Intermittent curing of fly ash geopolymer mortar

Intermittent curing of fly ash geopolymer mortar

Construction and Building Materials 110 (2016) 54–64 Contents lists available at ScienceDirect Construction and Building Materials journal homepage:...

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Construction and Building Materials 110 (2016) 54–64

Contents lists available at ScienceDirect

Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Intermittent curing of fly ash geopolymer mortar Amr Ibrahim Ibrahim Helmy ⇑ Civil Engineering Department, The British University in Egypt, BUE, The Structural Engineering Department, Faculty of Engineering, Ain Shams University, Cairo, Egypt

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Intermittent curing at 70 °C applied

on 4 steps 6 h each followed by 18 h at room temperature.  Intermittent curing improved the UCS at the end of each curing step.  The effect of resting period and added water content on UCS was investigated.  Increasing the AL-to-FA content is sensitive to the Na2O-to-SiO2 mole ratio.  Very low H2O-to-Na2O and high Na2O-to-SiO2 mole ratios have inverse effect on UCS.

INTERMITTENT CURING OF FLY ASH GEOPOLYMER MORTAR SiO2 55.9%, Al2O3 28.1%, Fe2O3 7.49%, CaO 2.71% Fly Ash-Type F

Natural Sand

Fly Ash Composition using XRF Results Fly Ash : Natural Sand = 1 : 2.75 Alkaline Liquid : Soluble Silicate= 1 : 2 AL : Fly Ash = 35% & 48.5% Na(OH) Sol Soluble Sodium Silicates Types SG 1.36 1.54 150 600 µ (cp) Na(OH) 8.9% 15% SiO2 27.5% 30% Water 72.5% 55% 8/12/16 mole

Experimental Work

24-hour Resting Period 72-hour Resting Period Intermittent Curing Regime

a r t i c l e

i n f o

Article history: Received 9 July 2015 Received in revised form 29 January 2016 Accepted 2 February 2016

Keywords: Fly ash Geopolymer Intermittent curing Mortar Compressive strength

The Effect of Na(OH) Mole, Resting Period, Added Water and AL% on UCS Gain

The Effect of Na2-to-SiO2 , H2O-to-Na2O and Water-toGeopolymer Solids on UCS

a b s t r a c t The research work focuses on the production of type F fly ash based geopolymer using intermittent curing. Two different types of soluble sodium silicate and Na(OH) solution with three different mole ratios were used with a fixed ratio. Two different fly ash-to-alkaline liquid activator ratios were used with and without additional water content. Two different resting periods were checked prior to starting the curing regime. The curing temperature was set at 70 °C applied intermittently on 4 steps for 6 h each per day followed by 18 h rest at ambient temperature. Twenty-one different geopolymer mixtures were cast using a mixture of fly ash and natural sand at a fixed ratio. The gain of compressive strength was checked at age 24, 48, 72, and 96 h and 7 days. Intermittent curing proved to increase the compressive strength of all geopolymer mortar at the end of each curing step. Thirteen geopolymer mixtures exceeded the Egyptian Code of Practice limit set for the 7-day compressive strength at 27 MPa. The UCS is directly proportional to the increase of the specific gravity of the soluble sodium silicate used, the age, the Na(OH) solution mole concentration, the alkaline liquid activator-to-fly ash ratio, the resting period and the Na2O-to-SiO2 mole ratio. Yet, it is inversely proportional to additional water content, H2O-to-Na2O mole ratio and the water-to-geopolymer solids ratio. Ó 2016 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/).

1. Introduction

⇑ Address: Room 204, Building A, The British University in Egypt, Al-Sherouk City, Cairo-Suez Desert Road, 11837, Egypt. E-mail address: [email protected]

Geopolymer is an inorganic polymer that results from polymerization of alumina-silicate raw materials using an alkaline activation. The alkaline liquid activator is mainly a combination of soluble sodium (or potassium) silicate and sodium (or potassium)

http://dx.doi.org/10.1016/j.conbuildmat.2016.02.007 0950-0618/Ó 2016 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/).

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hydroxide solution. The inorganic polymeric material was developed by Davidovits (1978) while geopolymer term is founded by him on 1990 [5,16]. Industrial waste materials containing a considerable amount of alumina and silica are fly ash, blast furnace slag, mine trailing, red mud. . . etc. Geopolymer [3,4,11,13,14] has high potential to compete with cement as a binder in the concrete technology. Cement production is one of the most highly intensive fuel consuming industries and thus highly environmentally nonfriendly material. The production of 1 ton of cement clinker produces 0.5 ton of carbon di-oxide due to the calcination of limestone and 0.45 tons due to burning fuel. Geopolymer may be considered an environmentally friendly material since the production of geopolymer cement does not contribute to the greenhouse gases while it uses environmentally polluting industrial waste materials as precursors. Geo-polymerization [3,11,13,14] process, although not fully understood, may be divided into three main phases; namely dissolution of Si and Al species found in the raw materials through the effect of hydroxide ions followed by condensation of precursor ions into monomers and finally polymerisation of monomers into three dimensional polymeric structures. These three steps can either take place simultaneously or concurrently with each other. Water is produced through the polymerisation process as discontinuous nono-pores in the paste. Water plays no role in the chemical reaction; it merely provides workability and initial reaction medium to the geopolymer. Curing temperature [1–4,6–9,12,15–22] plays an important role in the exothermic reaction of geopolymer. It represents the major hurdle in the in-situ use of geopolymer in the concrete industry. High temperature acts as reaction accelerator and influences the rate of gain of strength as well as the ultimate compressive strength of the geopolymer. Different curing schemes were investigated by numerous research works. Yet, they all have common characteristics namely; heat curing time is continuous at constant maximum temperature. Elevated curing temperature and longer curing time were proved to result in higher compressive strength and high rate of gain of strength. Most literature had revealed that continuous curing for 24–48 h at a maximum temperature between 60–90 °C, can produce high strength geopolymer binder. Intermittent curing regimes were never investigated. The laboratory elevated curing utilized constant heat curing inside the oven for the full period of the curing time.

