Ma e iales de Cons uCCión
Vol. 66, Issue 324, Oc obe –Decembe 2016, e098
ISSN-L: 0465-2746
h p://dx.doi.o g/10.3989/mc.2016.10215
Con ibu ions o he s udy o po osi y in ly ash-based geopolyme s.
Rela ionship be ween deg ee o eac ion, po osi y and comp essi e
s eng h
Y. Luna-Galianoa*, C. Fe nández-Pe ei aa, M. Izquie dob
a. Uni e si y o Se ille, School o Enginee ing, Chemical and En i onmen al Enginee ing Depa men . (Se ille, Spain)
b. School o Biosciences, Uni e si y o No ingham (Su on Boning on, Uni ed Kingdom)
*[email protected]
Recei ed 20 No embe 2015
Accep ed 14 Ma ch 2016
A ailable on line 20 Sep embe 2016
ABSTRACT: The main con ibu ion o his pape ela es o he de elopmen o a sys ema ic s udy in ol ing a
se o pa ame e s which could po en ially ha e an impac on geopolyme p ope ies: cu ing empe a u e, ype
o ac i a ing solu ion, alkali me al in solu ion, inco po a ion o slag (Ca sou ce) and ype o slag used. The
mic os uc u es, deg ees o eac ion, po osi ies and comp essi e s eng hs o geopolyme s ha e been e alu-
a ed. Geopolyme s p epa ed wi h soluble silica e p esen ed a mo e compac ed and closed s uc u e, a la ge
amoun o gel, lowe po osi y and g ea e comp essi e s eng h han hose p epa ed wi h hyd oxides. On he
o he hand, Na-geopolyme s we e mo e po ous bu mo e esis an han K-geopolyme s. Al hough he e is an
in e se ela ion be ween deg ee o eac ion and po osi y, be ween comp essi e s eng h and po osi y i is no
always in e sely p opo ional and could, in some cases, be masked by changes p oduced in o he in luencing
pa ame e s.
KEYWORDS: Fly ash; Blas u nace slag; Alkali-ac i a ed cemen ; Comp essi e S eng h; Po e size dis ibu ion
Ci a ion/Ci a como: Luna-Galiano, Y.; Fe nández-Pe ei a, C.; Izquie do, M. (2016) Con ibu ions o he s udy o
po osi y in ly ash-based geopolyme s. Rela ionship be ween deg ee o eac ion, po osi y and comp essi e s eng h.
Ma e . Cons ucc. 66 [324], e098. h p://dx.doi.o g/10.3989/mc.2016.10215.
RESUMEN: Con ibuciones al es udio de la po osidad de geopolíme os basados en cenizas olan es. Relación en e
g ado de eacción, po osidad y esis encia a comp esión. La p incipal con ibución de es e documen o es el desa -
ollo de un es udio sis emá ico implicando una se ie de pa áme os que pod ían a ec a a las p opiedades de
los geopolíme os: empe a u a de cu ado, solución ac i ado a, me al alcalino de la solución, inco po ación de
esco ias ( uen e de calcio) y ipo de esco ias. Se han e aluado: mic oes uc u a, g ado de eacción, po osidad y
esis encia a comp esión. Los geopolíme os p epa ados con silica os p esen a on un mic oes uc u a más densa
y compac a, una mayo can idad del gel geopolimé ico, meno po osidad y mejo es p opiedades mecánicas que
los p epa ados con hid óxidos. Los geopolíme os p epa ados con sales de sodio ue on más po osos pe o más
esis en es que los p epa ados con sales po asio. Aunque exis e una elación in e sa en e el g ado de eacción y
la po osidad, en algunos casos, la elación en e esis encia y po osidad es inexis en e ya que puede es a enmas-
ca ada po cambios p oducidos po o os pa áme os que a ec en a la eacción.
PALABRAS CLAVE: Ceniza olan e; Esco ia de al o ho no; Cemen o ac i ado alcalinamen e; Resis encia a la
comp esión; Dis ibución de amaño de po o
Copy igh : © 2016 CSIC. This is an open-access a icle dis ibu ed unde he e ms o he C ea i e Commons
A ibu ion License (CC BY) Spain 3.0.
2 • Y. Luna-Galiano e al.
Ma e iales de Cons ucción 66 (324), Oc obe –Decembe 2016, e098. ISSN-L: 0465-2746. doi: h p://dx.doi.o g/10.3989/mc.2016.10215
1. INTRODUCTION
Coal-based powe gene a ion p oduces la ge
amoun s o ly ash wo ldwide. An es ima ed 780
million onnes o coal ash a e p oduced e e y yea ,
bu less han 50% o global p oduc ion is u ilised
(1). The syn hesis o geopolyme s may be a suc-
cess ul al e na i e o e-use coal ash as added- alue
p oduc s esul ing in low cos and en i onmen ally
iendly ma e ials wi h cemen ing p ope ies esem-
bling hose o O dina y Po land Cemen (OPC) (2).
