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Procedía Engineering 171 (2017) 564 - 571
Procedía Engineering
www.elsevier.com/locate/procedia
Sustainable Civil Engineering Structures and Construction Materials, SCESCM 2016
Mechanical and durability performance of novel self-activating geopolymer mortars
Cheah Chee Bana, Part Wei Kena'*, Mahyuddin Ramlia
aSchool of Housing, Building & Planning, Universiti Sains Malaysia, 11800 Penang, Malaysia
Abstract
The research project aimed to explore the feasibility of activating fly-ash based geopolymer by the hybridization of fly-ash (FA) with high calcium wood ash (HCWA), a by-product from timber manufacturing industry, without the addition of conventional alkaline activators and post heat treatment curing regime. The raw materials namely FA and HCWA were characterized in term of their chemical and mineralogical phases by X-ray diffraction (XRD) and X-ray fluorescence (XRF). FA was substituted by HCWA at high replacement level of 50% to 100% at 10% incremental, by binder weight. Hardened geopolymer mortars samples were subjected to water curing and tested on the age of 7, 28 and 7 days + 24 hours hydrothermal treatment. Mechanical performance of the geopolymer mortars were assessed in term of compressive, flexural strength, ultrasonic pulse velocity (UPV) and dynamic modulus. Durability properties namely water absorption, vacuum porosity and capillary absorption were also investigated. Results were positive on the viability of hybridizing FA with HCWA to produce novel self-activating geopolymer mortars as mixtures with PFA replacement level of 50% and 60% showed enhanced mechanical and durability performance at all curing ages in comparison with other HCWA-PFA geopolymer mortar mixtures. Early strength development of HCWA-PFA geopolymer mortars was mainly contributed by the combination of hydraulic reaction of HCWA and geopolymerization of FA. On prolonged curing, strength development was due to the aforementioned reactions, plus pozzolanic reaction between the reactive silica from FA and portlandite formed from HCWA, producing additional secondary C-S-H gels. This experimental program showed positive findings in incorporating highly alkaline materials i.e. HCWA (12% K20) towards activating FA based geopolymer, thus eliminating the needs of external alkaline activator in conventional geopolymer mix design. © 2017 The Authors.PublishedbyElsevierLtd. Thisis an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of SCESCM 2016.
Keywords: geopolymer concrete; fly ash; self-activating; waste management; low embodied energy; high-calcium wood ash; low alkalinity.
* Corresponding author. Tel.: +60 0164871298. E-mail address: part.wei.ken@hotmail.com
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.Org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of SCESCM 2016.
doi:10.1016/j.proeng.2017.01.374
1. Introduction
Research in the field of geopolymer technology has gathered pace in the past decades owing to the environmental impact of Ordinary Portland Cement (OPC) production which raised concern amongst the practitioners regarding the sustainability of construction industry [1]. Various source materials have been employed with success such as pulverized fuel ash (PFA) or more commonly known as fly ash, ground granulated blast furnace slag (GGBS), rice husk as (RHA), palm oil fuel ash (POFA) etc in which the resultant geopolymer mixes exhibited similar or enhanced mechanical and durability performance if compared with OPC concrete [2-5]. However, the transition of geopolymer technology into mass production has not been successful. One of the major drawback of geopolymer system is the over reliance of alkaline activator in order to achieves the desired mechanical and durability performance [6]. The most common alkaline activator used is the synergy mixes of sodium hydroxide (NaOH) and sodium silicate (Na2Si03). The dosage of alkaline activators (NaOH + Na2Si03) in conventional geopolymer mix design ranged from 0.30-0.50 [7, 8] and moreover conventional geopolymeric system most often requires elevated temperature curing ranging from 60-90 0C [9, 10]. The two aforementioned factors have raised doubt on the practicability of geopolymer technology for industrial implementation as it significantly increased the effective cost and also the embodied energy of production of the resultant geopolymer mixes. Current laboratory investigation aims to tackle the aforementioned challenges by incorporating timber manufacturing waste material i.e. high calcium wood ash (HCWA) into fly ash based geopolymer mortar at high PFA replacement level i.e. > 50% of total binder mass without the need of alkaline activators and elevated curing temperature.
