Ci a ion: Padilla-Encinas, P.; Palomo,
A.; Fe nández-Jiménez, A. E ec o
Na e sus Ca Sul a e Sal s on he
Hyd a ion o Calcium
Sul oalumina e Clinke . Molecules
2023,28, 7651. h ps://doi.o g/
10.3390/molecules28227651
Academic Edi o : Hom Na h Dhakal
Recei ed: 7 Oc obe 2023
Re ised: 8 No embe 2023
Accep ed: 16 No embe 2023
Published: 18 No embe 2023
Copy igh : © 2023 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license (h ps://
c ea i ecommons.o g/licenses/by/
4.0/).
molecules
A icle
E ec o Na e sus Ca Sul a e Sal s on he Hyd a ion
o Calcium Sul oalumina e Clinke
Pila Padilla-Encinas, Angel Palomo and Ana Fe nández-Jiménez *
Edua do To oja Ins i u e o Cons uc ion Science, Na ional Resea ch Council (iETcc-CSIC), 28033 Mad id, Spain;
[email p o ec ed] (P.P.-E.); [email p o ec ed] (A.P.)
*Co espondence: [email p o ec ed]
Abs ac :
This pape examined how he amoun (5% o 20%) and ype (CaSO
4
o Na
2
SO
4
) o sulpha e
sal a ec he hyd a ion o calcium sulphoalumina e clinke (KCSA). The mechanical beha io o
he pas es was de e mined a 1, 3, 28, and 90 days, he hea low and o al hea we e measu ed
wi h iso he mal conduc ion calo ime y, and he eac ion p oduc s we e cha ac e ized using X- ay
di ac ion (XRD), di e en ial he mal analysis/ he mog a ime y (DTA/TG) and scanning elec on
mic oscopy (SEM). The esul s ob ained indica ed ha bo h he amoun o sulpha e sal (5% o 20%)
and i s ype (CaSO
4
o Na
2
SO
4
) a ec he hyd a ion kine ics, ype o eac ion p oduc s o med,
and de elopmen o mechanical s eng h. The inco po a ion o CaSO
4
has a posi i e e ec on he
de elopmen o he mechanical s eng h o KCSA. Howe e , Na
2
SO
4
can also p oduce ad e se side
e ec s a olde ages. The p esence o Na
2
SO
4
inc eases pH alues, which pa ly des abilizes he
e ingi e o med, he eby a o ing ca bona ion and hena di e p ecipi a ion, which can cause he
specimens o c ack and b eak.
Keywo ds: calcium sul oalumina e clinke ; alkaline sal s; mic os uc u e; calo ime y
1. In oduc ion
The p esence o a high alkali con en du ing KCSA hyd a ion has a nega i e e ec on
he de elopmen o mechanical s eng h. Con e sely, a ela i ely low alkali con en (0.1, 1
M) can ha e a posi i e e ec . This pape in es iga es how he p esence o solid chemical
alkali ac i a o s (Na
2
SO
4
ype sal s) a ec s KCSA hyd a ion. The in e es in knowing how
his sal a ec s KCSA hyd a ion s ems om he possibili y o p oducing hyb id alkaline
cemen s (HACs) [
1
–
3
], in which hese ypes o sal s a e used o inc ease he eac i i y o he
mine al addi ions ( ly ash, slag) [4–6].
Du ing he p oduc ion o Po land-cemen -based HACs, his neu al sal (Na
2
SO
4
)
in e ac s wi h he Ca(OH)
2
(p oduced by hyd a ion o he Po land cemen ) o gene a e
alkalini y in si u (see Equa ion (1)), which inc eases he eac i i y o he mine al addi ions.
Ca(OH)2+ Na2SO4+2H2O→CaSO4·2 H2O (s) + 2 Na+OH−(aq) (1)
In he hyd a ion o KCSA o calcium sul oalumina e (CSA), he o ma ion o Ca(OH)
2
is no a o ed, so we assume ha his eac ion does no ake place. The e o e, i is impo an
o know how he p esence o Na2SO4can a ec KCSA hyd a ion.
Many pape s [
7
–
10
] in he li e a u e ha e s udied he e ec o calcium sul a e on KCSA
hyd a ion. The p esence o calcium sul a e (anhyd i e o gypsum) leads o he o ma ion
o e ingi e and aluminum hyd oxide. Thus, i is es ima ed ha 15–25% by mass is he
app op ia e amoun o gypsum and/o anhyd i e o be added o calcium sul oalumina e
clinke o op imize he se ing ime, de elopmen o mechanical s eng h and olume
s abiliza ion and o ob ain a CSA.
In he hyd a ion p ocess o KCSA wi h calcium sul a e, Jansen e al. [
11
] dis inguished
he o ma ion o wo di e en ypes o e ingi e: (i) i s -gene a ion e ingi e, oge he wi h
Molecules 2023,28, 7651. h ps://doi.o g/10.3390/molecules28227651 h ps://www.mdpi.com/jou nal/molecules
Molecules 2023,28, 7651 2 o 20
o he compounds such as CAH
10
, amo phous AH
3
and monosul a e, which co esponded
o he i s peak and induc ion pe iod in he hea low cu es o iso he mal conduc ion
calo ime y; and (ii) second-gene a ion e ingi e, oge he wi h a mic ocys alline AH
3
,
which co esponded o he main peak in he hea low cu es o iso he mal conduc ion
calo ime y. The p esence o di e en shoulde s in he hea low cu es was a ibu ed
by he au ho s o he ec ys alliza ion p ocesses o mic oc ys alline AH
3
and he inal
adjus men o he wa e con en o he aluminum hyd oxide owa ds AH3.
Howe e , e y ew s udies ha e in es iga ed he e ec o Na
2
SO
4
on he hyd a ion o
KCSA o CSA. Sánchez-He e o e al. [
12
] analyzed he e ec o adding 4% Na
2
SO
4
and
8% Na
2
SO
4
on he hyd a ion o C
3
A. They obse ed ha he addi ion Na
2
SO
4
a o ed
he p ecipi a ion o he U-phase (C
4
A
s
NH
16
), which is mo phologically simila o calcium
monosul oalumina e. The same au ho s [
13
] s udied he e ec o 4% Na
2
SO
4
on he
hyd a ion o syn he ic ye’elimi e. They obse ed he o ma ion o he U-phase, which
sligh ly a o s he de elopmen o mechanical s eng h since he U-phase has he capaci y
o densi y he ma e ial. Rega ding he e ec o hese sal s on he hyd a ion o CSA, Palou
e al. [
14
] obse ed ha he hyd a ion o CSA is as e in he p esence o 1, 2, and 3% Na
2
SO
4
han wi h wa e . This beha io is associa ed wi h he o ma ion o a p ima y e ingi e,
p obably on he su ace o he ye’elimi e. Du lo e al. [
15
] s udied he e ec o 5% Na
2
SO
4
on he hyd a ion o CSA cemen . They obse ed wo opposi e e ec s: in he sho e m,
he hyd a ion o ye’elimi e is accele a ed, while in he long e m, he e is ep ecipi a ion o
hena di e, p ecipi a ion o gypsum and ca bona ion o he e ingi e.