The e m geopolyme was in oduced by Da ido i s
in he ea ly 1970s o desc ibe ino ganic ma e i-
als ob ained om he chemical eac ion o some
alumino-silica e p ecu so s wi h alkali silica es,
yielding polyme ic Si-O-Al bonds (3). The ne wo k
consis s o SiO4 and AlO4 e ahed on linked al e -
na ely by sha ing he oxygens. S udies conduc ed
in ecen yea s ha e esul ed in he de elopmen o
geopolyme s based on na u al ma e ials (mainly
kaolini e and me akaolin) o indus ial was es, wi h
pa icula emphasis on pul e ised coal combus-
ion ly ash (4–7). The syn hesis o geopolyme s
using coal ash may in ol e en i onmen al bene i s
including a educ ion in he consump ion o na u al
esou ces and a dec ease in ne p oduc ion o CO2.
I is es ima ed ha he geopolyme cemen syn hesis
emi s 5–6 imes less CO2 han Po land cemen (8).
Geopolyme iza ion has become a p omising ech-
nology ha o e s a ac i e possibili ies o com-
me cial applica ions, i.e. as ha dening, high and
ea ly comp essi e s eng h, op imal acid esis ance
and long e m du abili y (9–11). Howe e , he pe -
o mance o he inal p oduc in hese applica ions
is hea ily elian on he physical p ope ies o geo-
polyme s and mo e speci ically, he mic os uc u e
and po osi y. P e ious s udies ca ied ou by o he
au ho s sugges ha op imal dosages, manu ac u -
ing and cu ing condi ions a e he key in ob aining
geopolyme bodies wi h low po osi y and long- e m
s abili y (7, 12). Po osi y and po e size dis ibu ion
a e he main p ope ies o analyse in o de o s udy
co osion in geopolyme ic conc e e since hey de e -
mine he pe meabili y o he geopolyme ic ma ix
(13).
Me cu y In usion Po osime y (MIP) is ex en-
si ely used ac oss a numbe o applica ions o
which po osi y is a c i ical pa ame e , includ-
ing cemen i ious ma e ials de i ed om Po land
cemen . Howe e , he li e a u e on conc e e and
mo a s sugges s ha MIP may no always be highly
eliable. Some esea che s ound ha he po es mea-
su ed by MIP applied on cemen i ious ma e ials
can be smalle han he po es obse ed wi h SEM
(14–16). Despi e hese limi a ions, he MIP ech-
nique is gene ally conside ed an app op ia e me hod
o po osi y analysis and a use ul ool o examine
op imal dosages and cu ing condi ions o compa a-
i e pu poses.
A g ea deal o po osime y da a ela ed o ly
ash-based geopolyme s can be ound in he li e a-
u e, howe e , many s udies o en p esen inconsis-
en indings o a e con adic o y. As a p e ious s ep
o he expe imen al esea ch desc ibed in his pape ,
a s a e o he a s udy on he subjec was conduc ed
in o de o ind he mos ele an ac o s a ec ing
ash geopolyme po osi y.
Some esea ch clea ly e idences ha he wa e o
solids a io (w/s) o he mix u es has an impo an
e ec on he po e size dis ibu ion o he cu ed geo-
polyme s. Thus, o he same condi ions, as he w/s
inc eases, po osi y also inc eases (14, 17). Wa e is
consumed only ma ginally du ing he alkali ac i a-
ion p ocess in ash geopolyme s (like in me akaolin
geopolyme s). Fo his eason, he olume ac ion
o he liquid ac i a o go e ns he inal open po os-
i y o he geopolyme s (18). This explains why he
o al po osi y o me akaolin based geopolyme s,
which equi e la ge quan i ies o ac i a o o ob ain
a wo kable pas e, is o en highe han ha o ly ash
based geopolyme s (19).
Cu ing empe a u e and cu ing ime also play
an impo an ole in he de ini ion o geopolyme
po osi y. In gene al, a sys ema ic inc ease in he
o al olume o po es is obse ed when he cu -
ing empe a u e inc eases. Howe e , he esul s a e
no so conclusi e wi h ega d o po e size a ia-
ion. Inconsis en esul s ha e also been ound in
ela ion o he a ia ion o po osi y and po e size
dis ibu ion du ing he cu ing ime. Thus, in gen-
e al, po osi y declines o e ime (20). Addi ionally,
a e inemen o po es has been desc ibed in some
cases, whe eas in o he mix u es he po e size dis i-
bu ions and he o al po osi y sca cely changed o e
ime (19, 21).
Ano he ac o wi h a g ea in luence on ash geo-
polyme po osi y is he p esence o blas u nace
slag (BFS) in he geopolyme composi ion, which is
associa ed wi h he ole o calcium in he geopoly-
me ic eac ion. Thus, i has been e i ied ha as he
amoun o BFS in he mix u e inc eases, eplacing
ly ash, geopolyme po osi y educes lineally and he
po e size dis ibu ion is shi ed o a mic o-po osi y
(22, 23).
The in luence o he na u e o he ac i a ing
solu ion on geopolyme po osi y has also been s ud-
ied, compa ing esul s ob ained wi h and wi hou
soluble silica e. Mo eo e , i has been p o ed ha an
inc ease o Na2O in sys ems ac i a ed wi h sodium
silica e and NaOH p oduces dense ma ices wi h
small po es, p obably due o he g ea de elopmen
o he geopolyme ic eac ion (24). The in luence o
he Si/Al a io has also been s udied, al hough no
always showing he same esul s (25, 26).
