Effect of Curing Conditions on Strength of Fly ash-based Self-Compacting Geopolymer Concrete

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Effect of Curing Conditions on Strength of
Fly ash-based Self-Compacting Geopolymer  Concrete
J.Thomas Britto
Dept of Civil Engineering
This paper reports the results of an experimental work  conducted  to  investigate  the  effect  of  curing  conditions  on  the  compressive   strength   of   self-compacting   geopolymer   concrete  prepared by using fly ash as base material and combination of sodium  hydroxide and sodium silicate as alkaline activator. The experiments  were conducted by varying the curing time and curing temperature in  the  range  of  24-96  hours  and  60-90°C  respectively.  The  essential  workability    properties    of    freshly    prepared    Self-compacting  Geopolymer  concrete  such  as  filling  ability,  passing  ability  and  segregation   resistance   were   evaluated   by   using   Slump   flow, V-funnel,   L-box   and   J-ring   test   methods.   The   fundamental  requirements  of  high  flowability  and  resistance  to  segregation  as  specified  by  guidelines  on  Self-compacting  Concrete  by  EFNARC  were satisfied. Test results indicate that longer curing time and curing  the  concrete  specimens  at  higher  temperatures  result  in  higher  compressive  strength.  There  was  increase  in  compressive  strength  with  the  increase  in  curing  time;  however  increase  in  compressive  strength after 48 hours was not significant. Concrete specimens cured  at  70°C  produced  the  highest  compressive  strength  as  compared  to  specimens cured at 60°C, 80°C and 90°C.
Keywords—Geopolymer Concrete, Self-compacting Geopolymer  concrete, Compressive strength, Curing time, Curing temperature.
In the  growing  environmental  concerns  of  the  cement  industry,  alternative  cement  technologies  have  become  an  area  of  increasing  interest.  It  is  now  believed  that  new  binders  are  indispensable  for  enhanced  environmental  and  durability performance. In this aspect, geopolymer technology  is  one  of  the  revolutionary  developments  related  to  novel  materials    as    an    alternative    to    Portland    cement.    The  development  of  geopolymer  concrete  is  an  important  step  because of the potential application of geopolymers to a wide  variety  of  industrial  waste  materials  to  produce  added-value  construction      materials      resulting      in      low-cost      and  environmentally friendly  material with similar mechanical  performance and appearance properties to those from Portland  cement .  Geopolymer  concrete  is  produced  by  activating  different  alumino-silicate  based  waste  materials  with  highly  alkaline  solution.  Curing  of  freshly  prepared  geopolymer  concrete is the most crucial aspect and plays an important role  in   the   geopolymerisation   process  .   Proper   curing   of  concrete  has  a  positive  effect  on  the  final  properties  of  the  geopolymer   concrete.   Curing   of   geopolymer   concrete   is  mostly  carried  out  at  elevated  temperatures ,  however, curing  at  ambient  temperatures  has  also  been  carried  out.  At  ambient    temperatures;    the    reaction    of    fly    ash-based  geopolymeric  materials  is  very  slow  and  usually  show  a  slower  setting  and  strength  development.  It  is  believed  that   higher temperatures activate alumino-silicate phases in the fly  ash,    therefore,    they    are    generally    cured    at    elevated  temperatures between 60 0 C- 90 0 C.
Previous  research  has  shown  that  both  curing  time  and  curing  temperature  significantly  influence  the  compressive  strength of geopolymer concrete. Several researchers  have investigated the effect of curing time and curing  temperature on the properties of geopolymer concrete,  in  their  study  on  fly  ash-based  geopolymers  have  reported   that   the   curing   temperature   and   curing   time  significantly affected the mechanical strength of fly ash-based  geopolymers.  They  concluded  that  higher  curing  temperature  and longer curing time proved to result in higher compressive  strength. 
we  studied  the  interrelationship  of  certain parameters that affected the properties of fly ash-based geopolymer.  They  have  demonstrated  that  water  content, curing time and curing  temperature affected the properties of  geopolymers;  specifically  the  curing  condition  and  calcining  temperature   influenced   the   compressive   strength.   They  concluded  that  rapid  curing  and  curing  at  high  temperature  reduced the compressive strength and caused a negative effect  on the physical properties of the geopolymer. 
In  a  study  done  we   on  compressive  strength and microstructural characteristics of class C  fly ash  geopolymer, the authors have reported that curing temperature  had   a   significant   effect   on   the   compressive   strength  development.  Compressive  strength  began  to  decrease  after  curing for a certain period of time at higher temperature. They  revealed  that  prolonged  curing  can  break  down  the  granular  structure of the geopolymer mixture.
Iin  his  research  study  on  effect  of  curing  temperature  on  the  development  of  hard  structure  of  metakaolin-based   geopolymer,   has   reported   that   curing temperature had significant effect on the setting and hardening  of     metakaolin-based     geopolymer.     The     author     has  demonstrated  that  higher  curing  temperatures  and  longer    curing  time  increase  the  early  age  compressive  and  flexural  strengths. However, elevated temperature during early stage of  hardening  process  results  to  the  formation  of  larger  pores  consequently  increases  cumulative  pore  volume,  which  has  a  negative   effect   on   the   final   mechanical   properties   of  geopolymeric material.
This  study  aimed  to  analyze  the  effect  of  curing  time  and  curing  temperature  on  the  compressive  strength  of  fly  ash-based   self-compacting   geopolymer   concrete.   Test   results   indicate  that  longer  curing  time  and  curing  the  concrete  specimens at elevated temperature up to 70 0 C result in higher  compressive strength.
A.     Materials 
In the present study, Low-calcium (ASTM Class F) Fly ash  was used as a source material for the synthesis of SCGC. Fly  ash was obtained from Manjung power station, Lumut, Perak, Malaysia.The chemical composition of Fly ash as determined  by X-Ray Fluorescence (XRF) analysis is shown in Table I. 