Intermittent curing would have applied ‘‘a cyclic-type” of curing where high temperature is reached and maintained for a certain period of time followed by a ‘‘longer period” where temperature drops to a lesser value (ambient temperature), thus, utilizing a longer period of time. Several factors should be investigated such as molar ratios of constituents such as Na2O-to-SiO2, SiO2-to-Al2O3, Na2Oto-SiO2, and H2O-to-Na2O, water-to-geopolymer solids ratio, and resting period. An intermittent curing scheme should be investigated to shed some light on its effect on rate of gain of strength and ultimate compressive strength of geopolymer mortar. 2. Significance of the research This paper reports the experimental results for determining the compressive strength of geopolymer standard mortar cubes made out of fly ash type F after intermittent curing scheme at 70 °C for 4 steps for continuous 6 h followed by 18 h of rest at ambient temperature for each step. Parameters investigated include two different types of soluble sodium silicate, three different mole concentration ratios for the Na(OH) solution, one fixed ratio of Na(OH) solution-to-soluble sodium silicate, two different resting periods, two different alkaline liquid activator-to-fly ash ratios and using or omitting of additional water content. 3. The experimental programme The experimental program, shown in Table 1, was carried out to study the effect of intermittent curing regime on the compressive strength of standard mortar cubes 70.6  70.6  70.6 mm made out of fly ash geopolymer mixtures. All fly ash geopolymer mortar mixtures were cast using a fly ash-to-natural sand ratio of 1:2.75. This ratio was chosen to mimic the same ratio used in ordinary Portland cement OPC standard mortar mixture specified by the Egyptian Code of Practice ECP [10]. The alkaline liquid activator was made out of sodium hydroxide Na(OH) of 98% purity with a Na(OH) solution-to-soluble sodium silicate liquid at a constant ratio of 1:2 by weight. Three standard mole concentrations for Na(OH) solution were used, namely; 8, 12 and 16 mol by weight. All fly ash geopolymer mortar mixtures were subjected to an intermittent curing regime of 4 steps for 6 h each at 70 °C. Eighteen resting hours separated any two successive curing steps where the electric oven was shut down leaving the temperature to drop down gradually to the ambient temperature. Twenty-one fly ash based geopolymer mortar mixtures were cast and divided into three phases. In the first phase; two types of soluble sodium silicate were used; ‘‘low-viscous sodium silicate” LGW and ‘‘high viscous sodium silicate” HGW. The alkaline liquid activator-to-fly ash ratio was set at 35% of the fly ash content by weight. To mimic

Table 1 Geopolymer mortar mixture proportions. #

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Geopolymer mixture

GMI-LGW- 8M GMI-LGW-12M GMI-LGW-16M GMI-HGW-8M GMI-HGW-12M GMI-HGW-16M GMII-HGW-8MR GMII-HGW-12MR GMII-HGW-16MR GMIII-HGW-8M-35% GMIII-HGW-12M-35% GMIII-HGW-16M-35% GMIII-HGW-8M-48.5% GMIII-HGW-12M-48.5% GMIII-HGW-16M-48.5% GMIII-HGW-8MR-35% GMIII-HGW-12MR-35% GMIII-HGW-16MR-35% GMIII-HGW-8MR-48.5% GMIII-HGW-12MR-48.5% GMIII-HGW-16MR-48.5%

Fly ash

Sand

Na(OH)

Distilled water

Glass water

Add water

Resting period

g

g

g

g

g

G

hours

3000 3000 3000 3000 3000 3000 3600 3600 3600 600 600 600 600 600 600 600 600 600 600 600 600

8250 8250 8250 8250 8250 8250 9900 9900 9900 1650 1650 1650 1650 1650 1650 1650 1650 1650 1650 1650 1650

85 114 137 85 114 137 102 137 164 17 23 27 24 31 38 17 23 27 24 31 38

265 236 213 265 236 213 318 283 256 53 47 43 73 66 59 53 47 43 73 66 59

700 700 700 700 700 700 840 840 840 140 140 140 194 194 194 140 140 140 194 194 194

405 405 405 405 405 405 486 486 486 – – – –

24 24 24 24 24 24 72 72 72 24 24 24 24 24 24 72 72 72 72 72 72

Conversion factor: divide the weight in (g) by 453.6 to get weight in (lb).

– – – – – –

Total No. of Spec.

