1. INTRODUCTION
⌅The
production of Portland cement has been found to be a leading
contributor to the release of greenhouse gas emissions into the
atmosphere, particularly carbon dioxide (CO2). An estimated 0.95 tons of CO2 are released for each ton of the cement produced (11. Davidovits J. 2013. Geopolymer cement a review. Geopolymer Institute. 1–11. Retrieved from https://www.geopolymer.org/library/technical-papers/21-geopolymer-cement-review-2013/.
). It has been further found that of the total CO2 emissions associated with concrete production, a staggering 76-80% is attributed to the production of the cement (22.
Turner LK, Collins FG. 2013. Carbon dioxide equivalent (CO2-e)
emissions: A comparison between geopolymer and OPC cement concrete.
Constr. Build. Mater. 43:125–130. http://doi.org/10.1016/j.conbuildmat.2013.01.023.
, 33. Yang K, Song J, Song K. 2013. Assessment of CO2 reduction of alkali-activated concrete. J. Clean. Prod. 39:265–72. http://doi.org/10.1016/j.jclepro.2012.08.001.
). In fact, these CO2 emissions account for about 75% of all greenhouse gas emissions (44. Olivier JGJ, Peters JAHW. 2019. Trends in global CO2 and total greenhouse gas. Summary of the 2019 Report. PBL Netherlands
Environmental Assessment Agency PBL publication number: 4004.
Netherlands.
). This alarming rate of emission
necessitates the need to develop alternative binders to reduce the
environmental impact of cement production.
Geopolymer binder, which releases 80-90% lower CO2 emissions, currently represents one prospective alternative binder that can considerably minimize the environmental impact (11. Davidovits J. 2013. Geopolymer cement a review. Geopolymer Institute. 1–11. Retrieved from https://www.geopolymer.org/library/technical-papers/21-geopolymer-cement-review-2013/.
).
Geopolymer binder is created by activating precursors that contain
aluminosilicates like fly ash, silica fume, ground granulated blast
furnace slag, and metakaolin, with an alkaline activator such as sodium
hydroxide (SH), sodium silicate (SS), sodium sulfate or carbonate,
potassium hydroxide (KOH) and potassium silicate (K2SiO3) (5-75.
Tennakoon C, Shayan A, Sanjayan JG, Xu A. 2017. Chloride ingress and
steel corrosion in geopolymer concrete based on long term tests. Mater.
Des. 116:287–299. http://doi.org/10.1016/j.matdes.2016.12.030.
6.
Neupane K. 2016. Fly ash and GGBFS based powder-activated geopolymer
binders: A viable sustainable alternative of portland cement in concrete
industry. Mech. Mater. 103:110–122. http://doi.org/10.1016/j.mechmat.2016.09.012.
7.
Zannerni GM, Fattah KP, Al-Tamimi AK. 2020. Ambient-cured geopolymer
concrete with single alkali activator. Sustain. Mater. Techno.
23:e00131. https://doi.org/10.1016/j.susmat.2019.e00131.
).
The matrix and strength of this type of cement does not require the
formation of hydrated calcium silicate (C-S-H) gel, as with ordinary
Portland cement concrete (OPC), but instead relies on aluminosilicate
polycondensation (88.
Okoye FN, Durgaprasad J, Singh NB. 2016. Effect of silica fume on the
mechanical properties of fly ash based-geopolymer concrete. Ceram. Int.
42(2, Part B):3000–6. http://doi.org/10.1016/j.ceramint.2015.10.084.
).
When a low calcium precursor is used to create geopolymer binder, a
sodium aluminum silicate hydrate (N-A-S-H) gel with a three-dimensional
structure is produced (99.
Angulo-Ramírez DE, Valencia-Saavedra WG, Mejía de Gutiérrez R. 2020.
Alkali-activated concretes based on fly ash and blast furnace slag:
Compressive strength water absorption and chloride permeability. Ing.
Investig. 40(2):72–80. https://doi.org/10.15446/ing.investig.v40n2.83893.
).
Many
studies have focused on GPC made from low-calcium fly ash and using SS
and SH as an alkali activator. However, the moderate temperature curing
required for this type of geopolymer limits its practical use (10-1310.
Mehta A, Siddique R. 2016. An overview of geopolymers derived from
industrial by-products. Constr. Build. Mater. 127:183–98. http://doi.org/10.1016/j.conbuildmat.2016.09.136.
11.
Part WK, Ramli M, Cheah CB. 2015. An overview on the influence of
various factors on the properties of geopolymer concrete derived from
industrial by-products. Constr. Build. Mater. 77:370–395. http://doi.org/10.1016/j.conbuildmat.2014.12.065.
12.
Singh B, Ishwarya G, Gupta M, Bhattacharyya SK. 2015. Geopolymer
concrete: A review of some recent developments. Constr. Build. Mater.
85:78–90. http://doi.org/10.1016/j.conbuildmat.2015.03.036.
13.
Noushini A, Castel A. 2016. The effect of heat-curing on transport
properties of low-calcium fly ash-based geopolymer concrete. Constr.
Build. Mater. 112:464–477. http://doi.org/10.1016/j.conbuildmat.2016.02.210.
).
To address this issue, researchers have explored the potential of new
geopolymer formulas that can be cured at lower temperatures. One
possible solution is to increase the calcium content in the binder by
introducing slag, which has high calcium content, to the low-calcium fly
ash-based geopolymer mixture. This can enhance the mechanical and
microstructural properties of GPC by promoting the formation of hydrated
calcium aluminum silicate (C-A-S-H) gel, the primary product of
polymerization (14-1714.
Alanazi H, Hu J, Kim YR. 2019. Effect of slag silica fume and
metakaolin on properties and performance of alkali-activated fly ash
cured at ambient temperature. Constr. Build. Mater. 197:747–756. https://doi.org/10.1016/j.conbuildmat.2018.11.172.
15.
Cheng Y, Hongqiang M, Hongguang Z, Weijian L, Minglang X, Yilei Liu YG.
2018. Study on chloride binding capability of coal gangue based
cementitious materials. Constr. Build. Mater. 167:649–656. https://doi.org/10.1016/j.conbuildmat.2018.02.071.
16.
Rafeet A, Vinai R, Soutsos M, Sha W. 2019. Effects of slag substitution
on physical and mechanical properties of fly ash-based alkali activated
binders (AABs). Cem. Concr. Res. 122:118–35. https://doi.org/10.1016/j.cemconres.2019.05.003.
17.
Yousefi Oderji S, Chen B, Ahmad M.R, Shah SFA. 2019. Fresh and hardened
properties of one-part fly ash-based geopolymer binders cured at room
temperature: Effect of slag and alkali activators. J. Clean. Prod.
225:1–10. https://doi.org/10.1016/j.jclepro.2019.03.290.
).
Geopolymer
concrete with 20% fly ash substitution with slag has higher strength
than that made from 100% fly ash. One positive finding is that when
cured at room temperature, using 100% slag results in the highest
strength (77.
