The long term engineering properties of cementless building block work containing large volume of wood ash and coal fly ash

Construction and Building Materials 143 (2017) 522–536

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The long term engineering properties of cementless building block work containing large volume of wood ash and coal fly ash Chee Ban Cheah ⇑, Wei Ken Part, Mahyuddin Ramli School of Housing, Building and Planning, Universiti Sains Malaysia, 11800 Penang, Malaysia

h i g h l i g h t s
Standard consistency and setting time performance of HCWA-PFA binder paste.
Long term mechanical performance of HCWA-PFA block work up to 365 days.
Thorough characterization on the aluminosilicate bond framework of HCWA-PFA geopolymer.
Large volume reuse of biomass and coal energy sector by products for building block work production.

a r t i c l e

i n f o

Article history: Received 23 November 2016 Received in revised form 3 March 2017 Accepted 17 March 2017

Keywords: Recycling Mechanical properties Hybrid binder Geopolymer Load bearing block work

a b s t r a c t The reuse of pulverised fuel ash (PFA) and high calcium wood ash (HCWA) offers a two folds solution towards addressing the waste management problem and reduce the carbon footprint of concrete material. In order to enable sizeable application of such recycling method to produce block work for the construction industry, a comprehensive framework of knowledge on the mechanical strength performance of the concrete product containing large volume of PFA and HCWA needs to be established. Hence, it is the primary aim of the study to report a comprehensive assessment on the mechanical strength performance of concrete block work produced with the sole use of PFA and HCWA as the primary binder phase. The PFA and HCWA binder phase with small dosage of sodium silicate was characterized in terms of its standard consistency and setting times. Besides, a thorough assessment on the mechanical strength of block work produced with PFA-HCWA binder was conducted. Mechanical strength assessment covers the compressive strength, flexural strength and velocity of ultrasonic pulse propagation through the material. Besides, the fourier transformed infrared (FTIR) assessment was also performed to characterize the aluminosilicate bonds of the primary binder phase. The increasing mass ratio of HCWA was found to raise the water demand of the HCWA-PFA binder paste. Besides, the HCWA:PFA combination ratio of 50:50 and 40:60 has been found to exhibit the optimal flexural and compressive strength performance. It has also been established that the block work produced using HCWA:PFA combination ratio between 30:70 up to 90:10 meets the California Concrete Masonry Technical Committee requirement as class N concrete block work unit. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction OPC concrete brick is defined as concrete masonry unit made from Portland cement, water, and suitable aggregates, with or without the inclusion of other materials as per ASTM C55 Standard Specification for Concrete Brick. The most distinct advantage of OPC concrete brick over fired clay brick is the ability to attain the required strength and properties without the need of high temperature kiln firing. However, the utilization of OPC as sustainable ⇑ Corresponding author. E-mail address: [email protected] (C.B. Cheah). http://dx.doi.org/10.1016/j.conbuildmat.2017.03.162 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.

building material has come under heavy scrutiny in recent years due to the environmental impact by the clinker production [1,2]. In fact, the production of Portland Cement (PC) clinker from the cement production plants worldwide emit up to 1.5 billion tons of CO2 annually, which accounts for around 5% of the total manmade CO2 emission and if the undesirable trend continues, the figure will rise to 6% by year 2015 [3–5]. Demand for electricity in developing countries has increased by leaps and bounds in recent years due to the growth in world population and economic. Recent study forecasted the world energy consumption to be increased by 47% from 2007 to 2035 [6]. In Malaysia, only 5.5% of total electricity was generated using

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renewable energy sources and 94.5% of electricity was generated using fossil fuels such as natural gas, coal and oil in the year 2009. The wood processing industry in Malaysia represents a great potential to be used as renewable energy source to supplement the power/electricity demand as it is considered as one of the largest untapped biomass resource in Malaysia [7]. According to Vassilev et al. [8], wood wastes are the more preferable fuel for biomass furnaces as the incineration of wood waste yields relatively less residual and fly ash in comparison to other biomasses such as agricultural and herbaceous wastes. Though the aforementioned practices provide a great solution to the solid waste management of wood waste residues, the use of wood wastes such as sawdust, woodchips and offcuts as fuel have resulted in a significant amount of fine wood waste ash as a by-product material in which the industry found limited practical applications for the recycling of the fine particulate ash materials [9]. Pulverised fuel ash (PFA) or more commonly known as fly ash (FA), an industrial by-products of coal burning power plant industry, makes up of 75–80% of global annual ash production [10]. As 94.5% of total electricity generated in Malaysia is by the mean of fossil fuels such as natural gas, coal and oil, an enormous amount PFA is expected to be generated annually. The coal-fired power generation industry in Malaysia consumed approximately 8 million tons of coal annually. In 2010, the Malaysian government implemented a policy that eventually increased the national coal powered electricity by 40% due to the rapid economic growth and this has further increased the amount of PFA generated and this hazardous industrial by-product mandates a proper disposal or recycling methods [11]. Blended geopolymers is a new category of geopolymeric binder which is produced by the combination of two or more industrial waste ashes followed by subsequent stabilization and solidification using chemical activators. The dual advantages of waste utilization and more importantly the alteration in Si/Al and Ca/Si in geopolymer system prompted sudden rush in the amount of researches in the field of blended geopolymer during recent years [12–17]. Ever since it was known that C-S-H gel can be co-exist with geopolymeric gel in a single system and contributes to the overall strength gain [18], various researchers has utilized high calcium waste material such as GGBFS and ASTM class C fly ash to blend with ASTM class F fly ash in order to achieve a higher early strength gain and shorter setting time which is beneficial for the application in pre-cast industry [19,20]. Canfield et al. [21] investigated the role of calcium in fly ash based geopolymers by blending high and low calcium fly ash. It was found that calcium played two major roles during the geopolymerization of the blended fly ash geopolymer specimen, (i) calcium was found to aids the dissolution of silica and alumina species from the FA particles, yielding higher tetrahedral silicate and aluminate monomer concentration as shown in FTIR, XRD and TGA/DSC results, (ii) calcium also functions as a counter-balancing cation when incorporated into the geopolymer pore structure. In a separate study, The workability of fresh geopolymer concrete consisted of GGBFS and FA showed a decreasing trend with the increase in slag content and decrease of SS/SH ratio due to the enhancement in reactivity in the presence of GGBFS in FA based geopolymer system [12]. Other waste materials rich in silica such as rice husk ash (RHA) and palm oil fuel ash (POFA) have also found considerably interest amongst geopolymer researchers in the field of blended geopolymer [22–25]. Tyni et al. [26] had conducted a feasibility study to utilize fly ash and bottom ash fraction collected from circulating fluidize bed boiler combusting mixture of peat and wood as geopolymeric binder constituent. Results obtained from the scanning electron microscopy and X-ray diffraction assessments are indicative of the formation of amorphous geopolymer phases in the peat and wood blended fly ash paste activated with 5 M NaOH solution. Besides, it has also been

