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Effects of an eco-silica source based activator on functional alkali activated lightweight composites

10 Aug 2019-Construction and Building Materials (Elsevier)-Vol. 215, pp 686-695

AbstractIn this paper, alkali activated slag-fly ash lightweight composites with moderate densities between around 1200 and 1500 kg/m3 are prepared and characterized. An eco-olivine nano-silica is applied to prepare sustainable silicate based activators to replace commercial sodium silicates. Na2O contents of 2.0, 3.5 and 5.0 wt% are investigated in order to reach a suitable balance between performance, costs and application. The results show the positive effect of density and Na2O content on strength, while strength increment between Na2O dosage of 3.5% and 5.0% is limited. The reduction of Na2O content shows a dramatic delay of the reaction process up to around 3 d, but shows negligible effect on the typical Si-O bonds. An increased Na2O content benefits the formation of reaction products, including the contents of hydrotalcite and carbonates. Besides, the thermal conductivity and acoustical absorption properties of the lightweight products are characterized; the phase transition zones between lightweight aggregate and binder matrix are evaluated by SEM. The calculation on the carbon footprint shows an evident advantage of using alkali activated materials to replace Portland cement, also the utilization of olivine nano-silica further reduces the carbon emission of the activator by around 25%.

Topics: Portland cement (51%)

Summary (4 min read)

1. Introduction

  • Lightweight concrete has been widely applied as both structural and non-structural components in a wide range of weights and strengths for various applications [1,2], due to its properties such as low density, good thermal insulation and fire resistance [3].
  • In addition, Portland cement is commonly used as binding material for lightweight con- crete, but its production is responsible for around 7% of the global carbon emissions and high energy costs [5,6].
  • In order to reduce the negative environmental impacts, the development of sustainable alternatives such as alkali activated materials has been investigated because of the excellent mechanical properties, durability, thermal resistance together with low energy and carbon costs [7– 9].
  • The commercial process of sodium silicate production includes the melting of sodium carbonate and quartz sand around 1400–1500 C with carbon release of above 400 kg/ton [31].

2.1. Materials

  • The powder raw materials applied in the present work are blast furnace slag and Class F fly ash, and their major chemical compositions are shown in Table 1.
  • Commercial lightweight aggregates produced from natural expanded silicate with three particle sizes are applied: 0.5–1 mm, 1–2 mm and 2–4 mm, with particle densities of 600, 550 and 500 kg/m3, respectively.
  • CEN standard sand is also used as fine aggregate.
  • The SEM pictures of the lightweight aggregates’ surface and internal structure are provided in Fig. 9(A and B).
  • Analytical sodium hydroxide and laboratory prepared olivine nano-silica (19.04% SiO2 and 80.96% H2O by mass) were used to produce alkali activators.

2.2. Sample preparation

  • A fixed activator modulus of 1.4 and a slag/fly ash mass ratio of 8/2 were used based on the previous experiences [35,36].
  • The detailed mix proportions are presented in Table 2, for instance, sample with the label of D15-5.0 means it was designed to have a density level of 1500 kg/m3 and a Na2O content of 5.0%.
  • Specimens were prepared and poured into molds of 40 40 160 mm3, then covered with a plastic film to prevent the moisture loss.

2.3. Testing methods

  • The compressive strength was determined according to EN 196- 1 [37].
  • The early age hydration heat release was investigated by an isothermal calorimeter with TAM Air, Thermometric.
  • The thermal conductivity (k) and the mass heat capacity (c) were measured by Table 2 Mix proportions of alkali-activated slag-fly ash composites (kg/m3).
  • The acoustic absorption coefficient was measured according to EN 10534-2 [39].
  • Microstructure of the reaction products was identified by scanning electron microscopy (SEM) using a JEOL JSM-IT100 instrument, operated with high vacuum mode at an accelerating voltage of 10 kV.

3.1. Compressive strength

  • The relations between the oven dry density and strength are briefly depicted.
  • Fig. 2 depicts the effect of the equivalent Na2O content on 28 d compressive strength of mixtures with two density levels: 1500 and 1200 kg/m3, represented with sample label of D-15 and D-12 in the figure.
  • For mixtures with a Na2O content of 5% in this study, the additionally provided silicate from the activator accounts for around 14.9% of the total silicate within the system, and this activator dosage is commonly used in achieving a high strength [9,20,21,24–26].
  • On the one hand, increasing the alkalinity (Na2O %) will promote the activation of the binder that consequently leads to a higher strength from the aspect of the binder matrix; while on the other hand, the usage of lightweight aggregate limits the strength development by the relatively low crushing strength of the aggregate.