To de e mine he e ec o Na
2
SO
4
on he hyd a ion o KCSA, wo doses o his sal
(5% and 20%) ha e been selec ed o his s udy. The i s dose (5%) is he dosage no mally
used in Po land-cemen -based HACs [
16
–
18
] and he second (20%) is chosen o enable
compa ison wi h he CaSO
4
dosage used in CSAs [
9
,
10
]. CaSO
4
is used a he same dosage
(5% and 20%) o p o ide a e e ence, and some o he es s also included comme cial CSA
cemen as a con ol ma e ial.
2. Resul s
2.1. Iso he mal Conduc ion Calo ime y
The hyd a ion kine ics o he calcium sul oalumina e clinke we e analyzed using
iso he mal conduc ion calo ime y in he p esence o 5% and 20% Na
2
SO
4
and CaSO
4
( e e ence). This sec ion also includes he hyd a ion o a comme cial CSA ha should be
conside ed a con ol ma e ial. The liquid (wa e )/solid a io used in all cases was 0.5. The
hea elease a e esul s a e shown in Figu e 1.
The hea elease a e cu e o he wa e -hyd a ed KCSA pas e (KCSA-H) shows ou
peaks, occu ing, espec i ely, a 30 min, 1.4 h, 6.5 h, and 11 h (see Figu e 1). The cu e o
he wa e -hyd a ed cemen (CSA-H) also shows ou peaks [
15
]. The i s o hese, which
occu s a 30 min, is common o all he pas es. A e ha , wo peaks and one shoulde a e
o med, eaching hei highes alues a 1.7 h, 3.4 h, and 5.3 h, espec i ely. Compa ison o
he wo pas es (KCSA-H and CSA-H) shows ha cemen hyd a ion gene a es ema kable
hea elease o app oxima ely 20 h; du ing KCSA hyd a ion, he hea elease con inues o
a ound 30 h. While no induc ion pe iod is e iden in he cemen (CSA-H), he clinke pas e
(KCSA-H) p esen s an induc ion pe iod o 3 h.
When 5% CaSO
4
is added o he KCSA (KCSA-5CS), i e peaks a e o med (Figu e 1a).
The i s o hese occu s a 30 min, ollowed by wo peaks a 1.4 h and 4.1 h. A 13 h induc ion
pe iod is ollowed by one peak (22 h) and one shoulde (28 h). Inc easing he calcium
sul a e concen a ion o 20% (KCSA-20CS) esul s in h ee peaks and one shoulde . The
i s is common o all he p e ious pas es and is ollowed by wo peaks ha each hei
maximum alues a 1.5 h and 2.2 h, espec i ely. A e ha , one shoulde is o med a 4 h.
When compa ing he hea elease a e cu es o he ou pas es (CSA-H, KCSA-H,
KCSA-5CS, KCSA-20CS), he main di e ence lies in he p esence o an induc ion pe iod
be o e he hi d and ou h peaks in he pas es ha ha e ei he ze o calcium sul a e (KCSA-
H) o a low calcium sul a e con en (KCSA-5CS). Howe e , all ou pas es ha e a i s peak
Molecules 2023,28, 7651 3 o 20
ha appea s a a ound 30 min, ollowed by a second peak ha also appea s a simila imes
in all o hem (be ween 1.4 and 1.7 h). These esul s co obo a e hose ob ained by o he
au ho s [
9
,
10
], who indica e ha he p esence o CaSO
4
con en close o he s oichiome ic
a io (20–22%) accele a es he hyd a ion eac ion o he KCSA.
Molecules 2023, 28, x FOR PEER REVIEW 3 o 21
pas es (KCSA-H and CSA-H) shows ha cemen hyd a ion gene a es ema kable hea elease
o app oxima ely 20 h; du ing KCSA hyd a ion, he hea elease con inues o a ound 30 h.
While no induc ion pe iod is e iden in he cemen (CSA-H), he clinke pas e (KCSA-H) p e-
sen s an induc ion pe iod o 3 h.
When 5% CaSO4 is added o he KCSA (KCSA-5CS), i e peaks a e o med (Figu e 1a).
The i s o hese occu s a 30 min, ollowed by wo peaks a 1.4 h and 4.1 h. A 13 h induc ion
pe iod is ollowed by one peak (22 h) and one shoulde (28 h). Inc easing he calcium sul a e
concen a ion o 20% (KCSA-20CS) esul s in h ee peaks and one shoulde . The i s is com-
mon o all he p e ious pas es and is ollowed by wo peaks ha each hei maximum alues
a 1.5 h and 2.2 h, espec i ely. A e ha , one shoulde is o med a 4 h.
When compa ing he hea elease a e cu es o he ou pas es (CSA-H, KCSA-H,
KCSA-5CS, KCSA-20CS), he main di e ence lies in he p esence o an induc ion pe iod be o e
he hi d and ou h peaks in he pas es ha ha e ei he ze o calcium sul a e (KCSA-H) o a
low calcium sul a e con en (KCSA-5CS). Howe e , all ou pas es ha e a i s peak ha ap-
pea s a a ound 30 min, ollowed by a second peak ha also appea s a simila imes in all o
hem (be ween 1.4 and 1.7 h). These esul s co obo a e hose ob ained by o he au ho s [9,10],
who indica e ha he p esence o CaSO4 con en close o he s oichiome ic a io (20–22%) ac-
cele a es he hyd a ion eac ion o he KCSA.
When sodium sul a e is added ins ead o calcium sul a e, ou signals a e ob ained o
he KCSA-5NS pas e, and i e a e ob ained o he KCSA-20NS pas e (Figu e 1b). The i s wo
peaks eme ge a simila imes o hose in he pas es hyd a ed wi h wa e (a e 30 min and
be ween 1.4 and 1.8 h). In he pas e con aining 20% Na2SO4 (KCSA-20NS), he peak o med a
1.8 h is ollowed by a small shoulde a 2.9 h. The hyd a ion p ocess o he pas es con aining
he sal is hen slowed by a long induc ion pe iod, which is g ea es in he KCSA-5NS pas e.
The las peaks appea a 15 h and 25.7 h in he KCSA-5NS and a 10 h and 13 h in he KCSA-
20NS.
These esul s show ha he sul a e ype and concen a ion signi ican ly a ec he induc-
ion pe iod and he hi d and ou h peaks o he hea elease a e cu es. Rela i ely low sul a e
sal concen a ions delay he hi d and ou h signals while high concen a ions accele a e
hem. The i s and second peaks, howe e , which, acco ding o he li e a u e [11,19], a e as-
socia ed wi h he o ma ion o a p ima y e ingi e, do no appea o be se iously a ec ed.
Figu e 1. (a) Hea elease a e o he pas es con aining 5% and 20% CaSO4; (b) hea elease a e o
he pas es con aining 5% and 20% Na2SO4.
Figu e 1.
(
a
) Hea elease a e o he pas es con aining 5% and 20% CaSO
4
; (
b
) hea elease a e o he
pas es con aining 5% and 20% Na2SO4.
When sodium sul a e is added ins ead o calcium sul a e, ou signals a e ob ained
o he KCSA-5NS pas e, and i e a e ob ained o he KCSA-20NS pas e (Figu e 1b). The
i s wo peaks eme ge a simila imes o hose in he pas es hyd a ed wi h wa e (a e
30 min and be ween 1.4 and 1.8 h). In he pas e con aining 20% Na
2
SO
4
(KCSA-20NS), he
peak o med a 1.8 h is ollowed by a small shoulde a 2.9 h. The hyd a ion p ocess o he
pas es con aining he sal is hen slowed by a long induc ion pe iod, which is g ea es in
he KCSA-5NS pas e. The las peaks appea a 15 h and 25.7 h in he KCSA-5NS and a 10 h
and 13 h in he KCSA-20NS.