The e ec and ole o alkali me al (Na o K) used
o he syn hesis o geopolyme s ha e been a ma e
o discussion o a long ime (27–29). One o he
mo e epea edly s udied p ope ies is he s eng h
Con ibu ions o he s udy o po osi y in ly ash-based geopolyme s • 3
Ma e iales de Cons ucción 66 (324), Oc obe –Decembe 2016, e098. ISSN-L: 0465-2746. doi: h p://dx.doi.o g/10.3989/mc.2016.10215
achie ed by he geopolyme when NaOH, KOH,
Na2SiO3, K2SiO3 o mix u es M2SiO3+MOH (M:
Na o K) a e used as he ac i a ing solu ion, p e-
sen ing con adic o y esul s in many cases (30–32).
In ela ion o his, he use o sodium o po assium
silica es as alkaline ac i a o s is o special in e es
due o hei di e en ma ke p ices. O he au ho s
ha e concluded ha he use o po assium silica e
ins ead o sodium silica e imp o ed he mechanical
p ope ies o he inal p oduc s (33), which could
compensa e o i s highe cos .
The main con ibu ion o his pape ela es o
he de elopmen o a sys ema ic s udy, in ol ing
a se o pa ame e s which could po en ially ha e
an impac on geopolyme po osi y, i.e. he cu ing
empe a u e, ype o ac i a ing solu ion, inco po a-
ion o slag (Ca sou ce) o he mix u es and ype
o slag. Addi ionally, he comp essi e s eng h and
he deg ee o eac ion o he geopolyme specimens
ha e been measu ed and he possible co ela ions
among comp essi e s eng h, po osi y and deg ee
o eac ion ha e also been conside ed. Fu he mo e,
his esea ch has been comple ed wi h a mic os uc-
u al s udy o he ash geopolyme s manu ac u ed.
2. MATERIALS AND METHODS
2.1. Ma e ials
A low calcium ly ash (class F ASTM) om
one o he la ges coal powe s a ions in Sou he n
Spain (Los Ba ios, Cádiz (550 MW)) was used as
he main aluminosilica e sou ce. G ound g anu-
la ed blas u nace slag (BFS) and me allu gi-
cal slag (MS) we e used as calcium and silicon
addi ions and we e p o ided by Holcim S.A. and
Ace inox S.A., espec i ely. The chemical composi-
ion o hese cons i uen s (Table 1) was de e mined
a e chemical a ack and dissolu ion a 750 °C
(ASTM D-3682-78) (34) using a omic abso p ion
spec oscopy. The i eous phase o FA, BFS and
MS (Table 1) has been calcula ed as desc ibed in a
s udy by A juan (35). The aw ma e ials we e also
subjec ed o a deg ee o eac ion es (Table 1), a
me hod desc ibed by Fe nández-Jiménez (36, 37).
This pa ame e was calcula ed as he pe cen age
o soluble ma e ial a e a selec i e hyd ochlo ic
acid dissolu ion a ack o he samples las ing 3
hou s. The deg ee o eac ion gi es some idea o
he amoun o ly ash con e ed in geopolyme ic
cemen . This pa ame e was measu ed wi h a 0.5%
coe icien o a ia ion. The pa icle size dis ibu-
ion o aw ma e ials was de e mined by sie ing
and he esul s a e shown in Table 2. The D50 o
FA, BFS and MS ha e been calcula ed and a e 42.8
µm, 34.6 µm and 387 µm, espec i ely.
Di e en ac i a ing solu ions we e es ed. Two o
hem we e based exclusi ely on hyd oxides: (i) 8M
KOH and (ii) 8M NaOH, using sodium hyd oxide
and po assium hyd oxide pelle s wi h 99% and 85%
(w/w%) pu i y, espec i ely, and dis illed wa e . The
o he wo ac i a ing solu ions consis ed o a com-
bina ion o hyd oxide and soluble silica e i.e. (iii) a
mix u e o K-silica e (KSil) solu ion (SiO2 23 w/w%
and K2O 14.9 w/w%) (Indus ias Químicas del
Eb o) and enough KOH o inc ease he K2O/SiO2
mola a io up o 1.45 (pH o solu ion is 13.5); and
(i ) a mix u e o Na-silica e (NaSil) solu ion (SiO2
27 w/w% and Na2O 8 w/w%) (Me ck) and NaOH in
o de o inc ease he Na2O/SiO2 mola a io up o
0.98 (pH o solu ion is 13.5).