3.2. Reaction kinetics

  • The isothermal calorimeter test was performed on mixtures with the Na2O content of 2.0%, 3.5% and 5.0%, respectively, and lightweight aggregates were added with an aggregate/binder ratio of 0.8 (based on the mixture proportions shown in Table 2), in order to evaluate their effect on the early age reaction.
  • The induction period lasts more than 48 h; the main reaction peak exhibits an obviously broader covered area with a low peak intensity of about 0.34 mW/g, indicating a gradual and slow formation of the reaction products.
  • Thus it can be concluded that the reduction of the reaction process does not present a linear relation with the Na2O content, the shift of Na2O concentration effectively influences the characters of the reaction process such as induction time, reaction intensity, the location and duration of main reaction period.
  • Similar trends are also shown in samples with 3.5% Na2O content, indicating that the effect of lightweight aggregate on the early age reaction is rather limited, and those slight effects are probably attributed to the absorption of small amount of activator during the initial mixing.
  • Similar to the results shown in Fig. 3, as the Na2O content decreases, the effect of lightweight aggregate on heat release becomes more significant.

3.3. Gel structures

  • In order to investigate to the effect of the Na2O content on the gel compositions, the TG/DTG and FTIR analyses were performed and the results are shown in Figs. 5 and 6, respectively.
  • The mass loss between 600 C and 1000 C is partly attributed to the decomposition of reaction products, while the carbonates also play a role.
  • The infrared spectra of the unreacted slag and fly ash, and the reaction products with different Na2O contents are shown in Fig.
  • All mixes show a main absorption band around 950 cm 1, which is the vibration of a non-bridging Si-O bond [59], also commonly recognized as C-A-S-H type gels.
  • The fixed location of the typical bands together with the TG-DTG results indicate that the Na2O blends with different Na2O contents.

3.4. Thermal conductivity

  • In terms of the building materials, a low thermal conductivity contributes to an enhanced indoor thermal comfort, saving the energy cost and preventing the fire caused collapses; while lightweight concrete products based on alkali activated materials are capable of achieving those requirements with a further lowered environmental impact.
  • For mixtures with a density level of round 1500 kg/m3, the thermal conductivity (k-value) is 0.37 W/(m k), this value is lower than the ones from the obtained literatures.
  • This is because that besides the density, the differences in matrix composition, type of binder and aggregates also show an influence on the property of thermal insulation [64,65].
  • As can be seen from the mix proportions shown in Table 2, once the Na2O content is fixed, the difference between different mixes lies in the aggregate type and dosage.
  • Overall, the compressive strengths of around 20–30 MPa together with moderate densities and ideal thermal conductivities indicate a wide and promising application potential of this new lightweight concrete.

3.5. Acoustical absorption

  • Owing to the massive addition of the porous lightweight aggregates, the resulting alkali activated lightweight concrete is expected to exhibit good sound absorption behaviours.
  • Fig. 8 exhibits the sound absorption coefficient as a function of frequency, four mixtures with a Na2O content of 3.5% with different density levels were tested.
  • The mixture with label of D-15 refers to the sample with a density around 1500 kg/m3.
  • The peak absorption coefficient increases to around 0.35 and 0.52 in samples with a density about 1400 and 1300 kg/m3 respectively, while the main absorption frequency range remains similar.
  • It should be mentioned that the medium frequency usually refers to the sound from humans and daily life.

3.6. SEM analysis

  • Scanning electron microscopy images are used to characterize the applied lightweight aggregate and the interfacial transition zones (ITZ) of the reaction products.
  • Due to the fact that the effect of Na2O content on micro scale characteristics is not significant, the reaction products shown in Fig. 9 are having a constant Na2O content of 3.5%.
  • Fig. 9- B depicts a sectional view of the internal structure of the lightweight aggregate, micro pores with different sizes and shapes are clearly presented.
  • Table 3 Calculation on the carbon footprint (kg/m3).
  • This may lead to an increment of the density compared to the designed one, and may slightly reduce the thermal insulation and sound absorption properties.

3.7. Advantages in carbon footprint

  • Applying alkali activated materials shows an advantage in carbon emission towards Portland cement.
  • While within the alkali binder systems, the Na2O content is directly linked to the environmental issues.
  • The assumed recipes are based on the mix proportions shown in Table 2.
  • Moreover, when olivine nano-silica is applied as commercial waterglass replacement, the carbon emission in terms of activator can be further reduced by around 25%.