These esul s show ha he sul a e ype and concen a ion signi ican ly a ec he
induc ion pe iod and he hi d and ou h peaks o he hea elease a e cu es. Rela i ely
low sul a e sal concen a ions delay he hi d and ou h signals while high concen a-
ions accele a e hem. The i s and second peaks, howe e , which, acco ding o he
li e a u e [11,19]
, a e associa ed wi h he o ma ion o a p ima y e ingi e, do no appea
o be se iously a ec ed.
Figu e 2shows he o al hea as a unc ion o ime. I demons a es ha o he
i s wo hou s, all he sys ems sha e e y simila cu es. O e longe ime pe iods, he
wa e -hyd a ed clinke wi h no added sul a es (KCSA-H) eleases he mos o al hea .
The KCSA-5CS and KCSA-5NS pas es elease he leas ini ial hea due o hei delayed
eac ion a e, bu a ad anced ages, he hea eleased, al hough less han ha eleased by he
KCSA-H, is highe han ha eleased by he pas es wi h a 20% sul a e con en (KCSA-20CS
and KCSA-20NS). The samples wi h he g ea es calcium sul a e con en s (CSA-H and
KCSA-20CS) elease he mos hea ini ially, which is consis en wi h he accele a ion o he
eac ion p ocess obse ed in Figu e 1. Al hough he KCSA-20NS pas e ini ially eleases
sligh ly less hea han he o he pas es, i beha es simila ly o hem a ad anced ages. The
lowe amoun s o hea eleased by hese samples (CSA-H, KCSA-20CS, and KCSA-20NS)
a ad anced ages may be pa ly due o a dilu ion e ec .
Molecules 2023,28, 7651 4 o 20
Molecules 2023, 28, x FOR PEER REVIEW 4 o 21
Figu e 2 shows he o al hea as a unc ion o ime. I demons a es ha o he i s
wo hou s, all he sys ems sha e e y simila cu es. O e longe ime pe iods, he wa e -
hyd a ed clinke wi h no added sul a es (KCSA-H) eleases he mos o al hea . The
KCSA-5CS and KCSA-5NS pas es elease he leas ini ial hea due o hei delayed eac ion
a e, bu a ad anced ages, he hea eleased, al hough less han ha eleased by he KCSA-
H, is highe han ha eleased by he pas es wi h a 20% sul a e con en (KCSA-20CS and
KCSA-20NS). The samples wi h he g ea es calcium sul a e con en s (CSA-H and KCSA-
20CS) elease he mos hea ini ially, which is consis en wi h he accele a ion o he eac-
ion p ocess obse ed in Figu e 1. Al hough he KCSA-20NS pas e ini ially eleases
sligh ly less hea han he o he pas es, i beha es simila ly o hem a ad anced ages. The
lowe amoun s o hea eleased by hese samples (CSA-H, KCSA-20CS, and KCSA-20NS)
a ad anced ages may be pa ly due o a dilu ion e ec .
Figu e 2. To al hea o he e e ence samples (KCSA-H and CSA-H) and o he pas es con aining 5%
and 20% CaSO4 and Na2SO4.
2.2. Mechanical Beha io
Figu e 3 shows he comp essi e s eng h alues o he pas es a 1, 3, 28, and 90 days.
As can be seen, he KCSA-5CS pas e p esen s s eng h alues close o hose ob ained by
he KCSA-H, while he KCSA-20CS pas e p esen s highe s eng h alues a all ages, sim-
ila o hose ob ained by he cemen (CSA-H).
Figu e 3. Comp essi e s eng h o he pas es con aining 5% and 20% sal concen a ions; (a) CaSO4;
(b) Na2SO4.
010 20 30 40 50 60 70 80
0
50
100
150
200
250
300
350
400
0 2 4 6 8 10 12 14 16 18 20 22 24
0
50
100
150
200
250
300
350
400
0 2 4 6 8 10 12 14 16 18 20 22 24
0
50
100
150
200
250
300
350
400
Hea eleased (J/g)
Time (h)
H2O CSA
5% CaSO4 20% CaSO4
5% Na2SO4
20% Na2SO4
KCSA-H
CSA-HKCSA-20CS
KCSA-5CS KCSA-5NS
KCSA-20NS
Hea eleased (J/g)
Time (h)
H2O CSA
5% CaSO4 20% CaSO4
5% Na2SO4
20% Na2SO4
KCSA-H
CSA-H
KCSA-20CS
KCSA-5CS KCSA-5NS
KCSA-20NS
Hea eleased (J/g)
Time (h)
H2O CSA
5% CaSO4 20% CaSO4
5% Na2SO4
20% Na2SO4
KCSA-H
CSA-H
KCSA-20CS
KCSA-5CS
KCSA-5NS
KCSA-20NS
Figu e 2. To al hea o he e e ence samples (KCSA-H and CSA-H) and o he pas es con aining 5%
and 20% CaSO4and Na2SO4.
2.2. Mechanical Beha io
Figu e 3shows he comp essi e s eng h alues o he pas es a 1, 3, 28, and 90 days.
As can be seen, he KCSA-5CS pas e p esen s s eng h alues close o hose ob ained by he
KCSA-H, while he KCSA-20CS pas e p esen s highe s eng h alues a all ages, simila o
hose ob ained by he cemen (CSA-H).
Molecules 2023, 28, x FOR PEER REVIEW 4 o 21
Figu e 2 shows he o al hea as a unc ion o ime. I demons a es ha o he i s
wo hou s, all he sys ems sha e e y simila cu es. O e longe ime pe iods, he wa e -
hyd a ed clinke wi h no added sul a es (KCSA-H) eleases he mos o al hea . The
KCSA-5CS and KCSA-5NS pas es elease he leas ini ial hea due o hei delayed eac ion
a e, bu a ad anced ages, he hea eleased, al hough less han ha eleased by he KCSA-
H, is highe han ha eleased by he pas es wi h a 20% sul a e con en (KCSA-20CS and
KCSA-20NS). The samples wi h he g ea es calcium sul a e con en s (CSA-H and KCSA-
20CS) elease he mos hea ini ially, which is consis en wi h he accele a ion o he eac-
ion p ocess obse ed in Figu e 1. Al hough he KCSA-20NS pas e ini ially eleases
sligh ly less hea han he o he pas es, i beha es simila ly o hem a ad anced ages. The
lowe amoun s o hea eleased by hese samples (CSA-H, KCSA-20CS, and KCSA-20NS)
a ad anced ages may be pa ly due o a dilu ion e ec .
Figu e 2. To al hea o he e e ence samples (KCSA-H and CSA-H) and o he pas es con aining 5%
and 20% CaSO4 and Na2SO4.
2.2. Mechanical Beha io
Figu e 3 shows he comp essi e s eng h alues o he pas es a 1, 3, 28, and 90 days.
As can be seen, he KCSA-5CS pas e p esen s s eng h alues close o hose ob ained by
he KCSA-H, while he KCSA-20CS pas e p esen s highe s eng h alues a all ages, sim-
ila o hose ob ained by he cemen (CSA-H).
Figu e 3. Comp essi e s eng h o he pas es con aining 5% and 20% sal concen a ions; (a) CaSO4;
(b) Na2SO4.