Zannerni GM, Fattah KP, Al-Tamimi AK. 2020. Ambient-cured geopolymer
concrete with single alkali activator. Sustain. Mater. Techno.
23:e00131. https://doi.org/10.1016/j.susmat.2019.e00131.
).
The addition of slag also reduces pore size and porosity, ultimately
decreasing the amount of water it can absorb over a period of time (1818.
Provis JL, Myers RJ, White CE, Rose V, Van Deventer JSJ. 2012. X-ray
microtomography shows pore structure and tortuosity in alkali-activated
binders. Cem. Concr. Res. 42(6):855–64. http://doi.org/10.1016/j.cemconres.2012.03.004.
).
The reduction could be identified concerning the water absorption rate
with time. At first, the water absorption rate in GPC was relatively
high because of the absorption of pores at a size larger than 200 nm.
Over time, the rate diminishes. In contrast, the water absorption rate
in OPC specimens initially starts low due to fewer pores with a size
larger than 200 nm but continues to increase. Another report indicated
that, the water absorption rate of GPC containing 30% slag was
comparable to that of OPC specimens (1919.
Lee NK, Lee HK. 2016. Influence of the slag content on the chloride and
sulfuric acid resistances of alkali-activated fly ash/slag paste. Cem.
Concr. Compos. 72:168–79. http://doi.org/10.1016/j.cemconcomp.2016.06.004.
).
Other research showed that by using a higher slag percentage and lower
SS/SH ratio in GPC, shrinkage can be decreased, making it comparable to
OPC (2020.
Deb PS, Nath P, Sarker PK. 2015. Drying shrinkage of slag blended fly
ash geopolymer concrete cured at room temperature. Procedia. Eng.
125:594–600. http://doi.org/10.1016/j.proeng.2015.11.066.
).
It has been observed that as more fly ash substitution by slag in GPC,
the water absorption, sorptivity, and porosity of the material are all
reduced (2121.
Hu Y, Tang Z, Li W, Li Y, Tam VWY. 2019. Physical-mechanical properties
of fly ash/GGBFS geopolymer composites with recycled aggregates.
Constr. Build. Mater. 226:139–151. https://doi.org/10.1016/j.conbuildmat.2019.07.211.
, 2222.
Mehta A, Siddique R, Ozbakkaloglu T, Shaikh FUA, Belarbi R. 2020. Fly
ash and ground granulated blast furnace slag-based alkali-activated
concrete: Mechanical transport and microstructural properties. Constr.
Build. Mater. 257:1–10. https://doi.org/10.1016/j.conbuildmat.2020.119548.
).
This phenomenon is due to the slag and alkaline activator reaction,
resulting the formation of denser C-A-S-H gels than the N-A-S-H gels (1818.
Provis JL, Myers RJ, White CE, Rose V, Van Deventer JSJ. 2012. X-ray
microtomography shows pore structure and tortuosity in alkali-activated
binders. Cem. Concr. Res. 42(6):855–64. http://doi.org/10.1016/j.cemconres.2012.03.004.
). Furthermore, the smaller size of slag particles makes them function as micro fillers (2323.
Khan MZN, Shaikh F uddin A, Hao Y, Hao H. 2016. Synthesis of high
strength ambient cured geopolymer composite by using low calcium fly
ash. Constr. Build. Mater. 125:809–820. http://doi.org/10.1016/j.conbuildmat.2016.08.097.
).
Research
has shown that GPC has comparable or even superior physical and
mechanical properties to OPC. In terms of durability, GPC has been
studied for its resistance to aggressive environments, including sulfate
attack, chloride ingress, carbon dioxide penetration, and acids (2424.
Arbi K, Nedeljković M, Zuo Y, Ye G. 2016. A review on the durability
of alkali-activated fly ash/slag systems: advances issues and
perspectives. Ind. Eng. Chem. Re. 55(19):5439–5453. https://pubs.acs.org/doi/abs/10.1021/acs.iecr.6b00559.
).
Chloride ingress can destroy the passivation layer on the steel
reinforcement, trigger corrosion caused by electrochemistry, and
subsequently, decrease the structural strength of concrete. Although
chlorides can have such detrimental effect, they generally do not
disintegrate the concrete matrix since the rates of degradation caused
by chlorides are shown to be lower (2525.
Ismail I, Bernal SA, Provis JL, Nicolas RS, Nicolas RS, Brice DG, et
al. 2013. Influence of fly ash on the water and chloride permeability of
alkali-activated slag mortars and concretes. Constr. Build. Mater.
48:1187–1201. https://doi.org/10.1016/j.conbuildmat.2013.07.106.
).
Many researchers found that compared to GPC, OPC demonstrated a greater
capacity to bind chloride. This is because of the reaction among the
AFm phase (alumina, iron oxide, mono sulphate) in OPC with chloride
ions, forming Friedel salts. On the other hand, GPC has a minimal
ability to bind chloride as there is no visible reaction between the
geopolymer binder and chloride ions; however, the ability to bind
chloride is dependent on the adsorption or encapsulation of chloride in
the network of geopolymer pores (25-2725.
Ismail I, Bernal SA, Provis JL, Nicolas RS, Nicolas RS, Brice DG, et
al. 2013. Influence of fly ash on the water and chloride permeability of
alkali-activated slag mortars and concretes. Constr. Build. Mater.
48:1187–1201. https://doi.org/10.1016/j.conbuildmat.2013.07.106.
26.
Babaee M, Castel A. 2018. Chloride diffusivity chloride threshold and
corrosion initiation in reinforced alkali-activated mortars: Role of
calcium alkali and silicate content. Cem. Concr. Res. 111:56–71. https://doi.org/10.1016/j.cemconres.2018.06.009.
27.
Noushini A, Castel A, Aldred J, Rawal A. 2020. Chloride diffusion
resistance and chloride binding capacity of fly ash-based geopolymer
concrete. Cem. Concr. Compos. 105:103290. https://doi.org/10.1016/j.cemconcomp.2019.04.006.
).
Through
the use of the Rapid Chloride Permeability Test (RCPT), a comparative
study was conducted to analyze the chloride permeability of OPC and fly
ash-based GPC. The results showed that the permeability of OPC and fly
ash-based GPC is similar when compared using the same compressive
strength. Nevertheless, the addition of a higher proportion of slag to
the geopolymer concrete leads to a decrease in the total charge passed
and a change in the permeability classification of the concrete (2222.
Mehta A, Siddique R, Ozbakkaloglu T, Shaikh FUA, Belarbi R. 2020. Fly
ash and ground granulated blast furnace slag-based alkali-activated
concrete: Mechanical transport and microstructural properties. Constr.
Build. Mater. 257:1–10. https://doi.org/10.1016/j.conbuildmat.2020.119548.
, 2828.
