation of gypsum and ettringite in the surface
The length changes of plain PC bar specimens and mix- regions resulted from their relatively lower permeability.
tures containing 30 mass% SC were measured up to 75 days As it is seen, the mixtures containing 20 and 30 mass% SC
of exposure, whereas the measurements of length changes exhibit less expansion than the mixture containing 10 mass%
for mixtures containing 10 and 20 mass% SC continued up SC. This can be attributed to the replacement percentage and
to 60 days. The reason for this was the complete deteriora- its effect on calcium hydroxide content of the cement paste.
tion of the bar specimens due to strong sulfate attack under In fact, at relatively high replacement levels, the strong poz-
the condition of alternative wetting-drying cycles. In fact, mix- zolanic property of SC may cause a relatively fast consumption
tures containing 10 and 20 mass% SC were severely degraded of large amounts of calcium hydroxide in the cement paste
and shattered into smaller pieces after 60 days of exposure. resulting in a less permeable matrix, which less vulnerable
A similar trend occurred for reference bars and also mixtures to sulfate attack, before calcium hydroxide can participate in
containing 30 mass% SC after 75 days of exposure. A consid- the destructive formation reaction of gypsum and ettringite.
erable increase in the length of the bar specimens is due to However, differences observed between the binary cement
the formation of voluminous products such as gypsum and mixtures are difficult to explain because of the limited reliabil-
ettringite. From one side, the formation of gypsum and ettrin- ity and accuracy of the reported data from one side and lack of
gite is influenced by the concentration of reactants including additional complementary evidences from the other side. In
calcium hydroxide, calcium aluminate content of the cement such a study, the reliability and accuracy of the reported data
mixture, and the sulfate ion present in the solution. On the are limited because the non-uniform dimensional changes of
other hand, it is affected by the permeability of the hardened the exposed paste specimens significantly affect the accuracy
paste under the effect of capillary suction forces exerted by of the compressive strength and length changes.
alternative wetting-drying cycles [47,48].
If a pozzolanic additive, such as the SC, has a porous Mass changes
character, then the simultaneous effect of both pozzolanic
reaction and the permeability, can be complicated. At a The results of mass change measurements are presented in
relatively low permeability, the formation of gypsum and Fig. 8. As seen, the plain PC specimens showed continuous
ettringite is expected to be limited to surface regions, and the mass gain during the first 45 days of exposure time and after
expansion is proportional to the amount of ettringite. At a that continued exposure resulted in significant mass losses.
relatively high permeability, however, due to the formation of The specimens of the binary mixtures containing 10, 20, and
gypsum and ettringite not only in the surface regions, but also 30 mass% SC, however, exhibited continuous gradual and rel-
in relatively deep regions, a more expansion is expected. The atively small mass gains over the whole length of 75 days of
severity of the sulfate attack must also be taken into consider- measurements with average ultimate mass gains of 1.59, 2.50,
ation. If the sulfate attack is not severe, then the progression and 1.23%, respectively.
110 b o l e t í n d e l a s o c i e d a d e s p a ñ o l a d e c e r á m i c a y v i d r i o 5 8 (2 0 1 9) 103–114

C
P
P
P G
E G P
P P

Relative intensity (a.u.)

G C
E (b)

C: Calcite
E: Ettringite
G: Gypsum
P: Portlandite

(a)

5 10 15 20 25 30 35 40 45 50 55 60 65 70
2Theta angle (degree)

Fig. 9 – XRD patterns of paste specimens after 120 days of exposure to sulfate environment; (a) plain Portland cement and
(b) binary mixture containing 20 mass% RFCC spent catalyst.

The mass losses observed for plain PC specimens were are shown in Fig. 9. X-ray diffractometry analyses were per-
due to spalling of small pieces from the surfaces of the spec- formed on samples prepared from exposed surfaces. The two
imens. In fact, for plain PC specimens, the sulfate attack is XRD patterns are very similar showing the presence of Port-
mainly limited to the exposed surface regions. Intensive gyp- landite, gypsum, ettringite, and calcite in both samples. No
sum deposition in these areas gradually leads to intensifying sign of anhydrous cement phases or hydration products were
disintegrating stresses, which finally result in the spalling of observed probably due to relatively very high concentrations
pieces from surface regions [49]. In natural cases of sulfate of major crystalline phases. Reduced Portlandite content in
attack, this phenomenon usually does not occur during rel- binary cement mixture containing 20 mass% SC is due to its
atively short time periods about 45 days and here the main partial consumption in pozzolanic reactions in addition to
reasons for such a severe and fast attack are the type of sulfate its participation in the reactions with sulfate ions and also
and its relatively high concentration. with atmospheric carbon dioxide. Calcite is a secondary reac-
The continued mass gain of binary cement mixtures for tion product due to the application of wetting-drying cycles.
longer exposure times compared to plain PC is due to the When the specimens were exposed to open air atmosphere
presence of SC, which not only reduces the concentration of during drying stage, part of Portlandite present in surface
calcium hydroxide inside the hardened cement paste, but also layers of the specimens reacted with carbon dioxide result-
results in probably less concentrated expansion in the sur- ing in the formation of calcite. The formation of gypsum
face regions. It is reasonable to assume that the specimens of and ettringite due to the reaction of Portlandite with sul-
binary mixtures also undergo mass losses or shattering upon fate ion and carbonation of Portlandite due to its reaction
continued longer exposure times. with atmospheric carbon dioxide are common observations
as reported earlier by many researchers [1,3–18,41–43]. An
X-ray diffraction analysis important difference, however, lies in the kinetics of the dete-
rioration phenomenon. The kinetics depends on four main
The XRD patterns of plain PC and the mixture containing factors including: (1) differences in chemical and mineralog-
20 mass% SC after 120 days of exposure to sulfate solution ical compositions, (2) permeability of the cement paste, (3)