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found that the use of high concentration (10 M) NaOH activator solution produced semi-geopolymeric phases which also comprised of crystalline zeolite. Prior work done by the authors has established the potential for large volume reuse of high calcium wood ash (HCWA) and PFA as the primary binder phase in the fabrication of mortar block [27,28]. In the study [27], geopolymer mortar were fabricated with HCWA– PFA hybridization ratio of 100:0, 70:30, 60:40 and 50:50 using two different water/binder ratios of 0.30 and 0.35. The mechanical performance and durability performance of the HCWA–PFA mortar block was assessed. It has been established that the combination of HCWA-PFA at mass ratio of 70:30, 60:40 and 50:50 is optimum for self-induced hardening even without the presence of any alkali activator. This is largely contributed by the arcanite mineral compound which is inherently present in the HCWA material. It has also been established that the curing of HCWA-PFA mortar block under moist curing condition of relative humidity 80 ± 5% is favorable over water curing condition. This is evident as all the water cured test specimens exhibited inferior mechanical strength performance as compared to the moist cured equivalent test specimens largely due to de-polycondensation caused by water intrusion [28]. It has also been found that the mechanical strength development of HCWA-PFA geopolymeric system was mainly contributed by geopolymeric reaction which formed K-A-S-H geopolymeric framework at early age of hardening followed by the formation of the secondary C-S-H framework through pozzolanic reaction. Besides, the hydrothermal treatment regime of full immersion in hot water bath at 70 °C was found to accelerate the mechanical strength gain at early age of 7 days but impair the total porosity and subsequently water absorption property of the material. This is largely due to the induced formation of cracks at the interfacial transition zone of the paste and aggregate phase due to vast differences in thermal expansion coefficient of the two phases [29]. In general, though a substantial body of knowledge has been established on the properties of blended geopolymer, and specifically HCWA-PFA geopolymer, there are still areas which are scarcely explored namely the setting behaviour of the paste and long term mechanical strength development over a prolonged duration of curing. The present work is an extended investigation on the HCWAPFA paste standard consistency and setting times performance. Besides, new body of knowledge on the long term engineering performance of the HCWA-PFA building block up to 365 days with various HCWA-PFA combination ratio between 0:100 and 100:0 at 10% increments has also been established in the present study. This is done to establish a thorough understanding on the long term performance of the material for its intended application in the building construction industry.

2. Materials and methods 2.1. Materials 2.1.1. High calcium wood ash (HCWA) HCWA is a by-product acquired from an industrial scale fully automatic boiler unit (commercially known as Bio-Turbomax boiler) used in the rubber wood timber product manufacturing industry. The wood biomasses used as fuel in the boiler were derived from local rubber wood species dominantly Hevea Brasiliensis. The wood biomasses were incinerated under a self-sustained burning condition within an atmosphere with a turbulent air flow supplied by an in-built air pump unit. The temperature of incineration was maintained within the range of 800 ± 10 °C [30]. Raw wood ash extracted from the boiler unit was sieved through a laboratory sieve with an opening size of 600 mm to remove large agglomerated ash particles and carbonaceous materials. Resultant materials had a specific surface area of 5671 cm2/g and specific gravity of 2.43. The physical properties and chemical compositions of HCWA had been established and described deliberately in prior study of the authors [27] and are shown in Table 1.

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Table 1 Chemical composition and physical parameters of HCWA and PFA [27]. Chemical Composition and Physical Parameters

% by total mass

MgO Al2O3 SiO2 P2O5 SO3 Cl K2O CaO TiO2 MnO Fe2O3 ZnO SrO PbO CuO Rb2O C Na2O Median particle diameter, d50 (mm) Loss on ignition (%)

HCWA

PFA

8.70 1.30 2.70 2.70 2.80 0.10 12.00 61.00 0.11 0.86 1.30 0.10 0.22 Trace 0.01 0.05 6.70 – 8.39 18.00

5.94 17.61 43.22 0.23 – – 1.31 11.28 0.88 0.14 13.73 – – – – – 1.80 0.43 13.82 1.80

2.1.2. Pulverised fuel ash (PFA) Pulverised fuel ash (PFA) used in this study was collected from the precipitator unit of local coal fuelled power plant. Results obtained from Blaine fineness analysis indicated that PFA used in the study had a specific surface area of 3244 cm2/g. The specific gravity of PFA was determined to be 2.8. Similarly, the details on the physical and chemical properties of the PFA has been characterized and deliberated extensively in prior work of the authors [27] and are shown in Table 1. 2.1.3. Sodium silicate Commercial grade C140 of sodium silicate (Na2SiO3) solution with a modulus ratio of 2.1 (SiO2/Na2O, SiO2 = 36.5%, Na2O = 18.0%), specific gravity of 1.67 at 20 °C and a total solid content of approximately 52.75% was used as the sole alkaline activator in sample preparation. 2.1.4. Aggregates and mixing water Fine aggregates used were locally sourced quartzitic natural river sand in uncrushed form with a specific gravity of 2.65 and a maximum aggregate size of 5 mm. Fine aggregates were dried to saturated surface dry conditions for use as a constituent material in mortar mixes. Fine aggregates were graded in accordance to BS812: Part 102 [31] and the grading of fine aggregates used were in compliance with overall grading limits of BS 882 [32]. The fineness modulus of the fine aggregates was determined to be 3.26. Potable water from local water supply network was used as mixing water. The mix design for the geopolymer mortar block are shown in Table 2. 2.2. Methods 2.2.1. Mixture proportioning and mixing The binder: sand were maintained constant at 1:2.25 for all mortar mixes produced. The PFA was partially replaced using HCWA at high substitution levels of 0% to 100% by total binder weight at stepped increment of 10%. A corresponding set of hydrated ash paste was produced using the aforementioned ash hybridization ratio for the standard consistency and setting time performance assessment. The various mix proportions of HCWA-PFA geopolymer mortar are summarized in Table 3. Mix-

