Strength Properties of Self-compacting Mortar Mixed With GGBFS
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Transcript of Strength Properties of Self-compacting Mortar Mixed With GGBFS
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Corresponding author: [email protected] ; [email protected] Page 1
Original citation:
Lim, S.K., Ling, T.C., Hussin, M.W. (2012) Strength properties of self-compacting mortar
mixed with GGBFS. ICE-Construction Materials; 165(2):87-98.
http://www.icevirtuallibrary.com/content/article/10.1680/coma.10.00016
Strength Properties of Self-Compacting Mortar Mixed with GGBFS
Siong-Kang Lima, Tung-Chai Ling
b*, Mohd Warid Hussin
c
ABSTRACT
Self-compacting cement grout (SCCG) is one of the economical and an effective material usedfor repairing structural cracks. However, in terms of raw material cost, SCCG is higher than for
conventional concrete due to the high cement volumes at relatively low water-binder ratios to
achieve satisfactory combinations of high fluidity and stability. It is expected that ground
granulated blast furnace slag (GGBFS) can be used as an alternative material to replace high
volume of cement in preparing self-compacting repair mortar (SCRM). In this study, the
effects of GGBFS content on both fresh and hardened properties of SCRM were investigated.
The influence of different curing conditions on long term compressive strength was also
studied. In addition, the microstructure of some mixes at the age of 6 months was also observedby using scanning electron microscope. The results show that the workability and final
bleeding value of fresh SCRM decreased with the increase in GGBFS content. At early ages,
the compressive strength rate of SCRM incorporating GGBFS was lower but it increased with
time and became more pronounced at 30% to 50% replacement level. Thus, the maximum limit
of GGBFS replacement is suggested to be controlled at 50% to make the most excellent
development in long-term compressive strength. As for curing condition, specimens stored in
water showed higher gain in long-term strength than those samples exposed to air and natural
weather weathering conditions.
Keywords: Grouting/ Recycling & reuse of materials/ Strength and testing of materials
1. INTRODUCTION
The first grouting technique started 200 years ago to repair the structure damages of
Dieppa harbor by using grouting percussion pump in 1802 ( Bungey and Millardm 1996). In
England, Portland cement was used as cement grouting materials in 1838 during the
construction of the first Thames tunnel at Wapping. Soon after, cement grouting became
widely used in the early part of the last century ( Bungey and Millardm 1996). Nowadays,
several types of grout are used including cement, cement and sand, clay-cement, slag-cement,
resin gypsum-cement, clays asphalt, pulverized fuel ash and a large number of colloid and lowviscosity chemicals.
Cement-based grouts can self-compact under its own weight without segregation and
are easy to flow into place and have high filling ability. High fluidity characteristic of cement
grout is a prime requirement of high cohesion or segregation resistance during flow to formuniform and homogeneous grout (Bartos et al., 1996). As the fluid cement grout can be fully
compacted without vibration, the application of fluid cement grout can therefore reduce labour
and machinery, improve compaction and hence enhance durability of the critical cover zone of
structural member (Bartos et al., 1996). An essential feature of all grout systems is providing
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sufficient fluid to be injected into the void to be grouted and be set to a solid. This technology
also brings considerable advantages for concrete filling at narrow and complicated moulds
systems However, in terms of raw material cost; self-compacting cement grout (SCCG) is
higher than that of conventional concrete. The main reason is because of the use of chemical
admixtures and the use of large volumes of cement to achieve satisfactory combinations of
high fluidity and stability. In other words, SCCG requires high powder volumes at relatively
low water/binder ratios with significant quantities of superplasticizers.In order to address this scenario, in the past 10 years, several studies had been
conducted to utilize limestone powder and various types of supplementary cementing materials
as cement subsitute in self-compacting repair mortar (SCRM) (Felekoglu, 2008, Felekoglu et
al., 2006, Felekoglu et al., 2007, Turkel and Altuntas, 2009, Courard et al., 2002, Khayat and
Morin, 2002, OFlaherty and Mangat, 1999). Felekoglu (2008) conducted an extensive
laboratory study on the effects of using three types of limestone powders to replace cement in
self-compacting filling grouts (SCFG) products. The limestone powder used was obtained
from different quarries. He reported that the best performance at fresh and hardened properties
of SCFG could be achieved by using 10% special type of quarry dust as cement substitute.However, an optimum amount of limestone powder as cement replacement material may
depend on the application and construction purposes. Felekoglu et al. (2006) investigated and
compared the properties of SCRM containing fly ash and two types of limestone fillers. Thereplacement ratios by weight were varied from 20% to 60%, respectively. Based on the results
derived from early strength, both types of limestone powder were more effective than fly ash.