4. Conclusions

  • This paper evaluates the mechanical properties, thermal property, acoustical absorption and interfacial transition zones of ecofriendly alkali activated slag-fly ash lightweight composites (LWC) with different density classes.
  • The effect of Na2O contents and the utilization of alternative silica source on early age reaction, gel characteristics and carbon footprints on the designed LWC are assessed.
  • Mixtures with 28 d compressive strength up to 32.5 MPa and densities between 1200 and 1500 kg/m3 are resulted, and a direct correlation between strength and density is observed.
  • Compared to the commercial waterglass, the utilization of olivine nano-silica reduces the carbon footprint from activator by around 25%.
  • The lightweight concretes exhibit very low thermal conductivity between 0.16 and 0.37 W/(m k), as well as good sound absorption coefficient up to 0.7 for medium frequencies.

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Effects of an eco-silica source based activator on functional
alkali activated lightweight composites
Citation for published version (APA):
Gao, X., & Yu, Q. (2019). Effects of an eco-silica source based activator on functional alkali activated lightweight
composites.
Construction and Building Materials
,
215
, 686-695.
https://doi.org/10.1016/j.conbuildmat.2019.04.251
Document license:
TAVERNE
DOI:
10.1016/j.conbuildmat.2019.04.251
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Download date: 10. Aug. 2022

Effects of an eco-silica source based activator on functional alkali
activated lightweight composites
X. Gao
a
, Q.L. Yu
b,
a
School of Civil Engineering and Architecture, Wuhan University of Technology, Wuhan 430070, PR China
b
Department of the Built Environment, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
highlights
Eco-friendly functional alkali-activated lightweight composites are developed.
Effect of eco-silica source on alkali activation and carbon footprints is assessed.
Effect of alkali activator on acoustical property is studied.
Relationship between thermal conductivity and density is derived.
Effect of alkali content on compressive strength is evaluated.
article info
Article history:
Received 22 November 2018
Received in revised form 8 March 2019
Accepted 27 April 2019
Keywords:
Alkali activation
Lightweight aggregate composites
Eco-nano-silica
Thermal properties
Acoustic absorption
Carbon emission
abstract
In this paper, alkali activated slag-fly ash lightweight composites with moderate densities between
around 1200 and 1500 kg/m
3
are prepared and characterized. An eco-olivine nano-silica is applied to pre-
pare sustainable silicate based activators to replace commercial sodium silicates. Na
2
O contents of 2.0,
3.5 and 5.0 wt% are investigated in order to reach a suitable balance between performance, costs and
application. The results show the positive effect of density and Na
2
O content on strength, while strength
increment between Na
2
O dosage of 3.5% and 5.0% is limited. The reduction of Na
2
O content shows a dra-
matic delay of the reaction process up to around 3 d, but shows negligible effect on the typical Si-O bonds.
An increased Na
2
O content benefits the formation of reaction products, including the contents of hydro-
talcite and carbonates. Besides, the thermal conductivity and acoustical absorption properties of the
lightweight products are characterized; the phase transition zones between lightweight aggregate and
binder matrix are evaluated by SEM. The calculation on the carbon footprint shows an evident advantage
of using alkali activated materials to replace Portland cement, also the utilization of olivine nano-silica
further reduces the carbon emission of the activator by around 25%.
Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction
Lightweight concrete has been widely applied as both structural
and non-structural components in a wide range of weights and
strengths for various applications [1,2], due to its properties such
as low density, good thermal insulation and fire resistance [3].
Lightweight concrete can be categorized into three grades in gen-
eral, depending on the density: low density ones with densities
lower than 800 kg/m
3
, moderate ones with densities between
800 kg/m
3
and 1400 kg/m
3
and structural ones with densities
between 1400 kg/m
3
and 2000 kg/m
3
[4]. In addition, Portland
cement is commonly used as binding material for lightweight con-
crete, but its production is responsible for around 7% of the global
carbon emissions and high energy costs [5,6]. In order to reduce
the negative environmental impacts, the development of sustain-
able alternatives such as alkali activated materials has been inves-
tigated because of the excellent mechanical properties, durability,
thermal resistance together with low energy and carbon costs [7–
9]. Some efforts have also been spent on applying alkali activated
materials in producing lightweight products; including for
instance the study of design methodologies of ultra-lightweight
geopolymers by applying the particle packing approach, the effect
of key factors such as suitable ratios between binder, activator and
aggregates [10]; the development of lightweight geopolymers with
foaming agent especially for thermal insulating properties [11];
investigating the relations between density, mechanical properties
and thermal conductivity of geopolymers with medium to low
https://doi.org/10.1016/j.conbuildmat.2019.04.251
0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
Corresponding author.
E-mail address: q.yu@bwk.tue.nl (Q.L. Yu).
Construction and Building Materials 215 (2019) 686–695
Contents lists available at ScienceDirect
Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat

densities [12–14]; the utilization of different alkali binders and
unconventional aggregates [15–17].
In terms of the alkali activated binders, they are usually pro-
duced by mixing alkaline activator solutions with solid precursors,
and two typical binding systems can be classified: one is the Si and
Ca enriched system, having a chain structured C-A-S-H type gel as
main reaction product [18]; another is the Si + Al system, the reac-
tion product is mainly a three-dimensional N-A-S-H network [19].
Recently, the blended alkaline system, which is a mixed binder sys-
tem that combines two types of typical binders together, has
received increased attention due to the modified properties such
as setting times, workability, shrinkage, mechanical properties
and durability [20–23]. Synthesizing factors that are related to
key performances of blended system such as activator and raw
materials’ parameters, curing conditions on reaction kinetics, gel
characters, mechanical properties and durability issues have been
widely studied [24–28]. Those understandings make this type of
material a brighter future to applications.
In alkali activated materials, the activator plays an important
role on the activation process by providing necessary alkalinities
for the decomposition of raw materials and formation of reaction
products. The commonly used activators are alkali hydroxides, sil-
icates, carbonates, sulfates, aluminates or oxides; and it is widely
accepted that silicate based ones (M
2
OnSiO
2
+ MOH) usually result
in ideal mechanical properties and relatively low porosity [18,29].
The silicate based activator exhibits advantages by providing the
alkaline environment continuously and moderately, also the addi-
tionally provided silicates from the activator will participate into
the reaction products and lead to a refined microstructure [30],
and sufficient content of M
2
O promotes the activation efficiency
while exceeding dosage show limited further contribution. How-
ever, the primary drawback of silicate based activators is its nega-
tive environmental impact. The commercial process of sodium
silicate production includes the melting of sodium carbonate and
quartz sand around 1400–1500 °C with carbon release of above
400 kg/ton [31]. In order to achieve a more eco-friendly binder sys-
tem, a green olivine nano-silica is applied as alternative silica
source to prepare activator solutions in this study. The olivine
nano-silica is produced by dissolving olivine stone in acid under
temperatures lower than 95 °C, and this process shows advantages
in carbon emission, energy consumption and total costs [32].
Therefore an alkali activated product with further reduced overall
environmental impacts can be expected.
The objective of this study is to evaluate an eco-silicate source
based alkali activator in designing lightweight composites, using
blended alkali activated binder. The binder materials are chosen
to be ground granulated blast furnace slag and class F fly ash due
to their worldwide availability and relatively stable properties,
and the lightweight aggregate is based on a natural expanded sili-
cate. The lightweight composites are designed with the aim to have
moderate densities together with excellent mechanical, thermal
and acoustic properties. Special focuses are paid on the efficient
usage of activators and the effect of alkaline conditions on the
lightweight aggregate, because the silica based, porous structure
of lightweight aggregate would show an interaction with the