ing water was incorporated in the mortar mix at a predetermined w/b ratio of 0.30. This was to achieve the degree of mix consistency which is suitable for compaction using the hydraulic press while at the same time exhibit optimal mechanical strength performance [27]. The mortar samples are referred based on the percentage (%) of PFA replacement with HCWA and expressed as %HCWA. Meanwhile, the paste samples were demarcated with the nomenclature system of WAxFAy where x is the mass % of HCWA and y is the mass % of PFA. 2.2.2. Mixing, forming and curing Each batch of mortar was homogenized using an epicyclic type mechanical mixer complying with specifications in ASTM Standard C305 [33]. During the mixing of mortar mixes containing HWCA and PFA were initially dry mixed at a low mixing speed for 3 min prior to the addition of other constituent materials. Further mixing sequences and durations were performed in accordance to standard procedures prescribed in ASTM Standard C305 [33]. Upon completion of the mixing, mortar blocks with edge dimensions of 290  140  100 mm were formed with hydraulic press. The forming of the mortar block was performed using a constant volumetric compression ratio of 1.48. All the mortar blocks produced were then allowed to set in air for 24 h at ambient temperature of 28 ± 5 °C and relative humidity 80 ± 5%. Subsequently, the test specimens were further cured by wrapping the hardened HCWA-PFA geopolymer mortar blocks with a layer of HDPE film to prevent moisture loss till the testing ages of 7, 28, 90, 180 and 365 days. 2.2.3. Assessment on the standard consistency, initial and final setting time of HCWAPFA paste Water demand properties namely standard consistency and paste setting properties including initial and final setting time of HCWA-PFA blended geopolymer pastes with a fixed sodium silicate content were determined in accordance to testing methods specified in BS EN 196-3 [34] using the Vicat apparatus. The weighted proportion of HCWA and PFA was first dry blended using the standard epicyclic mixer for 3 min in order to obtain homogeneity of the two blended materials. Next, mixing water was added to the blended HCWA-PFA powder to a certain extent whereby the resultant HCWA-PFA geopolymer paste exhibited a penetration depth of 6 ± 2 mm between the plunger and base plate. The water content at that particular extent was then recorded as the water content to achieve HCWA-PFA geopolymer paste with standard consistence. HCWA-PFA geopolymer paste was then cured under moist condition for the subsequent initial and final setting time assessment. Periodical examination was done on the penetration of HCWA-PFA geopolymer paste. The initial setting time was determined as the time required for the HCWA-PFA geopolymer paste to obtain sufficient stiffness to provide resistance towards the penetration of Vicat needle such that the distance between the needle and the base plate is 6 ± 3 mm. For the determination of final setting time, the filled Vicat mould was inverted and subsequent testing was conducted on the face in which the mould was inverted. HCWA-PFA geopolymer paste was deemed to have achieved final set when the Vicat needle only penetrates the paste by only 0.5 mm or when the time where Vicat needle failed to leave a ring mark attachment onto the surface of HCWA-PFA geopolymer paste. 2.2.4. Flexural strength, compressive strength and ultrasonic pulse velocity assessment HCWA-PFA geopolymer mortar prisms with edge dimensions of 290  140  100 mm were tested as per procedures prescribed in ASTM Standard C348 [35] for the determination of flexure strength. The reported flexural strengths at given ages of mortar are the average of the three numbers of specimens tested. Subsequently, compressive strength test was performed using the test apparatus and method prescribed in ASTM C 349 [36]. The density or the compactness of HCWA-PFA geopolymer mortar block can be estimated by measuring the propagation velocity of a transmitted ultrasonic pulse across the cross sectional area of HCWA-PFA geopolymer mortar block. This is made possible by an ultrasonic pulse generator equipped with a transducer which complies with the standard specification prescribed in BS EN 12504-4 [37]. The test was conducted on three representative HCWA-PFA geopolymer mortar block samples measuring 290  140  100 mm dimension at various curing ages of 7, 28, 90,

Table 2 Mix proportion of HCWA-PFA geopolymer mortar block work. Mix Designation

PFA (kg/m3)

HCWA (kg/m3)

Sand (kg/m3)

Na2SiO3 (kg/m3)

Water (kg/m3)

Water/binder ratio

WA0FA100 WA10FA90 WA20FA80 WA30FA70 WA40FA60 WA50FA50 WA60FA40 WA70FA30 WA80FA20 WA90FA10 WA100FA0

656 66 131 197 262 328 394 459 525 590 0

0 590 525 459 394 328 262 197 131 66 656

1476 1476 1476 1476 1476 1476 1476 1476 1476 1476 1476

33 33 33 33 33 33 33 33 33 33 33

197 197 197 197 197 197 197 197 197 197 197

0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

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C.B. Cheah et al. / Construction and Building Materials 143 (2017) 522–536 Table 3 Standard consistency of HCWA-PFA geopolymer paste with various HCWA-PFA hybridization ratios. Mix Designation

Standard Consistency

WA0FA100 WA10FA90 WA20FA80 WA30FA70 WA40FA60 WA50FA50 WA60FA40 WA70FA30 WA80FA20 WA90FA10 WA0FA100

0.25 0.26 0.27 0.3 0.35 0.36 0.38 0.38 0.41 0.45 0.49

180 and 365 days of moist curing. The measurement of the ultrasonic pulse velocity was conducted on the constant path length of 290 mm through the cross section of HCWA-PFA geopolymer mortar block samples. The ultrasonic pulse generator record the time taken for the pulse to propagate through the aforementioned path length by the means of 52 kHz transducer of 50 mm diameter. In order to ensure high accuracy of measurement, petroleum gel was applied on the surface of the transducer and the surface of the mortar block sample before each measurement was taken. The result of UPV was taken as the average value of the three tested samples of HCWA-PFA geopolymer mortar block. 2.2.5. Fourier transformed infrared (FTIR) spectroscopy assessment The changes in terms of functional group and the bonding characteristic of HCWA-PFA geopolymer paste at various hybridization ratios and moist curing ages was assessed by the utilization of fourier transform infrared spectroscopy (FTIR) using the spectrometer Perkin Elmer system 2000 series model. The use of FTIR is intended to monitor the evolution and development of the Si-O-T (T: Si or Al) and Al-O bond within the geopolymer paste as the geopolymerization progress. This is an essential assessment to verify the formation of polysialate, poly-sialate-siloxo and poly-sialate-disiloxo phases within the binder system as the reaction progresses [38]. Similar application of FTIR method for such assessment had been practiced in a number of recent studies [39–41] related to characterization on the development of the geopolymeric framework. The KBr pellet method was utilized for sample preparation. KBr:HCWA-PFA powder ratio was made constant at 1:200 and was pelletized into 13 mm diameter disc using a hydraulic press at constant pressure of 8 tons/m2. The pelletized sample was then scanned from wavenumbers ranging from 400 to 4000 cm1 at 4 cm1 resolution.

3. Results and discussion 3.1. Stamdard consistency of HCWA-PFA blended geopolymer binder Standard consistency of HCWA-PFA geopolymer cement paste with various hybridization ratios is presented in Table 3. It is observed that gradual addition of HCWA content brings about corresponding increase in water demand in order to achieve standard consistency for HCWA-PFA geopolymer paste system. The aforementioned phenomenon is due to the angular particle shape and porous nature of HCWA if compared with PFA particle. The mixing water is trapped in the interstitial spacing between HCWA particles due to the angular effect and also within the pores that presents on the surface of HCWA particles. This imparts a higher degree of mixing water needed to wet the HCWA-PFA particles due to the higher contact surface area so that desirable lubricating effect which enable standard consistency to be achieved for HCWA-PFA geopolymer paste system that contains higher proportions of HCWA. On the other hand, the ball bearing effect due to the spherical and smooth nature of PFA particle resulted in significant lower water demand in order to achieve the desired standard consistency of HCWA-PFA geopolymer paste which contains higher proportion of PFA. 3.2. Initial and final setting time of blended geopolymer binder The setting properties namely initial and final setting time of HCWA-PFA geopolymer paste with 5% of Na2SiO3 addition is