15 15 15 15 15 15 18 18 18 3 3 3 3 3 3 3 3 3 3 3 3

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Table 2 Fly ash composition using XRF results (by weight%). Oxide

SiO2

Al2O3

Fe2Otot 3

CaO

TiO2

K2O

MgO

P2O5

SO3

% Na2O 0.215

55.819 MnO 0.103

28.112 ZrO2 0.078

7.488 SnO2 0.071

2.710 SrO 0.055

2.157 Cr2O3 0.034

1.515 Cl 0.033

0.846 ZnO 0.032

0.353 NiO 0.023

0.344 PbO 0.013

Fig. 1. Flay ash composition using XRF results. the preparation of OPC standard mortar specimens in the ECP [10], an additional water was used to reach water-cement ratio of 48.5%. A standard resting period of 24 h was used prior to the application of the intermittent curing regime. Six geopolymer mortar mixtures were cast in this phase, while the compressive strength was checked after the application of 6, 12, 18 and 24 h of elevated temperature curing that is at age of 1, 2, 3 and 4 days respectively and at 7 days of age. As the average compressive strength of all mixtures of HGW group, recorded consistently a substantially higher value than the mixtures of LGW group, then the following phases were cast using only the high-viscous sodium silicate HGW only. In the second phase, using only HGW, the resting period increased to 72 h prior to the application of the curing regime. Three geopolymer mortars mixtures were cast in this phase, while the compressive strength was checked after the application of 6, 12, 18, 24 and 30 h of elevated temperature curing that is at age of 3, 4, 5, 6 and 7 days respectively. An additional set was tested at 7 days after only 24 h of intermittent curing. In the third phase, again, the high viscous glass water HGW was used and the additional water to the geopolymer mortar was completely omitted. Two alkaline liquid activator-to-fly ash ratios were used namely 35% and 48.5%. Again, the latter ratio was chosen to mimic the w/c ratio used in determining the compressive strength of standard OPC mortar specimens. Two resting periods of 24 h and 72 h were investigated prior to the application of the intermittent curing regime. Twelve geopolymer mixtures were cast in this phase, where only the 7-day compressive strength was checked.

4. The experimental work 4.1. Chemical properties of fly ash The locally available fly ash in the Egyptian market was acquired from Geos, Cairo, Egypt. Chemical analysis using X-ray Fluorescence XRF analysis was carried out in the Central

Metallurgical Research and Development Institute, Helwan, Egypt. Table 2 and Fig. 1 present the XRF chemical analysis of the fly ash powder sample at ambient room temperature of 25 °C and 40% relative humidity. The chemical composition was predominantly made up of the silicon oxide (SiO2-55.819%), aluminium oxide (Al2O3-28.112%), total iron oxide (Fe2O3-7.488%), calcium oxide (CaO-2.710%), titanium oxide (TiO2-2.157%) and potassium oxide (K2O-1.515%), whereas other oxides such as magnesium, phosphorous, sulphur, sodium, manganese and other elements are also presented in a much fewer quantity. The fly ash is clearly classified as low calcium fly ash ASTM Class F with summation of silica and alumina content that exceeds 83.9% (required between 80–85%) and a lime CaO content that does not exceed 2.8% (required < 10%). The silica-to-alumina ratio is SiO2-to-AL2O3 = 1:2 that makes the fly ash suitable for the production of geopolymer mortars. 4.2. Alkaline liquid activator A mixture of sodium silicate solution and Na(OH) solution was used as the alkaline liquid activator. Sodium-based solutions were chosen because they were readily available in the Egyptian market and less costly than potassium-based solutions. All chemicals were obtained from Morgan Speciality Chemicals, Cairo. The sodium hydroxide solids were commercial grade, 1 mm spherical pellets, with a specific gravity of 2.13 and 98% purity. Two types of sodium silicate solution (glass water) were used where their chemical composition, specific gravity and viscosity are given in Table 3.

Table 3 The chemical composition, specific gravity and viscosity of sodium silicate. Type

High viscous Low viscous

Chemical composition (by mass) Na2O (%)

SiO2 (%)

Water (%)

15 8.9

30 17.5

55 72.5

Specific gravity

Viscosity Cp

1.54 1.36

600 150

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Fig. 2 presents the Na(OH) pellets, the high specific gravity glass water, and the preparation steps of the alkaline liquid. Alkaline liquid activator was prepared 24 or 72 h prior to mixing geopolymer mortar mixtures in order to prevent bleeding of geopolymer mortar mixtures [12]. This was carried out by dissolving the correct weight of Na(OH) pellets into the right amount of distilled water according to the required mole ratio 8, 12 or 16 mol. The process was accompanied with a sharp and sudden increase in the Na (OH) solution temperature. The Na(OH) solution was left for few minutes to cool off, then it was added to the correct weight of sodium silicate solution followed by stirring. Fresh alkaline liquid activator is mercy white color that clears out gradually. The weight of the alkaline liquid activator was recorded, covered and left to cool off until the day of mixing. A loss in weight of the solution on the day of mixing is detected because of water evaporation due to the exothermic reaction. The water loss was replenished by adding distilled water and vigorous stir to account for the weight loss. 4.3. Mixing, casting and compaction of geopolymer mortar Fig. 3 presents the mortar mixer, moulds, mortar vibrator, fresh geopolymer, mould strike off and the covered specimens during resting period. The natural sand used in the geopolymer mixtures passed from sieve size 850 lm and retained on sieve size 600 lm. It was washed and dried in the electric oven for 24 h at 110 °C and cooled down prior to being used in the geopolymer mortar mixtures. When additional mixing water was used, one third of the sand content was added to the mortar mixer followed by one half of the fly ash content followed by the second third of the sand followed by the remaining portion of the fly ash and finally topped with the remaining third of the sand. The alkaline solution activator, followed by the additional water content, was added immediately as the mixer is rotating on low speed. This protocol had prevented

