Effects of Magnesium Sulfate Attack on Ordinary Portland Cement (OPC) Mortars

Abstract

The chemical and mineralogical compositions in Ordinary Portland Cement (OPC) were determined and cement which is low of C 3A is more easily exposed to sulfate environment. The relation between physical strength of mortars sized 150 × 150 × 150 mm and the effects of different concentrations of sulfate solutions, SO 4 2- (1%, 3% and 5%) for 3, 14 and 28 days was found to be that stronger the sulphate concentrations, the greater the sulfate attack, but in a weaker concentration situation, the attack was more efficient as the comparable damaged being achieved with smaller amount of sulphate. The morphological studies were observed under scanning electron microscope (SEM) and the distribution of the main component, including Mg 2+ and SO 4 2- ions, was

Full text

Portugaliae Electrochimica Acta 26 (2008) 235-242
PORTUGALIAE
ELECTROCHIMICA
ACTA
Effects of Magnesium Sulfate Attack on Ordinary Portland
Cement (OPC) Mortars
M. M. Amin
1,*
, S. B. Jamaludin
1
, F. C. Pa
1
, K. K. Chuen
2
1
School of Materials Engineering, University Malaysia Perlis,
Complex Taman Muhibah UniMAP, 02600 Jejawi, Perlis, Malaysia
2
Department of Chemistry, Faculty of Science and Technology, University Malaysia
Terengganu, Kuala Terengganu, Malaysia
Received 23 May 2007; accepted 8 October 2007
Abstract
The chemical and mineralogical compositions in Ordinary Portland Cement (OPC) were
determined and cement which is low of C
3
A is more easily exposed to sulfate
environment. The relation between physical strength of mortars sized 150 × 150 × 150
mm and the effects of different concentrations of sulfate solutions, SO
42-
(1%, 3% and
5%) for 3, 14 and 28 days was found to be that stronger the sulphate concentrations, the
greater the sulfate attack, but in a weaker concentration situation, the attack was more
efficient as the comparable damaged being achieved with smaller amount of sulphate.
The morphological studies were observed under scanning electron microscope (SEM)
and the distribution of the main component, including Mg
2+
and SO
42-
ions, was
analysed by Energy Dispersive X-Ray Analyzer (EDS) to observe the cracks and
reactions.
Keywords: Ordinary Portland Cement (OPC), magnesium sulfate, sulfate attack, SEM
.
Introduction
Sulfate attack is consider one of the major deteriorative problems occurred when
the cement based materials, such as concrete, mortars and buildings, are exposed
to this environment. Sulphate ions in soil, ground water and sea water may cause
deterioration of reinforced concrete structures by provoking expansion and
cracking due to factors such as type of cement, sulphate cation type, sulphate
concentration and the period of exposure. Many structures affected by sulphate
degradation often need to be repaired or, in most severe cases, they need to be
*
Corresponding author. E-mail address
:
mmamin@unimap.edu.my; amin_56@yahoo.com