shown in Table 4. The setting properties of HCWA-PFA geopolymer paste system is a function of the degree of reactivity of the hybrid geopolymeric system. It can be observed that HCWA-PFA paste mixes WA30FA70WA50FA50 exhibited the lowest initial setting time if compared to other HCWA-PFA paste mixes. On the other hand, HCWA-PFA paste mixes WA0FA100-WA20FA80 and WA60FA40-WA0FA100 exhibited similar initial setting performance, whilst higher initial setting time was observed if compared with WA30FA70WA50FA50 group of HCWA-PFA paste mixes. The aforementioned phenomenon is due to the higher degree of reactivity in WA30FA70-WA50FA50 paste mixes that resulted in rapid development of geopolymeric products in the form of K-A-S-H, N-A-S-H and C-A-S-H gels. This in turns culminated in rapid hardening of the HCWA-PFA geopolymer paste and the subsequent observed lower initial setting time. For HCWA-PFA geopolymer paste that contains high PFA and HCWA content, namely WA0FA100WA10FA90 and WA90FA10-WA100FA0 mixes, it can be observed that higher PFA content mixes exhibited delayed initial setting properties if compared with mixes with high HCWA content. This is most probably due to the lower reactivity of PFA during the initial stage of curing, caused by the low alkaline activator and ambient temperature curing. On the other hand, the hydraulic properties in HCWA-PFA geopolymer paste with higher HCWA content are prevalent and this contributed towards the faster hardening rate and lower initial setting properties observed. Similar final setting performance of WA30FA70-WA50FA50 can be observed, where the aforementioned mixes exhibited lowest final setting time properties, culminated by the continuous formation of geopolymeric products and the subsequent hardening of the HCWA-PFA geopolymer paste. However, for HCWA-PFA geopolymer paste with high PFA and HCWA content, a contrasting final setting property is observed. WA90FA10 and WA100FA0 geopolymer paste mixes exhibited delayed final setting time if compared with WA0FA100 and WA10FA90 mixes. The aforementioned phenomena could be due to the higher dissolution of aluminosilicate species in PFA during the final setting period and resulted in a higher degree of geopolymeric products formation and subsequent lower final setting performance observed. It is also indicative of the continuous dissolution process of aluminosilicate materials even under mild alkaline activator and the ambient temperature curing condition.

3.3. Compressive strength The compressive strength development of HCWA-PFA geopolymer mortar blocks over a curing period up to 365 days is presented in Fig. 1. At 7 days of moist curing, compressive strength of HCWAPFA geopolymer mortar blocks increased with increasing HCWA

Table 4 Initial and final setting time of HCWA-PFA geopolymer paste with various HCWA-PFA hybridization ratios. Mix Designation

Initial Setting Time (min)

Final Setting Time (min)

WA0FA100 WA10FA90 WA20FA80 WA30FA70 WA40FA60 WA50FA50 WA60FA40 WA70FA30 WA80FA20 WA90FA10 WA100FA0

70 70 40 30 20 30 50 50 60 40 50

320 300 260 240 200 280 300 310 330 353 340

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framework will be that of C-A-S-H. This is due to the cation deficiencies phenomena where there is lack of charge balancing ions in the dissolved Si and Al species due to the relatively low amount of K2O present in HCWA (approximately 12%). Therefore, at this particular timeframe, Ca2+ ions will in turns act as charge balancing ions and resulted in the formation of geopolymeric precursor in the form of gehlenite framework (C-A-S-H). Upon long term moist curing exposure at 90, 180 and 365 days of moist curing, WA40FA60 remains the mortar block mix which possessed the highest compressive strength where its 90, 180 and 365 days compressive strength is 36.50, 37.87 and 37.98 MPa, respectively. It is also observed that the rate of long term compressive strength development is higher for HCWA-PFA geopolymer mortar blocks with higher PFA content. During long term curing, mortar mixes with higher PFA content contains higher amount of dissolved silicate and aluminate species, which in turns triggered the continuous formation of (Ca, K)-A-S-H geopolymeric framework and culminated in higher rate of long term compressive strength development of HCWA-PFA geopolymer mortar blocks with higher PFA content namely WA50FA50 to WA30FA70 mortar block mixes. Similar finding where (Ca, K)-A-S-H geopolymer gel constitute the long term geopolymeric product formation in high calcium geopolymer system was reported by Puligilla and Mondal [42]. Based on the compressive strength results, HCWA-PFA geopolymer mortar blocks mixes from WA20FA80 to WA80FA20 are qualified to be classified as normal weight solid load bearing unit with a minimum average 28 days compressive strength of 13.10 MPa prescribed under the specification ASTM C 90.

content up to 50% which is WA50FA50 mortar mix which exhibited compressive strength of 22.28 MPa. It is notable that there is a significant increment in terms of compressive strength between WA10FA90 and WA20FA80 mortar mixes where the compressive strength rose from 5.15 MPa to 13.16 MPa between the aforementioned mortar mixes. Beyond WA50FA50 mortar mix, compressive strength of HCWA-PFA geopolymer mortar blocks exhibited a steady decrease until WA100FA0 mortar mix. For mortar blocks with low HCWA content such as WA0FA100 and WA10FA90, compressive strength development is mostly governed by the formation of C-A-S-H geopolymeric gel resulted from the reaction of dissolved calcium, silicate, aluminate ions from the PFA source material and the soluble silicate from the chemical activator which is sodium silicate. Due to the scarce amount of calcium ions present in PFA (approximately 10%) and the relatively low alkalinity of the geopolymeric system, the compressive strength of HCWA-PFA geopolymer mortar block with low HCWA content is rather low. For mortar blocks with high HCWA content namely WA100FA0 and WA90FA10, compressive strength development is mostly governed by the hydraulic nature of HCWA and the formation of C-S-H mineral framework from the reaction of portlandite and soluble silicates from sodium silicate. It can be said that the optimum HCWA-PFA geopolymer mortar block mixes lie between WA40FA60 to WA60FA40 after 7 days of moist curing. The strength development in the aforementioned mixes is governed by the formation of potassium polysialates (K-A-S-H) geopolymer framework due to the higher dissolution rate of aluminosilicate compound from PFA where the alkalinity of the geopolymer system contributed by both potassium hydroxide (arcanite mineral) and sodium hydroxide (sodium silicate) which resulted in higher amount of soluble silicates and aluminates. Formation of C-S-H framework is also anticipated but in a lesser amount due to the higher reactivity of K+ ions compared with Ca2+ ions. Upon 28 days of moist curing, it is observed that the optimum compressive strength was exhibited by WA40FA60 mixes, a shift towards higher PFA content mortar blocks from the previous optimum mix at 7 days of moist curing for WA50FA50 mortar blocks which exhibited compressive strength of 29.76 MPa. It is anticipated that upon 28 days of moist curing, the primary geopolymeric

3.4. Flexural strength Flexural strength of cementitious composite is of importance for estimation of minimum bending load that one cementitious composite can sustain before the initiation of cracks. It is also a measurement of diagonal tensile resistance which is an important parameter of a masonry unit [43]. During service life of a masonry wall unit or mortar block, the most common defect will be that of sulphate attack where sulphate ions originated from underground

45.00 40.00

Compressive Strength (MPa)

35.00 30.00 7D

25.00

28D

90D

20.00

180D 365D

15.00 10.00 5.00 0.00

0

10

20

30

40

50

60

70

80

90

100

HCWA Content (%) Fig. 1. Compressive strength development of HCWA-PFA geopolymer mortar block up to 365 days of moist curing.