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fine particles in fly ash that usually got separated and lost as dust in the air. The total mixing time ranged between 90 and 120 s. When no additional water was used, the alkaline liquid activator and the fly ash were initially mixed to form the binder for 90 to 120 s. Natural sand was added gradually during mixing. Vigorous mixing continued for at least 120 s or until a homogenous mortar mixture is formed. All standard mortar moulds cubes were oiled and each mould was secured in its grip. Fresh geopolymer mortar mixture was light gray in color as can be seen from Fig. 3(c). It was noticed that a liquid black residue always appeared on the top surface of all specimens as can be seen from Fig. 3(d). All fresh geopolymer mortar mixtures were cohesive and plastic as can be seen from Fig. 3(c), except mixtures 10, 11, 12, 16, 17, and 18 where additional water was omitted. As can be seen from Fig. 3(d), these mixtures were extremely dry mixtures and exhibit a very low workability. Added water was omitted in all these mixture where alkaline liquid activator-to-fly ash content was 35%. Each mould was half filled with fresh geopolymer mortar mixture. It was compacted using standard cement mortar vibrator for 90 s. Then, the rest of the mold was filled with a second layer of fresh geopolymer mixture and re-vibrated using the same previously mentioned scheme. The mould surface was strike off and was wrapped inside a sealed polyethylene pp plastic bag for the full resting period. 4.4. Intermittent curing regime Fig. 4(a) and (b) present the intermittent curing regime applied for the 24-h and 72-h resting periods with respect to the date of casting and the date(s) of testing of specimens. The intermittent curing regime had started immediately after the resting period of either 24 or 72 h is completed. Fig. 5 shows the electric oven showing the temperature knob set at 70 °C, an inside view hosting the geopolymer specimens sealed inside their pp plastic bags and geopolymer specimen after curing.

(a) Na(OH)

(b) High SG glass water

(a) Na(OH) pellets

(b) Na(OH) solution

(c) Fresh alkalineliquid

(d) Alkaline liquid

Fig. 2. Preparation of alkaline liquid.

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(a) Mortar mixer

(b) Oiled moulds

(d) Fresh Geopolymer

(c) Mortar vibrator

(e) Mould strike off

(f) Sealed specimens

Fig. 3. Fresh geopolymer mortar mixtures.

(a) 24-hour resting period

(b) 72-hour resting period

Fig. 4. Intermittent curing regime.

(a) Electric oven

(b) Specimens inside the oven (c) Cured Geopolymer specimens Fig. 5. Curing of geopolymer mortar specimens.

4.5. Mould removal The geopolymer mortar specimens with high Na(OH) solution mole concentration ratios i.e. 12 and 16 mol, usually were still in the fresh state at the end of the 24-h resting period, i.e. did not solidify enough to be de-moulded without any significant damage

to the cube specimen. These specimens were left to cure inside its mould wrapped inside its sealed pp plastic bag in the first curing step12. The rest of the specimens were de-moulded and were kept wrapped inside its sealed pp plastic bag inside the heat treatment electric oven set at 70 °C for 4 steps for continuous 6 h each. At the beginning of each curing step, 2 ml of tap water were added to

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each plastic bag to prevent drying of any specimen during curing. At the end of each curing step, the oven was turned off and only three cubes were extracted from the oven for immediate testing. The rest of the specimens were left to cool inside the oven for the next 18 h. The curing step was repeated until the full 24 h of curing was completed.

Table 4 The molar ratio of mixtures and water-to-geopolymer solids ratio (per m3).

4.6. Testing of geopolymer mortar specimens Geopolymer mortar cube specimens were loaded gradually until failure in compression using a 2000 kN ELE compression testing machine equipped with a hemispherical head. The ultimate load carrying capacities were recorded and the modes of failure were photographed. Fig. 6 shows the modes of failure of geopolymer mortars where the failure mode is a typical shear failure of high compressive strength brittle materials. The shear failure surface is an hour-glass shape inclined by an angle 45 + U/2 where angle U is the angle of internal friction. Omitting additional water content had resulted in a steeper failure surface and had probably increased the angle of internal friction U. 5. Effect of intermittent curing on the development of compressive strength Table 4 presents the molar ratios of Na2O-to-SiO2, SiO2-to-Al2O3 and H2O-to-Na2O of geopolymer mortar mixtures and the waterto-geopolymer solids ratio by weight per 1 m3 of geopolymer mortar. Table 5 represents the average compressive strength of three tested cubes for all 21 mixtures.

SiO2

Al2O3

Na2O

H2O

Na2 O SiO2

SiO2 Al2 O3

H2 O Na2 O

Water* Solid

1 2 3 4, 7 5, 8 6, 9 10, 16 11, 17 12, 18 13, 19 14, 20 15, 21

5639 5639 5639 5914 5914 5914 5914 5914 5914 6168 6168 6168

1560 1560 1560 1560 1560 1560 1560 1560 1560 1560 1560 1560

426 494 548 539 607 661 539 607 661 740 833 909

12618 12385 12198 11254 11021 10834 7011 6778 6591 9715 9393 9133

0.0756 0.0876 0.0972 0.0912 0.1026 0.1118 0.0912 0.1026 0.1118 0.1199 0.1351 0.1473

3.62 3.62 3.62 3.79 3.79 3.79 3.79 3.79 3.79 3.95 3.95 3.95

29.6 25.1 22.2 20.9 18.16 16.39 13.00 11.17 9.97 13.14 11.27 10.05

0.362 0.350 0.341 0.310 0.299 0.291 0.1912 0.1813 0.1734 0.253 0.240 0.229

Water-to-geopolymer solids ratio is given by weight.