2.2. Geopolyme p epa a ion
The geopolyme mix u es we e p epa ed in a
common labo a o y mixe a 500 pm. Fi s ly, solid
ma e ials we e blended un il a homogeneous mix
was ob ained. Finally, an ac i a ing solu ion was
added o he solid mix u e un il a wo kable and
hixo opic pas e was achie ed. The mixing ime
was a ound 4 minu es. The eade can e e o Luna
e al. (38) o u he de ails. Rela i e p opo ions o
each componen in he geopolyme s a e de ailed in
Table 3. Cylind ical plas ic moulds (30 mm diam-
e e and 41 mm heigh ) we e illed wi h he pas e
ob ained. The pas es we e ib a ed o 5 minu es
Table 1. Chemical composi ion o aw ma e ials (w/w%)
Mois u e
105 °C
LOI
750 °C
Oxides Deg ee o
eac ion (%)
Vi eous
phase (%)Fe2O3CaO MgO SiO2Al2O3Na2O K2O
FA 0.05 3.32 5.86 3.94 1.84 63.9 21.5 0.68 1.67 17.7 72.3
BFS 0.24 0.31 0.49 45.8 6.52 32.1 15.0 0.32 0.26 96.1 57.1
MS 0.45 1.90 1.53 43.8 9.52 31.6 4.41 0.60 0.07 85.6 50.1
Table 2. Pa icle size dis ibu ion o aw ma e ials
A e age
diame e (μm)
Di e en ial pa icle size
dis ibu ion (w %)
FA BFS MS
> 400 1.21 - 48.5
225 5.53 - 30.3
112 9.82 1.44 15.6
59.5 5.42 6.86 3.52
22.5 78.0 91.7 2.11
4 • Y. Luna-Galiano e al.
Ma e iales de Cons ucción 66 (324), Oc obe –Decembe 2016, e098. ISSN-L: 0465-2746. doi: h p://dx.doi.o g/10.3989/mc.2016.10215
o elease en apped gas bubbles. The samples we e
cu ed in wo di e en condi ions: hal o he speci-
mens we e main ained a oom empe a u e and he
o he s we e cu ed in an o en a 60 ºC.
2.3. Physical and mechanical p ope ies o
geopolyme s
Comp essi e s eng h es s we e ca ied ou a e
7, 28 and 60 days o cu ing in acco dance wi h
ASTM C39 (39) using wo o h ee specimens o
each sample and a e aging he expe imen al alues
ob ained. The coe icien s o a ia ion we e <1%.
A Suzpeca , mod. MEM-102/50 , comp essi e
s eng h es ing ame was used. The mic os uc-
u e o he geopolyme s was s udied a e 7 days o
cu ing using a scanning elec on mic oscope (SEM-
FEG) Hi achi S-4800 coupled wi h an EDX de ec-
o . The deg ee o eac ion was also de e mined a e
7, 28 and 60 days o cu ing. The po osi y s udy was
ca ied ou a e 7 days o cu ing using an Au opo e
IV 9500 me cu y in usion po osime e a e d ying
he specimens a 60 ºC o 48 hou s. As he con ac
angle employed was 130º (as is usually adop ed o
e alua ing he po e s uc u e by me cu y pene a-
ion (40, 41)), he measu ed a e age po e sizes ange
heo e ically om 0.003-905 μm.
3. RESULTS
3.1. Geopolyme mic os uc u e
Fly ash based geopolyme s can be desc ibed
as a mul i-componen sys em ha consis s o (i)
a ela i ely dense packing o un eac i e pa icles
om s a ing ma e ials i.e. ly ash and slag, (ii)
an undi e en ia ed and homogeneous mass su -
ounding and cemen ing pa icles i.e. he geo-
polyme binde , and (iii) bo h in e g anula and
in ag anula po e ne wo ks. Sphe ical-shaped ly
ash pa icles a e p esen in all geopolyme sam-
ples cu ed a oom empe a u e, which sugges s
only pa ial ash dissolu ion o p oduce he geo-
polyme ma ix. The absence o ull dissolu ion
is consis en wi h low empe a u e ac i a ion and
has been epo ed by se e al au ho s (10). The
ela i e p opo ion o he geopolyme ma ix o e
he o he wo componen s (ash/slag and po osi y)
gi es an indica ion o he eac i i y o he sys-
em and he o e all deg ee o geopolyme iza ion.
Thus, geopolyme de elopmen seems o be lim-
i ed when hyd oxides a e used alone as ac i a o s,
ega dless o whe he hey a e NaOH (Figu e1b)
o KOH (Figu e 1c). In gene al, no signi ican
di e ences a e obse ed be ween he appea ance
o he wo geopolyme s om ha o he pa en
ly ash (Figu e 1a), exhibi ing limi ed ac i a ion
and low eac i i y. In gene al, poo de elopmen
o geopolyme binde is obse ed, wi h a highly
po ous mic os uc u e consis ing o ly ash ceno-
sphe es coa ed in a laye o a ine ma e ial com-
posed o Si and Al compounds.
The use o silica e as ac i a ing solu ion (wi h
alkali hyd oxide o inc ease he Na/Si a io) d a-
ma ically changes he mic os uc u e (Figu e 1d
and 1e). A subs an ial deg ee o geopolyme de el-
opmen is obse ed, wi h a la ge amoun o un e-
ac ed ly ash pa icles embedded in a homogeneous
amo phous ma ix, whils he majo i y o small
cenosphe es a e p esumably dissol ed and disap-
pea ed almos comple ely. The amo phous phase
is known as alkaline aluminium silica e hyd a e gel
(N-A-S-H gel, so-called since i is o med du ing
he ac i a ion wi h Na-silica e), a ich Si and Al
ma e ial p oduced du ing he eac ion be ween ly
ash and he ac i a ing solu ion. Figu e 1e shows a
cenosphe e enclosed in he ma ix and su ounded
by a laye o a di e en ma e ial in which mulli e is
de ec ed. Small laye s o sodium compounds and
Si and Ca ich a eas a e also obse ed. This sys em
appea s o be much less po ous han he hyd ox-
ide geopolyme s shown in Figu e 1b and 1c as he
newly o med homogeneous mass o geopolyme
binde illed in e s i ial po osi y. Howe e , pe sis-
en isola ed po osi y is p esen (Figu e 1d), which
is consis en wi h o he s udies (6, 12). Cenosphe es
accoun o mos isola ed esicles, bu andomly
dis ibu ed gas ca i ies a e also p esen .