pastes due to either dissolution of silica or absorption of activator
solution. The effect of activator dosages, addition of lightweight
aggregates on early age reaction, gel structure, and aggregates-
binder transition zones are investigated by using isothermal
calorimeter, Thermo-gravimeter (TG), Fourier transform infrared
spectroscopy (FITR) and Scanning electron microscopy (SEM) anal-
ysis. The mechanical properties, thermal conductivity and sound
absorption properties are evaluated and the effect of the alkali con-
tent on these properties is investigated.
2. Experiment
2.1. Materials
The powder raw materials applied in the present work are blast
furnace slag and Class F fly ash, and their major chemical compo-
sitions are shown in Table 1. Commercial lightweight aggregates
produced from natural expanded silicate with three particle sizes
are applied: 0.5–1 mm, 1–2 mm and 2–4 mm, with particle densi-
ties of 600, 550 and 500 kg/m
3
, respectively. CEN standard sand is
also used as fine aggregate. The SEM pictures of the lightweight
aggregates’ surface and internal structure are provided in Fig. 9(A
and B). Analytical sodium hydroxide and laboratory prepared oli-
vine nano-silica (19.04% SiO
2
and 80.96% H
2
O by mass) were used
to produce alkali activators. The olivine nano-silica was produced
by dissolving olivine in acid at low temperatures, followed by a
washing and filtering process [33,34]. Desired activator parameters
were obtained by mixing calculated dosage of sodium hydroxide,
olivine nano-silica and distilled water.
2.2. Sample preparation
Three levels of equivalent Na
2
O content were used in this study:
2.0%, 3.5% and 5.0% (by mass of the binder). A fixed activator mod-
ulus of 1.4 and a slag/fly ash mass ratio of 8/2 were used based on
the previous experiences [35,36]. Mixtures are designed to have
oven dry densities between 1200 and 1500 kg/m
3
, i.e. within the
density class of D1,4 and D1,6 according to EN 206-1 [70]. The
detailed mix proportions are presented in Table 2, for instance,
sample with the label of D15-5.0 means it was designed to have
a density level of 1500 kg/m
3
and a Na
2
O content of 5.0%. Speci-
mens were prepared and poured into molds of
40 40 160 mm
3
, then covered with a plastic film to prevent
the moisture loss. All specimens were demolded after 24 h of cur-
ing and cured at room temperature and a relative humidity of 95%
until testing age.
2.3. Testing methods
The compressive strength was determined according to EN 196-
1 [37]. Samples were tested at the ages of 7 and 28 d, respectively.
The early age hydration heat release was investigated by an
isothermal calorimeter with TAM Air, Thermometric. Measure-
ments were carried out for 72 h under a constant temperature of
20 °C. Fourier transform infrared spectroscopy (FTIR) study was
conducted in a Varian 3100 instrument with a resolution of
1cm
1
, and the applied wavenumbers were between 4000 and
600 cm
1
. The thermal analyses were performed in a STA 449 F1
instrument; the testing temperatures were up to 1000 °C with an
increment of 5 °C/min, protected by nitrogen gas. The thermal con-
ductivity (k) and the mass heat capacity (c) were measured by
Table 1
Major chemical composition of slag and fly ash.
Oxides (wt.%) FA GGBS
SiO
2
54.62 34.44
Al
2
O
3
24.42 13.31
CaO 4.44 37.42
MgO 1.43 9.89
Fe
2
O
3
7.21 0.47
Na
2
O 0.73 0.34
K
2
O 1.75 0.47
SO
3
0.46 1.23
LOI 2.80 1.65
X. Gao, Q.L. Yu / Construction and Building Materials 215 (2019) 686–695
687