The average 7-day compressive strength of all geopolymer mortar in the HGW group, was higher than the corresponding strength for the LGW group at the end of the curing regime by a ratio of 101, 104 and 233% for the Na(OH) solution mole concentration of 8, 12 and 16 mol respectively. The average compressive strength of geopolymer mortar increased by increasing the Na(OH) solution mole concentration from 8, 12 and 16 mol for the HGW soluble sodium silicate. For the LGW soluble sodium silicate, the average compressive strength of geopolymer mortar increased by changing the Na(OH) solution mole concentration from 8, 16 and 12 mol. The maximum compressive strength was recorded at Na(OH) solution mole concentration of 12 mol and 16 mol for LGW and HGW groups respectively. 5.2. The effect of resting period on average compressive strength

5.1. The effect of soluble sodium silicate on average compressive strength Fig. 7 presents a comparison between the compressive strength gain for mixtures # 1, 2 and 3 and mixtures # 4, 5 and 6 for LGW and HGW soluble sodium silicates respectively under a standard 24-h resting period. For each Na(OH) solution mole concentration, the compressive strength increased as the total number of curing hours increased for both types of LGW and HGW soluble sodium silicates. The average compressive strength of all mixtures for the HGW group was consistently and substantially higher than the corresponding strength for the LGW group at the end of any curing step for all Na(OH) solution mole concentration. This is due to that the molar ratios, presented in Table 4, of Na2O-to-SiO2 and SiO2-toAl2O3 for the LGW group is considerably less than the corresponding values for the HGW group while the H2O-to-Na2O molar ratio and the water-to-geopolymer solids for the LGW group is higher than the corresponding values for the HGW group.

(a) GPI

*

#

Fig. 8(a)–(c) present a comparison between the average compressive strength for mixtures # 4 and 7, # 5 and 8, and # 6 and 9 for Na(OH) mole concentration of 8, 12 and 16 mol respectively for HGW soluble sodium silicates under a 24-h and 72-h resting periods. The compressive strength increased as the total number of curing hours increased for both resting periods for all Na(OH) mole. The average compressive strength at any age increases as the Na (OH) mole concentration increases from 8, 12 and 16 mol for both resting periods. This may be due to that the Na2O-to-SiO2, H2O-toNa2O mole ratios and the water-to-geopolymer solids are reduced as the Na(OH) mole concentration increases from 8, 12 and 16 mol for both resting periods, while the SiO2-to-Al2O3 is constant. For each Na(OH) solution concentration, the SiO2-to-Al2O3, Na2O-toSiO2, H2O-to-Na2O molar ratios and the water-to-geopolymer solids ration are constant for both resting periods.

(b) GPII Fig. 6. Typical mode of failure of geopolymer mortar.

(c) GPIII

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Table 5 The average compressive strength of geopolymer mortar mixtures [MPa]. #

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Geopolymer Mixture

GMI-LGW-8 M GMI-LGW-12 M GMI-LGW-16 M GMI-HGW-8 M GMI-HGW-12 M GMI-HGW-16 M GMII-HGW-8 MR GMII-HGW-12 MR GMII-HGW-16 MR GMIII-HGW-8M-35% GMIII-HGW-12M-35% GMIII-HGW-16M-35% GMIII-HGW-8M-48.5% GMIII-HGW-12M-48.5% GMIII-HGW-16M-48.5% GMIII-HGW-8MR-35% GMIII-HGW-12MR-35% GMIII-HGW-16MR-35% GMIII-HGW-8MR-48.5% GMIII-HGW-12MR-48.5% GMIII-HGW-16MR-48.5%

Age (days) 1

2

3

4

5

6

7

7*

1.260 1.773 1.467 2.00 3.33 4.65 – – – – – – – – – – – – – – –

2.85 4.26 3.45 4.98 11.11 12.04 – – – – – – – – – – – – – – –

4.25 5.63 4.73 6.39 14.02 17.51 1.97 4.25 4.08 – – – – – – – – – – – –

4.81 6.42 5.37 7.77 16.91 22.8 3.68 8.71 16.91 – – – – – – – – – – – –

– – – – – – 5.26 11.42 20.3 – – – – – – – – – – – –

– – – – – – 8.99 14.31 28.0 – – – – – – – – – – – –

4.93 8.67 6.93 9.89 17.67 23.1 12.59 17.87 28.5 27.8 42.0 60.4 28.3 50.6 41.5 30.1 51.2 63.5 31.9 61.0 46.4

– – – – – – 13.66 20.9 37.3 – – – – – – – – – – – –

All results were based on average of 3 specimens. Conversion factor: divide the UCS in (MPa) by 6.9  10 3 to get UCS in (psi). * Specimens were subjected to 5 steps of intermittent curing.

the 72-h resting period was increased by adding a fifth curing step of 6 h by a ratio of 9, 17 and 31% for the Na(OH) solution mole of 8, 12 and 16 mol respectively. 5.3. The effect of intermittent curing on percentage of compressive strength gain

Fig. 7. The effect of soluble sodium silicate properties on average compressive strength.