010 20 30 40 50 60 70 80
0
50
100
150
200
250
300
350
400
0 2 4 6 8 10 12 14 16 18 20 22 24
0
50
100
150
200
250
300
350
400
0 2 4 6 8 10 12 14 16 18 20 22 24
0
50
100
150
200
250
300
350
400
Hea eleased (J/g)
Time (h)
H2O CSA
5% CaSO4 20% CaSO4
5% Na2SO4
20% Na2SO4
KCSA-H
CSA-HKCSA-20CS
KCSA-5CS KCSA-5NS
KCSA-20NS
Hea eleased (J/g)
Time (h)
H2O CSA
5% CaSO4 20% CaSO4
5% Na2SO4
20% Na2SO4
KCSA-H
CSA-H
KCSA-20CS
KCSA-5CS KCSA-5NS
KCSA-20NS
Hea eleased (J/g)
Time (h)
H2O CSA
5% CaSO4 20% CaSO4
5% Na2SO4
20% Na2SO4
KCSA-H
CSA-H
KCSA-20CS
KCSA-5CS
KCSA-5NS
KCSA-20NS
Figu e 3.
Comp essi e s eng h o he pas es con aining 5% and 20% sal concen a ions; (
a
) CaSO
4
;
(b) Na2SO4.
The p esence o sodium sul a e (Figu e 3b) esul s in low s eng h alues a all ages in
he s udy, wi h he KCSA-20NS pas e e en ually ac u ing a 90 days. Figu e 3includes a
pho og aph o he c acked pas e specimen a 90 days o which, due o he ac u e, i was
no possible o de e mine he mechanical s eng h. Simila esul s we e ob ained in ano he
s udy [
20
], in which KCSA clinke was hyd a ed wi h an 8 M NaOH solu ion, and also in
he pape s published by Tamba a e al. [
15
] on he hyd a ion o a CSA wi h Na
2
SO
4
, and
by Sánchez e al. [21] on he hyd a ion o ye’elimi e wi h 8 M NaOH.
Cemen pas e is a chao ic sys em wi h a e y complex s uc u e in which, in addi ion
o he size and olume o po es, he e a e geome ic cha ac e is ics ha a e di icul o
quan i y, i egula po e shapes and diso de ed spa ial dis ibu ions o po es. The s uc-
u e o he po es can be p edic ed indi ec ly h ough compu a ional models, applying
Molecules 2023,28, 7651 5 o 20
he concep s o ac al me hodology [
22
] o he modynamics. I can also be measu ed
di ec ly h ough expe imen al echniques such as nuclea magne ic esonance, ni ogen
adso p ion/deso p ion and me cu y in usion po osime y (MIP). In his wo k, he la e
me hod has been used. Al hough wi h his echnique he e a e pa ame e s ha a e no
aken in o accoun , such as he o uosi y o he sys em, i is conside ed ha he in o ma ion
ob ained may be su icien o he desi ed objec i e.
Figu e 4shows he po osi y alues and po e size dis ibu ion o he pas es con aining
5% and 20% CaSO
4
and Na
2
SO
4
a 1 and 28 days o hyd a ion. The i s obse a ion is ha ,
in e e y case, he po osi y dec eases wi h an inc ease in he hyd a ion ime. The igu e
also shows ha he pas es con aining CaSO
4
clea ly p esen lowe po osi y alues and
po e sizes han hose ob ained wi h Na
2
SO
4
. Based on he po e diame e in a ha dened
cemen pas e, po es can be classi ied as ollows: gel po es (<0.01
µ
m), small capilla y po es
(0.01–0.1
µ
m), la ge capilla y po es (0.1–1
µ
m), and ai po es (>1
µ
m) [
23
]. The numbe o
capilla y po es depends on he wa e /binde (w/b) a io and he deg ee o eac ion o he
pas e, which ends o dec ease as he deg ee o hyd a ion inc eases. The pas es con aining
calcium sul a e gene ally ha e a highe pe cen age o small capilla y po es (0.01–0.1
µ
m),
while la ge capilla y po es (0.1–1 µm) p edomina e in pas es con aining sodium sul a e.
Molecules 2023, 28, x FOR PEER REVIEW 5 o 21
The p esence o sodium sul a e (Figu e 3b) esul s in low s eng h alues a all ages
in he s udy, wi h he KCSA-20NS pas e e en ually ac u ing a 90 days. Figu e 3 includes
a pho og aph o he c acked pas e specimen a 90 days o which, due o he ac u e, i
was no possible o de e mine he mechanical s eng h. Simila esul s we e ob ained in
ano he s udy [20], in which KCSA clinke was hyd a ed wi h an 8M NaOH solu ion, and
also in he pape s published by Tamba a e al. [15] on he hyd a ion o a CSA wi h Na2SO4,
and by Sánchez e al. [21] on he hyd a ion o ye’elimi e wi h 8M NaOH.
Cemen pas e is a chao ic sys em wi h a e y complex s uc u e in which, in addi ion
o he size and olume o po es, he e a e geome ic cha ac e is ics ha a e di icul o
quan i y, i egula po e shapes and diso de ed spa ial dis ibu ions o po es. The s uc-
u e o he po es can be p edic ed indi ec ly h ough compu a ional models, applying he
concep s o ac al me hodology [22] o he modynamics. I can also be measu ed di ec ly
h ough expe imen al echniques such as nuclea magne ic esonance, ni ogen adso p-
ion/deso p ion and me cu y in usion po osime y (MIP). In his wo k, he la e me hod
has been used. Al hough wi h his echnique he e a e pa ame e s ha a e no aken in o
accoun , such as he o uosi y o he sys em, i is conside ed ha he in o ma ion ob ained
may be su icien o he desi ed objec i e.
Figu e 4 shows he po osi y alues and po e size dis ibu ion o he pas es con aining
5% and 20% CaSO4 and Na2SO4 a 1 and 28 days o hyd a ion. The i s obse a ion is ha ,
in e e y case, he po osi y dec eases wi h an inc ease in he hyd a ion ime. The igu e
also shows ha he pas es con aining CaSO4 clea ly p esen lowe po osi y alues and
po e sizes han hose ob ained wi h Na2SO4. Based on he po e diame e in a ha dened
cemen pas e, po es can be classi ied as ollows: gel po es (<0.01 μm), small capilla y po es
(0.01–0.1 μm), la ge capilla y po es (0.1–1 μm), and ai po es (>1 μm) [23]. The numbe o
capilla y po es depends on he wa e /binde (w/b) a io and he deg ee o eac ion o he
pas e, which ends o dec ease as he deg ee o hyd a ion inc eases. The pas es con aining
calcium sul a e gene ally ha e a highe pe cen age o small capilla y po es (0.01–0.1 μm),
while la ge capilla y po es (0.1–1 μm) p edomina e in pas es con aining sodium sul a e.
Figu e 4. Po osime y esul s a 1 and 28 days o he hyd a ed pas es con aining 5% and 20% CaSO4
and Na2SO4.
2.3. Mine alogical Cha ac e iza ion: Con inuous X- ay Di ac ion (XRD)
Figu e S1 shows he con inuous XRD (e e y 30 min) di ac og ams ob ained om
he hyd a ed pas es (KCSA-5NS, KCSA-5CS, KCSA-20NS, KCSA-20CS) in he i s 24 h.