using a heat transfer analyser (ISOMET 2104) [38]. The acoustic
absorption coefficient was measured according to EN 10534-2
[39]. Microstructure of the reaction products was identified by
scanning electron microscopy (SEM) using a JEOL JSM-IT100
instrument, operated with high vacuum mode at an accelerating
voltage of 10 kV.
3. Results and discussion
3.1. Compressive strength
The 7 and 28 d compressive strength of mixtures with different
density levels and a Na
2
O content of 3.5% are shown in Fig. 1. The
relations between the oven dry density and strength are briefly
depicted. It can be seen that as the density decreases, there is an
obvious reduction in strength at both 7 and 28 d. The highest 28
d strength reaches 30.7 MPa in mixtures with a density of
1471 kg/m
3
, and it gradually reduces to 20.6 MPa in samples with
a density of 1163 kg/m
3
. The relation between density and strength
in this study is in line with previous studies concerning the light-
weight aggregate based composites [40–44], while the strength
results shown here are higher in general than the alkali activated
lightweight composites reported in literatures [45–47]. Consider-
ing the fact that those mixtures are having the same binder con-
tent, the reduced compressive strength is mainly attributed to
the replacement of normal sand by lightweight aggregates (as
shown in Table 2), and a less overall capacity of the aggregates
against compressive loading is resulted. The 7 d strengths share a
similar tendency with the 28 d’s, which decreases from 27.4 MPa
to 18.5 MPa within the density range of around 1500–1200 kg/
m
3
. The 7 d compressive strengths shown in Fig. 1 are all above
88% of the 28 d strength. This is because of the nature of the alkali
activated binder system and the ceiling effect of the lightweight
aggregates, which presents a relatively fast reaction process and
exhibits large percent of the strength at early ages [1,9,35].It
should be noticed that key parameters such as activator type and
dosage, binder composition and fineness, curing conditions also
strongly influence the reaction process, and therefore the strength
development.
Fig. 2 depicts the effect of the equivalent Na
2
O content on 28 d
compressive strength of mixtures with two density levels: 1500
and 1200 kg/m
3
, represented with sample label of D-15 and D-12
in the figure. As stated in the introduction part, the alkali activator
contributes to a large fraction of the overall environmental issue of
alkali activated materials, thus an efficient usage of the activator is
of clear significance. For mixtures with a Na
2
O content of 5% in this
study, the additionally provided silicate from the activator
accounts for around 14.9% of the total silicate within the system,
and this activator dosage is commonly used in achieving a high
strength [9,20,21,24–26]. When reducing the Na
2
O content to
3.5% and 2.0%, the silicate fraction from the activator decreases
to 10.9% and 6.6%, respectively. Concerning its influence on
strength, as can be seen in Fig. 2 that there is an obvious increment
of strength when increasing the Na
2
O content from 2.0% to 3.5%,
and the increase of strength exhibits a limited scale when further
increasing the Na
2
O content to 5.0%. To specify, in mixtures with
Table 2
Mix proportions of alkali-activated slag-fly ash composites (kg/m
3
).
Sample Precursors Activator Sand Lightweight aggregates
Slag Fly ash Nano-s NaOH H
2
O 0–2 2–4 1–2 0.5–1
D15-5.0 384 96 170.8 30.9 77.7 370 200 150 30
D15-3.5 384 96 119.6 21.7 119.1 380 200 150 30
D15-2.0 384 96 68.3 12.4 170.7 390 200 150 30
D12-5.0 384 96 170.8 30.9 77.7 0 240 180 30
D12-3.5 384 96 119.6 21.7 119.1 10 240 180 30
D12-2.0 384 96 68.3 12.4 170.7 20 240 180 30
D14-3.5 384 96 119.6 21.7 119.1 260 230 140 30
D13-3.5 384 96 119.6 21.7 119.1 135 245 150 30
Fig. 1. Compressive strength of AA slag-fly ash lightweight composites with
different densities.
Fig. 2. Compressive strength of AA slag-fly ash lightweight composites with
different Na
2
O contents.
688 X. Gao, Q.L. Yu / Construction and Building Materials 215 (2019) 686–695