For Na(OH) solution mole concentrations of 8 and 12 mol, the average compressive strength of the 72-resting period is always less than the average compressive strength of the 24-resting period. The compressive strength of the former only exceeds the latter after the completion of the full curing regime. See Fig. 8(a) and (b). For the Na(OH) solution mole concentration of 16 mol, the average compressive strength of the 72-h resting period was less than the average compressive strength of the 24-h resting period until 18 h of elevated temperature curing. The compressive strength of the former exceeds the latter after 18 h of intermittent curing regime. See Fig. 8(c). Increasing the resting period may slow down the rate of gain of compressive strength, yet it has a positive impact by increasing the average 7-day compressive strength. The average 7-day compressive strength of any geopolymer mortar using the 72-h resting period, is higher than the corresponding strength for the 24-h resting period at the end of the curing regime by a ratio of 27, 1 and 23% for Na(OH) solution mole concentration of 8, 12 and 16 mol respectively. In addition, it was noticed that the average 7-day compressive strength of any geopolymer mortar using

Table 6 represents the percentage of compressive strength gain as a ratio from the 7-day average compressive strength of fully cured specimens for geopolymer mixtures # 1 to 9. The difference in percentage of compressive strength gain for any two successive values is obviously the effect of each curing step on the compressive strength gain. The average difference in percentage of increase in compressive strength is 23, 29, 21 and 10% of the 7-day compressive strength at the end of 6, 12, 18 and 24 h of curing respectively for the LGW group. The average difference in percentage of increase in compressive strength is 20, 34, 19 and 21% of the 7day compressive strength at the end of 6, 12, 18 and 24 h of curing respectively for the HGW group with 24 h resting period. The average difference in percentage of increase in compressive strength is 18, 28, 13, and 25% of the 7-day compressive strength at the end of 6, 12, 18 and 24 h of curing respectively for the HGW group with 72-h resting period. The percentage of increase of the compressive strength always recorded maximum after 12 h of intermittent curing, while it is lesser after 24, 18 and 6 h. Again, increasing the resting period reduces the rate of gain of strength at early ages and lower curing time, yet it compensates for that at late ages and longer curing time. 6. The effect of additional water content on the 7-day compressive strength The alkaline liquid-to-fly ash content ratio was set at 35% by weight for Geoploymer mortar mixtures # 4–12, 16, 17 and 18. Fig. 9 presents a comparison between the average 7-day compressive strength of the geopolymer mortar mixtures # 4, 5, and 6, cast with 13.5% additional water content and mixtures # 10, 11 and 12, cast without any added water content where both were subjected to a 24-h resting period. In addition, Fig. 9 presents a comparison

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A.I.I. Helmy / Construction and Building Materials 110 (2016) 54–64 Table 6 The effect of intermittent curing on rate of gain of compressive strength (%). #

Geopolymer mixture

1

GMI-LGW-8M

2

GMI-LGW-12M

3

GMI-LGW-16M

4

GMI-HGW-8M

5

GMI-HGW-12M

6

GMI-HGW-16M

7

GMII-HGW-8MR

8

GMII-HGW-12MR

9

GMII-HGW-16MR

Age (days)

% Difference % Difference % Difference % Difference % Difference % Difference % Difference % Difference % Difference

1

2

3

4

5

6

7*

26 26 20 20 21 21 20 20 19 19 20 20 –

58 32 49 29 50 29 50 30 63 44 52 32 –





























97 11 74 10 78 10 79 14 96 23 99 23 29 13 49 25 59 45





86 28 65 24 68 18 65 15 79 16 76 24 16 16 24 24 14 14

42 13 64 15 71 12

71 29 80 16 100 29

100 3 100 21 100 22 100 21 100 4 100 1 100 29 100 20 100 0

* All percentages shown are calculated with respect to the 7-day average compressive strength.

Fig. 9. The effect of added water on the average 7-day compressive strength.

Fig. 8. The effect of resting period on compressive strength.

between the average 7-day compressive strength of the geopolymer mortar mixtures # 7, 8, and 9, cast with 13.5% additional water content and mixtures # 16, 17 and 18, cast without any added water content where both were subjected to a 72-h resting period. Recall, omitting the added water content had resulted in dry and extremely stiff geopolymer mortar that required different protocol during mixing and special care during compaction of fresh geopolymer mortar in the moulds, while, the additional water content had resulted in cohesive mixtures that were easily mixed, cast in the moulds and compacted. As can be seen from Table 4, SiO2-to-Al2O3 is constant for all 12 mixtures at 3.79 while Na2O-to-SiO2 increased and H2O-to-Na2O and the water-to-geopolymer solids reduced as the Na(OH) solution mole concentration increased from 8, 12, and 16 mol. As can be seen from Fig. 9, omitting added water content for mixtures # 10, 11 and 12 for the Na(OH) solution mole concentration of 8,

12 and 16 mol with 24-h resting period had resulted in an increase in the 7-day compressive strengths over mixtures # 4, 5 and 6 of 181%, 138% and 162% respectively. Likewise, omitting added water content for mixtures # 16, 17 and 18 for the Na(OH) solution mole concentration of 8, 12 and 16 mol with 72-h resting period had resulted in an increase in the 7-day compressive strengths over mixtures # 7, 8 and 9 of 139%, 186% and 123% respectively. As can be seen from Table 4, omitting additional water had resulted in substantial reduction in H2O-to-Na2O mole ratio and the water-to-geopolymer solids ratios while the Na2O-to-SiO2 mole ratio is constant for each Na(OH) solution mole concentration of 8, 12, and 16 mol. The 72 h resting period had consistently resulted in increasing the 7-day compressive strength for all Na(OH) solution mole concentration with or without additional water.