At 28-day compressive strength, 20% fly ash replacement gave higher value as compared to
other fillers but slightly lower than control mixes. However, at later strength beyond 28 days,
SCRM containing fly ash gave higher strength than the control mixtures due to the reactivity of
pozzolanic reaction by fly ash.
The application of steel fiber reinforcement in SCRM was made by Felekoglu et al.
(2007). It was noticed that no deleterious effect on compressive strength could be detected for
an additional of 2% steel fiber by volume due to the better compaction and homogeneity of
fiber distribution. Moreover, a strong improvement of 28-day flexural strength by 19% and
abrasion resistance by 42% of reinforced SCRM was detected. A most recent study conducted
by Turkel and Altuntas (2009) aims to compare the effects of limestone powder on the
properties of SCRM with other mineral additives such as silica fume, fly ash and combination
of both. In general, combinations of mineral additives at different proportions showed a higher
strength than using single mineral additive alone. The compressive strength results indicated
that 30% silica fume replacement ratio of cement obtained the maximum strength due to the
pore-filling effect and improved bonding between mortar matrix.
GGBFS is a potential hydraulic binder. The traditional usage of GGBFS as cement
replacement in conventional concrete or mortar decreases the early strength but improved in
late strength and mechanical properties (Robins et al.,1992, Olorunsogo, 1998, Atis and Bilim,
2007, Olorunsogo and Wainwright, 1998, Ozkan et al., 2007, Cakir and Akoz, 2008, Felekoglu,
2008, Shariq et al., 2008). While cement is used as a major composition in the cement grout, it
is expected that the GGBFS can be utilized as cement substitute to produce cost-effectiveSCRM with added environmental and technical benefits. The effect of GGBFS on long term
strength properties and curing behaviour particularly for SCRM is not yet studied and well
established. This present study is therefore designed to utilize high volume of GGBFS as a
replacement of commonly used Portland cement in SCRM. The effects of GGBFS content on
the flowability and bleeding of fresh stage and long-term strength under different curing
conditions of hardened properties are studied.
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2. EXPERIMENTAL PROGRAM
2.1 Materials
2.1.1 Ordinary Portland cement
Ordinary Portland Cement (OPC) complied with the Malaysian Standard MS 522: Part
1(2002), equivalent to the Type I Portland cement as per ASTM C 150 (2006) was used as a
cementing material to produce SCRM in this study. The OPC used with the brand of
SELADANG was obtained from Tenggara Cement Manufacturing Sdn. Bhd. Table 1 showsthe chemical compositions and physical properties of the OPC.
2.1.2 Ground granulated blast furnace slag
Ground granulated blast furnace slag (GGBFS) used in this study was a by-product
obtained from a local steel industry, Slag Cement (Southern) Sdn. Bhd, YTL. The GGBFS was
first sieved through 600 m in order to remove larger size particles and litter, if any. After
going through the sieving process for around one hour, only the slag that passed through a
45m sieve was collected. The slag activity index of this material was then conducted
following ASTM C989 (2009). Two cubes of reference cement (100% Portland cement mix)
and 50-50- slag cement-reference mortars from single batches were prepared on the same day,
respectively. The compressive strength (activity index) of GGBFS was determined on the 7th
and 28th day. Based on the results shown in Table 1, the GGBFS used in this study wasclassified as a category 100 slag according to ASTM C 989 (2009). The chemical compositions,
physical and mechanical properties of GGBFS are shown in Table 1.