Locally  available  crushed  coarse  aggregate  of  maximum  size  14  mm  having  specific  gravity  of  2.66  was  used  in  the  preparation  of  all  test  specimens.  The  coarse  aggregate  was  used in saturated surface dry (SSD) condition. Natural Malaysian sand having specific gravity of 2.61 and the  fineness  modulus  of  2.76  was  used  as  fine  aggregate.  Fine  aggregate was sieved  for the  size less than 5mm  and  used  in  dry condition.

For   the   alkaline   activator,   a   combination   of   sodium  hydroxide  and  sodium  silicate  solution  was  used.  Sodium  hydroxide  in  pellets  form  with  99%  purity,  supplied  by QuickLab  Sdn  Bhd,  Malaysia  and  Sodium  silicate  solution (Grade A53 with Na2O = 14.26%, SiO2 = 29.43% and water = 56.31%) obtained from Malay-Sino Chemical Industries Sdn  Bhd,  Malaysia  were  used.  To  prepare  sodium  hydroxide  solution, sodium hydroxide pellets were dissolved in ordinary  drinking water. Both the liquid solutions were mixed together  and alkaline solution was prepared.

To  achieve  higher  workability  and  required  flowability  of  the  fresh  concrete,  a  commercially  available  superplasticizer  (Sika  Viscocrete-3430)  supplied  by  Sika  Kimia  Sdn  Bhd,  Malaysia,  and  a  specified  amount  of  extra  water  (other  than  the  water  used  for  the  preparation  of  sodium  hydroxide  solution) was also used. 

Oxide                                                    (%) by mass
Silicon dioxide (SiO 2 )                                    51.3
Aluminum oxide (Al 2 O 3 )                 30.1
Ferric oxide (Fe 2 O 3 )                                      4.57
Total SiO 2  + Al 2 O 3  + Fe 2 O 3       85.97
Calcium oxide (CaO)                               8.73
Phosphorus pentoxide (P 2 O 5 )                         1.6
Sulphur trioxide (SO 3 )                                       1.4
Potassium oxide (K 2 O)                          1.56
Titanium dioxide (TiO 2 )                        0.698
B.     Design of Mix Proportion 
In  this  experimental  work,  a  mix  proportion  with  the  content  of  400  kg/m 3   of  fly  ash  was  designed  to  study  the  effect   of   curing   time   and   curing   temperature   on   the compressive strength of self-compacting geopolymer concrete. Four levels of curing time i.e. 24, 48, 72 and 96 hours and four  ranges of curing temperature i.e. 60°C, 70°C, 80°C and 90°C  were  used.  The  details  of  the  mix  proportion  are  given  in  Table II. In order to attain required workability characteristics  of  self-compacting  geopolymer  concrete,  a  water  content  of  12% and superplasticizer dosage of 7% by mass of the fly ash  were also used. The alkaline solution-to-Fly ash ratio was kept  0.5 whereas the ratio of  sodium  silicate  to  sodium  hydroxide  and  concentration  of  sodium  hydroxide  were  2.5  and  12  M  respectively.