7. The effect of alkaline liquid-to-fly ash content on the 7-day compressive strength According to the ECP [10], a standard OPC mortar is mixed using a water/cement ratio of 48.5%. Thus the alkaline liquid-to-fly ash content ratio was set at 48.5% by weight for Geoploymer mortar

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mixtures # 13, 14 and 15 and 19, 20 and 21 for 24-h and 72-h resting period respectively. As can be seen from Table 4, SiO2-to-Al2O3 is constant for all 6 mixtures at 3.85 while Na2O-to-SiO2 increased and H2O-to-Na2O and the water-to-geopolymer solids reduced as the Na(OH) solution mole concentration increased from 8, 12, and 16 mol. Fig. 10 presents a comparison between the average 7-day compressive strength of the geopolymer mortar mixtures # 10, 11, and 12, and mixtures # 13, 14 and 15, cast with 35 and 48.5% alkaline liquid-to fly ash content where both were subjected to a standard 24-h resting period. In addition, Fig. 10 presents a comparison between the average 7-day compressive strength of the geopolymer mortar mixtures # 16, 17, and 18, and mixtures # 19, 20 and 21, cast with 35% and 48.5% alkaline liquid-to fly ash content where both were subjected to a 72-h resting period. Recall, omitting the added water content for the 35% alkaline liquid content had resulted in dry and extremely stiff geopolymer mortar that required different mixing protocol and special care during compaction of fresh geopolymer mortar in the moulds, while for the 48.5% alkaline liquid content, the geopolymer mortar was cohesive and plastic mixture that was easily mixed, cast and compacted. As can be seen from Fig. 10, increasing the alkaline liquid content from 35% to 48.5% of the fly ash content for mixtures # 13, 14 and 15 for the Na(OH) solution mole concentration of 8, 12 and 16 mol with 24-h resting period had resulted in a change in the 7-day compressive strengths over mixtures # 10, 11 and 12 of 1.58%, 20.5% and 31.3% respectively. Likewise, increasing the alkaline liquid content from 35% to 48.5% of the fly ash content for mixtures # 19, 20 and 21 for the Na(OH) solution mole concentration of 8, 12 and 16 mol with 72-h resting period had resulted in a change in the 7-day compressive strengths over mixtures # 16, 17 and 18 by 6%, 19.1% and 26.9% respectively. Increasing the Na(OH) solution mole concentration ratio beyond 12 mol into 16 mol had resulted in reverse effect and decreased the compressive strength. As can be seen from Fig. 10 and Table 4, increasing the alkaline liquid percentage from 35% into 48.5% of the fly ash content, for both resting periods, is extremely sensitive to the combination of molar ratios of constituents. It is worth to compare the molar ratios for geopolymer pairs # 12 and 18, and 15 and 21, where the former pair recorded the highest compressive strengths and the latter pair recorded the adverse effect in strength reduction. The Na2O-to-SiO2, H2O-to-Na2O molar ratios and the water-to-geopolymer solids ratio were 0.1118, 9.97

and 0.1734 and 0.1473, 10.05 and 0.229 for the geopolymer pairs # 12 and 18, and 15 and 21 respectively. The recorded adverse effect on compressive strength is associated with highest value for Na2Oto-SiO2. It is worth to mention that this observation was also reported in the literature. As a rule of thumb, increasing the resting period from 24-h into 72-h had resulted in an increase in the average 7-day compressive strength of the all previously mentioned geopolymer mortars. 8. The effect of molar ratios of constituents on the 7-day compressive strength Fig. 11(a)–(c) give a graphical representation for the effect of molar ratio of Na2-to-Sio2 and H2O-to-Na2O and the water-togeopolymer solids by weight for various SiO2-to-Al2O3 mole ratios on the average 7-day compressive strength for geopolymer mixtures. As the Na2-to-SiO2 mole ratio increases the average 7 day compressive strength increases either with or without additional water content and under any resting period until a thresh hold between 13.51 and 14.73% corresponding to Na(OH) solution mole concentration between 12 and 16 mol. See Fig. 11(a). As the H2Oto-Na2O mole ratio and water-to-geopolymer solids increased, the average 7 day compressive strength decreased either with or without additional water content and under any resting period. See Fig. 11(b) and (c). This phenomenon is similar to the effect of water/cement ratio in the concrete technology. Although both mixture # 15 and 21 possessed the lowest H2O-to-Na2O mole ratio, they suffered from an adverse effect on the compressive strength due to the high Na2-to-SiO2 mole ratio. The Egyptian Code of Practice ECP [10] specified limit for the minimum 7-day compressive strength of OPC standard mortar is not less than 27 MPa calculated as the average of three specimens. As can be seen from Fig. 11, the 7-day strength of all mixtures from # 1 to 8 for phase I, and II cast with 13.5% additional water content were less than the ECP limit specified. Only mixture # 9 with Na (OH) solution mole concentration of 16 mol, cast with 13.5% additional water content and a 72-h resting period recorded an average 7-day compressive strength higher than the ECP limit specified. The average 7-day compressive strength of all mixtures from # 10 to 21, cast with 35% or 48.5% of alkaline liquid-to-fly ash content, with no additional water content and under both resting periods of 24 and 72 h were higher than the ECP specified limit. 9. Conclusions and recommendations The experimental results of the current research program suggest the following conclusions:

Fig. 10. The effect of alkaline liquid-to-fly ash percentage on the average 7-day compressive strength.

