2015 World of Coal Ash (WOCA) Conference in Nasvhille, TN - May 5-7, 2015
http://www.flyash.info/
Compressive Strength of Chemically and
Mechanically Activated Aluminosilicate systems.
Grizelda du Toit1, 2, Elizabet M. van der Merwe2, Elsabé P. Kearsley3,
Mike McDonald1 and Richard A. Kruger4
1
Afrisam SA, C/o Main Reef & Elias Motsoaledi Roads, Roodepoort, Johannesburg,
1724; Departments of 2Chemistry and 3Civil Engineering, University of Pretoria,
Lynnwood Road, Pretoria, 0002; 4Richonne Consulting, PO Box 742, Somerset Mall,
7137, South Africa
CONFERENCE: 2015 World of Coal Ash – (www.worldofcoalash.org)
KEYWORDS: fly ash, hybrid cement, chemical activation, mechanical activation
ABSTRACT
The aim of this study is to evaluate the reaction products, performance and suitability of
activated, high fly ash containing cement blends in an effort to reduce CO2 emissions by
reducing clinker factors; and to optimally utilize South African fly ash in blended
cements.
Due to the high energy demand and the emission of greenhouse gasses during clinker
production, it is common practice to utilize appropriate supplementary cementitious
materials (SCMs) to offset environmental impact. These materials are usually industrial
by-products that must otherwise be stockpiled or disposed.
The purpose of this research is to produce hybrid-alkali activated aluminosilicate
systems containing up to 70% fly ash, by means of both mechanical and chemical
activation of siliceous coal fly ash.
Three siliceous fly ashes produced from coal combustion at the same power station,
differentiated by fineness and/or particle shape (due to mechanical activation), have
been chemically activated by addition of varying amounts of alkali activator.
These specimens were subjected to physical testing, including both mortar and
concrete compressive strengths at different curing ages. Combined activation of fly ash
indicated significant improvements in hybrid cements with regards to compressive
strength in mortar and concrete specimens, especially in early age strength
development.
INTRODUCTION
Blended cements and concrete containing high volumes of SCMs, have become a
pertinent research topic in recent years. The reason for the increased interest in gaining
more knowledge and a better understanding of this subject matter is driven by the
cement industries’ need to produce more environmentally friendly products. It is well
known that clinkering, a key step in cement production is energy intensive (kiln
temperatures of up to 1450 °C), and produces large amounts of greenhouse gasses
which are emitted into the atmosphere. The production of blended cements provide an
alternative option of improving the environmental impact, by making use of suitable byproducts like coal fly ash that would otherwise be disposed or stockpiled1-3.
The particular SCMs under consideration in this specific study, is siliceous coal fly ash
from a coal-fired power station in South Africa, Due to the country’s significant coal
reserves the generation of electricity (and fly ash), is dominated by coal-fired power
stations.
Eskom, one of the top utilities in the world, is the state-owned power utility and supplies
approximately 95% of the electricity consumed in the country. In 2011, Eskom used 125
million tons of coal and produced 36.2 million tons of ash from their coal-fired stations.
Currently, two new supercritical coal-fired 6 x 794 MW (gross) power stations are being
constructed, and are the largest ever ordered by Eskom. Once these utilities are
completed, Eskom will generate an estimated 45 million tons of coal ash per year.
Blended cements
The production of blended cement is the predominant application of fly ash in the South
African building and construction industry, with 72% of the SCM volume being fly ash.
The domestic market for coal ash for non-cementitious applications is very small4.
The current South African National Standard (SANS 50197-1:2013) specification for the
composition and conformity criteria of common cements, was adopted from the
European Standard (EN 197-1:2011). It lists 27 common cements, of which eight allow
for the addition of siliceous fly ash, and two for calcareous fly ash (not available in South
Africa).
The maximum level of replacement of clinker with fly ash is for CEM IV/B cement, which
allows up to 55% fly ash5. This study aims to go beyond the specified 55% level of
clinker replacement, and investigates the activation and performance of blended
cements containing substantially higher amounts (up to 70%) of fly ash. These
formulations are also known as hybrid cements6.
An understanding of the activation mechanism and physical performance of such
activated hybrid aluminosilicate systems, as well as characterization of the reaction
products, will provide much needed and valuable information towards the production of
low carbon footprint cementitious products.
