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The effect of cement mortar composition on the pH value

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Cement-based materials (CBMs) initially start their life at a high pH of about 12.5- 13.5, which is further reduced due to several factors. These include ageing, penetration of moisture, carbonation, chloride ingress, alkali leaching, corrosion, and other biodegradation processes. A less reported factor enhancing the above reduction is incorporation of supplementary cementitious materials (SCMs) as partial replacement of cement due to the consumption of Ca(OH)2 in their pozzolanic reaction. In this study, pH and effect of Ca(OH)2 contents of cement mortars having 50% of fly ash and ground granulated ballast furnace slag were studied up to the age of 150 days. The results obtained strongly indicate that pH is not only dependent on the Ca(OH)2 content in cement mortars as specified in the most previous studies.
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The effect of cement mortar composition on the pH value
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8th Global Conference on Materials Science and Engineering (CMSE2019)
IOP Conf. Series: Materials Science and Engineering 770 (2020) 012026
IOP Publishing
doi:10.1088/1757-899X/770/1/012026
1
The effect of cement mortar composition on the pH value
P Shafigh1,2, S Yousuf 3, J C Lee4,5 and Z Ibrahim3
1Department of Building Surveying, Faculty of Built Environment, University of
Malaya, 50603 Kuala Lumpur, Malaysia
2Center for Building, Construction & Tropical Architecture (BuCTA), Faculty of Built
Environment, University of Malaya, 50603 Kuala Lumpur, Malaysia
3Department of Civil Engineering, Faculty of Engineering, University of Malaya,
50603 Kuala Lumpur, Malaysia
4Department of Civil Engineering, Faculty of Engineering, UCSI University, Cheras,
56000 Kuala Lumpur, Malaysia
E-mail: leejc@ucsiuniversity.edu.my
Abstract. Cement-based materials (CBMs) initially start their life at a high pH of about 12.5-
13.5, which is further reduced due to several factors. These include ageing, penetration of
moisture, carbonation, chloride ingress, alkali leaching, corrosion, and other biodegradation
processes. A less reported factor enhancing the above reduction is incorporation of
supplementary cementitious materials (SCMs) as partial replacement of cement due to the
consumption of Ca(OH)2 in their pozzolanic reaction. In this study, pH and effect of Ca(OH)2
contents of cement mortars having 50% of fly ash and ground granulated ballast furnace slag
were studied up to the age of 150 days. The results obtained strongly indicate that pH is not only
dependent on the Ca(OH)2 content in cement mortars as specified in the most previous studies.
1. Introduction
Cement-based materials (CBMs), such as concrete, mortar and paste, start their life at a high pH of about
12.5 to 13.5 due to the presence of portlandite (CH or Ca(OH)2 contents). The portlandite is by-product
of cement hydration process and the main reason of high pH of CBMs [1,2]. The pH of CBMs does not
remain constant and varies with time due to several factors. These factors include carbon dioxide, acidic
gases, chlorides and moisture that can penetrate into the embedded reinforcement through the process
of infiltration, diffusion and capillary action [3,4].The main processes involved in pH reduction of
concrete are, carbonation, corrosion, chloride ingress, biodegradation and acid attack [5].
The durability, strength and service life of concrete structures is directly affected by their pH values.
Czarnecki and Woyciechowski [6] stated that CH contents and other alkaline hydroxides present in the
concrete maintain the durability of concrete structures. According to Alotaibi [7], the high pH of
concrete offers the best safety to embedded reinforcement against destructive agents. The high pH of
concrete can protect passive layer of reinforcement for hundreds of years from damaging.
Both, low and high pH than normal pH value of concrete are dangerous for the durability and long
service life of concrete structures. The pH of concrete does not remain constant and it may change due
to the penetration of carbon dioxide, chlorides and moisture. The main processes involved in pH
reduction of concrete are, carbonation [8], corrosion [9], chloride ingress, biodegradation and acid attack
[5]. However, high pH of concrete may also cause deterioration in concrete such as alkali silica reaction,
porosity and moisture related damages [10].
8th Global Conference on Materials Science and Engineering (CMSE2019)
IOP Conf. Series: Materials Science and Engineering 770 (2020) 012026
IOP Publishing
doi:10.1088/1757-899X/770/1/012026
2
Currently, the use of supplementary cementitious materials (SCMs) such as fly ash (FA) and ground
granulated blast-furnace slag (GGBFS) as partial replacement of ordinary Portland cement (OPC) has
been increasing due to commercial, environmental and sustainable issues [11,12]. The SCMs contain
high amounts of alumina and amorphous silica. Therefore, they are used to improve the durability of
CBMs through filler effect and pozzolanic reaction [13,14].