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soil propagate through the mortar block masonry wall unit by capillary movement, forming expansive crystals such as ettringite within the matrix structure and causes internal expansion which leads to the formation of micro-cracks and subsequent failure of the whole masonry structure. The rate at which the sulphate ions penetrate through the matrix structure depends on various factors, one of those is the presence of cracks of the masonry unit, where the flexural strength of the masonry unit plays an important role in controlling the cracks formation in the aforementioned masonry unit. Flexural strength behaviour of HCWA-PFA geopolymer mortar blocks with various hybridization ratios over a prolonged curing period is shown in Fig. 2. At 7 days of curing ages, HCWA-PFA geopolymer mortar blocks with low HCWA and PFA content namely. WA0FA100, WA10FA90, WA20FA80, WA80FA20, WA90FA10 and WA100FA0 mortar mixes exhibited significantly low flexural strength as compared to other mixes. This is probably due to the weak bonding within the binder paste and also the bonding between binder matrix and the aggregate particle. On the other hand, WA30FA70, WA40FA60, WA50FA50, WA60FA40 and WA70FA30 geopolymer mortar block mixes exhibited enhanced flexural strength in comparison to other mortar mixes. The governing factor for the aforementioned phenomena is due to the higher degree of geopolymerization where the formation of K-A-S-H geopolymeric gel resulted in a denser geopolymer matrix with stronger bonds between the binder paste matrix and aggregate particle, thus enhanced the resistance of the aforementioned HCWA-PFA geopolymer mortar mixes towards tensile rupture. HCWA-PFA geopolymer mortar mix WA60FA40 exhibited the highest flexural strength of 1.48 MPa. Upon 28 days of moist curing, it is observed that all the HCWAPFA geopolymer mortar blocks exhibited gradual increase in flexural strength. However, it is noted that upon 28 days of moist curing, WA40FA60 geopolymer mortar blocks exhibited highest flexural strength amongst other mortar blocks, a shift toward geopolymer mortar blocks with higher PFA content, indicating continuous and higher degree of geopolymeric reaction taking place in HCWA-PFA geopolymer mortar blocks with higher PFA content such as mixes with higher amount of silicate and aluminate. Due to the cation deficiency phenomena as per discussed in the previous section, it is expected that the main geopolymer product upon

28 days of moist curing of HCWA-PFA geopolymer mortar blocks will be that of (Ca,K)-A-S-H geopolymer gels. Upon 28 days of moist curing, HCWA-PFA geopolymer mortar block mixes WA30FA70-WA90FA10 meet the minimum flexural strength requirement of 0.93 MPa prescribed by California Concrete Masonry Technical Committee to be qualified as class N concrete block work [43]. Upon prolonged moist curing up to. 90 and 180 days, WA40FA60 geopolymer mortar blocks consistently outperformed other mortar blocks mixes in terms of flexural strength performance, consistent with the 28 days flexural strength results. It is observed that during this particular timeframe, the flexural strength development of HCWA-PFA geopolymer mortar blocks with higher HCWA content namely WA70FA30, WA80FA20, WA90FA10 and WA100FA0 geopolymer mortar blocks was stagnant with no significant increment in flexural strength. During prolonged moist curing, continuous geopolymeric reaction is expected to take place mainly due to the ambient temperature curing approach utilized throughout the whole research project, albeit in a slower rate if compared to conventional geopolymer mixes which utilized elevated temperature curing. Thus, the dissolution process of silicate and aluminate species from PFA will continue for a prolonged period of curing if the geopolymer mixes were subjected to ambient temperature curing. The continuous formation of (Ca,K)-A-S-H geopolymer gels in geopolymer mortar blocks with higher readily soluble silicate and aluminate species in mortar blocks with higher PFA content culminated in a higher flexural strength development observed during moist curing period from 90 to 180 days. Focusing on the flexural strength development of HCWA-PFA geopolymer mortar blocks upon 365 days of moist curing, all the fabricated geopolymer mortar blocks exhibited constant/enhanced flexural strength if compared with corresponding test specimens upon 180 days of moist curing. HCWA-PFA geopolymer mortar blocks with higher HCWA content such as WA70FA30, WA80FA20, WA90FA10 and WA100FA0 mortar blocks maintained their flexural strength upon moist curing for 365 days, indicating there is little/no geopolymeric reaction taking place in the aforementioned mortar block mixes upon prolonged curing period. For other HCWA-PFA geopolymer mortar blocks, it is observed that there is still gradual increase in terms of flexural strength even after

3.00

Flexural Strength (MPa)

2.50

2.00 7D 28D

1.50

90D 180D

1.00

365D 0.50

0.00

0

10

20

30

40

50

60

70

80

90

100

HCWA Content (%) Fig. 2. Flexural strength development of HCWA-PFA geopolymer block work up to 365 days of moist curing.

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365 days of moist curing, indicating a continuous long term geopolymeric reaction taking place in the aforementioned mortar block mixes, most notably in mixes with high PFA content such as WA30FA70 and WA40FA60 mortar blocks mixes where the average increase in flexural strength from 180 days moist cured samples is around 30%. The continuous increase in flexural strength of HCWA-PFA geopolymer mortar block over prolonged moist curing period up to 365 days is indicative of the enhancement in the formation of geopolymeric product due to the addition of mild alkaline activator of 5 wt.% of sodium silicate. This is because for during initial trial mix of HCWA-PFA geopolymer mortar block without the addition of alkaline activator, flexural strength of the hardened mortar block decrease during prolonged moist curing period up to 90 days, mainly due to the increase in surface porosity of the hardened mortar block. A review on the compressive strength and flexural strength results are indicative that the optimum range of HCWA composition is 20–60% by total binder weight. When the HCWA content is below 20% by mass, the hybrid binder system would be deficient of the arcanite mineral which is inherently present in the chemical composition of HCWA and essential to initiate the geopolymerization reaction especially at early age of the test specimens. On the other hand, when the HCWA content is in excess of 60%, the displacement of the PFA glassy aluminosilicate phase from the binder system could have resulted in deficiency of the glassy aluminosilicate mineral for continuous geopolymerization and secondary hydration reaction to form additional binder framework. This resulted in a significantly lowered rate of the formation of new geopolymeric phases and hydration product which is reflected in the low rate of mechanical strength development for test specimens with HCWA content below 20% or in excess of 60%. The finding is consistent with the observation in prior studies [27–29] whereby a reversal in the uptrend of the mechanical strength was observed when the HCWA content was increased beyond 60% by total mass of binder. 3.5. Ultrasonic pulse velocity of propagation across the HCWA-PFA block work Ultrasonic pulse velocity (UPV) test is one of the most established non-destructive test methods for estimation of concrete strength. Although there is no physical relationship between ultrasonic pulse velocity and the strength of concrete, due to its relation with the density of concrete, ultrasonic pulse velocity test was viewed as the most rationale and reliable non-destructive test method for preliminary strength indication of hardened concrete. Besides, UPV test is also useful for detecting cracks, voids, deterioration and the uniformity of concrete [44]. The UPV of HCWA-PFA geopolymer mortar blocks subjected to continuous moist curing up to 365 days of exposure is presented in Fig. 3. At 7 days of moist curing, HCWA-PFA geopolymer mortar blocks with higher PFA content, namely WA0FA100 and WA10FA90 exhibited inferior UPV value in comparison to other HCWA-PFA geopolymer mortar blocks. This indicates the highly porous nature of the two aforementioned mortar blocks due to the low degree of geopolymeric reaction which resulted in a low density of the internal matrix during early age of curing. The optimum HCWA-PFA geopolymer mortar blocks for UPV test lies between WA40FA60, WA50FA50 and WA60FA40 mortar blocks, consistent with the compressive strength results shown in the earlier section. Upon 28 days of moist curing, all the HCWA-PFA geopolymer mortar blocks showed gradual increment in terms of UPV value, with the exception of WA0FA100 and WA10FA90 geopolymer mortar blocks, which exhibited an increase of 82% and 22% in comparison with the UPV value after 7 days of moist curing, respectively.