M.M. Amin et al. / Portugaliae Electrochimica Acta 26 (2008) 235-242
236
reconstructed [1-3]. Three main factors are reported [4-5]: properties; aggressive
medium and environmental atmosphere.
The present investigation was carried out to determine the chemical compositions
in cement and the effect of magnesium sulphate of different concentrations on
the surfaces of mortars, followed by observing the changes of strength by
measuring through compression tests. The morphological structures of the
mortars exposed in sulphate solutions were observed using scanning electron
microscope (SEM) and studied by EDS (Energy Dispersive X-Ray Analyzer).
The investigations help to clarify the great importance of understanding the
physical strength of mortars exposed in sulphate environments and there are not
many reported results. The spallation and cracking are produced due to sulphate
ions by exposure to containing inorganic salts atmospheres.
Experimental
Mortar cubes (150 × 150 × 150 mm) established from cement, sand and water
ratio of 2.5, 7.5 and 1.0, respectively, with the sand size 20 mm (fine aggregates
clean river sand) were prepared and exposed in magnesium sulfate solution
(10,000 mg/L, 30,000 mg/L and 50,000 mg/L) for 3, 14 and 28 days at room
temperature. The mortar cubes were moulded in metal moulds which are not
attacked by cement or sand. The cube specimens were rigidly constructed in such
a way as to facilitate the removal of the moulded specimen without damage.
After the exposure period, the mortars were taken out for compressive strength,
followed by SEM and EDS tests.
Chemical and mineralogical analysis of the cement were carried out to determine
the main components such as: SiO
2
, R
2
O
3
, CaO, MgO, Fe
2
O
3
, Al
2
O
3
, SO
3
. Based
on the oxide components, the Bogue analysis was determined. Table 1 shows the
main components of Ordinary Portland Cement and the calculation of the oxides
composition.
Table 1. Main components of Ordinary Portland Cement [3].
Name of compound Oxide composition Abbreviation
Tricalcium silicate
Dicalcium silicate
Tricalcium aluminate
Tetracalcium
aluminoferrite
3CaO.SiO
2
2CaO.SiO
2
3CaO.Al
2
O
3
4CaO.Al
2
O
3
.Fe
2
O
3
C
3
S
C
2
S
C
3
A
C
4
AF
C
3
S = 4.07 (CaO) – 7.60 (SiO
2
) – 6.72 (Al
2
O
3
) – 1.43 (Fe
2
O
3
) – 2.85 (SO
3
)
C
2
S = 2.87 (SiO
2
) – 0.75 (3CaO.SiO
2
)
C
3
A = 2.65 (Al
2
O
3
) – 1.69 (Fe
2
O
3
)
C
4
AF = 3.04 (Fe
2
O
3
)

M.M. Amin et al. / Portugaliae Electrochimica Acta 26 (2008) 235-242
237
The morphological studies of the mortars were conducted using scanning
electron microscope (SEM) and combined with energy dispersive X-ray analyser
(EDS) to observe the distribution of ions and the attack.
Results and discussion
Chemical and mineralogical analysis
Chemical and mineralogical analysis results are presented in Table 2.
Table 2. Chemical and mineralogical analysis of the OPC.
Oxides Ordinary Portland Cement (OPC) / %
SiO
2
21.47
R
2
O
3
6.51
CaO 60.77
MgO 0.17
Fe
2
O
3
2.34
Al
2
O
3
4.17
SO
3
2.71
Insoluble residue 0.39
Loss on ignition 1.31
Bogue analysis
C
3
S 45.07
C
2
S 27.82
C
3
A 7.10
C
4
AF 7.11
Compressive strength analysis
The compressive strength of mortars, illustrated in Fig. 1, indicates that after long
exposure time periods into the weaker solution (1%), the sulphate attack was
more efficient than when subjected to stronger sulphate concentrations during
short periods.

M.M. Amin et al. / Portugaliae Electrochimica Acta 26 (2008) 235-242
238
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
20
3 days 14 days 28 days
Exposure Period, days
Relative Compressive
Strength, (%)
1% 3% 5%
Figure 1. Relative compressive strength / (%) after exposure in 1, 3 and 5% MgSO
4
solution for 3, 14 and 28 days.
Surface analysis at different concentrations
Precipitation of salts in the inner part of the mortars is shown in Fig. 2 (a and b).
It is observed that the precipitating of salts around the specimen at 5% solution is
greater than in the 3% solution.
Morphological analysis under SEM
Scanning electron microscope was used to observe the morphological structure
on the surface layer and inner part of the specimens under EDS. Besides, it was
determined the formation of thaumasite and ettringite in the specimen after
exposure.
Figure 2. Surface of mortars with precipitation of MgSO4 salts after exposure in (a)
30,000 mg/L and (b) 50,000 mg/L solution for 28 days.