Table 3. Rela i e p opo ion (by mass) o each componen o each se ies o geopolyme s
FA BFS MS NaSil NaOH KSil KOH
NaOH
solu ion 8M
KOH
solu ion 8M
NaOH 8M 1.4 - - - - - - 0.56 -
NaSil NaOH 1.4 - - 0.8 0.16 - - - -
NaSil NaOH BFS 1.4 0.3 - 0.8 0.16 - - - -
NaSil NaOH MS 1.4 - 0.3 0.8 0.16 - - - -
KOH 8M 1.4 - - - - - - 0.56
KSil KOH 1.4 - - - 0.8 0.42 - -
KSil KOH BFS 1.4 0.3 - - 0.8 0.42 - -
KSil KOH MS 1.4 - 0.3 -  0.8 0.42 - -
Con ibu ions o he s udy o po osi y in ly ash-based geopolyme s • 5
Ma e iales de Cons ucción 66 (324), Oc obe –Decembe 2016, e098. ISSN-L: 0465-2746. doi: h p://dx.doi.o g/10.3989/mc.2016.10215
As can be seen, he addi ion o slag o he mix-
u e does no signi ican ly change he mic os uc u e
(Figu e 1 , 1g and 1h). A well-de eloped geopoly-
me ic ma ix is obse ed, al hough his seems o be
less homogeneous and uni o m han ha obse ed
when no slag is used (Figu e 1d and 1e). The p es-
ence o lamina s uc u es ich in Ca, Mg and Si is
dis inc i e o MS-con aining geopolyme s. The gel is
coa ed in a c ys alline laye o med by Si and O (sil-
ica), which is p e ailing in MS-geopolyme s, and by
pa icles con aining Ca and Si in BFS-geopolyme s.
In Figu e 1h, he geopolyme ic gel can be clea ly
obse ed. Hollow cenosphe es wi h geopolyme ic
gel illing in ag anula po es p o ide e idence o
pa ial dissolu ion aking place du ing he alkaline
a ack. This enables he en y and g ow h o he
newly o med geopolyme binde wi hin he ash
pa icle. Fu he e idence o he ex en o ac i a-
ion is he in e nal im o he cenosphe e (Figu e
1i), which is an a acked zone o amo phous na u e,
ich in Al, K and Si (gel K-A-S-H o med du ing he
ac i a ion o he ma e ial wi h K-silica e).
3.2. Deg ee o eac ion
The de e mina ion o he deg ee o eac ion was
ca ied ou in all geopolyme samples. Figu es 2a
and 2b show he alues ob ained a e 7, 28 and
60 days o cu ing a oom empe a u e (25 ºC) and
60ºC, espec i ely.
As can be seen, K-silica e geopolyme s p esen
highe deg ees o eac ion han Na-silica e geopoly-
me s. The wo silica e solu ions (K o Na) p esen
simila pH alues, bu he concen a ion o alkali
is di e en in he solu ions: SiO2/K2O mola a io
o 0.689 in K-silica e (KSil) and SiO2/Na2O mola
a io o 1,02 in Na-silica e (NaSil). As ema ked
by Duxson (27), solu ions wi h low concen a-
ions o silicon imp o e he a e o mass anspo
and p omo e dissolu ion in he geopolyme iza ion
eac ion, so he pe cen age o un eac ed ma e ial
is low. The e o e, he low Si/K a io could explain
he la ge amoun o geopolyme ic gel shown by he
K-silica e sys ems. Sys ems wi h hyd oxide show he
same beha iou : KOH geopolyme s always display
a highe deg ee o eac ion alues han NaOH geo-
polyme s. Sindhuna a (9) men ioned ha sys ems
ac i a ed wi h K we e mo e e icien in he dissolu-
ion s age o he aluminosilica e han sys ems ac i-
a ed wi h Na, bu did no conclude i K-sys ems
we e mo e eac i e han Na-sys ems. In he ecen
wo k, i has been con i med ha he a ack wi h HCl
dissol es mo e amoun o gel in he K-geopolyme s
han in Na-geopolyme s.
An inc ease in eac i i y is also clea ly app ecia-
ble in geopolyme s con aining slag. Na-silica e ly
ash based geopolyme cu ed a oom empe a u e
o 7 days shows a deg ee o eac ion o 46%, which
indica es ha 54% is un eac ed ly ash. The addi-
ion o BFS o he mix u e p oduces an inc ease
Figu e 1. SEM Images o a) Fly ash, b) NaOH-geopolyme , c) KOH-geopolyme , d) and e) NaSil NaOH-geopolyme s, ) NaSil
NaOH MS-geopolyme , g) and h) NaSil NaOH BFS-geopolyme s, i) KSil KOH BFS-geopolyme .

6 • Y. Luna-Galiano e al.