Table 1:Chemical compositions, physical and mechanical properties of OPC and GGBFS
OPC GGBFS
Chemical ConstituentsSilicon dioxide (SiO2) (%) 20.1 28.2
Aluminium oxide (Al2O3) (%) 4.9 10.0
Ferric oxide (Fe2O3) (%) 2.5 1.8Calcium oxide (CaO) (%) 65.0 50.4
Magnesium oxide (MgO) (%) 3.1 4.6
Sulphur oxide (SO3) (%) 2.3 2.2Sodium oxide (Na2O) (%) 0.2 0.1Potassium oxide (K2O) (%) 0.4 0.6
Titanium oxide (TiO2) (%) 0.2 -Phosphorous oxide (P2O2) (%)
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ASTM C 637 (2009). This range of particle size was classified as grading 1 in fine aggregate
according to the ASTM C637 (2009) and is suitable to be used as filler in cement mortar.
2.1.4 WaterTap water used was clean, neutraland contained limited substances that does not no
cause any harm to the process of cement hydration and durability of produced self-compacting
repair mortars.
2.2 Mixture proportions
In this study, the SCRM composition was designed based on its consistency flow
without segregation using the flow cone method. Binder to sand ratio was fixed at 1 to 1 ratio
throughout this study. GGBFS was replaced from 30% to 60% of total cement by weight in an
increment of 10%. Table 2 shows the composition of SCRM with various replacement levels of
GGBFS. In accordance to ASTM C 827 (1987), the efflux or flow time needed to produce fluid
mixture should be controlled in the range of 10 to 30 seconds. Therefore, in order to fixed the
efflux time of self-compacting mortar at the range of 212 second, water to binder ratios of allthe mixtures in the range of 0.55 to 0.60 were obtained.
2.3 Specimens preparationThe mixing procedure of fluid SCRM was carried out in accordance to ASTM C 1107
(2008). The mixing procedures are described as follow:- First, OPC, GGBFS and graded dry
sand were mixed in a concrete mixer for three minutes until the materials were blended
intimately and uniformly. Water was then added into the dry mix and mixed for another three
minutes until it was uniformed. The dimension of 70.6 mm 70.6 mm 70.6 mm cube
samples in accordance to BS 1881: part 116 (1993) were used to determine the determination
of compressive strength. Prism measuring 100 mm 100 mm 300 mm in accordance toRILEM PC-2 (1975) was used to determine the flexural strength.
After 24 hours of casting, the specimens were demoulded and subjected to the
respective curing condition until the day of testing. Three curing conditions were adopted to
assess the influence of GGBFS content on the flexural and compressive strengths development.
The three curing conditions were described as below:
i) Air curing in laboratory at 27-300C average temperature with relative humidity of 65%.
ii) Natural weathering outside laboratory at temperature ranged from 26-360C with relative
humidity ranged from 65% to 90%.
iii) Continuous water curing at constant temperature of 260C.
2.4 Test methods
The flowability of fresh self-compacting cement grout was determined in accordance to
ASTM C 939 (2006). The flowability of the cementitious grout complied with the physical
requirements in accordance to ASTM C 937 (2006). The fresh cement grout was poured into a
clean moistened flow cone without any compaction and vibration until the grout surface rose
till the point gauge (17255ml). Once the flow cone was fully filled, unplug the cone, the conewas unplugged and simultaneously the time was recorded until it was time to empty the cone
(when light inside the flow cone was visible). For each mix property, two consecutive test run
was employed. The influence of GGBFS on the bleed characteristic of SCRM was investigated
in accordance to ASTM C 940 (2003). The percentage of bleed water was drawn at 15 minute
intervals during the first hour and at 30 minute intervals thereafter until cessation of bleeding.
The compressive strength test was performed in accordance to BS 1881: part 116
(1993).
For each mix property, three cubic samples were used for each age and curing
conditions. The compressive strength was determined on the 7th
day, 14th
day, 28th
day, 3
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months, 6 months, and 9 months after casting using a TONIPAC 300 testing machine with a
maximum capacity of 3000 kN. The flexural strength test was carried out in accordance to
ASTM C 78 (2009). In this test, only SCRM incorporating 40% and 50% GGBFS including
control mix exposed to three different curing conditions were tested at the age of 6 months. The
flexural strength reported represents the mean of three specimens. After the mechanical
strengths testing, some samples were selected to examine the microstructure as well as nature
of hydration product in the pastes.