When used in blended cement systems, the effect of the pozzolanic reaction between
fly ash and portlandite is delayed. The end result being that the early strength gain is
compromised and the influence of the pozzolanic reaction is only observed at later ages
of curing.
Cement activation
As is demonstrated in this investigation the strength development characteristics are
significantly improved by subjecting the fly ash to a combination of mechanical (milling
of the ash) and chemical (Na2SO4) activation. These hybrid cements were water cured
at room temperature.
The early age hydration of hybrid cement activated by means of Na2SO4 as an alkali
activator has been studied before7,8. Depending on reaction conditions and composition
of the raw material used these authors postulate that the following principal reactions
may occur during early hydration:
Na2SO4 + Ca(OH)2
CaSO4.xH2O + C3A
→
→
2 NaOH + CaSO4.xH2O
ettringite (Aft)
(1)
(2)
The presence of SO42- enhances the initial dissolution of alite as well as the formation of
ettringite, resulting in a denser matrix and an increase in early compressive strength7.
There is however uncertainty regarding the structure and composition of the main
reaction products of these hybrid cements.
It is generally accepted that the main reaction product is a tetrahedrally coordinated
Na2O.Al2O3.SiO2.nH20 gel (N-A-S-H) or a (N,C)-A-S-H-type gel, where the calcium (C)
content is determined by the local availability of calcium. It is the cross-linked nature of
the N-A-S-H-type gel which is believed to be responsible for the increase in early
strength1,7,9.
It has been reported that mechanical activation of fly ash leads to increased reactivity,
especially when the median particle size (d50) is reduced to less than 5-7 µm; the critical
particle size for silicates below which mechanical activation begins to manifest itself10-12.
Jueshi et al13 proved that the combination of grinding and the addition of Na2SO4
produced higher compressive strength compared to any single method of activation for
lime-fly ash systems.
MATERIALS AND METHODS
Materials
Three different fly ash products were used in this study. These are:
 an unclassified ash (d50 ~ 50-60 µm),
 a very fine classified ash (d50 < 5 µm),
 and a mechanically activated (milled) residue (d50 < 5-7 µm) of the unclassified
ash.
The cement used contained 8% limestone and had a density of 3.14 g/cm3.
The chemical and mineralogical composition of the fly ash samples and cement are
presented in Tables 1 and 2.
From the compositional analysis, it is evident that all three fly ash samples contain a low
amount of volatiles, which is evident from the low loss on ignition (LOI) values. The
South African National Standard on fly ash (SANS 50450-1:2014), adopted from EN
450-1:2012, specifies LOI not greater than 5% by mass.
High loss on ignition values due to unburnt carbon, may lead to numerous issues in
concrete mixes for example higher water – and activator demands.
Table 1. Chemical composition of raw materials (wt. %).
Unclassified
Classified
Mechanically activated
fly ash
fly ash
unclassified fly ash
LOI
0.85
1.23
1.52
SiO2
54.06
55.21
54.54
Al2O3
34.78
31.15
30.38
CaO
4.58
4.86
6.10
Fe2O3
3.09
3.61
3.83
MgO
1.27
1.14
1.20
K2O
0.73
0.62
0.63
Na2O
0.23
0.17
0.17
TiO
1.76
1.57
1.54
Mn2O3
0.03
0.03
0.03
P2O5
1.01
0.57
0.55
SO3
0.40
0.28
0.31
Total
102.79
100.44
100.80
Table 2. Mineralogical composition of raw materials (wt.% normalized).
Unclassified
Classified
Mechanically activated
Fly Ash
Fly Ash
unclassified Fly Ash
Anhydrite
Belite
1.04
Alite
1.41
Brownmillerite
Calcite
Gypsum
Hematite
1.35
0.30
0.81
Mullite
36.09
26.32
29.02
Quartz
14.84
3.93
11.49
Amorphous
47.73
69.45
56.24
Cement
3.72
20.22
4.54
65.14
2.54
1.71
0.47
0.11
0.42
0.14
0.09
2.73
101.83
Cement
2.26
7.51
48.00
14.26
7.47
0.92
1.21
18.38
The fly ash samples consist predominantly of an amorphous alumina silica glass phase,
which is higher in classified ash than in unclassified ash. The two major crystalline
phases are mullite, the most abundant, and a lesser amount of quartz. After milling the
unclassified ash both the mullite and the quartz decreased with a concomitant increase
in the amorphous phase. It is believed that the small amount of belite and alite evident
in the mechanically activated fly ash was due to contamination during the milling
process.