According to some researchers [15-17], the SCMs decrease pH of CBMs by consuming CH contents
in their pozzolanic reaction and small amount of pure cement in the mixes. However, the values of
reduction in pH of CBMs due to addition of SCMs has not been reported in the present literature.
Therefore, the aim of this study is to investigate the influence of using high volumes of FA and GGBFS
on the pH of cement mortars with passage of time up to 150 days. In addition, the effect of CH contents
on the pH of mortars have been studied in detail.
2. Experimental program
2.1. Materials
Ordinary Portland cement (OPC) with a specific gravity of 3.12 was used in all the mixes. The specific
surface area (SSA) of the OPC based on the Blaine and B.E.T. tests were determined to be 351 m2/kg
and 2667.24 m2/kg, respectively. The class F fly ash (FA) had specific gravity of approximately 2.29.
The color of FA was whitish grey. According to the B.E.T. test, the SSA of FA was determined to be
2858.6 m2/kg.
The ground granulated ballast furnace slag (GGBFS) had specific gravity of approximately 2.83. The
color of GGBFS was off-white. According to the B.E.T. test, the SSA of GGBFS was determined to be
3197.2 m2/kg. Local mining sand having maximum grain size of 4.75 mm and specific gravity of 2.68
was used in this study. The water from the pipeline of the lab was used in all mixes and for curing of
the samples. The chemical composition of OPC, FA, GGBFS was determined by ‘‘X-ray fluorescence
spectrometry (XRF)” and shown in table 1.
Table 1. Chemical compositions and LOI of OPC, GGBFS and FA (% by mass).
Chemical composition
OPC
GGBFS
FA
CaO
60.68
40.88
12.78
SiO2
20.46
35.98
40.10
Al2O3
3.86
13.47
17.05
Fe2O3
3.38
0.43
15.05
MgO
3.10
5.42
6.68
P2O5
0.06
0.01
0.20
TiO2
0.17
0.63
0.88
K2O
0.26
0.36
1.05
SO3
2.20
1.75
0.63
SrO
0.03
0.05
0.07
MnO
0.15
0.20
0.21
LOI
2.23
0.72
0.70
*LOI = Loss on ignition
2.2. Mix proportions and mixing procedures
A total of three different mortar mixes were produced. In a control mortar having cement to sand ratio
of 1:3 and water to cement ratio of 0.48, the OPC was replaced with 50% of FA and GGBFS by weight.
Table 2 shows the mix proportions of all mortars in one batch. All mortars had a flow of 210±10 mm.
However, for GGBFS mortar SP was needed to achieve the same flow.
For mixing, at first, binder and sand were dry mixed for 2 minutes. Afterwards, the mixture of SP
and about 70% of the mixing water were added to mixture and mixing was done for further 3 minutes.
Subsequently, the remaining water was added to mixture and mixing was done for further 5 min to get
homogeneous mixing. Then, the workability was performed using flow table test.
8th Global Conference on Materials Science and Engineering (CMSE2019)
IOP Conf. Series: Materials Science and Engineering 770 (2020) 012026
IOP Publishing
doi:10.1088/1757-899X/770/1/012026
3
Table 2. Mix proportions of mortars in one batch.
Binder
Water (kg)
w/b
Sand (kg)
SP (% of binder)
OPC (kg)
FA (kg)
GGBFS (kg)
12.50
-
-
6
0.48
37.5
1
6.25
6.25
-
6
0.48
37.5
-
6.25
-
6.25
6
0.48
37.5
1.5
Fresh mortar was then cast into 50 mm cube steel molds in two layers. Each layer was compacted
using vibrating table. One day after casting, all the cube specimens were demolded and cured under
water curing at room temperature (WC), until the samples were used for compressive strength test at the
ages of 2, 28 and 150 days.
2.3. Test methods
Flow table test was used to control the workability of mixtures. All mixtures were maintained in a good
workability with a flow of 210±10 mm. The calculated compressive strength for each mix was the
average of four tested samples. The compressive strength measurements were carried out by using ELE
testing machine press with a capacity of 3000 kN and loading rate 0.5 kN/s. Compressive strength tests
have been done according to BS [18].
For pH measurement, initially, the inner portions of the cube were taken after crushing the specimen
with compression machine. Then, these portions were grinded using grinding machine [19]. The 20g of
prepared powder was used for the pH measurement. This powder was mixed with 40 g water (dilution
ratio of 1:2) as recommended by Grubb et al. [20]. This solution was stirred using magnetic stirrer for
15 minutes. Then, the solution was filtered using no. 40 filter paper with a 110mm diameter. Finally,
three pH readings were taken by digital pH meter. The prepared solution was not stirred during the
measurement process. For each time of the pH reading, the pH value was recorded after pH meter
showed a stable reading.