2. Materials and methods
2.1. Materials
ASTM Class F PFA was sourced from local coal-fired power plant in Manjung, Perak, Malaysia. The specific gravity and specific surface area ofPFA were determined to be 2.8 and 3244 cm2/g, respectively.
High Calcium Wood Ash (HCWA) is a by-product from local timber manufacturing industry which utilized wood-waste such as saw dust, woodchips etc in the boiler unit to generate energy for wood drying purposes. Freshly extracted HCWA was sieved through laboratory sieve of 600^m to remove carbonaceous and large agglomerated particles before being used as constituent materials in the geopolymer mortars fabrication. HCWA was found to have specific gravity of 2.43 and specific surface area of 5671 cm2/g. The detailed chemical composition characterization ofPFA and HCWA can be found in author's previous publication [11, 12].
Locally available natural siliceous river sand with specific gravity of 2.65 and maximum aggregate size of 5mm was used as fine aggregate throughout the laboratory investigation. Potable water from local water supply network was used as mixing water.
2.2. Methods
2.2.1. Mixture proportioning, mixing and curing
In the experimental program, PFA was replaced by HCWA at 50-100% by binder weight, at 10% step incremental. The sand to binder ratio was held constant at 2.25 throughout the program. The workability of the fresh geopolymer mortars was controlled at between 120-150mm for adequate compaction using the flow table apparatus, thus the water to binder ratio for each mixes varied. The mixture proportioning of HCWA-PFA geopolymer mortars were shown in Table 1.
HCWA-PFA geopolymer mortar mixes were homogenized using epicyclic mixer. The freshly casted mortar samples were left to cure in ambient temperature curing condition i.e. 260C and 85% RH for 24 h before being subjected to water curing until the designated testing ages. All the hardened HCWA-PFA geopolymer mortar samples were tested at the age of 7, 28 and 7 + 24 h hydrothermal treatment.
Table 1. Mixture proportioning of HCWA-PFA geopolymer mortars.
Batch Designation HCWA (kg/m3) PFA (kg/m3) Sand (kg/m3) Water (kg/m3) w/b Flow (mm)
WA50FA50 322 322 1449 219 0.34 145
WA60FA40 386 258 1449 238 0.37 140
WA70FA30 451 193 1449 245 0.38 140
WA80FA20 515 129 1449 258 0.4 140
WA90FA10 580 64 1449 271 0.42 140
WA100FA0 644 0 1449 271 0.42 145
2.2.2. Mechanical and durability tests
The mechanical performance of the hardened HCWA-PFA geopolymer mortars was assessed in terms of compressive, flexural, ultrasonic pulse velocity (UPV) and dynamic modulus. Compressive and flexural strength assessment was done in accordance to the testing methods prescribed in BS EN 196-1 and the reported results were based on the average of three representative samples. UPV values of the hardened samples were determined using an electrical pulse generator which measured the propagation velocities of a transmitted ultrasonic pulse and the testing procedure was done in accordance to BS EN 12504-4. Dynamic modulus of the hardened sample was measured by application of non-destructive stress in the longitudinal mode of vibration on the 100 x 100 mm end face of a prism with a path length of 500 mm is done in accordance to the methods prescribed in ASTM C 215.
Three testing parameters were employed to assess the durability performance of HCWA-PFA geopolymer mortars namely water absorption, total porosity and capillary absorption tests. Water absorption test was done based on the testing procedure prescribed in BS 1881-122 while vacuum saturation method recommended by RILEM was employed to measure the total porosity of the hardened mortar samples. Finally, the pore size distribution of the mortar sample was determined using the capillary absorption method prescribed in French Standard NF P 18354.