100 µm
100 µm

Fig. 10 – SEM micrographs of hardened pastes after 120 days of curing in tap water (Left: plain Portland cement, Right:
mixture containing 20 mass% spent catalyst).
 b o l e t í n d e l a s o c i e d a d e s p a ñ o l a d e c e r á m i c a y v i d r i o 5 8 (2 0 1 9) 103–114 111

a b

20 µm 20 µm

Ca
Ca

S
S

Ca Ca

Fig. 11 – SEM micrographs and EDX elemental analyses of gypsum crystals formed after 120 days of exposure to sulfate
solution in the paste specimens of (a) plain Portland cement and (b) mixture containing 20 mass% spent catalyst.

a b

20 µm 20 µm
Ca
Ca

S S
Al
Al
Ca Ca

Fig. 12 – SEM micrographs and EDX elemental analyses of ettringite crystals formed after 120 days of exposure to sulfate
solution in the paste specimens of (a) plain Portland cement and (b) mixture containing 20 mass% spent catalyst.
112 b o l e t í n d e l a s o c i e d a d e s p a ñ o l a d e c e r á m i c a y v i d r i o 5 8 (2 0 1 9) 103–114

a b

20 µm
100 µm

Fig. 13 – SEM micrographs of microcracks formed after 120 days of exposure to sulfate solution in the paste specimens of (a)
plain Portland cement and (b) the mixture containing 20 mass% spent catalyst.

the type of attacking sulfate and its concentration, and (4) continuation of this process leads to enlarged microstructural
exposure conditions including the exertion of capillary suc- cracks and finally dimensional expansion, mass changes (gain
tion forces by wetting-drying cycles. Therefore, the kinetics and loss), and loss of compressive strength.
still requires extensive research activities to well under-
stand.
Conclusions

Microstructural studies by SEM

Experimental results showed that paste specimens of binary
mixtures incorporating different levels of 10, 20, and 30 mass%
Fig. 10 depicts SEM micrographs of hardened pastes of plain
of ground RFCC spent catalyst exhibiting considerably higher
PC and the binary mixture containing 20 mass% SC after 120
compressive strengths were deteriorated faster and deeper
days of curing in tap water at a magnification of 300×. As
than plain Portland cement when exposed to accelerated
can be seen, the microstructures are similar and no signif-
10% magnesium sulfate attack. This was due to the effect of
icant difference between them can be observed, although it
capillary suction forces exerted by alternative wetting-drying
is expected the microstructure of the cement paste specimen
cycles. Such an odd behavior, when compared to the other
containing the SC has more uniformity and compactness due
pozzolanic materials, can be attributed to the effect of highly
to the pozzolanic reactions and the partial consumption of
porous microstructure of RFCC spent catalyst in increasing
Portlandite.
the permeability of the hardened Portland cement paste. The
In order to track the formation and deposition of gyp-
results of this study clearly prove the important role of the
sum and ettringite crystals in the microstructure of exposed
porous microstructure of RFCC spent catalyst on permeability
specimens, it was necessary to study the microstructure of
of the blended Portland cement paste as an important dura-
the paste samples at relatively high magnifications along
bility determining. Despite an adverse effect of the addition
with the application of EDX analysis. Gypsum crystals formed
of RFCC spent catalyst on the sulfate resistance of PC, the
in the paste specimens of plain PC and the mixture con-
results of the present study are important in three respects
taining 20 mass% SC after 120 days of exposure to sulfate
including: (1) proposing a promising method for the reuse
solution are shown in Fig. 11. As can be seen, elemental
of RFCC spent catalyst as a heavy metal-polluted industrial
composition of these crystals consists of Ca and S ele-
waste in the preparation of blended cements for application
ments, confirming the certainty of the presence of relatively
in sulfate-free or very low sulfate content environments, (2)
large gypsum crystals in the microstructure of the paste
the RFCC spent catalyst significantly improves the compres-
specimens.
sive strengths due to its relatively strong pozzolanic property,
Fig. 12 shows SEM micrographs and the corresponding EDX
and (3) clarifying the fact that a higher compressive strength
elemental point analyses performed on needle-like crystals
does not necessarily mean a better durability performance.
formed and deposited in the paste specimens of plain PC and
the mixture containing 20 mass% SC after 120 days of expo-
sure to sulfate solution. The needle-like morphology and the Conflict of interests
chemistry of Al, S, and Ca confirm the presence of ettringite
The authors have no competing interests.
crystals.
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