In WA0FA100 and WA10FA90 mortar blocks, the strength development is mostly governed by the formation of geopolymeric gel, in this case N-A-S-H gels due to the scarcity of calcium ion (low HCWA content). In addition, the slow strength development during early ages is due to the utilization of ambient temperature curing and also the relatively low alkalinity of the aforementioned mortar block mixes due to the low HCWA content. Therefore, it can be said that for HCWA-PFA geopolymer mortar blocks with low HCWA content, significant geopolymeric reaction is only taken place after 7 days of moist curing condition. It is also notable that upon 28 days of moist curing, HCWA-PFA geopolymer mortar block which showed the highest UPV value is WA40FA60 mortar block mix, a shift from WA50FA50 if compared to 7 days UPV results, consistent with the compressive strength result. The rigorous formation of C-A-S-H geopolymeric gel filled the voids and interfacial transition zone in mortar block mixes with higher PFA content during 28 days of moist curing, thus resulted in a denser microstructure. During prolonged moist curing from 90 to 365 days, there is still a gradual increase in UPV value for all the HCWA-PFA geopolymer mortar blocks, with the exception of WA70FA30, WA80FA20, WA90FA10 and WA100FA0 mortar block mixes, which showed little or no increase in terms of UPV value for the prescribed moist curing time frame. Generally, the optimum HCWA-PFA geopolymer mortar blocks is WA40FA60 to WA60FA40 which exhibited a steady increase in UPV value in the range of 2–5% for the moist curing period from 90 to 365 days. For HCWA-PFA geopolymer mortar blocks containing higher PFA content especially WA0FA100 and WA10FA90 mortar block mixes, a significant increase in UPV value was observed as 87.6% and 29.6% increment, respectively. This reaffirms the compressive strength result which showed similar trend and finding. Even during prolonged curing period (90– 365 days of moist curing), (Ca,K)-A-S-H geopolymeric gel was continuously formed and filled up the capillary pore and interstitial zones of HCWA-PFA paste matrix and HCWA-PFA pasteaggregate interface which resulted in a denser microstructure, especially for HCWA-PFA geopolymer mortar blocks with high PFA content. 3.6. The empirical correlation between compressive strength and flexural strength of HCWA-PFA block work From the laboratory investigation, it was observed that an empirical correlation exists between the compressive strength (fcu) and flexural strength (fcf) of HCWA-PFA geopolymer mortar blocks at moist curing ages up to 365 days. Regression analysis was performed on the individual HCWA-PFA geopolymer mortar block mixes utilizing the compressive strength and flexural strength data acquired and the following correlations was obtained:

WA0FA100 : f cf ¼ 0:0279f cu þ 0:0393; R2 ¼ 0:989

ð1Þ

WA10FA90 : f cf ¼ 0:0323f cu þ 0:1227; R2 ¼ 0:948

ð2Þ

WA20FA80 : f cf ¼ 0:0683f cu  0:0962; R2 ¼ 0:937

ð3Þ

WA30FA70 : f cf ¼ 0:0517f cu þ 0:265; R2 ¼ 0:960

ð4Þ

WA40FA60 : f cf ¼ 0:0484f cu þ 0:3455; R2 ¼ 0:958

ð5Þ

WA50FA50 : f cf ¼ 0:0822f cu  0:4684; R2 ¼ 0:876

ð6Þ

WA60FA40 : f cf ¼ 0:1012f cu  0:6488; R2 ¼ 0:951

ð7Þ

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4000.00

Ultrasonic Pulse Velocity (m/s)

3500.00 3000.00 2500.00

7D

2000.00

28D 90D

1500.00

180D 1000.00

365D

500.00 0.00

0

10

20

30

40

50

60

70

80

90

100

HCWA Content (%) Fig. 3. Ultrasonic pulse velocity development of HCWA-PFA geopolymer mortar block up to 365 days of moist curing.

WA70FA30 : f cf ¼ 0:0542f cu  0:4663; R2 ¼ 0:992

ð8Þ

WA80FA20 : f cf ¼ 0:0978f cu  0:4558; R2 ¼ 0:839

ð9Þ

WA90FA10 : f cf ¼ 0:1709f cu  0:8707; R2 ¼ 0:853

ð10Þ

2

WA100FA0 : f cf ¼ 0:1675f cu  0:4454; R ¼ 0:903

WA0FA100 : f cu ¼ 4  106 V 2  0:0077V þ 6:3602; R2 ¼ 0:977

ð11Þ

The regression analysis indicated a strong linear relationship between the compressive strength and flexural strength of HCWA-PFA geopolymer mortar blocks cured at moist curing ages up to 365 days. The high R2 value of the Eqs. (1)–(11) which ranged between 0.839 and 0.992 indicated high degree of reliability to predict the compressive strength of HCWA-PFA geopolymer mortar block with known flexural strength, or vice versa. The relationship between compressive strength and flexural strength of individual HCWA-PFA geopolymer mortar block mixes at various moist curing ages up to 365 days is presented in Fig. 4. In such an empirical correlation is derived for the prediction of the flexural strength performance of the HCWA-PFA block work as given in a general form in Eq. (12). The empirical model is useful for the purpose of crack development prediction of block work structure under both static and seismic load [45].

f cf ¼ xfcu  y

pressive strength and UPV of HCWA-PFA geopolymer mortar blocks at any given moist curing ages. Thus, compressive strength of HCWA-PFA geopolymer mortar blocks at various moist curing ages can be predicted with high degree of precision using the equations as stated below:

ð13Þ WA10FA90 : f cu ¼ 1  105 V 2  0:0393V þ 45:948; R2 ¼ 0:990 ð14Þ WA20FA80 : f cu ¼ 7  105 V 2  0:4589V þ 726:42; R2 ¼ 0:849 ð15Þ WA30FA70 : f cu ¼ 6  105 V 2  0:4053V þ 667:38; R2 ¼ 0:958 ð16Þ WA40FA60 : f cu ¼ 0:0001V 2 þ 0:9612V  1783:4; R2 ¼ 0:919 ð17Þ WA50FA50 : f cu ¼ 0:0004V 2 þ 2:875V  5060:2; R2 ¼ 0:906

ð12Þ

where x = multiplier coefficient and y = subtraction constant. 3.7. Relation between UPV and compressive strength of HCWA-PFA geopolymer mortar blocks The relationship between compressive strength (fcu) and ultrasonic pulse velocity (V) for HCWA-PFA geopolymer mortar blocks is shown in Fig. 5. It can be observed that for any given UPV value, WA30FA70-WA60FA40 HCWA-PFA geopolymer mortar blocks exhibited higher compressive strength if compared with others HCWA-PFA geopolymer mortar blocks namely WA0FA100WA20FA80 and WA70FA30-WA100FA0 mixes. Regression analysis was done on the individual HCWA-PFA geopolymer mortar block mixes and is presented and the empirical relationship is presented in Eqs. (13)–(23). The high R2 values for Eqs. (13)–(23) which ranged between 0.849 and 0.998 implied that there is a strong quadratic relationship exists between the com-

ð18Þ WA60FA40 : f cu ¼ 0:0002V 2 þ 1:5461V  2734; R2 ¼ 0:998 ð19Þ WA70FA30 : f cu ¼ 3  105 V 2  0:1451V þ 171:97; R2 ¼ 0:867 ð20Þ WA80FA20 : f cu ¼ 2  105 V 2  0:1003V þ 148:85; R2 ¼ 0:872 ð21Þ WA90FA10 : f cu ¼ 2  105 V 2  0:1365V þ 205:22; R2 ¼ 0:925 ð22Þ WA100FA0 : f cu ¼ 2  105 V 2  0:1334V þ 183:92; R2 ¼ 0:984 ð23Þ

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2.5 WA0FA100 WA10FA90

Flexural Strength (MPa)

2

WA20FA80 WA30FA70

1.5

WA40FA60 WA50FA50

1

WA60FA40 WA70FA30 WA80FA20

0.5

WA90FA10 0

WA100FA0 0

10

20

30

40

50

Compressive Strength (MPa) Fig. 4. Relationship between compressive strength and flexural strength of individual HCWA-PFA geopolymer mortar block mixes at various moist curing ages up to 365 days.

45

WA0FA100

Compressive strength (MPa)

40

WA10FA90

35

WA20FA80

30

WA30FA70 WA40FA60

25

WA50FA50

20

WA60FA40 15

WA70FA30

10

WA80FA20

5 0

WA90FA10 WA100FA0 0

500

1000

1500

2000

2500

3000

3500

4000

Ultrasonic pulse velocity (m/s) Fig. 5. Relationship between compressive strength and ultrasonic pulse velocity of individual HCWA-PFA geopolymer mortar block mixes at various moist curing ages up to 365 days.

3.8. Aluminosilicate bond analysis by fourier transformed infrared spectroscopy The FTIR spectra of HCWA-PFA geopolymer pastes cured at various ages were shown in Figs. 6–9. The major bands associated with WA0FA100 paste are located at 478, 778, 1020, 1453, 1640 and 3438 cm1. For WA40FA60 geopolymer paste major bands were detected at 458, 618, 712, 875, 996, 1117, 1429, 1636 and 3438 cm1. Finally, peaks corresponding to WA100FA0 geopolymer paste were detected at 446, 618, 712, 871, 965, 1047, 1429, 1460, 1632 and 3461 cm1. In WA0FA100 geopolymer paste, the visible band at 478 cm1 is associated with the bending vibration of Si-O-Si and O-Si-O while the band at approximately 778 cm1 is corresponding to the symmetric stretching vibration of Si-O-Si. The broad and distinct band centered at 1020 cm1 is due to the assymetric stretching vibration of Si-O-T (T = Al, Si) non-bridging bond, which indicate that the

reaction product is mainly consist of chain structure C-A-S-H gel. Visible band centered at 1453 cm1 is associated with the assymetric stretching vibration of O-C-O, which indicates the presence of carbonates in WA0FA100 geopolymer paste. The carbonates were believed to be that of sodium carbonate due to the atmospheric carbonation of the mild sodium silicate (5% of binder weight) alkaline activator media [14]. While distinct bands observed at 1640 and 3438 cm1 was associated with the bending vibration of HO-H and stretching vibration of O-H. The aforementioned bands were due to the weakly bound water molecules which adhered on the surface or trapped in cavities between the rings of the polysialate-siloxo geopolymeric products [46,47]. As for WA100FA0, distinct difference in terms of peak intensity was observed on the assymetric stretching vibration of O-C-O where WA100FA0 has a significantly higher degree of intensity of carbonate compound if compared with WA0FA100 geopolymer paste. This is most probably due to the high amount of CaO present

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Fig. 6. FTIR spectra of HCWA-PFA geopolymer paste upon 7 days of moist curing.

Fig. 7. FTIR spectra of HCWA-PFA geopolymer paste upon 28 days of moist curing.

in HCWA which exist as calcite mineral as per the XRD diffractogram of HCWA raw material. The visible sharp band at around 871 cm1 is also indicative of the presence of carbonates in the

WA100FA0 geopolymer paste system [48]. Besides, there is no major band associated with the assymetric stretching vibration of Si-O-T (T = Al, Si). Instead, WA100FA0 geopolymer paste showed

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Fig. 8. FTIR spectra of HCWA-PFA geopolymer paste upon 90 days of moist curing.

Fig. 9. FTIR spectra of HCWA-PFA geopolymer paste upon 365 days of moist curing.

two visible bands centered at 1047 and 965 cm1, which is most probably due to the low amount of silicate and aluminate in HCWA. Upon hybridization between HCWA and PFA such as in WA40FA60 geopolymer paste mix, it can be seen that the intensity

of bending vibration of H-O-H and stretching vibration of O-H increased if compared with the IR spectra of WA0FA100 and WA100FA0. The increase in the amount of weakly bounded water molecules in WA40FA60 geopolymer paste is indicative of higher reactivity and the formation of geopolymeric products. Further-