3. RESULTS AND DISCUSSION
3.1 Flowability
Flowability is an important design parameter of self-compacting cement grouts. In
field, free flow and high filling ability of SCRM is preferred in grouting works to penetrate
cracks, fine pores and fissures in concrete completely. For this reason, sufficient water
(optimum w/b) for each mix proportion to produce a grout efflux time of 21 2 s was
determined in accordance to ASTM C 938 (2002). Based on the results in Table 2, it was
observed that as the GGBFS content increased, it is possible to restrict the efflux time specified
by ASTM C 938 (2002) by increasing the amount of water. The replacement of OPC at 30%,
40%, 50% and 60% with GGBFS increased the w/b ratio from 0.55 to 0.57, 0.58, 0.59 and0.60, respectively. In other words, the fluidity of SCRM mixtures decreased as the GGBFS
content was increased. Ozkan et al. (2007) also reported similar observations that replacement
of cement by GGBFS decreased the workability of fresh concrete. This may be attributed to the
fineness of GGBFS which leads to an increase in total area surface per unit volume.Table 2: Mix proportions of SCRM
Mix IngredientsSCRM mix compositions
CG-CTR CG-30 CG-40 CG-50 CG-60
Binder : Sand Ratio 1:1 1:1 1:1 1:1 1:1
Water : Binder Ratio 0.55 0.57 0.58 0.59 0.60
OPC Content (%) 100 70 60 50 40
GGBFS Content (%) 0 30 40 50 60
Flowability Fulfilled the requirement of ASTM C 938 (2002)
3.2 Bleeding control
Table 3 shows the results of percentage of bleed water at prescribed interval and final
bleeding rate of self-compacting repair mortars incorporating 0%, 30%, 40%, 50% and 60%
GGBFS. As seen from the table, the total bleeding percentage of all the SCRM mixtures is less
than 2% which satisfy the requirement of ASTM C 937 (2002). The results show that controlmix without GGBFS possessed the highest final bleeding value. Nevertheless, as the
percentage of GGBFS increased, it decreased the final bleeding value of SCRM. The reason for
this could be associated to the relatively higher fineness of GGBFS as compared with OPC.
According to ACI 226 (1987), the bleeding rate of concrete mix is governed by the ratio
of the surface area of solids to the volume of water. Therefore it can be concluded that the use
of high fineness GGBFS for optimum hydration will not have any considerable inverse effect
on the workability and strength properties of concrete. Additionally, by incorporating GGBFS
in repair mortar, it may reduce the risk of excessive bleeding of freshly placed mix, thus,
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resulting in better bonding strength along cracks and the interfaces between the coarse
aggregate and cement paste can be provided.
Table 3: Percentage of bleed water of SCRM mixtures
Bleeding at prescribed interval (%) CG-CTR CG-30 CG-40 CG-50 CG-60
1st
15 minutes 0.62 0.60 0.60 0.50 0.371
st30 minutes 1.24 1.24 1.25 1.23 0.87
1st
45 minutes 1.49 1.47 1.50 1.23 0.99
1st
hour 1.74 1.60 1.62 1.38 0.99
1 hour until 3 hours 1.99 1.76 1.62 1.38 0.99
Final bleeding (3 hours) (%) 1.99
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However, it should be noted that the compressive strength of air and natural weather
cured samples was rapidly leveled out after 14 days as shown in Figs 2 and 3. This is because
the water was intensively eliminated and evaporated by the scorching sunshine as well as hot
temperature in the tropical climate of Malaysia. The evaporation faced by air cured samples
can be noticed by the lighter density as compared to water cured samples. The high rate of
water evaporation through capillary pores in cement matrixes may result in insufficient water
for hydration of cement. The samples under natural weather might have either higher or lowercompressive strength and density as compared to the samples cured under air. During hot days
with low humidity and higher temperature, the moisture content inside the samples faced
higher evaporation through the diffusion mechanisms. On the other hand, during rainy days,
the natural weather cured samples provided a suitable condition to maintain adequate water for
hydration process as the inlet and outlet pores structure of the sample were restricted by rain
water. From the above mentioned study, it can be concluded that initial water curing for early
7th
day to 14th
day is important to provide adequate hydration for early strength development.