1. The intermittent curing scheme at 70 °C for 4 steps for continuous 6 h of heat curing in each step followed by 18 h of ambient temperature proved to improve the geopolymer mortar compressive strength at the end of each curing step with no adverse effect on the strength. 2. Thirteen geopolymer mortar mixtures had resulted in 7-day compressive strength that is higher than the Egyptian Code of Practice specified minimum limit of 27 MPa. The use of high specific gravity soluble sodium silicate, an alkaline liquid percentage of 35% or 48.5% with Na(OH) solution concentration mole of 8, 12 or 16 mol, with no added water and a resting period of either 24 or 72 h may guarantee a 7-day compressive strength above 27 MPa. 3. The rate of gain of compressive strength suggested that highest gain of compressive strength took place at the end of 12 h of intermittent curing. A tailored intermittent curing scheme should be investigated to maximize the compressive strength

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5. The added water content played an important role in facilitating the mixing, pouring and compaction of geopolymer mortar. All geopolymer mortar mixtures, cast with added water content of 13.5% of fly ash content, produced a cohesive and fully plastic geopolymer mixture. While, omitting added water content produced dry and stiff geopolymer mortar mixtures. On the other hand the latter mixtures exhibit higher 7-day compressive strength than the former. The highest average 7-day compressive strength recorded was 63.5 MPa for lowest H2O-to-Na2O mole ratio of 9.97. The UCS is inversely proportional to the H2O-to-Na2O mole ratio and the water-to-geopolymer solids ratio. Different methods to facilitate concrete technology may be investigated such as using water reducers type A or superplasticizers type G or type F (ASTM C494) rather than adding extra water content. 6. The 72-h resting period had proved to produce higher 7-day compressive strength than the 24-h resting period under the intermittent curing scheme used with or without additional water content. The top two average 7-day compressive strengths recorded was 63.5 and 60.4 MPa for lowest H2O-toNa2O mole ratio of 9.97 subjected to 72-h and 24-h resting periods respectively. Resting periods between 72 and 96 h were reported to enhance the compressive strength of geopolymer. It may have been the same case with the intermittent curing scheme used. 7. Increasing the alkaline liquid-to-fly ash content from 35% into 48.5% had proved to be very sensitive to the Na2O-to-SiO2 mole ratio in the mixture. The research work suggested that a thresh hold for that ratio, between 13.51 and 14.73% corresponding to Na(OH) solution mole concentration between 12 and 16 mol, is probably the limit for maximum compressive strength beyond which an adverse effect on the compressive strength can take place resulting in reducing the 7-day compressive strength. A fixed ratio of soluble sodium silicate-to-Na(oH) solution of 2 and two fixed ratios of alkaline liquid-to-fly ash content namely; 35% and 48.5% were used in the research programme. It may be beneficial to apply the same intermittent curing scheme on geopolymer mixtures with different ratios of soluble sodium silicate-to-Na(oH) solution and alkaline liquid-to-fly ash content. A good correlation between alkaline liquid content and Na2O-to-SiO2 mole ratio must be carefully considered.

Acknowledgements

Fig. 11. The effect of molar ratios on the average 7-day compressive strength.

gain at this particular period by increasing the maximum temperature of that period. Other intermittent schemes may be investigated by changing either the number of steps, duration of each step and the maximum temperature for the heat treatment. 4. The geopolymer compressive strengths were very sensitive to the soluble sodium silicate solutions used in the experimental programme. High specific gravity, high coefficient of viscosity and low water content of the soluble silicate solution are extremely important to exclude unsuitable activator. It may be beneficial to investigate powder type soluble silicates to have more control on H2O-to-NaO2 mole ratio and water-to-geopolymer solids ratio and to change into potassium-based solutions.

The experimental work, reported herein, was carried out in the Concrete Materials Laboratory in the Civil Engineering Department at the Faculty of Engineering, The British University in Egypt, BUE, Al-Sherouk city, Egypt. The author is thankful for the generous donation of the fly ash material, offered by Dr. Nagi Riad, CEO of GEOS Egypt that enabled carrying out this research work successfully. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.conbuildmat. 2016.02.007. References [1] M.R. Nagral, T. Ostwal, M.V. Chitawadagi, Effect of curing temperature and curing hours on the properties of geopolymer concrete, Int. J. Comput. Eng. Res. (IJCER) 04 (9) (2014). ISSN (e): 2250-3005, . [2] W.I.W. Mastura, H. Kamarudin, I.K. Nizar, A.M.M. Al Bakri, H. Mohammed, Effect of curing system on properties of fly ash-based geopolymer bricks, Int. Rev. Mech. Eng. (IREME) 7 (1) (2013).

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Amr I. I. Helmy is an Associate Professor at the Civil Engineering Department, the British University in Egypt, BUE, on sabbatical from the Structural Engineering Department, Faculty of Engineering, Ain Shams University, Cairo, Egypt. He received his BSc (1984) and MSc. (1988) from the Structural Engineering Department, Faculty of Engineering at Ain Shams University, Cairo, Egypt. He received his M.A.Sc. (1993) and Ph.D. (1998) from the Civil Engineering Department at the University of Toronto, Toronto, Ont., Canada. His research interests include Geopolymer, Nano materials in concrete technology, Repair and strengthening of RC members using FRP, off-shore concrete structures and shell structures.