Ma e iales de Cons ucción 66 (324), Oc obe –Decembe 2016, e098. ISSN-L: 0465-2746. doi: h p://dx.doi.o g/10.3989/mc.2016.10215
in deg ee o eac ion om 46% o 57%. This inc e-
men could be due o he di e en eac i i y o aw
ma e ials when hey a e subjec ed o he HCl a ack
(Table 1), slags eac i i y (BFS 96.1% and MS
85.6%) is highe han ly ash eac i i y (17.1%). In
addi ion, pa icle size dis ibu ion could also a ec
his, mainly in BFS-geopolyme s, since BFS con-
ains he ines pa icle size (Table 2). The use o he
me allu gical slag (MS) also p oduces an inc ease
in he geopolyme ic gel, bu his inc emen is lowe
han ha obse ed in he BFS-geopolyme s, pos-
sibly due o he lowes eac i i y and he highes
po e size dis ibu ion o MS. Howe e , he deg ee
o eac ion alue o 46% in he ly ash based geo-
polyme canno be explained only by hese wo
pa ame e s ( eac i i y o po e size dis ibu ion).
The i eous phase pe cen age, ano he a iable
ha a ec s he ac i a ion p ocess, should also be
aken in o accoun . An accele a ion o he geopo-
lyme iza ion eac ion and an inc ease in he deg ee
o eac ion a e obse ed in geopolyme s based on
a high pe cen age o i eous phase aw ma e ials
(42). As can be seen in Table 1, he ly ash i e-
ous phase (72.3%) is highe han he co esponding
alue o BFS (57.1%) and MS (50.1%). Examining
he esul s o Figu e 2a and 2b, i seems ha he
i eous phase is he pa ame e ha con ols he
deg ee o eac ion in he ac i a ion o ly ashes, bu
he eac i i y and pa icle size dis ibu ion mus
also be aken in o accoun in eac ions wi h slags. In
any case, he p esence o slag esul s in a high in e -
connec ed gel-like ne wo k wi h p ecipi a es o cal-
cium silica e hyd a e gel, which can emb ace a small
pe cen age o aluminium in i s s uc u e, o ming
calcium aluminium silica e hyd a e gel (43), which
ac s as nuclea ion si es (44, 45).
Figu e 2a. Deg ee o eac ion (%) o geopolyme s cu ed a oom empe a u e.
Figu e 2b. Deg ee o eac ion (%) o geopolyme s cu ed a 60 ºC.
Con ibu ions o he s udy o po osi y in ly ash-based geopolyme s • 7
Ma e iales de Cons ucción 66 (324), Oc obe –Decembe 2016, e098. ISSN-L: 0465-2746. doi: h p://dx.doi.o g/10.3989/mc.2016.10215
Rega ding he cu ing empe a u e, in all he
cases he highes deg ees o eac ion a e ob ained
when geopolyme s a e cu ed a 60 ºC. As i is well
known, he mal ac i a ion could accele a e geo-
polime iza ion eac ions (46) and could cause an
inc ease in he ex en o eac ion. The deg ee o
eac ion also changes o e he ime s udied (7-60
days). The alues inc ease up o 9%, especially in
ce ain Na-geopolyme s, bu his inc ease is no as
impo an as he inc ease which occu ed in he i s
s ages. Howe e , he deg ee o eac ion should con-
inuously g ow un il he o al a ailable ma e ial is
consumed by he ac i a ing solu ion.
3.3. Po osi y
Po osi y o geopolyme s measu ed as o al me -
cu y in usion olume anged om 0.0766 mL∙g−1
o 0.202 mL∙g−1. A e age po e diame e s a ied o e
he ange 0.03–0.99 µm (Table 4).
As an icipa ed wi h SEM obse a ions in Figu e
1b and c, geopolyme s ac i a ed exclusi ely wi h
hyd oxides p esen a mic os uc u e o a non- eac i e
sys em, wi h ly ash pa icles loosely connec ed, so
he emp y space be ween pa icles is highe han in
eac i e sys ems. The e o e, he po e olume and he
po e diame e in hyd oxide-ac i a ed geopolyme s
a e g ea e han in silica e–ac i a ed geopolyme s.
Fo a gi en composi ion and cu ing empe a u e,
geopolyme s p oduced wi h Na-ac i a o s a e mo e
po ous and p esen highe a e age po e diame e s
han hose wi h K-ac i a o s. Lloyd e al. (14) ound
ha geopolyme s ob ained using K o Na+K ac i-
a o s we e mo e po ous han hose manu ac u ed
using Na-ac i a o s, al hough he au ho s did no
ind a clea explana ion o his. Con e sely, K i en
and Bell (47) s udied me akaolin based geopolyme s
ac i a ed wi h sodium silica e con aining NaOH,
KOH o mix u es o bo h in di e en p opo ions
and ound ha he mean po e size dec eases almos
mono onically as sodium was p og essi ely eplaced
by po assium in he geopolyme mix u es. In he
cu en wo k, K-silica e geopolyme s show a highe
deg ee o eac ion han Na-silica e geopolyme s
(Figu e 2a o 2b), so i is possible ha sys ems wi h
a high ex en o eac ion ha e a la ge amoun o
geopolyme ic gel, also p esen small spaces be ween
un eac i e pa icles and he e o e a low po osi y.