Due to availability of buffer solutions for this experimental work, the calibration of pH meter was
done by buffer solutions of pH 7.01 and 4.01. It should be noted that based on the different experimental
results of various CBMs done by the authors, the difference due to calibration with buffer solutions of
pH 7.01, 10.01 and pH 7.01,4.01 was about 0.6.
The thermal gravimetric analysis (TGA) test was used to determine the CH contents for all mortars
at the age of 2, 28 and 150 days. The TGA testing is used to measure the weight changes in relation to
temperature changes. It highlights the point at which the weight loss is the most apparent, provides the
decomposition rate and is helpful for evaluating the mass loss steps accurately. The obtained curves
consist of mass loss (%) and derivative weight (%/min). During testing, sample of around 100 mg was
heated at 10°C/min from about 30-1000°C in the nitrogen atmosphere at constant rate of 20 ml/min.
According to several reports [21-23], the CH content is determined from the percentage weight loss
between around 300-550°C.
3. Results and discussion
3.1. Compressive strength
The compressive strength results of control, FA-50 and GGBFS-50 are shown in table 3. The results
showed reduction in compressive strength of mortars due to incorporation of FA and GGBFS as
described by the previous studies [24, 25]. There was improvement in compressive strength of SCMs
blended cement mortars with time due to pozzolanic reaction of FA and GGBFS. The strength of
GGBFS-50 mortar was comparable with control mix at the ages of 28 and 150 days.
8th Global Conference on Materials Science and Engineering (CMSE2019)
IOP Conf. Series: Materials Science and Engineering 770 (2020) 012026
IOP Publishing
doi:10.1088/1757-899X/770/1/012026
4
Table 3. Compressive strength test results.
Compressive strength (MPa)
2-day
28-day
150-day
24.3
46.9
62.7
9.2
25.1
37.8
20.5
47.1
57.5
3.2. The pH measurements and CH quantity
The measured pH values of control, FA-50 and GGBFS-50 mortars at the ages of 2, 28 and 150 days
are given in table 4. The CH quantities of all mortars obtained by TGA testing are given in table 5. The
obtained results showed reduction in pH of blended cement mortars due to FA and GGBFS with age in
accordance to the previous studies [15,26,27]. However, the TGA CH results showed increase in CH
contents of FA-50 mix with time. It might be due to more dominant filler effect of FA than its pozzolanic
effect [28]. The reduction in CH contents of GGBFS-50 mix showed pozzolanic reaction of GGBFS
with passage of time [29]. It can be concluded from the obtained results that pH is not only dependent
on the CH contents. Therefore, other factors affecting pH of SCMs-blended cement mortars should be
investigated.
Table 4. The pH measurement results of mortars.
Mix Name
pH value at different ages
2-day
28-day
150-day
Control
12.5
12.3
12.1
FA-50
12.4
12.2
11.7
GGBFS-50
12.4
12.2
11.9
Table 5. The CH quantities of mortars.
Mix Name
Quantity of CH
2-day
28-day
150-day
Control
1.468
1.754
2.609
FA-50
1.163
1.423
1.572
GGBFS-50
1.360
1.252
1.223
4. Conclusions
This paper presents a detailed study on the pH of cement mortars containing 50% of fly ash (FA) and
ground granulated ballast furnace slag (GGBFS) with passage of time. The following conclusions can
be drawn from the study:
There is reduction in pH of cement mortars due to incorporation of FA and GGBFS with passage
of time. However, this reduction is not significant, and pH is in the safe range to avoid corrosion
of rebars and any other durability related problems.
The pH of blended cement mortars is not only dependent on the CH contents. Therefore, the
other factors affecting pH of cement mortars should be investigated.
Acknowledgment
The authors would like to acknowledge to UCSI University Pioneer Scientist Incentive Fund, Malaysia
(Project code: Proj-2019-In-FETBE-066) for providing research facilities and materials.
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Supplementary cementitious materials (SCMs) have been widely used all over the world in ready-mixed concrete due to their economic and environmental benefits; hence, they have drawn much attention in recent years. Whether deriving from industrial waste, agro-waste or by-products, supplementary cementitious materials can be mixed with blended cement to enhance concrete strength. Supplementary cementitious materials may contain fly ash (FA), silica fume (SF), ground granulated blast furnace slag (GGBFS), rice husk ash (RHA), metakaolin (MK) and palm oil fuel ash (POFA), to name a few. The utilization of these materials in concrete can partially reduce the consumption of Portland cement, which, in turn, can lessen construction costs, providing materials suppliers, contractors and engineers with substantial advantages. Furthermore, despite the drawbacks of their binary blends, the combination of supplementary cementitious materials can lead to many advantages, such as optimized strength, workability and durability. Unfortunately, these advances have not been fully taken into consideration in state specifications. Hence, by adopting a review approach, this study aimed to provide new insights into the effect of the incorporation supplementary cementitious materials on the properties of mortar and concrete.
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