Bellum RR, Muniraj K, Madduru SRC. 2020. Influence of slag on
mechanical and durability properties of fly ash-based geopolymer
concrete. J. Korean Ceram. Soc. 57(5):530–45. https://doi.org/10.1007/s43207-020-00056-7.
).
The reduction in concrete permeability is due to the addition of
hydrated compounds based on calcium, which act as micro aggregates and
coexist with the geopolymeric components to create a dense
microstructure that impedes the infiltration of water from the capillary
pores. Furthermore, slag is more reactive than fly ash particles in
alkaline media, making it easier for the abundantly available alumina
and silica ions to dissolve and improve the polycondensation mechanism (2222.
Mehta A, Siddique R, Ozbakkaloglu T, Shaikh FUA, Belarbi R. 2020. Fly
ash and ground granulated blast furnace slag-based alkali-activated
concrete: Mechanical transport and microstructural properties. Constr.
Build. Mater. 257:1–10. https://doi.org/10.1016/j.conbuildmat.2020.119548.
, 2929.
Değirmenci FN. 2018. Utilization of natural and waste pozzolans as an
alternative resource of geopolymer mortar. Int. J. Civ. Eng.
16(2):179–88. https://doi.org/10.1007/s40999-016-0115-1.
).
The total charge passed decreases with extended curing time, as a
denser matrix of geopolymer is created, impeding the penetration of
chloride ions (3030.
Lee WH, Wang JH, Ding YC, Cheng TW. 2019. A study on the
characteristics and microstructures of GGBS/FA based geopolymer paste
and concrete. Constr. Build. Mater. 211:807–13. https://doi.org/10.1016/j.conbuildmat.2019.03.291.
).
GPC
is much more resistant to chloride attack, showing a longer time to
experience cracking due to chloride-induced corrosion in comparison to
OPC. At 40 mm depth, GPC’s chloride content is ten times lesser than
that of OPC, which is insufficient to initiate chloride-induced
corrosion (3131.
Reddy DV, Edouard JB, Sobhan K. 2013. Durability of fly ash–based
geopolymer structural concrete in the marine environment. J. Mater. Civ.
Eng. 25(6):781–7. https://doi.org/10.1061/(asce)mt.1943-5533.0000632.
). Additionally, the pore structure of GPC becomes disconnected as it matures (55.
Tennakoon C, Shayan A, Sanjayan JG, Xu A. 2017. Chloride ingress and
steel corrosion in geopolymer concrete based on long term tests. Mater.
Des. 116:287–299. http://doi.org/10.1016/j.matdes.2016.12.030.
). With increasing curing time of GPC, the porosity is reduced owing to the space-filling properties of the C-(N)-A-S-H phase (1818.
Provis JL, Myers RJ, White CE, Rose V, Van Deventer JSJ. 2012. X-ray
microtomography shows pore structure and tortuosity in alkali-activated
binders. Cem. Concr. Res. 42(6):855–64. http://doi.org/10.1016/j.cemconres.2012.03.004.
).
The main reaction products of GPC with slag content are C-S-H, C-A-S-H,
N-A-S-H, and the hydrotalcite phase, with an increase of slag content
resulting in greater amounts of C-S-H and C-A-S-H, as well as an
increase in the hydrotalcite phase. C-A-S-H has a higher ability than
N-A-S-H to bind chloride ions, reducing their entry (2525.
Ismail I, Bernal SA, Provis JL, Nicolas RS, Nicolas RS, Brice DG, et
al. 2013. Influence of fly ash on the water and chloride permeability of
alkali-activated slag mortars and concretes. Constr. Build. Mater.
48:1187–1201. https://doi.org/10.1016/j.conbuildmat.2013.07.106.
). Hydrotalcite (Mg6Al2(OH)16CO34H2O)
is a compound that has a double-layered hydroxide structure. This
layered crystal structure consists of an outer layer with hydroxides and
an inner layer with anions and water molecules. The addition of
hydrotalcite to externally contaminated chloride-contaminated concrete
results in a reduction in the chloride content in the concrete compared
to without the hydrotalcite structure (3232.
Kayali O, Khan MSH, Sharfuddin Ahmed M. 2012. The role of hydrotalcite
in chloride binding and corrosion protection in concretes with ground
granulated blast furnace slag. Cem. Concr. Compos. 34(8):936–45. http://doi.org/10.1016/j.cemconcomp.2012.04.009.
).
To
date, research on fly ash-slag-based GPC has predominantly employed
sodium hydroxide solution with moderate to high molarity (1616.
Rafeet A, Vinai R, Soutsos M, Sha W. 2019. Effects of slag substitution
on physical and mechanical properties of fly ash-based alkali activated
binders (AABs). Cem. Concr. Res. 122:118–35. https://doi.org/10.1016/j.cemconres.2019.05.003.
, 2222.
Mehta A, Siddique R, Ozbakkaloglu T, Shaikh FUA, Belarbi R. 2020. Fly
ash and ground granulated blast furnace slag-based alkali-activated
concrete: Mechanical transport and microstructural properties. Constr.
Build. Mater. 257:1–10. https://doi.org/10.1016/j.conbuildmat.2020.119548.
, 33-3533.
Mallikarjuna Rao G, Gunneswara Rao TD. 2018. A quantitative method of
approach in designing the mix proportions of fly ash and GGBS-based
geopolymer concrete. Aust. J. Civ. Eng. 16(1):53–63. https://doi.org/10.1080/14488353.2018.1450716.
34.
Nagajothi S, Elavenil S. 2021. Effect of GGBS addition on reactivity
and microstructure properties of ambient cured fly ash based geopolymer
concrete. Silicon. 13(2):507–516. https://doi.org/10.1007/s12633-020-00470-w.
35.
El-Hassan H, Ismail N. 2018. Effect of process parameters on the
performance of fly ash/GGBS blended geopolymer composites. J. Sustain.
Cem. Mater. 7(2):122–40. https://doi.org/10.1080/21650373.2017.1411296.
), resulting in significant CO2 emissions during the production of alkaline activator solution (22.
Turner LK, Collins FG. 2013. Carbon dioxide equivalent (CO2-e)
emissions: A comparison between geopolymer and OPC cement concrete.
Constr. Build. Mater. 43:125–130. http://doi.org/10.1016/j.conbuildmat.2013.01.023.
, 36-3836.
Habert G, D’Espinose de Lacaillerie JB, Roussel N. 2011. An
environmental evaluation of geopolymer based concrete production:
Reviewing current research trends. J. Clean. Prod. 19(11):1229–38. http://doi.org/10.1016/j.jclepro.2011.03.012.
37. Yang KH, Song JK, Song KIl. 2013. Assessment of CO2 reduction of alkali-activated concrete. J. Clean. Prod. 39:265–72. http://doi.org/10.1016/j.jclepro.2012.08.001.
38.
Mclellan BC, Williams RP, Lay J, Riessen AV, Corder GD. 2011. Costs and
carbon emissions for geopolymer pastes in comparison to ordinary
portland cement. J. Clean. Prod. 19(9–10):1080–1090. http://doi.org/10.1016/j.jclepro.2011.02.010.