5CS 1d
5CS 28d
20CS 1d
20CS 28d
5NS 1d
5NS 28d
20NS 1d
20NS 28d
0
5
10
15
20
25
30
35
40
32.23%
28.53%
34.31%
33.83%
11.05%
16.93%
15.66%
19.53%
Po osi y (%)
P<0.01 0.1>P>0.01 1>P>0.1
10>P>1 100>P>10 P>100
Figu e 4.
Po osime y esul s a 1 and 28 days o he hyd a ed pas es con aining 5% and 20% CaSO
4
and Na2SO4.
2.3. Mine alogical Cha ac e iza ion: Con inuous X- ay Di ac ion (XRD)
Figu e S1 shows he con inuous XRD (e e y 30 min) di ac og ams ob ained om
he hyd a ed pas es (KCSA-5NS, KCSA-5CS, KCSA-20NS, KCSA-20CS) in he i s 24 h.
Di e en colo s a e used in he di ac og ams depending on he ime in e als es ablished
o each signal ob ained du ing calo ime y (Figu e 1).
The educ ion in he in ensi y o he peaks associa ed wi h he ye’elimi e is g ea e in
he calcium sul a e pas es (KCSA-5CS and KCSA-20CS (Figu e S1a,b) han in he sodium
sul a e pas es (Figu e S1c,d). The in ensi y o he peaks associa ed wi h he e ingi e,
meanwhile, is g ea e .
As ega ds he sul a e con en , g ea e ye’elimi e consump ion appea s o occu in
he pas es wi h 20% sul a e con en (KCSA-20CS and KCSA-20NS) han in he pas es wi h
5% sul a e con en (KCSA-5CS and KCSA-5NS). This con i ms he esul s obse ed using
calo ime y, whe e he pas es wi h 20% sul a e con en begin o eac ea lie han hose wi h
5% sul a e con en did.
Fo a clea e ep esen a ion o pas e hyd a ion, Figu e 5shows he con inuous XRD
di ac og ams selec ed as a unc ion o he ime a which he peaks we e ob ained using
calo ime y (Figu e 1). The o ma ion o small e ingi e peaks was obse ed a 30 min in all

Molecules 2023,28, 7651 6 o 20
ou pas es (Figu e 5). In he pas es con aining calcium sul a e (Figu e 5a,b), he in ensi y
o he e ingi e peaks inc eases om 1.5 h onwa ds. In he KCSA-5CS pas e, e ingi e
becomes he main p oduc om 22 h onwa ds, while in he KCSA-20CS pas e, in addi ion o
e ingi e being o med a 1.5 h, small peaks co esponding o AH
3
a e also obse ed. These
peaks o AH
3
also s a o eme ge a e 4.1 h in he pas e con aining 5CS. In addi ion, he
KCSA-5CS pas e shows small peaks co esponding o calcium ca boalumina e (C
4
A
c
H
11
).
Molecules 2023, 28, x FOR PEER REVIEW 7 o 21
Figu e 5. Con inuous XRD di ac og ams as a unc ion o calo ime y ime. Legend: Y: ye’elimi e;
a: anhyd i e; b: beli e; B: b ediggi e; P: pe iclase; A: AH3; e: e ingi e; o: C4A𝑐H11; d: CAH10; L: mi a-
bili e (Na2SO4 · 10 H2O).
2.4. Mine alogical Cha ac e iza ion: Discon inuous XRD
Figu e 6 shows a po ion o he di ac og ams (zone spanning alues 2–5 a 30°)
o he clinke and hyd a ed pas es (KCSA-5CS, KCSA-20CS, KCSA-20NS, and KCSA-
5NS) a 1, 3, 28, and 90 days.
In bo h he clinke and he calcium sul oalumina e cemen , ye’elimi e (C4A3S) p e-
sen s as he main phase. In addi ion, he cemen also con ains anhyd i e and, in bo h ma-
e ials, beli e (C2S) and b ediggi e (Ca7Mg(SiO4)4), gehleni e (Al2Ca2O7Si), C3A, and pe i-
clase (MgO) p esen as seconda y phases.
The di ac og ams o he pas es con aining calcium sul a e (KCSA-5CS and KCSA-
20CS, Figu e 6) show ha as he hyd a ion ime inc eases, he in ensi y o he peaks asso-
cia ed wi h he ye’elimi e and anhyd i e dec eases. The main hyd a ion p oduc , alongside
AH3 and calci e, is e ingi e. In he pas e wi h 5% con en (KCSA-5CS), calcium ca boalu-
mina e (C4AcH11) also p esen s a 90 days. In he pas e wi h 20% con en (KCSA-20CS),
he p esence o calcium ca boalumina e is de ec ed a e 24 h alongside small peaks a -
ibu able o calcium monosul oalumina e.
Figu e 5.
Con inuous XRD di ac og ams as a unc ion o calo ime y ime. Legend: Y: ye’elimi e;
a: anhyd i e; b: beli e; B: b ediggi e; P: pe iclase; A: AH
3
; e: e ingi e; o: C
4
A
c
H
11
; d: CAH
10
; L:
mi abili e (Na2SO4·10 H2O).
In he case o he pas es con aining sodium sul a e (Figu e 5c,d), he di ac og ams
show ha e ingi e becomes he main hyd a ion p oduc om 1.5–2 h onwa ds, wi h he
peaks being mos in ense in he KCSA-20NS pas e. The o ma ion o peaks o AH
3
and
small peaks co esponding o calcium alumina e (CAH
10
) is also obse ed a e 24 h in
he KCSA-5NS pas e. Meanwhile, in he KCSA-20NS pas e, he o ma ion o small peaks
iden i ied as co esponding o he mi abili e (Na
2
SO
4·
10 H
2
O) is obse ed om 30 min
onwa ds, inc easing in in ensi y wi h ime [24].
I all he con inuous di ac og ams o he pas es con aining calcium sul a e a e ana-
lyzed, he main phase in bo h pas es (KCSA-5CS and KCSA-20CS) is e ingi e, wi h he
mos in ense peaks appea ing o be hose gene a ed by he 5% sample.
In he di ac og ams o he pas es con aining sodium sul a e (KCSA-5NS and KCSA-
20NS), he o ma ion o e ingi e as he main eac ion p oduc is also obse ed in he 5%
and 20% samples. Bo h pas es also show peaks co esponding o AH
3
ha inc ease in
in ensi y wi h an inc ease in he hyd a ion ime om 15 h onwa ds.
2.4. Mine alogical Cha ac e iza ion: Discon inuous XRD
Figu e 6shows a po ion o he di ac og ams (zone spanning alues 2–5
Φ
a 30
◦
) o
he clinke and hyd a ed pas es (KCSA-5CS, KCSA-20CS, KCSA-20NS, and KCSA-5NS) a
1, 3, 28, and 90 days.
Molecules 2023,28, 7651 7 o 20
Molecules 2023, 28, x FOR PEER REVIEW 8 o 21
Figu e 6. Di ac og am o he KCSA pas e con aining he di e en p opo ions o sal s: (a) 5%
CaSO4, (b) 20% CaSO4, (c) 5% Na2SO4, (d) 20% Na2SO4 a 1, 3, 28, and 90 days. Legend: Y: Ye’elimi e
(C4A3𝑠, PDF-33-0256); b: beli e (C2S PDF-86-0398); e: e ingi e (C6A𝑠3H32 PDF-41-1451); A: AH3 (AH3
PDF-74-1775); T: hena di e (Na2SO4 PDF-37-1465); o: C4A𝑐 H 11 (C4A𝑐 H11 PDF-87-0493); c: calci e
(CaCO3 PDF-86-2334); d: CAH10 (CAH10 PDF-12-0408); m: calcium monosul oalumina e (C4A𝑠H12 PDF-
83-1289); a: anhyd i e (C𝑠 PDF-72-0916).