3. Results and discussions
3.1. Mechanical Properties 3.1.1. Compressive strength
Fig. 1 showed the compressive strength development of HCWA-PFA geopolymer mortars at various curing ages. It can be seen that upon 60% of HCWA replacement level, the compressive strength generally decreased with the increase in HCWA content. During early age of curing i.e. 7 days, strength development of HCWA-PFA geopolymer mortars were mostly governed by geopolymeric reaction resulted from the dissolution of aluminosilicate compounds of PFA, though the hydraulic reaction of HCWA itself is also expected to contribute towards the hardening of the resultant mortar samples. The anticipated geopolymeric gel formation in HCWA-PFA geopolymer mortars is K-A-S-H geopolymer gel since K+ ions are more reactive than Ca2+. When the dry blended materials of HCWA and PFA are in contact with water, the significant amount of Arcanite mineral (12% by weight) inherently present in HCWA dissolved into potassium hydroxide (KOH), this highly alkaline mineral acts as dissolution agent and dissolve the aluminosilicate species of PFA. The dissolved aluminate and silicate ions will undergoes geopolymerization with the reactive K+ ions to form the geopolymeric framework in the forms ofK-A-S-H gel. Upon 28 days of curing age, similar trend was observed with the exception of the compressive strength difference between WA50FA50 and WA60FA40 mixes, where the WA50FA50 mixes exhibited higher compressive strength in comparison with WA60FA40 mixes which is a total contrast in comparison with the 7 days compressive strength results. Similar trend was observed for the accelerated curing series. During prolonged curing, besides geopolymerization and hydraulic reaction of HCWA, the strength development of HCWA-PFA geopolymer mortars is also governed by the formation of secondary C-S-H gels resulted from the pozzolanic reaction between the reactive silica from PFA and portlandite mineral formed from hydration of HCWA [13]. Therefore, higher rate of
long term strength development of HCWA-PFA geopolymer mortars can be anticipated in mixes with higher PFA content.
9.00 8.00 7.00 6.00 5.00 4.00 3.00 2.00 1.00
HCWA Content (%)
* 7 Days Water Curing
H 7 Days Water Curing + 24H Accelerated Curing
28 Days Water Curing
Fig. 1. Compressive strength development of HCWA-PFA geopolymer mortars.
3.1.2. Flexural Strength
Flexural strength of HCWA-PFA geopolymer mortars at various HCWA replacement level is shown in Fig. 2. Disregarding the curing ages, upon 60% of HCWA replacement level, flexural strength exhibited a decreasing trend. Also, WA60FA40 mixes showed the highest flexural strength for all the curing ages. At 7 days of curing, the difference in flexural strength for WA60FA40 and WA50FA50 is 29.53%. However, upon 28 days and accelerated curing, the difference is narrowed down significantly, at 4.24% and 13.69%, respectively. The aforementioned phenomenon further suggests that mixes with higher PFA content exhibited higher long term strength development, mainly due to the continuous formation of geopolymeric product and also pozzolanic reaction between the reactive silica from PFA and portlandite from HCWA which resulted in the formation of secondary C -S-H gels. Accelerated curing treatment, on the other hand, was found to have a more pronounce effect with mixes with higher PFA content i.e. WA50FA50, WA60FA40 and WA70FA30, where the flexural strength increment is comparatively higher than other mixes.
Fig. 2. Flexural strength development ofHCWA-PFA geopolymer mortars.
3.1.3. Ultrasonic Pulse Velocity (UPV)
Table 2 showed the UPV of HCWA-PFA geopolymer mortars at various HCWA replacement level and curing ages. Consistent with compressive strength result, WA60FA40 mixes exhibited highest UPV value of 3120 m/s at 7
days of curing ages. On the contrary, upon 28 days of curing period, highest UPV value of 3467 m/s was exhibited by WA50FA50 instead. Early age strength development was mostly governed by the hydraulic reaction of HCWA and the formation of geopolymeric products in the form of K-A-S-H gels. Upon 28 days of curing, mixes with higher PFA will trigger higher degree of pozzolanic reaction which yields the formation of secondary C -S-H gels, coupled with the continuous formation of geopolymeric products, resulted in a much denser microstructure hence higher UPV value was obtained.