The addi ion o BFS is ound o exe an in lu-
ence on po osi y. Fo a gi en dosage and cu ing em-
pe a u e, geopolyme s p epa ed wi h BFS a e less
po ous han geopolyme s wi hou BFS. This is in
line wi h he indings o P o is e al. (22) who ound
ha he use o BFS ins ead o ly ash in geopolyme s
leads o a dec ease in po osi y, possibly due o he
di e en chemis y exis ing in ly ash and BFS geo-
polyme s. Puligilla and Mondal (48) p oposed ha
he simul aneous o ma ion o he geopolyme ic
gel and he calcium aluminium silica e hyd a e gel
( o med in p esence o Ca) may help o ill he gaps
be ween he di e en hyd a ed phases and he un e-
ac ed pa icles, esul ing in a mo e dense and homo-
geneous ma ix. Howe e , he e a e no ho ough
conclusions abou he con ibu ion o he di e en
gels, so o he pa ame e s should p obably be aken
in o accoun . This gene al end is no obse ed o
MS geopolyme s, since he po osi y alues change
e y ma ginally, because MS slag has a highe a e -
age pa icle size (Table 2) and a lowe eac i i y han
BFS slag (Table 1).
Geopolyme po osi y is la gely a ec ed by he
he mal ac i a ion du ing cu ing. In gene al, geo-
polyme s cu ed a 60 °C show lowe a e age po e
sizes and highe o al po e olumes han hose
cu ed a oom empe a u e. O he au ho s epo ed
a simila e ec (19, 49, 50), concluding ha mos
o he po osi y o geopolyme s cu ed a a high
empe a u e was associa ed wi h small po es (less
han 0.01 µm), while geopolyme s cu ed a oom
empe a u e we e domina ed by po es in he ange
0.1-1 µm. Two e ec s mus be aken in o accoun
in o de o explain his beha iou . The i s is he
e ec o empe a u e on he deg ee o eac ion.
Samples cu ed a oom empe a u e show he low-
es deg ee o eac ion (Figu e 2a and 2b), which
could be ela ed o he high po osi y o he geo-
polyme s cu ed a oom empe a u e. The second
has o do wi h he syne esis phenomenon, which
Table 4. Po osi y (as o al in usion olume) and a e age po e diame e o geopolyme s

To al in usion
olume (mL∙g−1)
A e age po e
diame e (µm) 
To al in usion
olume (mL∙g−1)
A e age po e
diame e (µm)
NaOH 8M AMB 0.165 0.246 KOH 8M AMB 0.135 0.118
NaOH 8M 60 ºC 0.185 0.999 KOH 8M 60 ºC 0.144 0.296
NaSil NaOH AMB 0.095 0.077 KSil KOH AMB 0.090 0.028
NaSil NaOH 60 ºC 0.099 0.056 KSil KOH 60 ºC 0.093 0.089
NaSil NaOH BFS AMB 0.077 0.053 KSil KOH BFS AMB 0.059 0.059
NaSil NaOH BFS 60 ºC 0.094 0.042 KSil KOH BFS 60 ºC 0.074 0.061
NaSil NaOH MS AMB 0.131 0.066 KSil KOH MS AMB 0.107 0.066
NaSil NaOH MS 60 ºC 0.202 0.047 KSil KOH MS 60 ºC 0.171 0.041
8 • Y. Luna-Galiano e al.
Ma e iales de Cons ucción 66 (324), Oc obe –Decembe 2016, e098. ISSN-L: 0465-2746. doi: h p://dx.doi.o g/10.3989/mc.2016.10215
p o okes he expulsion o apped wa e du -
ing con ac ion o he gel and subsequen po e
gene a ion.
To imp o e ou unde s anding o how empe a-
u e and he ac i a ing solu ion a ec he po osi y,
a po e size dis ibu ion (PSD) s udy (in usion Log-
di e en ial olume (mL∙g−1) e sus po e diame e
(µm)) was ca ied ou (Figu es 3).
When cemen pas es a e s udied using he MIP
echnique, wo di e en peaks co esponding o he
gel po es and capilla y po es appea in he po e size
dis ibu ion. The small po es a e o med du ing he
calcium silica e hyd a e o ma ion and he po e size
diame e anges om 0.0005 o 0.01 µm. The capil-
la y po es e e o he space le by he wa e ha
does no eac du ing he hyd a ion o cemen and
ange om 0.01 o 10 µm (51).
Geopolyme s p epa ed wi h hyd oxide (Na o K)
as ac i a o (Figu e 3a) p esen he same po e size
dis ibu ion, wi h only one peak a ound 1 µm ( ange
o capilla y po es). In he SEM images (Figu e 1b
and 1c) poo de elopmen o he geopolyme gel
is obse ed, so hese capilla y po es a e possibly
o med by he space be ween un eac ed ly ash pa -
icles. Cu ing a 60 ºC p oduces he same dis ibu-
ion, wi h a sligh inc ease in peak wid h.