a density level of 1500 kg/m
3
, the 28 d strength is increased from
24.0 MPa to 30.7 MPa when shifting the Na
2
O dosage from 2.0%
to 3.5%, namely a relative increase of 27.9%, and this value is
32.5 MPa in mixtures with a Na
2
O content of 5.0%, with a relative
increase of only 5.9%. This phenomenon reveals that both Na
2
O
content and density show an important influence on strength. On
the one hand, increasing the alkalinity (Na
2
O %) will promote the
activation of the binder that consequently leads to a higher
strength from the aspect of the binder matrix; while on the other
hand, the usage of lightweight aggregate limits the strength devel-
opment by the relatively low crushing strength of the aggregate.
Investigating the effect of both density and alkalinity provides
insights concerning how the strength would vary within the
designed parameter ranges, which also gives information on tailor-
ing the strength with certain densities and reasonable activator
dosages for different application requirements.
3.2. Reaction kinetics
The isothermal calorimeter test was performed on mixtures
with the Na
2
O content of 2.0%, 3.5% and 5.0%, respectively, and
lightweight aggregates were added with an aggregate/binder ratio
of 0.8 (based on the mixture proportions shown in Table 2), in
order to evaluate their effect on the early age reaction. Fig. 3 illus-
trates the normalized heat flows of samples with and without
lightweight aggregates during the first 6 d; the heat flow is normal-
ized by mass of the binder (slag and fly ash). The sample labelled as
‘‘LWA-5.0” refer to the one with Na
2
O content of 5.0, and ‘‘LWA-5.0
+Agg” means lightweight aggregates are included in the test. It is
generally agreed that reaction process of alkali activated materials
can be classified into four typical stages [18,48,49]: the initial dis-
solution period, induction period, acceleration/deceleration period
and stable period. The first stage refers to the initial contact of the
solid components with liquid solution, and the dissolution of the
precursors. The induction period refers to the temporary induction
of heat release before the main reaction takes place. The accelera-
tion and deceleration periods are attributed to the massive forma-
tion of the reaction products, and the stable period indicates that
the main reaction process comes to the end while minor reaction
may still occur in the long term.
It can be seen from Fig. 3 that the mixtures with a Na
2
O content
of 5% exhibit a main reaction peak at around 16 h, with a peak
intensity of 1.35 mW/g. The induction stage shows an induction
period between around 6 and 10 h, followed by an evident increase
of heat flow representing the intensive chemical reaction [50].
When reducing the Na
2
O content to 3.5%, a significantly retarded
reaction process can be observed. The induction period appears
at around 9–22 h, the beginning of this stage is 3 h later than the
one in sample with a Na
2
O content of 5%, also the duration of this
stage is more than 3 times longer. The location of the main reaction
peak is also delayed to 37 h, which is 21 h later than the 5% Na
2
O
mixtures. Besides, the peak intensity is reduced to 0.63 mW/g
and the deceleration period exhibits a more moderate reduction
of heat release. A further lowered Na
2
O dosage of 2.0% results in
a dramatic delay of the reaction process. For instance, the induc-
tion period lasts more than 48 h; the main reaction peak exhibits
an obviously broader covered area with a low peak intensity of
about 0.34 mW/g, indicating a gradual and slow formation of the
reaction products. Until the testing age of 144 h, the deceleration
stage is still ongoing. Thus it can be concluded that the reduction
of the reaction process does not present a linear relation with
the Na
2
O content, the shift of Na
2
O concentration effectively influ-
ences the characters of the reaction process such as induction time,
reaction intensity, the location and duration of main reaction per-
iod. Similar tendencies are also shown in previous researches
regarding the effect of activator modulus (usually with a constant
Na
2
O dosage) [35,49], while the reduction of peak intensity and the
delay of the main reaction process is rather limited compared to
the case of this study, which suggests that the activator characters
such as additional silicate content and the alkalinity strongly con-
trol the activation process of the precursors.
The reaction kinetics results can be correlated to the compres-
sive strength results in this study, where mixtures with a Na
2
O
content of 2.0% show an obviously lower strength than the ones
with higher Na
2
O contents. It is concluded that the delayed reac-
tion process together with lower intensities provide a reduced acti-
vation on the starting materials, resulting in an overall lower
reaction degree and strength. The heat release curves with dash
lines refer to the mixtures with lightweight aggregate additions.
For mixes with a Na
2
O content of 5.0%, the aggregate incorporation
slightly reduces the intensity of the main reaction peak without
significant effect on its location, and there is also a very slight delay
of the induction period. Similar trends are also shown in samples
with 3.5% Na
2
O content, indicating that the effect of lightweight
aggregate on the early age reaction is rather limited, and those
slight effects are probably attributed to the absorption of small
amount of activator during the initial mixing. Mixes with a Na
2
O
content of 2.0% exhibits an evident delay of the main reaction peak,
which indicates that under low alkalinity conditions, the effect of
lightweight aggregate (activator absorption) is magnified.
The cumulative heat release of all mixtures is shown in Fig. 4.
Differences in heat release processes caused by Na
2
O content and
aggregate addition are clearly depicted. Mixtures with a Na
2
O con-
tent of 5.0% present the highest cumulative heat of about 166 J/g
till the testing age of 144 h. Samples with 3.5% Na
2
O show a
slightly lower heat release of 153 J/g, while the heat release of
mixes with a Na
2
O dosage of 2.0% is significantly lower: 106 J/g,
which is also partly because that the main reaction process is still
processing. When lightweight aggregate is incorporated, similar
heat release is shown in 5% Na
2
O mixtures. Similar to the results
shown in Fig. 3, as the Na
2
O content decreases, the effect of light-
weight aggregate on heat release becomes more significant.
3.3. Gel structures
In order to investigate to the effect of the Na
2
O content on the
gel compositions, the TG/DTG and FTIR analyses were performed
and the results are shown in Figs. 5 and 6, respectively. Samples
with the label of ‘‘LWA-2.0” refer to the ones produced with
Fig. 3. Normalized heat flows of AA slag-fly ash pastes with different Na
2
O contents
and lightweight aggregates.
X. Gao, Q.L. Yu / Construction and Building Materials 215 (2019) 686–695
689