3.1.4. Dynamic Modulus
Dynamic modulus of HCWA-PFA geopolymer mortars are shown in Table 3. For all curing ages, upon 70% of HCWA replacement level, the dynamic modulus of the geopolymer mortars decreased significantly up until 100% HCWA replacement level, indicating a weak internal structure of mixes beyond 70% of HCWA replacement. Similar to compressive strength results, during early curing age, WA60FA40 showed the highest dynamic modulus value of 18.3 GPa, followed by WA70FA30 and WA50FA50 mixes with dynamic modulus value of 17.9 and 16.8
GPa, respectively. Upon 28 days of curing age, WA50FA50 exhibited highest dynamic modulus value of 21.0 GPa, followed by WA60FA40 and WA70FA30, with dynamic modulus value of 20.6 and 19.8 GPa, respectively. Similar trend was observed for accelerated curing mixes, suggesting higher geopolymeric reaction occurre d in mixes with higher PFA content due to elevated heat treatment, which resulted in higher formation of K -A-S-H gel within the matrix interface and subsequently higher dynamic modulus was observed.
Table 2. Ultrasonic pulse velocity of HCWA-PFA geopolymer mortars at various curing ages.
Mix Designation 7 Days Water Curing 28 Days Water Curing 7 Days Water Curing + 24H Accelerated Curing
WA50FA50 3046 3467 3217
WA60FA40 3120 3394 3276
WA70FA30 3001 3409 3216
WA80FA20 2984 3320 3078
WA90FA10 2907 3162 2908
WA100FA0 2670 3022 2785
Table 3. Dynamic modulus ofHCWA-PFA geopolymer mortars at various curing ages.
Mix Designation 7 Days Water Curing 28 Days Water Curing 7 Days Water Curing + 24H Accelerated Curing
WA50FA50 16.8 20.97 20.93
WA60FA40 18.3 20.60 19.40
WA70FA30 17.9 19.77 19.82
WA80FA20 15.2 18.10 17.94
WA90FA10 10.2 14.80 11.03
WA100FA0 7.4 10.90 9.50
3.2. Durability Properties 3.2.1. Water Absorption
Water absorption test employed primarily to assess the surface porosity of the hardened HCWA-PFA geopolymer. As can be seen in Fig. 3, at early curing age i.e. 7 days, upon 60% of HCWA replacement level, water absorption of HCWA-PFA geopolymer mortars increased as the HCWA replacement level increased. The higher geopolymeric reaction in mixes with higher PFA content triggered higher amount of K-A-S-H geopolymer gel formation and resulted in a denser geopolymer matrix with lower surface permeability. Similar trend was observed during 28 days of curing and also accelerated curing regime, with the exception WA50FA50 mix exhibited lower
water absorption value in comparison with WA60FA40 mix, a total reverse trend if compared with 7 days curing result, suggesting that during prolonged curing, mixes with higher PFA content not only have higher rate of enhancement in mechanical properties, also enhanced durability performance was also observed for HCWA-PFA geopolymer mortars mixes with higher content of PFA. An interesting phenomenon was observed if comparing the water absorption trend in regards with curing ages. Upon 28 days of curing and accelerated curing regime, all the HCWA-PFA geopolymer mortar mixes exhibited an increase of water absorption value if compared to their 7 days cured counterpart. It appeared that prolonged water curing resulted in inferior durability performance of HCWA-PFA geopolymer mortars. During water curing, ions interchange occurred between the water curing medium and also the alkaline pore solution presents in HCWA-PFA geopolymer mortars. As a result, the alkalinity of pore solution of the geopolymer mortars decreased and resulted in disruption of the dissolution rate of the essential geopolymer precursor species i.e. silica and alumina and hence the geopolymerization process in all. Also, it should be noted that during polycondensation reaction, excess water was expelled and it is believed that those expelled water molecules which resided near the surface of the mortar mixes resulted in a higher water penetratio n and hence the higher water absorption value. Similar observation was reported in author's previous publication [12].
50 60 70 80 90 100
HCWA Content (%)
Fig. 3. Water absorption of HCWA-PFA geopolymer mortars at various ages.