M.M. Amin et al. / Portugaliae Electrochimica Acta 26 (2008) 235-242
239
The SEM image in Fig. 3 shows mortars specimen exposed in the sulfate
concentrations of 50,000 mg/L for 28 days. The specimen used in the
micrograph vacuum is taken from an area close to the edge of the mortar at low
vacuum and shows the morphology of the inner and outer parts of the specimen
which then were used to obtain the sub-micron volume of the specimen at the
area 006 and 007 using EDS (Energy Dispersive X-Ray Analyzer).
It is observed the presence of a cracked area at the left bottom (marked in colour)
along the paste interfaces which is caused by the formation of ettringite. It is not
filled in with white colour materials because the exposure time period was not
long enough for the formation of effringite; the white material filling in the
cracked area consists of needle-like crystals with lengths ranging between 2 and
6 µm. EDS analysis were carried out to determine ettringite or thaumasite [4].
Figure 3. Morphology of specimen attacked by 5% MgSO
4
solutions for 28 days. The
region (006) and (007) are the inner part and the outer part, respectively.
Table 3. Mass in percentage of the main chemical components at 006 and 007 regions.
Region
C O Mg Si S Ca Total
006 6.44 56.77 25.93 0.72 0.41 9.73 100.00
007 6.21 54.04 13.49 2.02 0.46 23.78 100.00
Fig. 4 shows the presence of Ca, Si and S, whereas Al is almost absent. Thus, it
can be concluded that thaumasite is present. The presence of high Mg and O
indicates that the specimen is attacked by the magnesium ions with the formation
of magnesium hydroxide (brucite) and the conversion of C-S-H into magnesium
silicate hydrate (M-S-H) occurred.

M.M. Amin et al. / Portugaliae Electrochimica Acta 26 (2008) 235-242
240
Figure 4. EDS spectrum at 006 and 007 regions with component peaks.
The mechanism of magnesium sulfate attack with cement hydrates as follows [6]:
MgSO
4
(aq)
+ Ca(OH)
2
CaSO
4
. 2H
2
O + Mg(OH)
2
MgSO
4 (aq)
+ C-S-H CaSO
4
. 2H
2
O + M-S-H
The presence of high Mg in the region is indicative of carbonation and of the
region where the brucite deposits. Furthermore, in the magnesium sulfate
solution, the increase in concentration led to higher rate of expansion. Comparing
the values in Table 3, it results that the magnesium peak is higher in the 006
region with 25.93% of mass than in the 007 region with 13.49%, which shows a
difference of 12.44%. This is because of the magnesium ions tend to attack the
deeper surface and replacing the calcium.
Ca
2+
and OH
-
are provided initially by dissolution of CH and the Si/Ca ratio of
the C-S-H begins to increase only when the CH has been depleted; it also implies
the use of Ca
2+
in the formation of gypsum
[6]. By comparing mass percentage of
regions 006 and 007, it is seen that when the area with high magnesium
precipitates, the percentage of calcium becomes lesser and the magnesium
percentage being low, suggests a high calcium percentage.
Fig. 5 shows a near-surface microstructure of a mortar after being immersed in
MgSO
4
solution of concentration 50,000 mg/L during 28 days and the
distribution of the main components. As a result, based on the distribution of

M.M. Amin et al. / Portugaliae Electrochimica Acta 26 (2008) 235-242
241
magnesium ions, shown in green, the magnesium cation tends to attack the inner
area of the mortar which is about 2 mm depth. At the area with highest
magnesium concentration, it can be seen that the calcium started to become lesser
at the same spot.
Figure 5. Distribution of Mg, S, Ca, Si, Al and O in the microstructure using EDS
elemental mapping.
Mg
2+
and SO
4
2-
ions independently make a line of attack into different regions,
which consists of calcium at the deeper region of the surface, exhibiting the

M.M. Amin et al. / Portugaliae Electrochimica Acta 26 (2008) 235-242
242
formation of the crystalline salt at the first stage. The following stages are the
formation of ettringite and gypsum which play an important role in softening the
materials, causing cracking. The formation of gypsum has a close relationship
with the Ca(OH)
2
content [7].
Conclusions
Based on the results of this study, it is concluded:
The chemical and mineralogical composition in the Ordinary Portland Cement
(OPC) were determined, following SEM and EDS techniques.
The formation of ettringite at the early stage causes cracking, thus being related
to the expansion which is the cause of damage in the cement paste.
Damage caused by the sulphate attack is attributed to decalcification, which
weakens the C-S-H matrix and partly the formation of ettringite, which causes
cracking and expansion.
The resistance of mortars to sulfate attack was influenced by the content of the
interfacial zone, which means the higher the content of interfacial zone, the faster
the cement mortar expanded. In order to improve the structure of the interfacial
zone, pre-treated quartz aggregate, which is composed of hydraulic surface layer
and inert core, can be used.
Mg
2+
and SO
4
2-
ions make a line of attack into different regions which consists
of calcium at the deeper region of the surface, resulting on the formation of the
crystalline salt.
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