).
To address this issue, the present study proposes using a combination
of SS and SH molarity of 2M as an alkaline activator to represents a
more sustainable GPC that can be cured at room temperature. The use of
sodium hydroxide with a molarity of 2M is based on preliminary tests on
fly ash-slag-based geopolymer paste and mortar. This test uses
variations in SH molarity of 1M, 2M, 4M, 6M, 8M, and 10M, a slag
percentage of 30%, the SS/SH ratio of 0.5, 1.0, and 1.5, and the Al/Bi
ratio of 0.40, 0.45, and 0.50. Based on the setting time with an SH
molarity of 2M, the results were obtained at SS/SH ratios of 1.0 and
1.5, and at all Al/Bi ratios, the final setting time was around 100-160
minutes. Based on the compressive strength of the mortar, the results
showed that sodium hydroxide with molarity of 2M had a compressive
strength similar to other SH molarities.
Based on these preliminary tests, this research used SH molarity 2M to assess the transport properties of the GPC, focusing on porosity, sorptivity, and chloride permeability properties. The investigation examines the impact of various mixture compositions, such as slag percentages, alkali aktivator to binder (Al/Bi) ratios, and SS/SH ratios. The results offer insight for engineers seeking to determine the optimal composition of durable and environmentally friendly GPC for practical applications.
2. MATERIALS AND METHODS
⌅2.1. Materials
⌅In this study, the fine aggregate was sourced from Yogyakarta, Indonesia, and coarse aggregate with a nominal diameter of 12.7 mm was obtained from Central Java, Indonesia. The binder materials consisted of type F fly ash from Tanjung Jati B PLTU, Central Java, Indonesia, and slag obtained from PT. Krakatau Semen Indonesia, Banten, Indonesia. Both fly ash and slag must pass sieve no. 200 with a diameter of 75 μm, according to cement fineness specifications (SNI 15-2530-1991). X-ray fluorescence (XRF) testing was employed to determine the chemical compositions of fly ash and slag, and the results are shown in Table 1. The fly ash had a low calcium oxide (about 8.64%) but a high total content of SiO2, Al2O3, and Fe2O3 (about 83.8%), so it includes type F fly ash according to ASTM C618 standards. On the other hand, the slag had a high calcium oxide content (about 62.1%).
Component % | Fly Ash | Slag |
---|---|---|
SiO2 | 41.00 | 23.50 |
Al2O3 | 15.00 | 8.20 |
Fe2O3 | 26.94 | 0.95 |
CaO | 8.64 | 62.10 |
MgO | 0.74 | 0.30 |
SO3 | 0.50 | 0.94 |
K2O | 2.43 | 0.10 |
TiO2 | 1.71 | 1.20 |
Others | 2.5 | 2.61 |
LoI | 0.54 | 0.1 |
Alkaline activators were prepared using a mixture of SH and SS, with varying ratios of SS/SH at 1.0, 1.5, and 2.0. The SH solution was created by dissolving 98% pure SH flakes in distilled water, maintaining a constant concentration of 2M across all mixtures. The SS solution used in this study was a commercially available gel with the production code BE-58. Figure 1 and Figure 2 show the raw materials and alkaline activators used in the manufacture the geopolymer concrete.
2.2. Preparation, casting, and curing of specimens
⌅In
order to achieve a 25 MPa design strength for normal concrete, the
study employed a mixture design that followed the guidelines of SNI
7656:2012 (3939.
Badan Standarisasi Nasional. 2012. SNI 7656:2012: Tata cara pemilihan
campuran untuk beton normal beton berat dan beton massa. badan
standarisasi nasional jakarta indones.
), a modified version of ACI 211.1-91 (4040.
Committee 211 ACI. 2009. ACI 211.1-91: Standard practice for selecting
proportions for normal heavyweight and mass concrete. ACI Stand.
)
for Indonesian context. The GPC production involved replacing the
cement with fly ash and slag, with the slag to total binder percentages
ranging from 10% to 50%. To substitute the water-cement ratio, an
alkaline activator was utilized with varying Al/Bi ratios of 0.40, 0.45,
and 0.50. The aggregates and total binder were kept constant throughout
the study. In Table 2, the results of the concrete compressive strength test according to the mix design are presented.
Mix ID | Designation | Concrete Mixture Quantity (kg/m3) | |||
---|---|---|---|---|---|
Aggregate (CAg/FAg) | Binder (Fly ash/Slag /Cement) | Activator (SS/SH/Water) | Compressive Strength (MPa) | ||
GPC1 | S10-1.5-0.45 | 858/863 | 360/40/ - | 108/72/ - | 23.930 |
GPC2 | S20-1.5-0.45 | 858/863 | 320/80/ - | 108/72/ - | 36.793 |
GPC3 | S30-1.5-0.45 | 858/863 | 280/120/ - | 108/72/ - | 46.667 |
GPC4 | S40-1.5-0.45 | 858/863 | 240/160/ - | 108/72/ - | 50.402 |
GPC5 | S50-1.5-0.45 | 858/863 | 200/200/ - | 108/72/ - | 56.076 |
GPC6 | S30-1.0-0.45 | 858/863 | 280/120/ - | 90/90/ - | 39.620 |
GPC7 | S30-2.0-0.45 | 858/863 | 280/120/ - | 120/60/ - | 38.531 |
GPC8 | S30-1.5-0.40 | 858/863 | 280/120/ - | 96/64/ - | 35.704 |
GPC9 | S30-1.5-0.50 | 858/863 | 280/120/ - | 120/80/ - | 42.839 |
OPC | - | 858/863 | - / - /400 | - / - /216 | 27.068 |
Abbreviations: CAg: Coarse Aggregate; FAg: Fine aggregate; SS: Sodium Silicate; SH: Sodium Hydroxide
Ten mixtures were produced, consisting of nine GPCs and one OPC as a control (Table 2). The mixtures were designated as GPC1 to GPC9, each with different components. GPC1 to GPC5 were created to investigate the impact of slag percentage. Mixtures GPC3, GPC6, and GPC7 focused on the variations in the SS/SH ratio, while GPC3, GPC8, and GPC9 examined the effects of varying the Al/Bi ratio. The GPC mix designation represented the components of the mixture, for instance, S30-1.5-0.50, indicating a GPC mix with a 30% slag content, the ratio of SS/SH 1.5, and the ratio of Al/Bi 0.50. No additional water or superplasticizer was used in the mixtures.
The GPC mixing process began with the preparation of the necessary materials. A two-molar SH solution was prepared the day before its use, while the alkaline activator, comprising of SS and SH, was mixed and left to settle until it had reached room temperature. After that, a container was filled with a homogeneous mixture of precursor materials, which are fly ash and slag. Then, the GPC constituents were blended in a specific order: sand and coarse aggregate were mixed together for one minute, followed by the addition of a fly ash and slag binder and a two-minute mixing time. The alkaline activator solution was then added according to a precise ratio, and the mixture was given five minutes of blending time to ensure thorough homogeneity.