Silica e-geopolyme s (Figu e 3b) show simila
po e size dis ibu ion o hyd oxide-geopolyme s
(Figu e 3a), wi h one peak in a po e size o 0.5-0.7µm,
less han 1 µm (hyd oxide-geopolyme s). The me -
cu y in usion olume (a ea unde he cu e) is
lowe oo, in acco dance wi h he highe amoun
o he geopolyme ic gel and i s subsequen oid
illing e ec as silica es a e used ins ead o hyd ox-
ides. The di e ences be ween he use o K-silica es
and Na-silica e a e small. The K-silica e solu ion
only p oduces a sligh shi o he lowes po e sizes,
possibly due o he ex en o eac ion in K-silica e
sys ems, which is sligh ly highe han in Na-silica e
(Figu e 2a o 2b).
The addi ion o blas u nace slag (Figu e 3c)
esul s in a wide po e size dis ibu ion, showing a
ce ain bimodali y, wi h a signi ican peak in he
capilla y ange (0.1-10 µm) and an inc ease in po e
olume in he zone o gel po es (0.005-0.1 µm),
which is in acco dance wi h he highe deg ee o
eac ions o mix u es wi h BFS.
In gene al, Table 4 shows ha he e ec o he
alkali elemen is mo e clea ly app eciable when slag
is added o he mix u e. In o de o unde s and he
e ec o alkali, he cumula i e po e a ea e sus he
po e size diame e o K-silica e and Na-silica e geo-
polyme s wi h BFS o bo h cu ing condi ions a e
ep esen ed (Figu e 4a and 4b).
Figu e 4a ep esen s he cumula i e po e a ea
o geopolyme s con aining BFS wi h bo h silica es
and bo h cu ing condi ions. A oom empe a u e,
cu es o bo h alkalis p esen a pla eau in a simila
po e size (0.015-0.004 µm) ollowed by an inc ease in
a ea up o 0.005 µm. Inc easing he empe a u e, an
ele a ed inc emen o po e a ea is obse ed in po es
less han 0.01 µm. The only di e ence be ween he
use o Na and K is ha he a ea unde he cu e is
g ea e in Na-silica e han in K-silica e geopolyme .
Subs an ial change was obse ed in o al po e
a ea wi h a ia ion in cu ing empe a u es, inc eas-
ing he o al a ea wi h he he mal ac i a ion a
60ºC combined wi h a gene al shi o he mic o-
po e zone (less han 0.01 µm).
Figu e 4b shows he cumula i e po e a ea o
K-silica e geopolyme s wi h o wi hou slags. The
h ee cu es p esen a pla eau. This pla eau is
shi ed o he smalle po es in BFS-geopolyme s
and shi ed o he bigge po es in MS-geopolyme s.
Geopolyme s wi hou slag show in e media e
beha iou . The use o MS p oduces geopolyme s
Figu e 3a. Po e size dis ibu ion (In usion Log di e en ial (mL∙g−1) s Po e size diame e ) o hyd oxide-geopolyme s.
Con ibu ions o he s udy o po osi y in ly ash-based geopolyme s • 9
Ma e iales de Cons ucción 66 (324), Oc obe –Decembe 2016, e098. ISSN-L: 0465-2746. doi: h p://dx.doi.o g/10.3989/mc.2016.10215
wi h a g ea e po e a ea, possibly due o i s coa se
po e size dis ibu ion.
3.4. Comp essi e s eng h
Comp essi e s eng h esul s o geopolyme s
a e 7, 28 and 60 days o cu ing a oom empe a u e
and 60 ºC a e shown in Figu e 5 (Na-geopolyme s)
and Figu e 5b (K-geopolyme s).
The in luence o he ype o alkali in geopoly-
me ic ac i a o s has been widely discussed in he
li e a u e wi h some con o e sy since some au ho s
show he posi i e e ec o Na and o he s o K. In he
cu en wo k, he comp essi e s eng hs a e highe
in geopolyme s p epa ed wi h sodium ac i a o s
han in hose p epa ed wi h po assium ac i a o s,
using ei he hyd oxide o silica e. These esul s a e
di icul o explain compa ing he alues o deg ee
o eac ion and he po osi y (calcula ed a 7 days)
since K-silica e geopolyme s show a highe deg ee
o eac ion and lowe po osi ies han Na-silica e
geopolyme s. On he one hand, K is mo e alkaline
han Na, so he dissolu ion a e o aluminosilica es
is highe (9) and he deg ee o eac ion and he
amoun o geopolyme ic gel (wi h lowe po osi-
ies) a e g ea e . On he o he hand, he size o K
is la ge han Na, so K could hinde he e olu ion
p ocess o he p ezeoli ic gel (52). Mo eo e , he
K-A-S-H gel is mo e chemically uns able han he
N-A-S-H gel (52), which may explain he lowe al-
ues o comp essi e s eng h o K-geopolyme s han
Na-geopolyme s. In any case, in o de o unde -
s and he esul s, he a io M2O/SiO2 (whe e M is
Na o K) in he ac i a ing solu ion mus be aken
in o accoun . Acco ding o some au ho s, he e is an
op imum M2O/SiO2 a io o maximum comp essi e
Figu e 3b. Po e size dis ibu ion (In usion Log di e en ial (mL∙g−1) s Po e size diame e ) o silica e-geopolyme s.
Figu e 3c. Po e size dis ibu ion (In usion Log di e en ial (mL∙g−1) s Po e size diame e ) o silica e+BFS-geopolyme s.