Based on the results shown in Figs 2 and 3, water cured samples acquired a slightly
higher compressive strength than air and natural weather cured samples. The continuousincrease in compressive strength with time for water cured samples after 14 days will have
risen due to sufficient water for ongoing hydration of the reactive siica in GGBFS and
remaining CaOH2. The 14 and 28-day compressive strength of water cured SCRM samplescontaining 30% and 40% GGBFS were slightly higher than the control SCRM, and for all the
replacement level beyond 40%, the relationships were below than that of the control SCRM.
The glassy compounds in GGBFS reacted slowly with water and it took time to obtain
hydroxyl ions from the hydration product of Portland cement to break down the glassy slag
parcels at this period of age. It was noted that additional C-S-H gel was formed as a result of the
pozzolanic reaction of CaOH2 in cement, with reactive SiO2 in GGBFS.
Fig 2: 14-day compressive strength of SCRM with varying percentage of GGBFS
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Fig 3: 28-day compressive strength of SCRM with varying percentage of GGBFS
Figs 4-6 show that as the curing age increased from 3 months to 9 months, the presence
of 30% to 50% GGBFS was highly beneficial for samples cured under water and air with the
compressive strength exceeding control mix samples. This indicated that GGBFS can achieve
sufficient early compressive strength, while providing higher long term strength. In order to
find a plausible reason for that, the SEM observation at the age of 6 months of water cured
CG-CTR and CG-50 specimens were performed. The SEM images obtained with different
magnification are shown in Fig. 7. As can be seen from Fig. 7 (a), there are great deals of
rod-like crystals of ettringite or monosulfate, and relatively larger pore were observed inCG-CTR (without GGBFS). On the other hand, Fig. 7 (b) shows a denser microstructure in
CG-50 sample, with improved pore structure by a certain amount of rod-like ettringgite and
lots of cotton-shaped CH cover throughout. Therefore it can be concluded that at 6 months of
age, GGBFS hydration and pozzonalic reaction were almost completed for all samples stored
in water. The improved compressive strength reflected the strengthening effect of fine GGBFS
on the mechanical properties of SCRM. However, a noticeable reduction in compressive
strength was observed at all ages as the content of GGBFS increased to 60%. Thus, the
maximum limit of GGBFS should be controlled at 50% to make the most excellent in long term
compressive strength development of SCRM.
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Fig 4: 3-month compressive strength of SCRM with varying percentage of GGBFS
Fig 5: 6-month compressive strength of SCRM with varying percentage of GGBFS
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Fig 6: 9-month compressive strength of SCRM with varying percentage of GGBFS
Table 4 shows the compressive strength development of SCRM expressed as
percentage of 28-day compressive strength being subjected to different curing conditions. The
results indicate that beyond 28 days of air curing, the compressive strength of control mix
gradually decreased due to inadequate moisture content in samples for hydration process. It can
be seen from the table with the age from 3 months up to 9 months, there is a general trend of
decreasing compressive strength of air cured SCRM samples, regardless of GGBFS content.
On the other hand, the compressive strength development of SCRM samples exposed to natural
weathering crucially depends on humidity and temperature of tropical climate thatinconsistently changed. Samples cured under inconsistent wet-dry cycles were considered as
severe environmental condition. A better compressive strength development may be achieved
with adequate moist-cured during rainy season and vice-versa during hot days. This is because
the rain water can be absorbed by the SCRM samples through capillary and pore structures to
compensate water loss via evaporation and maintain sufficient water in samples for both
hydration and pozzolanic reactivity.