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Abstract: A mechanistic model accounting for reduced structural reorganization and densification in the microstructure of geopolymer gels with high concentrations of soluble silicon in the activating solution has been proposed. The mechanical strength and Young's modulus of geopolymers synthesized by the alkali activation of metakaolin with Si/Al ratio between 1.15 and 2.15 are correlated with their respective microstructures through SEM analysis. The microstructure of specimens is observed to be highly porous for Si/Al ratios ≤1.40 but largely homogeneous for Si/Al ≥1.65, and mechanistic arguments explaining the change in microstructure based on speciation of the alkali silicate activating solutions are presented. All specimens with a homogeneous microstructure exhibit an almost identical Young's modulus, suggesting that the Young's modulus of geopolymers is determined largely by the microstructure rather than simply through compositional effects as has been previously assumed. The strength of geopolymers is maximized at Si/Al = 1.90. Specimens with higher Si/Al ratio exhibit reduced strength, contrary to predictions based on compositional arguments alone. The decrease in strength with higher silica content has been linked to the amount of unreacted material in the specimens, which act as defect sites. This work demonstrates that the microstructures of geopolymers can be tailored for specific applications.

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Abstract: Concrete for construction has traditionally been based on an Ordinary Portland Cement (OPC) binder. Geopolymers, an alternative binder based on fly ash (a fine waste collected from the emissions liberated by coal burning power stations) that is activated by an alkaline activator, have potential to lower the significant carbon footprint of OPC concrete. This paper presents the results of comprehensive carbon footprint estimates for both geopolymer and OPC concrete, including energy expending activities associated with mining and transport of raw materials, manufacturing and concrete construction. Previous studies have shown a wide variation of reported emission estimates: the results of this study are benchmarked with data from those studies.

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Abstract: The activation of fly ash/slag pastes with NaOH solutions have been studied. The parameters of the process studied are: activator concentration (NaOH 2 and 10 M), curing temperature (25°C and 65°C), and fly ash/slag ratios (100/0, 70/30, 50/50, 30/70, and 0/100). The equations of the models describing the mechanical behaviour of these pastes have been established as a function of the factors and levels considered. The ratio of fly ash/slag and the activator concentration always result to be significative factors. The influence of curing temperature in the development of the strength of the pastes is lower than the contribution due to other factors. At 28 days of reaction, the mixture 50% fly ash/50% slag activated with 10 M NaOH and cured at 25°C, develop compressive mechanical strengths of about 50 MPa. The nature of the reaction products in these pastes has been studied by insoluble residue in HCl acid, XRD, FTIR and MAS NMR. It has been verified that slag reacts almost completely. It has also been determined that the fly ash is partially dissolved and participates in the reactive process, even in pastes activated at ambient temperature. The main reaction product in these pastes is a hydrated calcium silicate, like CSH gel, with high amounts of tetracoordinated Al in its structure, as well as Na ions in the interlayer spaces. No hydrated alkaline alumino-silicates with three-dimensional structure characteristics of the alkaline activation of fly ashes were formed.

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Abstract: There are two main models of alkali-activated cements, one is the case of the activation of slag (Si + Ca) and the other is activation of metakaolin (Si + Al). This paper reviews current knowledge about the comparison between alkali-activated slag (Si + Ca) and metakaolin (Si + Al) cements, including the general properties of slag and metakaolin, hydration products reaction mechanisms, and the role of Ca and Al.

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Abstract: Upon heating, the cement paste undergoes a continuous sequence of more or less irreversible decomposition reactions. This paper reports studies on a cement paste fired to various temperature regimes up to 800 °C in steps of 100 °C for a constant period of 24 h. This work has been carried out using thermal analysis technique to study the effect of temperature in the mineralogical composition of cement hydration products. The thermal decomposition of the cement paste is analysed with the thermogravimetric analysis (TGA) and the derivative thermogravimetric analysis (DTG) curves. Such techniques can be used to determine fire conditions and the consequent deterioration expected in the cement paste. Therefore, the aim of this work is to have a better knowledge of the reactions that take place in a cement paste during a fire and thus to be able to determine the temperature history of concrete after a fire exposure. The results show that even if the dehydroxylation reaction is reversible, the portlandite formed during the cooling has an onset temperature of decomposition lower than the original portlandite and can thus be considered as a tracer for determining the temperature history of concrete after a fire exposure.

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Frequently Asked Questions (1)
Q1. What are the contributions in "Effects of an eco-silica source based activator on functional alkali activated lightweight composites" ?

In this paper, alkali activated slag-fly ash lightweight composites with moderate densities between around 1200 and 1500 kg/m are prepared and characterized. The calculation on the carbon footprint shows an evident advantage of using alkali activated materials to replace Portland cement, also the utilization of olivine nano-silica further reduces the carbon emission of the activator by around 25 %.