Fly Ash (Kg/m 3 )                                                                     400
Fine Aggregate (Kg/m 3 )                                                         850
Coarse Aggregate (Kg/m 3 )                                                     950
Sodium Hydroxide (Kg/m 3 )                                                   57
Concentration of NaOH Solution (Molarity)                            12
Sodium Silicate (Kg/m 3 )                                                         143
Super plasticizer (%)                                                                  7
Extra water (%)                                                                          12
Sodium Silicate/Sodium hydroxide by mass                             2.5
Alkaline to Fly ash ratio                                                            0.5
Curing time (hrs)                                                                       24-96 
Curing temperature (°C)                                                            60-90

C.     Mixing Procedure 

Mixing was carried out in two stages. Initially, Fly ash, Fine  aggregate (in dry condition) and coarse aggregate (in saturated  surface dry condition) were mixed in a pan mixture for about
2.5  minutes. At the end of  this  mixing, the  liquid component  of   the   geopolymer   concrete   mixture   comprising   alkaline  solution, superplasticiser and the extra water, was added to the  solid particles and the mixing continued for another 3 minutes.To  ensure  the  mixture  homogeneity,  fresh  concrete  mix  was  hand  mixed  for  further  2  to  3  minutes.  The  freshly  prepared  concrete  mix  was  then  assessed  for  the  essential  workability  tests   required   for   characterizing   self-compacting   concrete(SCC). Tests such as slump flow, slump flow at T 50 , V-funnel, L-box, and J-ring were performed for this purpose.

D.     Casting and Curing of Test Specimens 
After  assessing  the  necessary  workability  properties  as  guided  by  EFNARC  [11],  the  fresh  concrete  was  placed  in  steel moulds of dimensions 100 x 100 x 100 mm and allowed  to  fill  all  the  spaces  of  the  moulds  by  its  own  weight.  Three  cubes  were  prepared  for  each  test  variable.  After  casting  the  moulds,  without  any  delay,  they  were  kept  in  the  oven  at  a  specified   temperature   for   a   specified   period   of   time   in  accordance with the test variables selected. At the end of the  curing  period,  the  moulds  were  taken  out  from  the  oven  and  left undisturbed for about 15 minutes. The test specimens were  removed  from  the  moulds  and  left  to  air  dry  in  the  room  temperature  conditions  until  tested  for  direct  compression  at  the specified age. 

A.        Fresh Properties of SCGC 
The properties of fresh SCC differ significantly from that of  conventional  fresh  concrete.  There  are  three  distinct  fresh  properties  of  SCC  which  are  fundamental  to  its  performance  both in fresh and hardened state. According to EFNARC [11],  a   concrete   mix   can   only   be   classified   as   SCC   if   the  requirements   for  all  the  three  workability  properties  are  fulfilled. The three essential fresh properties required by SCC  are filling ability, passing ability and resistance to segregation. To accomplish the workability properties, tests such as slump  flow,  slump  flow  at  T50,  V-funnel,  L-box,  and  J-ring  were  carried  out.  All  the  tests  were  performed  by  following  the  European  Guidelines  for  SCC.  The  test  results  of  fresh  properties of SCGC are presented in Table III. The results of   the quantitative measurements and visual observations showed  that  freshly  prepared  concrete  mix  had  good  flow,  filling  & passing ability, and produced desired results and were within  the EFNARC range of SCC.

Workability Test Results                                        Acceptance Criteria for SCC
As per EFNARC [11]
Minimum         Maximum
Slump flow (mm)                            710                650 mm           800 mm
T 50 cm  Slump flow (sec.)                         4.0                   2 sec.               5 sec.
V-Funnel Flow time (sec.)               7                      6 sec.               12 sec.
L-Box (H 2 /H 1 )                            0.96                 0.8                   1.0
J-Ring (mm)                                      5                     0 mm               10 mm

B.     Compressive Strength of SCGC
Compressive strength is one of the most common measures  used  to  evaluate  the  quality  of  hardened  concrete  and  is  considered   as   the   characteristic   material   value   for   the  classification   of   concrete.   Many   researchers   have   used  compressive  strengths measurements  as  a  tool  to  assess  the  success   of   geopolymerisation   process   [12].   Compressive  strength test was performed in accordance with BS EN 12390-3:2002  using 2000  KN  Digital  Compressive  &  Flexural  Testing Machine. At the end of specified oven curing period, a  set of three cubes for each test variable was tested at the ages  of 1, 3, 7 and 28 days. The compressive strength test results  are presented in Table IV. The reported compressive strength  is the average strength of three specimens.  