Commercially available Na2SO4 powder (99% purity) was used in all the chemically
activated mixes, and was added in powder form directly to each mix. The Na2SO4
content is reported as a percentage of the cementitious mass.
Analytical Methods
The oxide composition (except SO3) of the raw materials was determined by fused bead
analysis on XRF equipment (PANalytical Axios Cement). In order to produce the fused
bead, 1 g of sample was mixed with 5g of fluxing agent (100% Li2B407) and fused at
1050 °C. The fine powdered samples were heated to 1050 °C and the mass loss used
to determine loss on ignition (LOI). Following the de-carbonation, all samples were
manually pulverized.
The SO3 content was determined by combustion in a LECO S-144DR instrument.
XRD measurements were carried out by using a PANalytical X’Pert Pro powder
Diffractometer an X’Celerator detector and variable divergence- and fixed receiving slits,
with Fe filtered Co-Kα radiation (λ=1.789Å).
The phases were identified using X’Pert Highscore plus software. After addition of 20%
Si (Aldrich 99% pure) and milling in a McCrone micronizing mill, the samples were
prepared for XRD analysis using back loading preparation method. The relative phase
amounts (weight %) were estimated using the Rietveld method (Autoquan Program).
The respective particle size distribution analyses were determined utilizing a
Mastersizer 2000 laser particle size analyzer fitted with a Scirocco 2000 sample
handling unit from Malvern. Scattered light data were recorded for 25 seconds and
25 000 measurement snaps. Size data collection was performed within the mandatory
obscuration in the range of 10-20%.
To study morphology of the fly ash samples, images were obtained with a Zeiss Ultra
SS (Germany) field emission scanning electron microscope (FESEM), operated at an
acceleration voltage of 1 kV under high-vacuum conditions. The FA powder was
mounted on double-sided carbon tape by dipping carbon stubs into the samples.
Excess material was removed by gentle blowing with compressed nitrogen.
Heat of hydration was measured using a TAM Air microcalorimeter from TA
Instruments, at 25 °C and 0.5 water:binder ratio.
Blends for all of the physical test work (mortar and concrete) consisted of 70% of the
relevant fly ash product and 30% Portland cement. The blends were chemically
activated by adding 1%, 3% and 5% Na2SO4 respectively.
Mechanical activation of unclassified fly ash was carried out in a laboratory ball mill (Lab
43) in 20 kg batches, until a d50 < 5 µm was achieved (4 hours). The stainless steel balls
used as the milling media consisted of approximately 50% of 15 mm and 40 mm each.
The material to media ratio of 1:9.9 was maintained throughout.
Preparation, mixing and compressive strength testing of all mortar blends were done as
per SANS 50196-1: Methods of testing cement Part 1: Determination of strength.
Setting time was determined according to Part 3: Determination of setting times and
soundness.
Compressive strength testing of all concrete blends were done as per SANS 5863:2006
Compressive strength of hardened concrete. Consistency was determined according to
SANS 5862-1:2006 Consistence of freshly mixed concrete – slump test. All the concrete
mixes were prepared at a 0.67 water:binder ratio, and contained 90 kg/m3 cement and
210 kg/m3 fly ash product. Sets of two 100 mm cubes were crushed for compressive
strength purposes.
RESULTS AND DISCUSSION
Effect of mechanical activation on particle size distribution (psd) and morphology
of coal fly ash
Even though the chemical and mineralogical composition of the unclassified ash did not
change drastically after mechanical activation, significant changes were observed
regarding the physical nature of the ash. It is clearly evident from Table 3 and Fig. 1,
that mechanical activation reduces the particle size. One can also see the similarities in
mean particle size for classified and mechanically activated ash at both d10 and the
median size d50, however, as can be observed from the d90 value, the milled ash has
more coarse particles and therefore a broader particle size distribution than the
classified ash.
Table 3. Particle size analysis indicators for the three fly ashes (µm).
Unclassified
Classified
Mechanically activated
fly ash
fly ash
unclassified fly ash
d10
4.44
0.53
0.55
d50
52.46
3.07
4.79
d90
224.22
6.67
19.01
Figure 1. Particle size distribution for the three fly ashes.