In he case o he pas es con aining sodium sul a e (Figu e 6), he in ensi y o he
ye’elimi e also dec eases wi h an inc ease in he hyd a ion ime. In he KCSA-5NS pas e,
he main p oduc o med is e ingi e, which inc eases in in ensi y up o 28 days be o e
dec easing a 90 days, he age a which he p esence o o he phases such as AH3, C4A𝑐H11,
and calci e is obse ed along wi h small peaks o CAH10. In he KCSA-20NS pas e, he
p esence o C4A𝑐H11 is also obse ed a 90 days, while he AH3 and calci e a e obse ed a
3 days and 1 day, espec i ely. In his pas e (KCSA-20NS), CAH10 is no de ec ed.
One impo an inding o no e in he KCSA-20NS pas e is ha , a 1 day, he p esence
o Na2SO4 was no obse ed, no was i obse ed in he con inuous XRD analyses (see
Figu e 5). Howe e , his phase was de ec ed a mo e ad anced ages, indica ing ha so-
dium sul a e dissolu ion ollowed by e-p ecipi a ion in he o m o hena di e occu s. The
peaks associa ed wi h he p esence o hena di e inc ease in in ensi y wi h an inc ease in
he hyd a ion ime. A his poin , i should be no ed ha hese pas es exhibi ed signi ican
mechanical s eng h losses (and e en c acking) be ween 28 and 90 days (see Figu e 3).
2.5. Di e en ial The mal Analysis and The mog a ime y (DTA/TG)
Figu e 7 p esen s he DTA/TG esul s o he hyd a ed KCSA pas es con aining 5%
and 20% CaSO4 and Na2SO4 a 1, 3, 28, and 90 days.
Figu e 6.
Di ac og am o he KCSA pas e con aining he di e en p opo ions o sal s: (
a
) 5% CaSO
4
,
(
b
) 20% CaSO
4
, (
c
) 5% Na
2
SO
4
, (
d
) 20% Na
2
SO
4
a 1, 3, 28, and 90 days. Legend:
Y
: Ye’elimi e
(C
4
A
3s
, PDF-33-0256);
b
: beli e (C
2
S PDF-86-0398);
e:
e ingi e (C
6
A
s3
H
32
PDF-41-1451);
A
: AH
3
(AH
3
PDF-74-1775);
T
: hena di e (Na
2
SO
4
PDF-37-1465);
o
: C
4
A
c
H
11
(C
4
A
c
H
11
PDF-87-0493);
c
: calci e (CaCO
3
PDF-86-2334);
d
: CAH
10
(CAH
10
PDF-12-0408);
m
: calcium monosul oalumina e
(C4AsH12 PDF-83-1289); a: anhyd i e (CsPDF-72-0916).
In bo h he clinke and he calcium sul oalumina e cemen , ye’elimi e (C
4
A
3
S) p esen s
as he main phase. In addi ion, he cemen also con ains anhyd i e and, in bo h ma e ials,
beli e (C
2
S) and b ediggi e (Ca
7
Mg(SiO
4
)
4
), gehleni e (Al
2
Ca
2
O
7
Si), C
3
A, and pe iclase
(MgO) p esen as seconda y phases.
The di ac og ams o he pas es con aining calcium sul a e (KCSA-5CS and KCSA-
20CS, Figu e 6) show ha as he hyd a ion ime inc eases, he in ensi y o he peaks
associa ed wi h he ye’elimi e and anhyd i e dec eases. The main hyd a ion p oduc ,
alongside AH
3
and calci e, is e ingi e. In he pas e wi h 5% con en (KCSA-5CS), calcium
ca boalumina e (C
4
A
c
H
11
) also p esen s a 90 days. In he pas e wi h 20% con en (KCSA-
20CS), he p esence o calcium ca boalumina e is de ec ed a e 24 h alongside small peaks
a ibu able o calcium monosul oalumina e.
In he case o he pas es con aining sodium sul a e (Figu e 6), he in ensi y o he
ye’elimi e also dec eases wi h an inc ease in he hyd a ion ime. In he KCSA-5NS pas e,
he main p oduc o med is e ingi e, which inc eases in in ensi y up o 28 days be o e
dec easing a 90 days, he age a which he p esence o o he phases such as AH
3
, C
4
A
c
H
11
,
and calci e is obse ed along wi h small peaks o CAH
10
. In he KCSA-20NS pas e, he
p esence o C
4
A
c
H
11
is also obse ed a 90 days, while he AH
3
and calci e a e obse ed a
3 days and 1 day, espec i ely. In his pas e (KCSA-20NS), CAH10 is no de ec ed.
One impo an inding o no e in he KCSA-20NS pas e is ha , a 1 day, he p esence
o Na
2
SO
4
was no obse ed, no was i obse ed in he con inuous XRD analyses (see
Figu e 5). Howe e , his phase was de ec ed a mo e ad anced ages, indica ing ha sodium
sul a e dissolu ion ollowed by e-p ecipi a ion in he o m o hena di e occu s. The peaks
Molecules 2023,28, 7651 8 o 20
associa ed wi h he p esence o hena di e inc ease in in ensi y wi h an inc ease in he
hyd a ion ime. A his poin , i should be no ed ha hese pas es exhibi ed signi ican
mechanical s eng h losses (and e en c acking) be ween 28 and 90 days (see Figu e 3).
2.5. Di e en ial The mal Analysis and The mog a ime y (DTA/TG)
Figu e 7p esen s he DTA/TG esul s o he hyd a ed KCSA pas es con aining 5%
and 20% CaSO4and Na2SO4a 1, 3, 28, and 90 days.
Molecules 2023, 28, x FOR PEER REVIEW 9 o 21
Figu e 7. Di e en ial he mal analysis (DTA) and he mog a ime ic (TG) analysis o he KCSA
pas es con aining di e en sal s.
The DTA cu es show he p esence o a se ies o signals ha co espond o he de-
composi ion o he di e en phases men ioned abo e and iden i ied using XRD. The i s
signal (signal 1), a a ound 100 °C, co esponds o he p esence o e ingi e. In he KCSA-
20NS pas e, his signal (signal 1) is de ec ed a ~95 °C and spli s, c ea ing ano he signal
(signal 5) a a ound 65 °C o he pas es a 1, 3, and 28 days. Al hough signal 5 is no longe
de ec ed a 90 days, ano he signal (signal 2) is obse ed a 125 °C. Signal 5 may be due o
he decomposi ion o he calcium ca boalumina e while signal 2 may be due o he decom-
posi ion o he hyd a ed calcium alumina e, which is no clea ly obse ed in he KCSA-
20NS pas e using XRD [25].
A hi d signal (signal 3) hen appea s a close o 245 °C and shi s o 267 °C as he
hyd a ion ime inc eases. This signal is associa ed wi h he decomposi ion o he alumi-
num hyd oxide phase (AH3) p e iously obse ed wi h XRD. In he KCSA-20NS pas e, a
28 days, signal 3 spli s in wo, s anding a 247 °C and 261 °C, espec i ely. In he KCSA-
Tempe a u e
Tempe a u e
Figu e 7.
Di e en ial he mal analysis (DTA) and he mog a ime ic (TG) analysis o he KCSA
pas es con aining di e en sal s.