After the mixing process, the specimens underwent slump testing and were subsequently cast in cylindrical molds with dimensions of 100 mm in diameter and 200 mm in height. The casting was performed in two layers, ensuring compaction in each layer to avoid any cavities. The specimens were allowed to remain in the molds for 24 hours prior to being taken out and cured. For the OPC, curing is done by submerging the specimen in water, while GPC curing involves storing it in an airtight plastic container to maintain moisture at room temperature. Prior to testing, all specimens were shaped into cylindrical forms with a diameter of 100 mm and a height of 50 mm, a process that occurred after 28 days of curing.
2.3. Testing procedure
⌅2.3.1. Porosity test
⌅The porosity test is specified in ASTM C642-97 (4141.
ASTM International. 1997. ASTM C642-97: Standard test method for
density absorption and voids in hardened concrete. ASTM Int United
States.
). The specimen has a cylindrical shape with a height of 50 mm and a 100 mm diameter. The Equation [1] is used to calculate the porosity.
where:
A = Oven-dried specimen mass (g). The test is carried out by drying the sample in an oven at a temperature of 100-110C for no less than 24 hours. Then it is removed from the oven and allowed to cool to a temperature of 20-25C and then the mass is determined.
C = Surface-dry specimen mass in air (g). After immersion and boiling for 5h, let it cool to a temperature of 20-25C. Then remove surface moisture with a dry towel and determine the mass.
D = Apparent mass of specimen in water (g). After the sample has been soaked, boiled and cooled, the next step is to determine the mass of the sample in the water.
2.3.2. Sorptivity test
⌅The
sorptivity test, following the procedure outlined in ASTM C1585-13, is
conducted to measure the rate of water absorption and sorptivity of a
cylindrical specimen. This test involves monitoring the weight change of
the specimen as it absorbs water over time (4242.
ASTM International. 2018. ASTM C1585-13: Standard test method for
measurement of rate of absorption of water by hydraulic cement
concretes. In: ASTM International.
). The specimens used in this test have a diameter of 100 mm and a height of 50 mm.
To perform the test, the specimen is positioned on supports within a pan filled with tap water. The water level is adjusted to be 1-3 mm above the bottom edge of the specimen, as depicted in Figure 3. The weight of the specimen is then recorded at regular intervals for up to 9 days, following the guidelines of ASTM C1585. These measurements are used to determine the amount of water absorbed, and subsequently calculate the sorptivity value and the rate of water absorption using Equation [2].
).
where:
I = the absorption,
m t = the increase in specimen weight (g), at time t,
a = the area of exposed surfaces (mm2)
d = the water density (g/mm3)
Initial sorptivity (mm/s1/2) is identified as the slope of the line acquired from plotting water absorption, I, to the square root of time (s1/2). This slope is determined by utilizing least squares from linear regression, using all of the data from 1 minute to 6 hours. The secondary sorptivity (mm/s1/2) is identified as the slope of the line obtained from plotting water absorption, I, to the square root of time (s1/2) utilizing all the points from 1 to 7 days. The slope was determined using least-square linear regression.
2.3.3. Rapid Chloride Permeability Test (RCPT)
⌅The
Rapid Chloride Permeability Test, as specified by ASTM C1202-12, is
used to measure the electrical current passing through cylindrical
specimens with a diameter of 100 mm and a height of 50 mm. The test
duration is six hours. In this test, a direct current (DC) voltage of
60V is applied to the ends of the specimen. One side of the specimen is
submerged in a sodium hydroxide (NaOH) solution, while the other side is
submerged in a sodium chloride (NaCl) solution. The overall charge
passed through the specimen in Coulombs is then calculated to determine
the resistance of the specimen to the penetration of chloride ions (4343.
ASTM International. 2012. ASTM C1202-12: Standard test method for
electrical indication of concrete’s ability to resist chloride ion
penetration. American Society for Testing and Materials.
). As an additional measure, a solution of silver nitrate (AgNO3) is sprayed onto the exposed surface of the concrete to assess the penetration depth of chloride ions. Figure 4 illustrates the configuration of the RCPT test.
3. RESULTS AND DISCUSSION
⌅This research aims to examine how various independent variables, such as different levels of slag in place of fly ash, variations in Al/Bi ratios, and variations in SS/SH ratios at a constant SH molarity of 2M, influence the transport properties of geopolymer concrete (GPC). The results and discussion will be presented separately, based on the type of tests carried out, including porosity, sorptivity, and chloride permeability.
3.1. Porosity
⌅The influence of various independent variables on porosity are presented in Figure 5. Based on Figure 5 (a), an increased in the slag content causes the porosity to decrease. The highest reduction value of 10.4% occurs at a slag percentage of 10% and the lowest (7.5%) occurs at 50% slag. The porosity value in OPC is 14.4%. Compared with the OPC, the GPC with slag recorded lower porosity.
The
decrease in porosity with increased slag content is likely due to the
binder reactions. The formation of C-A-S-H gel is due to the presence of
slag, which is denser than geopolymer type gels, resulting in a less
porous structure (1414.
Alanazi H, Hu J, Kim YR. 2019. Effect of slag silica fume and
metakaolin on properties and performance of alkali-activated fly ash
cured at ambient temperature. Constr. Build. Mater. 197:747–756. https://doi.org/10.1016/j.conbuildmat.2018.11.172.
, 2121.
Hu Y, Tang Z, Li W, Li Y, Tam VWY. 2019. Physical-mechanical properties
of fly ash/GGBFS geopolymer composites with recycled aggregates.
Constr. Build. Mater. 226:139–151. https://doi.org/10.1016/j.conbuildmat.2019.07.211.
). The slag used in this study contains a significantly higher CaO content than fly ash, at 62.1% and 8.6%, respectively (see Table 1).
By increasing the amount of slag used in place of fly ash, more C-A-S-H
gel will form, which will, in turn, decrease the number of pores in the
concrete. In addition, at similar SS/SH ratio, GPC based on fly
ash-slag has a larger amount (79%) of 10nm or smaller pores than GPC
based on fly ash (16%). This can be attributed to the formation of C-S-H
through slag activation in geopolymer concrete, which can fill most of
the capillary pores (1414.
Alanazi H, Hu J, Kim YR. 2019. Effect of slag silica fume and
metakaolin on properties and performance of alkali-activated fly ash
cured at ambient temperature. Constr. Build. Mater. 197:747–756. https://doi.org/10.1016/j.conbuildmat.2018.11.172.
).
On the other hand, the increased porosity of the fly ash-based GPC is
probably related to the gel’s binding, mainly in the form of N-A-S-H,
which is less dense and more porous (1313.
Noushini A, Castel A. 2016. The effect of heat-curing on transport
properties of low-calcium fly ash-based geopolymer concrete. Constr.