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more, it was observed that the assymetric stretching vibration of Si-O-T (T = Al, Si) band shifted to lower frequencies wavenumber, indicating a more polymerized framework was formed, consistent with the mechanical performance explained in the previous chapter. It is also indicative that structural reorganization and formation of new geopolymeric products is taking place in the WA40FA60 geopolymer paste during the alkali activation process [49]. The intensity of carbonates peak centered at around 1429 cm1 and 875 cm1 was also found to reduce as a result of the reduction of HCWA content in WA40FA60 geopolymer paste. FTIR spectra of WA0FA100, WA100FA0 and WA40FA60 geopolymer paste at various moist curing ages is presented in Figs. 10–12, respectively. For WA0FA100 geopolymer paste (Fig. 10), gradual increase in the band intensity of the assymetric stretching vibration of Si-O-T (T = Al, Si) can be observed from 7 to 365 days of moist curing. This can be attributed to the continuous formation of geopolymeric product over prolonged moist curing duration. It is also notable that the band shifted toward lower frequency number as the moist curing ages increased, which is an indication of the formation of new reaction product and subsequent structural reorganization during the alkali activation process. The other possibility of the aforementioned phenomenon could be due to the increasing number of tetrahedral aluminium atoms in WA0FA100 geopolymer paste system as the stretching mode are sensitive towards the Si:Al composition of the geopolymeric framework [50]. The narrow band centered at around 470 cm1 which is associated with the bending vibration of Si-OSi and O-Si-O exhibited the same phenomenon, where the increased in the band’s intensity is more pronounce upon 90 and 365 days of moist curing. The observed phenomenon could be due to the increased formation of geopolymeric product namely N-A-S-H gels during prolonged moist curing duration, consistent with the mechanical performance of WA0FA100 geopolymer mortar block, where there is a significant increase in terms of compressive strength between 90 to 365 days of moist curing duration.

533

As for WA100FA0 geopolymer paste (Fig. 11), the distinct difference in terms of IR spectra over 365 days of moist curing duration is the increase in band’s intensity at 1436 cm1 which is associated with the assymetric stretching vibration of O-C-O, which is indicative of carbonate in the system. The increase in intensity is more notable during the early age of curing of 7–28 days of moist curing. This is most probably due to the formation of calcite as a result of the carbonation reaction of Ca2+ ions. There is also increase in intensity in visible peaks at around 1120 cm1 and 980 cm1 which is associated with the assymetric stretching vibration of Si-O-T (T = Al, Si). The reaction between Ca(OH)2 and K2O liberated as a result of HCWA hydration and the sodium silicate resulted in the formation of K-A-S-H and C-A-S-H type geopolymer gel [51]. Over prolonged curing duration of 90–365 days, IR spectra of WA100FA0 paste do not exhibit significant changes in terms of both intensity and wavenumber. This implied that most of the reaction product development in WA100FA0 paste system occurred during the early age of moist curing of 7–28 days. The finding is consistent with the mechanical strength performance of WA100FA0 geopolymer mortar block where upon 90 days of moist curing, the compressive strength development remains constant until 365 days. In WA40FA60 geopolymer paste system (Fig. 12), it can be seen that the bands associated with the stretching vibration of O-H and bending vibration of H-O-H which is 3639 cm1 and 1639 cm1, respectively exhibited an increase in intensity over the duration of moist curing up to 365 days. The increase in intensity signified the increase in the amount of weakly bound water molecules which is adsorbed or trapped in large cavities between the chains of geopolymeric products which in turn implied the continuous formation of geopolymeric products in WA40FA60 geopolymer paste system over moist curing duration up to 365 days. In addition, the aforementioned bands exhibited a shift in wavenumber to a higher frequency number of 3439 cm1 to 3474 cm1 for stretching vibration of O-H and 1639 cm1 to 1650 cm1 from 7

Fig. 10. FTIR spectra of WA0FA100 geopolymer paste at various moist curing ages.

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Fig. 11. FTIR spectra of WA100FA0 geopolymer paste at various moist curing ages.

Fig. 12. FTIR spectra of WA40FA60 geopolymer paste at various moist curing ages.

to 365 days of moist curing. This is an indication of the formation of highly polymerized geopolymeric network [49] and is consistent with the compressive strength results discussed in the previous chapter.

4. Conclusions Based on the results of the laboratory investigation, the following conclusions of study can be derived.

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1. In HCWA-PFA hybrid geopolymer paste system, as the HCWA content increased, the amount of mixing water required to achieve standard consistency increased proportionally due to the angular particle shape and the porous nature of HCWA in comparison with PFA. 2. HCWA-PFA geopolymer paste WA30FA70-WA50FA50 showed higher rate of initial and final setting characteristic, if compared with WA0FA100-WA20FA80 and WA60FA40-WA100FA0. The faster rate of initial and final setting is due to the higher degree of reactivity in WA30FA70-WA50FA50 geopolymer paste which resulted in rapid development of geopolymeric products and resulted in subsequent hardening of the aforementioned geopolymer paste. 3. HCWA-PFA geopolymer mortar blocks WA50FA50 and WA40FA60 showed highest compressive and flexural strength performance during the early age of moist curing between 7 and 28 days. Over prolonged moist curing between 90 days and 365 days, WA30FA70 consistently showed optimum compressive and flexural strength performance amongst the HCWA-PFA geopolymer mortar block mixes. 4. Based on the 28 days compressive strength results of HCWAPFA geopolymer mortar blocks, WA20FA80 to WA80FA20 geopolymer mortar block mixes can be qualified as normal weigth solid load bearing unit with a minimum average 28 days compressive strength of 13.10 MPa prescribed under the specification ASTM C 90. On the other hand, HCWA-PFA geopolymer mortar block mixes WA30FA70-WA90FA10 meet the minimum flexural strength requirement of 0.93 MPa prescribed by California Concrete Masonry Technical Committee to be qualified as class N concrete masonry unit. 5. HCWA-PFA geopolymer mortar block WA40FA60-WA60FA40 exhibited highest UPV value during both early and prolonged moist curing duration up to 365 days. Highest long term development rate of UPV was observed for HCWA-PFA geopolymer mortar block with higher PFA content such as WA0FA100 and WA10FA90 while mortar block mixes with high HCWA content such as WA90FA10 and WA100FA0 showed negligible enhancement in terms of UPV value over prolonged moist curing period. The enhancement in UPV value for HCWA-PFA geopolymer mortar block is due to the formation of various phases of geopolymeric products which resulted in a denser microstructure. 6. The hybridization between HCWA and PFA in WA40FA60 paste brings about an increase in the intensity of IR spectra associated with the bending vibration of H-O-H and stretching vibration of O-H, which corresponded to the amount of weakly bound water. This indicates the rigorous polycondensation process in WA40FA60 where large amount of weakly bound water was expelled. The high degree of geopolymerization taking place in WA40FA60 paste can also be justified by the increase in peaks associated with the assymetric stretching vibration of Si-O-T (T = Al, Si) over prolonged moist curing condition. 7. The blockwork produced using the primary binder phase derived from the combination of HCWA and PFA is viable for use in building construction.

Acknowledgement The author would like to acknowledge the Ministry of Science, Technology and Innovation (MOSTI) – Malaysia, Malaysian Ministry of Higher Education and Universiti Sains Malaysia for funding the study under the MOSTI Sciencefund (Project No.06-01-05SF0848). Fundamental Research Grant Scheme (FRGS) (203/ PPBGN/6711471) and Research University (RU) Grant (Grant no. 1001/PPBGN/814212 and 1001/PPBGN/814211).

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