The DTA cu es show he p esence o a se ies o signals ha co espond o he
decomposi ion o he di e en phases men ioned abo e and iden i ied using XRD. The
i s signal (signal 1), a a ound 100
◦
C, co esponds o he p esence o e ingi e. In he
KCSA-20NS pas e, his signal (signal 1) is de ec ed a ~95
◦
C and spli s, c ea ing ano he
signal (signal 5) a a ound 65
◦
C o he pas es a 1, 3, and 28 days. Al hough signal 5 is
no longe de ec ed a 90 days, ano he signal (signal 2) is obse ed a 125
◦
C. Signal 5 may
be due o he decomposi ion o he calcium ca boalumina e while signal 2 may be due o
he decomposi ion o he hyd a ed calcium alumina e, which is no clea ly obse ed in he
KCSA-20NS pas e using XRD [25].
Molecules 2023,28, 7651 9 o 20
A hi d signal (signal 3) hen appea s a close o 245
◦
C and shi s o 267
◦
C as he
hyd a ion ime inc eases. This signal is associa ed wi h he decomposi ion o he aluminum
hyd oxide phase (AH
3
) p e iously obse ed wi h XRD. In he KCSA-20NS pas e, a 28 days,
signal 3 spli s in wo, s anding a 247
◦
C and 261
◦
C, espec i ely. In he KCSA-5CS and
KCSA-20CS pas es, a simila spli occu s a 3 days and 28 days, espec i ely [
23
]. The signal
de ec ed a 250 ◦C in he KCSA-20CS pas e is associa ed wi h his phase (AFm).
Finally, a alues abo e 600
◦
C, he signal (signal 4) ob ained co esponds o he loss
o he CO
2
in he calcium ca boalumina e and he calci e, wi h he peak a 650
◦
C being
a ibu able o he C4AH11 and he peak a a ound 720 ◦C being a ibu able o he calci e.
Rega ding he TG cu es in he 50–200
◦
C in e al (co esponding o he phases
C
4
A
s3
H
32
, CAH
10
, C
4
A
c
H
11
and C
4
A
s
H
12
), i can be obse ed how, in all he pas es, he
wa e loss inc eases up o 28 days and dec eases a 90 days; mo eo e , his wa e loss
inc eases as he concen a ion o he sal s inc eases. Fo he nex in e al (200–400
◦
C),
whe e he AH
3
phase is ound, he weigh loss inc eases wi h an inc ease in he hyd a ion
ime and dec eases wi h inc easing CaSO
4
concen a ion, while in he case o he Na
2
SO
4
sal , he wa e loss is g ea e han 20%. The ange (600–800
◦
C) is associa ed wi h he loss
o CO2 om he ca boalumina es and calci e.
2.6. Scanning Elec on Mic oscopy (SEM/EDX)
The KCSA pas es con aining he di e en sal s a 5% and 20% concen a ions we e
analyzed a 3 and 90 days using SEM (Figu es 8–11).
Molecules 2023, 28, x FOR PEER REVIEW 11 o 21
Figu e 8. SEM mic og aphs o KCSA con aining CaSO4; (a) KCSA-5CS a 3 days; (b) KCSA-5CS a
90 days; (c) KCSA-20CS a 3 days; (d) KCSA-20CS a 90 days.
Figu e 9. High- esolu ion iew o he mic og aphs showing e ingi e in he KCSA-CS pas es.
SEM analysis o he pas es con aining di e en concen a ions o sodium sul a e like-
wise e eals e ingi e needles (Figu e 10a) and aluminum hyd oxide pla es (Figu e 10b)
in he pas e con aining 5% (KCSA-5NS). When he concen a ion o sodium sul a e
(KCSA-20NS) inc eases, in addi ion o he abo e-men ioned phases, p isma ic pa icles
co esponding o hena di e (Figu e 10c) a e de ec ed. A 90 days, in addi ion o he he-
na di e, calcium ca bona e pla es a e also de ec ed (Figu e 10d). The p esence a 90 days
Figu e 8.
SEM mic og aphs o KCSA con aining CaSO
4
; (
a
) KCSA-5CS a 3 days; (
b
) KCSA-5CS a
90 days; (c) KCSA-20CS a 3 days; (d) KCSA-20CS a 90 days.
Molecules 2023,28, 7651 16 o 20
e ec becomes less isible a 28 and 90 days and e en diminishes. As ega ds AH
3
, he
da a show how i s p esence inc eases sligh ly wi h an inc ease in he hyd a ion ime.
The ma e ials con aining sodium sul a e a e highly po ous (see Figu e 4), howe e , and
la ge capilla y po es (0.1–1
µ
m) a e clea ly p edominan , which explains he low mechanical
s eng h alues. This g ea e po osi y is pa ly associa ed wi h nonhyd a ed a eas and
pa ly wi h he c acks c ea ed by he o ma ion o c ys alline phases in he ha dened pas e.
The c ys alliza ion (see he SEM mic og aphs, Figu es 10 and 11) causes he samples o
expand, leading o c acking.
Ano he impo an aspec o highligh in he KCSA-20NS pas es is he appa en
dec ease in e ingi e a ad anced ages, a o ing he o ma ion o calci e and, abo e all,
hena di e. The c acking o he specimens a 90 days is ela ed o he in e nal s ess caused
by p ecipi a ion o calci e and hena di e in he ma ix and he appa en decomposi ion o
he e ingi e.
The beha io obse ed in he KCSA-20NS pas es closely esembles ha o he hyd a ed
pas es wi h high alkali con en s (KCSA-4M and KCSA-8M) [
36
]. This sugges s ha he
high sodium sul a e con en eleases sul a e ions ha eac o o m AF , as well as eleasing
Na
+
ions ha may con ibu e o he inc ease in pH le el. When he pH inc eases, he
e ingi e’s s abili y dec eases, a o ing ca bona ion [
15
,
21
]. As e ingi e is uns able a
high pH le els, i b eaks down and eleases sul a e ions, which, wi h he Na
+
p esen in he
medium, e-p ecipi a e in he o m o hena di e. SEM (Figu e 10) analysis shows a high
p esence o hena di e and calcium ca bona e (calci e) pla es when he hyd a ion ime o
he KCSA-20NS pas e inc eases om 3 o 90 days. This combina ion o des abiliza ion and
c ys alliza ion o sul a e sal s (Na
2
SO
4
) is esponsible o he change in he olume o he
ma e ial and i s subsequen c acking [37].
As ega ds he pH le el o he po ous solu ion o a KCSA o CSA pas e, his alue is
known o be a ound 11, clea ly lowe han ha o a Po land cemen pas e (pH le el o
~12.5). A his poin , an addi ional s udy was conduc ed o de e mine how he pH o he
KCSA-20NS pas e e ol ed in he i s 24 h. Fo his pu pose, pas es we e p epa ed in a
2:1 a io and he liquid phase was ex ac ed using a acuum pump. This liquid phase was
la e used o measu e he pH alue [
38
,
39
]. The esul s ob ained a e shown in Table 1. I
can be seen ha in he i s 24 h, he pH ises o alues g ea e han 13.