Build. Mater. 112:464–477. http://doi.org/10.1016/j.conbuildmat.2016.02.210.
, 4444.
Aiken TA, Kwasny J, Sha W, Soutsos MN. 2018. Effect of slag content and
activator dosage on the resistance of fly ash geopolymer binders to
sulfuric acid attack. Cem. Concr. Res. 111:23–40. https://doi.org/10.1016/j.cemconres.2018.06.011.
).
The findings presented in Figure 5 (b) suggest that the Al/Bi ratio plays an integral role in porosity. When
the ratio reaches 0.45, the minimum porosity of 8.82% is achieved. This
can be attributed to the OH- ions from the alkaline activator, which act as catalysts in the geopolymerization process and facilitate the release of Si4+ and Al3+ ions from the binder. If the amount of alkaline activator used is less,
the leaching process of the aluminum silicate will also be slower,
resulting in a poor geopolymeric structure with larger pores and lower
strength (2828.
Bellum RR, Muniraj K, Madduru SRC. 2020. Influence of slag on
mechanical and durability properties of fly ash-based geopolymer
concrete. J. Korean Ceram. Soc. 57(5):530–45. https://doi.org/10.1007/s43207-020-00056-7.
).
Meanwhile, at a higher Al/Bi ratio, the amount of alkaline activator
used also increases so that the mixture is more dilute. The excess
alkaline activator not utilized in the geopolymerization process will
eventually become a pore in the concrete after it has hardened. The
increasing number of pores causes the porosity to increase and, when
compared with OPC, all GPC samples with various Al/Bi ratios have lower
porosity.
Figure 5 (c) depicts an increased SS/SH ratio that reduces porosity. The highest
porosity of 9.2% was observed for the SS/SH ratio of 1.0. On the other
hand, porosity decreased to 8.8% and 7.6% for the SS/SH ratios of 1.5
and 2.0, respectively. The decrease in porosity is due to the higher
SS/SH ratio and the higher proportion of sodium silicate results in a
greater provision of free silicate. Increasing the amount of silicate in
the mixture can increase the polymerization reaction and will form a
N-A-S-H gel. Besides that, the presence of high calcium content in the
slag will form a denser C-(A)-S-H gel. This is in line with the finding
by other researcher (4545.
Puertas F, Palacios M, Manzano H, Dolado JS, Rico A, Rodríguez J. 2011.
A model for the CS-H gel formed in alkali-activated slag cements. J.
Eur. Ceram. Soc. 31(12):2043–2056. http://doi.org/10.1016/j.jeurceramsoc.2011.04.036.
),
who stated that the smaller pores at high SS/SH ratios were due to the
increased silica content resulted in a denser polymerized gel with
excellent mechanical properties.
According to the description above, porosity is closely related to the number of pores in the concrete. GPC with low molarity SH (2M) and varying slag percentages, Al/Bi ratios, and SS/SH ratios show lower porosity than OPC. This indicates that GPC has fewer pores and a denser structure, which will increase the mechanical properties of concrete.
3.2. Sorptivity
⌅The sorptivity test results indicate that the water absorption rate on the concrete surface and the initial and secondary sorptivity have a strong relationship with the time1/2. This can be seen from the data presented in Figure 6, which demonstrates the correlative relationship between the two variables.
The rate
of water absorption can be seen in two different phases. Initially, a
faster rate of absorption process take place and started to decrease
after 1 day. This condition is most likely caused by the pores in the
concrete, which are partially saturated (4646.
Kristiawan SA, Sunarmasto, Ridlo MM. 2018. Sorptivity of
self-compacting concrete with high volume fly ash and its
eco-mechanical-durability performance. IOP Conf. Ser. Mater. Sci. Eng.
442(1):012002. https://doi.org/10.1088/1757-899X/442/1/012002.
). In Figure 6(a),
the initial absorption of OPC is higher than the other GPC mixtures.
However, after 1 day it showed that GPC with 10% slag content (GPC1) had
higher absorption than OPC. Based on the data presented in Figure 6(b) and 6(c),
it can be observed that all GPC, regardless of the Al/Bi ratios and
SS/SH ratios, exhibit lower water absorption values compared to OPC.
This trend is observed both at the initial stage of water absorption and
throughout the extended water absorption period. This shows that the
proportion of the GPC2-GPC9 mixture has resulted smaller pores than OPC.
In accordance with ASTM C1858, the slopes of the graphs in Figure 6 may be used to determine the initial and secondary sorptivity values. However, some requirements must be met, where the graph of the relationship of water absorption with time must show a linear relationship, with a coefficient of correlation of more than 0.98. The sorptivity values obtained through the analysis of these slopes are shown in Figure 7.
Figure 7(a) confirms that a higher percentage of slag used will result in lower initial and secondary sorptivity. Samples with 20%, 30%, 40%, and 50% have initial sorptivities of 10.4, 9.6, 8.2, and 6.0 x 10-3 mm/s1/2, respectively. The proportion of GPC1 mixture with 10% slag has a slightly different sorptivity value, which has an initial sorptivity of 9 x 10-3 mm/s1/2, lower than the initial sorptivity of GPC2 and GPC3. However, GPC1 has a greater secondary sorptivity compared to other GPC mixtures. The secondary sorptivity of GPC is much lower than OPC with a very significant difference, except for GPC1. Because GPC1 has the lowest slag content, relatively less C-A-S-H gel is formed, which causes the concrete to be less dense. As a result, GPC1 has the highest secondary sorptivity when compared to other geopolymer concretes. The secondary sorptivity of GPC2, GPC3 and GPC4 is 3.0, 2.0, and 0.8 x 10-4 mm/s1/2, respectively. Meanwhile, GPC1 has a secondary sorptivity of 12 x 10-4 mm/s1/2, almost the same as the secondary sorptivity of OPC of 13 x 10-4 mm/s1/2. Unfortunately, the coefficient of correlation of the GPC5 graph is less than 0.98, so it isn’t possible to figure out its secondary sorptivity.
The
decline in sorptivity value as the slag content increases signifies
that the inclusion of slag in the fly ash-based GPC mixture enhances the
microstructural performance, which, in turn, makes the concrete last
longer, in terms of water access so that the sorptivity value becomes
low (2828.
Bellum RR, Muniraj K, Madduru SRC. 2020. Influence of slag on
mechanical and durability properties of fly ash-based geopolymer
concrete. J. Korean Ceram. Soc. 57(5):530–45. https://doi.org/10.1007/s43207-020-00056-7.
).
The increase of slag will cause the formation of denser C-A-S-H gel
with finer pore sizes, thereby preventing water from entering deeper
into the concrete (1818.
Provis JL, Myers RJ, White CE, Rose V, Van Deventer JSJ. 2012. X-ray
microtomography shows pore structure and tortuosity in alkali-activated
binders. Cem. Concr. Res. 42(6):855–64. http://doi.org/10.1016/j.cemconres.2012.03.004.
).