The differences in the morphology of the three fly ash products are also clearly evident
from the scanning electron microscopy (SEM) images portrayed in Fig. 2.
Figure 2. Scanning electron micrographs for the 3 fly ash products used. From left to
right: unclassified fly ash, fine classified fly ash, and milled unclassified fly ash.
Setting time and heat of hydration
Table 4 lists the setting time of the three different fly ashes at the corresponding sulfate
additions. The setting time reduces with increasing sulfate addition for all three chemical
activated fly ash blends. It is also worth noting that the results for the mortars containing
unclassified ash shows a trend of shortened setting time after being subjected to
mechanical activation. The classified ash blends have significantly prolonged setting
times compared to the corresponding milled and unclassified blends.
Table 4. Mortar setting times (minutes).
0% Na2SO4
1% Na2SO4
3% Na2SO4
5% Na2SO4
Unclassified fly ash blend
386
351
276
260
Classified fly ash blend
712
612
526
406
Milled unclassified fly ash blend
351
316
218
230
The cumulative heat (J/g) versus time, and heat flow (J/g.h) versus time profiles for the
respective samples, taken for the initial 48 hours (early age hydration), as well as at 23
days are shown in Figures 3-4. The maximum heat flow and total heat values are
presented in Table 5. For all the graphs, the following abbreviations are used: sodium
sulfate (SSF), unclassified fly ash (U-FA), classified fly ash (C-FA) and milled
unclassified fly ash (MU-FA).
Figure 3. Heat flow rates, 48 hours and 23 days.
Figure 4. Cumulative heat, 48 hours and 23 days.
Table 5. Calorimetric data.
Maximum heat flow (J/g.h)
0% Na2SO4
5% Na2SO4
Unclassified fly
ash blend
Classified fly ash
blend
Milled unclassified
fly ash blend
Cumulative heat (J/g)
0% Na2SO4
5% Na2SO4
Flow
Hours
Flow
Hours
48 hrs
23 days
48 hrs
23 days
3.53
17.14
4.42
12.27
85.78
178.45
103.80
217.25
3.56
28.00
4.17
14.19
81.13
172.59
113.18
199.59
2.97
17.21
5.97
9.79
98.22
218.56
158.42
242.89
It is clearly evident from Fig. 3 that the addition of sulfates to the hybrid cement blends
increases both the rate of hydration, as well as the peak heat evolution intensities for all
of the represented scenarios. When they investigated the effect of sulfate addition on
hybrid cements, Donatello et al7 concluded from similar calorimetric data that in the
presence of SO42-, the initial dissolution of alite is enhanced, setting times shortened
and early compressive strength increased.
The cumulative heat graph (Fig. 4) shows that combined activation not only increases
the rate of hydration, but also continues to have the highest heat output up to 23 days.
This result agrees with combined activation blends also producing the highest mortar
compressive strength, especially at early curing ages.
Compressive strength of mortar bars
With increasing sulfate addition, mortar compressive strength for the unclassified ash
blends (Fig. 5) indicates an increase in strength for all curing ages up to 90 days.
However, at 180 days of curing, the strength decreases with increasing sulfate addition.
At 3% sulfate addition, there is a 3.2% drop in strength between 90 days and 180 days,
and a 16.8% drop at 5% sulfate addition for the same two curing ages.
The classified fly ash blends (Fig. 6) also show that, up to 90 days, increasing the level
of sulfate increases the compressive strength. In contrast to the unclassified ash, the
strength at 180 days is not negatively affected with increasing sulfate addition.
However, at 3% sulfate addition, there is a 3.6% drop in strength evident from 90 days
to 180 days.
Figure 5. Mortar compressive strength for unclassified fly ash blends.
Figure 6. Mortar compressive strength for classified fly ash blends.
The blends consisting of both mechanically and chemically activated fly ash (Fig. 7),
followed a similar strength trend as the normal unclassified fly ash blends up to 28 days.
Regardless of the amount of Na2SO4 added, the combination of chemical activation and
milling was the most effective approach and enhanced mortar strengths up to 90 days.
This finding also correlates with work done by Jueshi et al13 on lime-fly ash blends up to
28 days of curing, where the combination of grinding and addition of sulfate gave higher
strength than any single activation method investigated.
After 90 days, mortar strength appears to reach a plateau when the amount of Na2SO4
exceeds 1%.