3.2.2. Total Porosity
Fig. 4 showed the total porosity value of HCWA-PFA geopolymer mortars at various curing ages. At 7 days of curing age, WA60FA40 mix showed a reduction of 1.46% in total porosity value if compared to WA50FA50 mix, mainly due to the geopolymerization reaction and hydraulic reaction of HCWA itself. Upon 60% of HCWA replacement level, the total porosity value of the geopolymer mortars increased with the increasing of HCWA content up to 90% of total binder weight, before a reduction in total porosity value was observed for mortar mix with 100% HCWA content. The higher total porosity value observed for WA80FA20 and WA90FA10 mixes compared with WA100FA0 mix is most probably due to the low amount of PFA content in the mortar mixes which impart a low degree of geopolymerization and hence less dense microstructure. Upon 28 days and accelerated curing, all the HCWA-PFA geopolymer mortar mixes exhibited an increase in total porosity value, consistent with the results shown in water absorption test earlier. The total porosity results implied that not only does the deleterious water curing method affect the surface porosity of the geopolymer mortars, it also adversely affects the total porosity of the resultant geopolymer mortar mixes. It should be noted that upon 28 days and acc elerated curing, WA50FA50 mix showed a lower total porosity value if compared with WA60FA40 mix, a total contrast to 7 days cured mixes. The aforementioned observation further suggests that there is a higher degree of microstructure refinement during long term curing for mortar mixes with higher PFA content, mainly due to the continuous geopolymerization and also formation of secondary C-S-H gels from the pozzolanic reaction between portlandite from HCWA and reactive silica from PFA. The total porosity results were in full agreement with the mechanical properties test discussed earlier.
Fig. 4. Total porosity of HCWA-PFA geopolymer mortars at various curing ages.
Fig. 5. Capillary absorption of HCWA-PFA geopolymer mortars.
3.2.3. Capillary Absorption
The pore size distribution of HCWA-PFA geopolymer mortars is shown in Fig. 5. There are two phases of pore filling phenomena that can be interpret from the graph i.e. filling of bigger pores during the initial linear phase which lasted up to 2 days and subsequent non-linear phase which lasted up to 7 days of testing period, correspond to the filling of smaller/finer pores. Generally, for the filling of bigger pores, HCWA-PFA geopolymer mortar mixes with 90% of HCWA content showed the highest capillary absorption value, followed by mixes with 100%, 80%, 50%, 70% and 60% of HCWA content. It can be seen that HCWA-PFA geopolymer mortar mixes with small amount of PFA i.e. 10% exhibited higher degree of bigger pores filling, as compared with 0% PFA mortar mix. This implied that the addition of PFA in small amount brings about negative effect to the pore structure of the resultant matrix, in line with the porosity results which show similar trend. On the other hand, HCWA-PFA geopolymer mortar mixes with 100% HCWA content showed highest capillary absorption value for finer pore filling, followed by mixes with 90%, 80%, 70%, 50% and 60% of HCWA. Thus, it can be said that for the curing period of 7 days, WA60FA40 geopolymer mortar mix exhibited the most desired pore structure amongst all the HCWA-PFA geopolymer mortar mixes, with the lowest capillary absorption value for both the bigger and finer pore filling phases.
4. Conclusions
Following the laboratory investigation, following conclusions can be derived:
• HCWA-PFA geopolymer mortars with HCWA content of 50 and 60% consistently exhibit optimum mechanical and durability performance,
• Strength development of HCWA-PFA geopolymeric system was mainly contributed by geopolymeric reaction which culminated in K-A-S-H geopolymer gels formation during the early ages and pozzolanic reaction in which secondary C-S-H gels were formed during prolonged curing.
• HCWA-PFA geopolymer mortars with higher content of PFA i.e. WA50FA50 mortar mix exhibited enhanced long term mechanical and durability performance, mainly due to the higher degree of pozzolanic reaction.
• Prolonged water curing resulted in inferior durability performance for all HCWA-PFA geopolymer mortars.
• Arcanite mineral inherently presents in HCWA proved crucial in activating PFA by functioning as both dissolution agent for silica and alumina species in PFA and also as binding agent for the formation of geopolymer product i.e. K-A-S-H gels.
Acknowledgements
Fundings from Malaysian Ministry of Higher Education (203/PPBGN/6711347) and University Sains
Malaysia (1001/PPBGN/814211, 304/PPBGN/6312106) are highly appreciated.
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