Compressive Strength Test Results
     Mix             1-Day              3-Day              7-Day              28-Day
S1                45.01               45.85               46.94               48.53
S2              51.03                51.98               52.26               53.80
S3                51.41               52.20              52.69               53.92
S4                51.68               52.33               52.72               53.99
S5                44.81               45.64               45.98               47.54
S6                51.03               51.98               52.26               53.80
S7                48.56               49.22               49.80               50.77
S8                47.99               48.83               49.67               50.42

1.      Effect of Curing Time on Compressive Strength
Fig.   1   shows   the   influence   of   curing   time   on   the  compressive strength of self-compacting geopolymer concrete.The test specimens were cured in the oven at a temperature of  70°C.  The  curing  time  varied  from  24  hours  to  96  hours (4  days).  It  is  believed  that  longer  curing  time  improved  the  geopolymerisation  process resulting  in  higher  compressive  strength.  The  test  results  shown  in  Fig.  1  indicate  that  the  compressive  strength  increases  with  increase  in  curing  time. The  test  specimens  cured  at  70°C  for  a  period  of  96  hours  produced the highest compressive strength at all ages. The rate  of increase in strength was rapid up to 48 hours of curing time; however,   the   gain   in   strength   beyond   48   hours   is   not           significant.  The  results  shown  in  Fig.  1  clearly  demonstrate  that  longer  curing  time  does  not  produce  weaker  material  as  claimed .  The  trend  of  these  test  results  is  similar  to  those  observed  by   in their study on Fly ash-based geopolymer concrete.

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2.      Effect of Curing Temperature on Compressive Strength
Curing  temperature  plays  a  significant  role  in  the  setting  and hardening of the geopolymer concrete [2]. Hardjito et al. [14], in their study on low-calcium Fly ash-based geopolymer  mortar   have   reported   that   curing   temperature   plays   an  important  role  in  the  geopolymerization  process  of  Fly  ash-based geopolymer. They have concluded that higher the curing  temperature,  higher  will  be  the  rate  of  geopolymerization  process,   which   eventually   accelerates   the   hardening   of
geopolymer mortar.

The   effect   of   curing   temperature   on   the   compressive  strength  is  illustrated  by  the  test  data  shown  in  Fig.  2.  As  curing of fly ash-based geopolymer concrete is usually carried  out  at  an  elevated  temperature  in  the  range  of  60-90°C;  therefore,  in  this  experimental  study  the  curing  temperature  was varied from 60 to 90°C. The test specimens were cured in  the oven at a temperature of 60, 70, 80 and 90°C for a period  of 48 hours. All the other test parameters were kept constant. 

Compressive  strength  results  shown  in  Fig.  2  indicate  that  higher curing temperature does not ensure higher compressive  strengths  as  claimed  by   in  their  study  on  Fly ash-based geopolymer concrete. Test results show that an  increase   in   the   curing   temperature   from   60°C   to   70°C  increases the  compressive strength of  the concrete. However, increasing the  curing temperature beyond 70°C decreases the  compressive strength of self-compacting geopolymer concrete.

It  is  believed  that  increase  in  the  curing  temperature  from  60°C to 70°C increased the rate and extent of reaction through  an increase in the heat of reaction; consequently increased the  compressive strength of the concrete.

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Fig. 2 Effect of Curing Temperature on Compressive Strength

In this experimental work, the effect of curing conditions on  the  compressive  strength  of  fly  ash-based  self-compacting  geopolymer  concrete  was  investigated.  Test  results  indicate  that curing time and curing temperature significantly affect the  compressive strength of hardened concrete. Based on the test  results reported here, the following conclusions can be drawn.

1.  Longer   curing   time   improves   the   geopolymerisation  process  resulting in  higher compressive strength. Increase  in  compressive  strength  was  observed  with  increase  in  curing  time.  The  compressive  strength  was  highest  when  the  specimens  were  cured  for  a  period  of  96  hours; however,  the  increase  in  strength  after  48  hours  was  not  significant.

2.  Compressive   strength   of   concrete   increased   with   the  increase   in   curing   temperature   from   60°C   to   70°C;however  an  increase  in  the  curing  temperature  beyond  70°C    decreased    the    compressive    strength    of    self-  compacting  geopolymer concrete.

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