In general, the presence of C-A-S-H and N-A-S-H gels in fly ash/slag
concrete describes the beneficial behavior that occurs in this material.
The formation of N-A-S-H gel during the geopolymerization process is
instrumental in reducing the porosity of concrete. This gel fills the
voids in the paste and interfaces between the paste and the aggregate,
leading to a denser material. The C-A-S-H gel is associated with greater
densification and compactness, due to the fact that fly ash/slag
concrete has fewer porous capillaries or pores (99.
Angulo-Ramírez DE, Valencia-Saavedra WG, Mejía de Gutiérrez R. 2020.
Alkali-activated concretes based on fly ash and blast furnace slag:
Compressive strength water absorption and chloride permeability. Ing.
Investig. 40(2):72–80. https://doi.org/10.15446/ing.investig.v40n2.83893.
).
In addition, the less porous matrix is also associated with a smaller
slag particle size than the fly ash particles which develops a micro
filler effect (2222.
Mehta A, Siddique R, Ozbakkaloglu T, Shaikh FUA, Belarbi R. 2020. Fly
ash and ground granulated blast furnace slag-based alkali-activated
concrete: Mechanical transport and microstructural properties. Constr.
Build. Mater. 257:1–10. https://doi.org/10.1016/j.conbuildmat.2020.119548.
).
As shown in Figure 7(b), the sorptivity coefficient exhibits an increase in conjunction with the rise in the Al/Bi ratio. This can be attributed to the higher volume of liquid present, which results in a more diluted mixture. After the concrete hardens, this dilution leads to the formation of additional capillary pores. The lowest initial sorptivity value of 8.1 x 10-3 mm/s1/2 was achieved at an Al/Bi ratio of 0.40, followed by Al/Bi ratios of 0.45 and 0.50, which yielded initial sorptivity values of 9.6 x 10-3 mm/s1/2 and 10.6 x 10-3 mm/s1/2, respectively. When compared with the initial sorptivity of OPC, GPC has a lower value, but the difference is not significant. This indicates that GPC and OPC have almost the same ability to absorb water at the beginning of the period. In GPC, variations in the Al/Bi ratio have no effect on secondary sorptivity, whereas the three variations have the same secondary sorptivity. Meanwhile, the value of secondary sorptivity on GPC and OPC differs very significantly. The GPC with all variations of the Al/Bi ratio has a much lower secondary sorptivity of 2 x 10-4 mm/s1/2, while the OPC is 13 x 10-4 mm/s1/2. This is because GPC has fewer pores and a denser structure which is indicated by a lower rate of water absorption value than OPC, as seen in Figure 6(b).
Figure 7(c) illustrates the effect of the SS/SH ratio on the sorptivity
coefficient. As the SS/SH ratio increases, the initial sorptivity
decreases. This decrease was caused by the fact that the alkaline
activator was mostly composed of sodium silicate which has a high
viscosity. In the end, it leaves fewer pores after the concrete hardens.
In addition, the smaller pores at high SS/SH ratios are because the
increased silica amount produces denser polymerized gels with superior
mechanical characteristics (4545.
Puertas F, Palacios M, Manzano H, Dolado JS, Rico A, Rodríguez J. 2011.
A model for the CS-H gel formed in alkali-activated slag cements. J.
Eur. Ceram. Soc. 31(12):2043–2056. http://doi.org/10.1016/j.jeurceramsoc.2011.04.036.
).
In contrast, the secondary sorptivity shows a direct relationship with
the SS/SH ratio. As the SS/SH ratio increases, the secondary sorptivity
tends to increase as well, although the observed increase is not
substantial. It is worth noting that all GPCs with varying SS/SH ratios
exhibited lower initial and secondary sorptivity values compared to OPC.
When the SS/SH ratio was 1.0, 1.5, and 2.0, the initial sorptivity
value was 9.8, 9.6, and 7.2 x 10-3 mm/s1/2 and the secondary sorptivity was 2.0, 2.0, and 3.0 x 10-4 mm/s1/2, respectively. The OPC has an initial and secondary sorptivity value of 10.6 x 10-3 mm/s1/2 and 13 x 10-4 mm/s1/2, respectively.
Based on the previous description, it can be seen that the sorptivity coefficient is closely related to the level of water absorption in concrete. The higher the water absorption level, the more the sorptivity coefficient increases. In general, GPC with low molarity SH (2M) and with variations in slag percentage, Al/Bi ratio, and SS/SH ratio has lower sorptivity than OPC. This shows that GPC has a lower rate of water absorption, which is related to the smaller number of pores. So it can be concluded that the sorptivity coefficient is directly proportional to the porosity of the concrete.
3.3. Chloride Permeability
⌅Through the RCPT test, the chloride permeability of GPC was evaluated, as illustrated by the total charge passed value in Figure 8. To further ascertain the depth of chloride penetration, silver nitrate solution (AgNO3) was sprayed on the GPC surface, and the results of this test are displayed in Figure 9.
As seen in Figure 8(a),
when the proportion of slag increases as a replacement for fly ash, the
chloride permeability of the GPC decreases. This is indicated by the
smaller total charged passes. Mixes with slag content of 10%, 20%, 30%,
40%, and 50%, the total charged successively passed was 845.33 C, 739.80
C, 527.18 C, 438.08 C, and 379.13 C, respectively. The presence of
calcium-based hydrated materials is responsible for the decreased
chloride permeability as the percentage of slag increases, which form
together with the geopolymer gel and produce a strong and compact
microstructure, thus preventing the entry of chloride ions through pore
capillaries. In addition, in the presence of slag, alumina and silica
ions (which are very soluble and widely available) will be more easily
soluble, thereby increasing the polycondensation mechanism (2222.
Mehta A, Siddique R, Ozbakkaloglu T, Shaikh FUA, Belarbi R. 2020. Fly
ash and ground granulated blast furnace slag-based alkali-activated
concrete: Mechanical transport and microstructural properties. Constr.
Build. Mater. 257:1–10. https://doi.org/10.1016/j.conbuildmat.2020.119548.
).
The matrix formed with a decrease in pore size diameter is also
associated with a smaller slag particle size than fly ash particles,
which develops a micro filler effect (2323.
Khan MZN, Shaikh F uddin A, Hao Y, Hao H. 2016. Synthesis of high
strength ambient cured geopolymer composite by using low calcium fly
ash. Constr. Build. Mater. 125:809–820. http://doi.org/10.1016/j.conbuildmat.2016.08.097.
).
The correlation between chloride ion penetration and other pore
characteristics, such as tortuosity, is evident according to (1818.
Provis JL, Myers RJ, White CE, Rose V, Van Deventer JSJ. 2012. X-ray
microtomography shows pore structure and tortuosity in alkali-activated
binders. Cem. Concr. Res. 42(6):855–64. http://doi.org/10.1016/j.cemconres.2012.03.004.
).