Figure 7. Mortar compressive strength for milled unclassified fly ash blends
Compressive strength concrete
As expected from the different morphologies for the three different fly ashes, the
addition of mechanically activated unclassified fly ash with its irregular particle shape,
substantially reduced the workability of the concrete (Table 6). Mixes changed from
being very flowable for unclassified ash and classified ash, to less workable for the
milled ash blends.
Table 6. Concrete slump retentions (mm).
0% Na2SO4
1% Na2SO4
3% Na2SO4
5% Na2SO4
Unclassified fly ash blend
150
155
160
170
Classified fly ash blend
160
170
175
170
Milled unclassified fly ash blend
70
70
75
75
Fig. 8 to 10 portray the concrete compressive strength results for the respective curing
ages. It can be seen that the same trend is evident for concrete as for mortar.
Irrespective of the amount of chemical activator added or fineness of fly ash used, the
samples always indicate an increase in concrete compressive strength, with an increase
in chemical activation up to 28 days of curing when compared to the respective control
samples.
It is only when samples reach the 90 day curing mark and onwards, that results become
erratic and deviate from the above mentioned trend for both of the unclassified ash
blends. However, unlike the findings for the mortar testing, the fine classified fly ash
samples still produce definite increases in compressive strength with an increase in
chemical activation up to 180 days of curing.
Regarding the chemical activation of unclassified fly ash (Fig. 8), little activation takes
place at 24 hours for different sulfate additions when compared to the control sample.
Figure 8. Concrete compressive strength for unclassified fly ash blends.
If the fine classified fly ash is considered (Fig. 9), the activation effect is evident for all
five curing ages presented on the graph. It is possible that the consistent enhancement
in strength may be due to the very fine nature of the classified fly ash, which can result
in enhanced “filler effect” in the concrete. The filler effect of the fine spherical particles
can result in improved particle packing and workability, as well as the provision of
additional nucleation sites on the surface of the fly ash for the cement hydrates (seeding
effect), and the increase of effective water-to-cement ratio (w/c), when the water-to-solid
ratio is kept constant whereby the hydration of cement is promoted14.
Figure 9. Concrete compressive strength for classified fly ash blends.
The concrete strength for mechanically and chemically activated unclassified fly ash
blends are presented in Fig. 10. Considering the previous two fly ash blends discussed
which included only chemical activation, one can clearly see the benefit, especially at
early strength ages, which the combined activation of the ash provides.
Figure 10. Concrete compressive strength for milled unclassified fly ash blends.
SUMMARY AND CONCLUSIONS
The findings of the investigation on the use of combined activation techniques for hybrid
cements, and their effect on compressive strength, can be summarized as follows:
-
Increased sulfate addition, as well as mechanical activation, reduces setting time
of mortar blends, which is due to increased hydration and reaction rates,
especially at early curing ages. This finding correlates well with the faster heat
flow rates and higher cumulative heat from the calorimetric data, as well as
increased early compressive strength.
-
Combined activation of hybrid cement produced the most enhanced compressive
strength in mortar up to 90 days of curing, and up to 28 days of curing for
concrete mixes. This activation method is especially effective for early ages
(1 day to 28 days) of both mortar and concrete compressive strength.
-
Mechanical activation of the unclassified fly ash significantly reduces the
workability of concrete mixes.
-
Enhanced filler effect from chemically activated, classified fly ash results in
continuous concrete compressive strength gain over all five curing ages
presented.
RECOMMENDATIONS
These results form part of an on-going PhD study, and the following recommendations
will still be included as part of future investigations:
-
A repeatability study on mortar and compressive strength gain for combined
activation of hybrid cement systems.
-
Reporting on specimens cured for 365 days to assist with the determination of
optimum activation conditions.
-
Complete durability studies on the concrete mixes and include results for 365
days of curing.
-
Investigate the reason for the plateau/drop in strength at later curing ages.
-
In depth study on the characterization of the reaction products formed during
combined activation of hybrid cements.
ACKNOWLEDGEMENTS
The author acknowledges the financial aid and study opportunity from AfriSam (South
Africa) (Pty) Ltd., and also for making their laboratory resources and equipment
available. Ms Wiebke Grote (University of Pretoria) is acknowledged for performing the
XRD analyses, and the University of Pretoria Laboratory for Microscopy and
Microanalysis for assistance with SEM.
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