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Fig. 7: Observation of SEM images of 6 months water cured SCRM samples (a) CG-CTR (b)
CG-50 at different magnification
CH
CH
CH
CH CH
CH
Ettringite
CH
CH
Ettringite
CH
CH
(a2)
(a3)
(a4)
(b2)
(b3)
(b4)
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Table 4: Compressive strength development of SCRM expressed as percentage of 28-day
compressive strength when being subjected to different curing conditions
Age Mix notation
Compressive strength development as percentage of
28-day compressive strength
Air Natural weather Water
7 days
CG-CTR 79 74 76
CG-30 79 67 62
CG-40 75 73 59
CG-50 80 66 60
CG-60 83 72 64
14 days
CG-CTR 96 93 84
CG-30 90 80 83
CG-40 86 83 79
CG-50 90 87 83
CG-60 88 88 75
28 days
CG-CTR 100 100 100CG-30 100 100 100
CG-40 100 100 100
CG-50 100 100 100
CG-60 100 100 100
3 months
CG-CTR 92 122 125
CG-30 111 133 135
CG-40 105 119 124
CG-50 121 113 144
CG-60 111 140 107
6 months
CG-CTR 91 142 135
CG-30 104 121 134
CG-40 102 134 125
CG-50 114 119 166
CG-60 121 121 108
9 months
CG-CTR 87 128 128
CG-30 109 139 140
CG-40 98 118 137
CG-50 111 137 174
CG-60 125 139 113
3.4 Flexural Strength
Fig. 8 shows the results of a 6-month flexural strength. Based on the results, both
SCRM samples containing 40% and 50% GGBFS cured under air and natural weather
conditions exhibited a lower flexural strength than that of control mix. However, it can be
noticed that under water curing condition, SCRM samples containing 40% and 50% GGBFS
exhibited higher flexural strength than control mix. 40% of GGBFS replacement achieved 10.9
MPa while 50% of GGBFS replacement achieved 9.4 MPa after a 6 months period of water
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curing, which is about 27% and 17% higher than that of the control mix. The improved flexural
strengths may be attributed to the sufficient water for cement hydration process and provide
better bond between paste-aggregate. Therefore, adequate water is also important for long term
flexural strength development of SCRM.
Fig 8: 6-month flexural strength of SCRM with varying percentage of GGBFS under different
curing conditions (selected mixes)
4. CONCLUSIONS
Based on the results obtained in the present investigation, the following conclusions are
made.1. Incoporating GGBFS in the SCRM decreased the flowbility due to its fineness and
higher amount of water is needed to compensate for the loss in workability.
2. The increased content of GGBFS from 30% to 60% decreased the bleeding rate of freshSCRM mixtures, reflecting the beneficial effect of GGBFS for repairing work.
3. There was a systematic decrease in compressive strength with the increase in GGBFScontent during the early age, however, beyond 28 days and up to 9 months, the presence
of 30% to 50% GGBFS in SCRM exceeded the strength of the control mix because of
its pozzolanic reactivity.
4. A noticeable reduction in compressive strength was observed at all ages as the contentof GGBFS reached 60%, thus the maximum use of GGBFS is suggested to be 50% or
less to make the most excellent in long term compressive strength development of
SCRM.
5. Water curing condition reflected the best performance in long term compressivestrength development of SCRM as compared to air and natural weather curing
conditions.
6. A 6-month flexural strength of water cured SCRM made with 40% and 50% GGBFSexceeded that of the SCRM made with pure OPC due to sufficient water for long-term
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hydration process and pozzolanic activity. On the contrary, the air and natural cured
SCRM made with 40% and 50% GGBFS showed a lower flexural strength gain than
SCRM made with pure OPC.
Overall test results in this study demonstrate that it is beneficial to use GGBFS as
cement replacement to prepare self-compacting repair mortars for grouting works. However,
due to very high fine material contents in SCRM, further investigation on shrinkage cracking is
required before it can be introduced to the construction industry. As expected, the positive
influence of GGBFS can be more pronounced in these mixtures even at higher replacement
ratios of GGBFS when superplasticizers are used. Therefore, the use of superplasticising
admixtures in SCRM mixtures with lower w/b ratios for further investigation can be
considered.
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