When fly ash is replaced with slag, the tortuosity of the pore
structure increases, which could be a major factor in the decrease of
chloride penetration. According to the category in ASTM C 1202-12, all
GPC mixtures with varying percentages of slag and OPC have chloride
permeabilities in the very low category because they have total charged
passes of less than 1000C.
Figure 8(b) shows that the Al/Bi ratio 0.45 produces the least chloride
permeability, as indicated by the charged passed value of 527.18 C while
the charged passed values of 0.40 and 0.50 Al/Bi ratios were 565.43 C
and 628.43 C, respectively. The general trend is that higher Al/Bi
ratios will lead to higher porosity and a higher rate of chloride
penetration (4747.
Zhu H, Zhang Z, Zhu Y, Tian L. 2014. Durability of alkali-activated fly
ash concrete: Chloride penetration in pastes and mortars. Constr.
Build. Mater. 65:51–9. http://doi.org/10.1016/j.conbuildmat.2014.04.110.
).
However, as stated in the sub-discussion on porosity, at an Al/Bi ratio
of 0.40, the amount of alkaline activator is insufficient for the
geopolymerisation process to produce a geopolymeric structure with
larger pores (2828.
Bellum RR, Muniraj K, Madduru SRC. 2020. Influence of slag on
mechanical and durability properties of fly ash-based geopolymer
concrete. J. Korean Ceram. Soc. 57(5):530–45. https://doi.org/10.1007/s43207-020-00056-7.
).
Likewise, if the Al/Bi ratio increases from 0.45 to 0.50, the excess
alkaline activator that is not used in the geopolymerisation process
will leave pores with a larger size and volume. This condition causes
higher chloride permeability.
When the SS/SH ratio is increased, the chloride permeability is lessened, as illustrated in Figure 8(c). The highest total charged passed of 560.25 C was noted at SS/SH 1.0, followed by SS/SH 1.5 and SS/SH 2.0, with the total charged passed of 527.18 C and 353.03 C, respectively. At the same SS/SH ratio, the reduction in chloride permeability is in line with the decrease in porosity and sorptivity in concrete. This is because a high SS/SH ratio will increase the silica content and cause the gel to polymerize into a denser matrix, making the pores smaller. In GPC, with all variations of the SS/SH ratio and OPC, it has a very low chloride permeability category because it has a charged passed value of less than 1000 C. When compared to OPC, which has a total charged passed of 472.50 C, the mixture of GPC with slag percentages of 40%, 50%, and SS/SH ratio of 2.0 have lower chloride permeability.
Based on Figure 9, the depth of penetration of chloride at various percentages of slag, variations in Al/Bi ratios, and variations in SS/SH ratios is directly proportional to the value of porosity, rate of water absorption, and chloride permeability.
As illustrated in Figure 9(a), an increase in slag content as a replacement for fly ash leads to a decrease in chloride penetration depth. The depth was found to be the greatest (31.5 mm) when 10% slag was used and the lowest (5.7 mm) when 50% slag was used. This phenomenon can be attributed to the dense matrix that slags with a high CaO content creates, resulting in a reduction in the size and number of the concrete pores. Subsequently, the slag in GPC promotes a lower amount of chloride attack since the chlorides penetrate concrete through capillary pores. When compared to OPC, GPC based on fly ash-slag has better durability in a chloride environment, as evidenced by a much lower depth of chloride penetration (OPC has a chloride penetration depth of 39.6 mm).
The Al/Bi ratio that produced the lowest chloride penetration depth is 0.45, with a depth of 16.1 mm. At 0.40 and 0.50 ratios of Al/Bi, the GPC has chloride penetration depths of 24.6 mm and 18.0 mm, respectively, as shown in Figure 9(b). Meanwhile, Figure 9(c) shows that geopolymer concrete with a minimum chloride penetration depth of 16.1 mm is noted at an SS/SH ratio of 1.5. The SS/SH ratios of 1.0 and 2.0 result in deeper penetration depths of 24.3 mm and 18.2 mm, respectively. Compared with OPC, which has a chloride penetration depth of 39.6 mm, GPC exhibits a much lower penetration depth for all variations of the Al/Bi ratio and SS/SH ratio. These levels of chloride penetration depth can be seen in Figure 10.
The depth of chloride ion penetration in concrete relates to the amount and size of pores, water absorption, and the presence of external pressure. Based on the previous description, GPC with SH 2M molarity has fewer and smaller pores, as well as lower water absorption. As a result, GPC has a lower chloride ion penetration depth and greater resistance to chloride attack than OPC.
CONCLUSIONS
⌅In this paper, the results of the research are presented for the effect of low molarity SH (2M) with variations in slag percentages, variations in SS/SH ratios, and variations in Al/Bi ratios on porosity, sorptivity and chloride permeability in fly ash-slag-based GPC. Based on the results of the study it can be concluded that:
-
The higher the percentage of slag as a substitute for fly ash, and the higher the SS/SH ratio, the lower the porosity of the concrete. A slag content of 50% and SS/SH ratio of 2.0 resulted in the lowest porosities (7.53% and 7.65%, respectively). Meanwhile, the Al/Bi ratio that produces minimum porosity, is the Al/Bi ratio of 0.45 (8.82%). All GPC has lower porosity than OPC.
-
Increasing the slag percentage and ratio of SS/SH reduces the level of water absorption and sorptivity. The lowest sorptivity was 6 x 10-3 mm/s1/2 and 7.2 x 10-3 mm/s1/2, obtained at 50% slag content and an SS/SH ratio of 2.0. The increase in the Al/Bi ratio causes the sorptivity of concrete to increase. Meanwhile, the lowest water absorption rate was obtained at an Al/Bi ratio of 0.45. All variations of GPC mixes have lower levels of water absorption and sorptivity than OPC.
-
The higher the slag percentage and ratio of SS/SH, the lower the chloride permeability. The lowest chloride permeability was obtained at 50% slag content and an SS/SH ratio of 2.0, with a total charged passed of 379.13 C and 352.03 C, respectively. The optimal Al/Bi ratio is 0.45, which produces the lowest total charged passed of 527.18 C.
-
When compared to OPC, the mixture of GPC with slag percentage of 40%, 50%, and SS/SH ratio of 2.0 have lower chloride permeability. All mixtures, both GPC and OPC have chloride permeabilities in the very low category because they have total charged passes of less than 1000C.
-
The lowest chloride penetration depth was obtained at a 50% slag content (GPC5) of 5.7 mm and the highest at a 10% slag content (GPC1) of 31.5 mm. All chloride penetration depths in GPC are lower than OPC, with a chloride penetration depth of 39.6 mm.
-
The recommended proportions of fly ash-slag based GPC mixes with porosity, sorptivity, and chloride permeability lower than OPC are GPC4 (slag 40%, SS/SH ratio 1.5, Al/Bi ratio 0.45); GPC5 (slag 50%, SS/SH ratio 1.5, Al/Bi ratio 0.45); and GPC7 (slag 30%, SS/SH ratio 2.0, Al/Bi ratio 0.45).