Table 1. pH alues o he aqueous phase o he KCSA-20NS pas e.
pH MEASUREMENTS
30 min 1 h 2 h 5 h 7 h 24 h
KCSA-20NS 11.39 11.40 11.64 12.42 12.51 13.19
The s abili y ange o e ingi e as a unc ion o pH emains unde deba e. Based on
s udies conduc ed on di e en ypes o cemen s, se e al au ho s [
15
,
21
,
36
] indica e ha
e ingi e is s able a pH le els be ween 9 and 13. I has been obse ed how, in syn he ic
sys ems and a high pH le els (>13), he mo phology o he AF changes om needle o
sphe ical o m. This beha io is e y simila o ha obse ed in he KCSA-20NS pas es
(see Figu e 11). A highe pH alues, he e ingi e becomes uns able [33].
In sho , he p esence o sodium sul a e a ec s he hyd a ion o KCSA clinke di -
e en ly depending on he sal concen a ion. A 5%, he mechanical s eng h is lowe
han ha ob ained wi h ei he he e e ence ma e ial (H
2
O) o calcium sul a e. In addi-
ion, he a e o hyd a ion slows and he main hyd a ion p oduc is e ingi e, ollowed
by AH
3
and hena di e. A a 20% sal con en , he a e o hyd a ion inc eases bu he
mechanical s eng h dec eases, p oducing specimen ac u e a 90 days due o he inc eased
p ecipi a ion o hena di e.

Molecules 2023,28, 7651 17 o 20
4. Ma e ials and Me hods
4.1. Ma e ials
In his s udy, a calcium sul oalumina e cemen clinke (KCSA) and an indus ial
calcium sul oalumina e cemen (CSA)—ma ke ed by i. ech ALI PRE GREEN (supplied
by Heidelbe g Cemen Hispania)—we e used. Table 2shows he chemical composi ion
de e mined using X- ay luo escence and he mine alogical composi ion ob ained using X- ay
di ac ion, including quan i a i e analysis using he Rie eld me hod in he Topas applica ion.
Table 2. Chemical and mine alogical analysis o he KCSA clinke .
Chemical Composi ion Mine alogical Composi ion
Oxides KCSA CSA 2ICDD PDF KCSA CSA
CaO 40.82% 41.50% Ye’elimi e, C4A3s, (PDF = 33-0256) 68.4 ±0.9% 52.6 ±0.4%
SiO29.51% 8.14% B ediggi e, Ba0.293Ca13.467Mg1.81Mn0.43O32Si8
(PDF = 36-399) 7.40 ±0.4% 5.0 ±0.5%
Al2O329.37% 23.20% Beli e, C2S, (PDF = 86-0398) 16.90 ±1.0% 14.9 ±0.4%
Fe2O31.32% 1.05% Pe iclase, MgO, (PDF = 4-829) 3.67 ±0.2% 2.8 ±0.3%
MgO 4.13% 3.22% Gehleni e, Al2Ca2O7Si, (PDF = 35-755) 1.90 ±0.4% 1.6 ±0.5%
Na2O 1.06% 0.86% C3A, (PDF = 70-0839) 1.75 ±0.4% 1.0 ±0.2%
K2O 0.47% 0.44% Anhyd i e, CaSO4(PDF = 72-0916) 22.1 ±0.4%
TiO20.39% 0.32%
SO39.92% 18.36%
O he s 1.70% 1.35%
1LOI 1.31% 1.45% C ys allog aphy Open Da abase
1Loss on igni ion; 2In e na ional Cen e o Di ac ion Da a.
Figu e S2 shows he g anulome ic analyses o he calcium sul oalumina e clinke
(KCSA) and he calcium sul oalumina e cemen (CSA). These lase di ac ion analyses
we e pe o med using a COULTER LS 130 g anulome ic analyze wi h a measu emen
ange o 0.1–900.0
µ
m. In o al, 90% o he pa icles in he KCSA we e smalle han 35
µ
m;
in he CSA, 90% o he pa icles we e smalle han 11.51
µ
m. The clinke had a densi y
o 2.83 g/mL and he cemen had a densi y o 2.90 g/mL. The Blaine inenesses we e
408.82 m2/kg and 474.6 m2/kg o he KCSA and CSA, espec i ely.
4.2. P ocedu e
To he KCSA, 5 and 20% Na
2
SO
4
and CaSO
4
we e inco po a ed ia solid subs i u ion.
To de e mine he e olu ion o he mechanical s eng h, pas es we e p epa ed om solid
mix u es o KCSA wi h he abo emen ioned sal s, wi h a cons an wa e /binde a io o 0.5.
A p isma ic mold (6 es specimens o 1
×
1
×
6 cm
3
) was hal - illed and 60 s okes we e
gi en on he shaking able so ha he pas e was dis ibu ed homogeneously and no po es
we e le in he es specimen. The mold was hen illed and he 60 s okes we e epea ed.
Once he molds we e illed, he pas e was cu ed in a clima ic chambe o 24 h a 21
◦
C wi h
a 95% ela i e humidi y. The es specimens we e hen demolded, labelled and s o ed again
in he clima ic chambe un il he es age (1, 3, 28 and 90 days). A he selec ed age, he
specimens we e b oken using comp ession. The equipmen used o his was an Ibe es
p ess (Au o es -200/10-SW).
A e b eaking he specimens, pa o he ma e ial ob ained was g ound in an aga e
mo a and mixed wi h isop opanol o hal he hyd a ion p ocess. The mix u e was hen
shaken o 3 min and il e ed unde acuum. Nex , o emo e he le o e isop opanol,
he mix u e was d ied in a acuum desicca o o 3 days, a e which i was cha ac e ized
using XRD, DTA/TG, and MRI. Some o he ung ound pieces we e imme sed in a ja
o isop opanol o 7 days o la e use in SEM mic os uc u al analysis and me cu y
in usion po osime y.
The po osime y equipmen used was a MICROMERITICS Au opo e IV 9500 capable
o eaching p essu es o up o 3200 Psi, he equi alen o a po e size o 0.0067 µm.
Molecules 2023,28, 7651 18 o 20
The con inuous XRD equipmen used was a BRUKER D8 ADVANCE made up o
a 3 kW high- ol age gene a o , an X- ay ube wi h a coppe anode (Cu K
α1,2
adia ion,
1.540 Å) ha usually ope a es a 40 kV and 50 mA, and a Lynxeye de ec o wi h a 3 mm
an isca e sli and a Ni K-be a (0.5%) il e wi hou a monoch oma o (does no emo e
K
α2
). The KCSA and sal mix u e was shaken o 3 min and he sample holde was illed
and co e ed wi h Kap on. Sample eadings we e aken e e y 30 min in a 5–45◦ ange a a
ime/s ep o 0.9 s wi h a s ep size o 0.01973◦.
Discon inuous XRD analysis was pe o med using he same equipmen as o con in-
uous XRD. Analysis was pe o med using a 6 mm a iable di e gence sli in a 5–60
◦
2
θ
ange a a ime/s ep o 0.5 s wi h a s ep size o 0.02◦.
DTA/TG was pe o med using a TA SDT Q 600. The eco ding condi ions we e as
ollows: hea ing om oom empe a u e o 1000
◦
C a a a e o 10
◦
C/min in a pla inum
c ucible unde a ni ogen a mosphe e. The measu emen sensi i i y was 0.001
◦
C o DTA
and 0.1 µg o TG.
Scanning elec on mic oscopy and ene gy-dispe si e X- ay (SEM/EDX) we e pe -
o med using a HITACHI S-4800 mic oscope. The scanning mic oscope employed a
ield emission gun (FEG) wi h a esolu ion o 1.4 nm. This de ice was equipped wi h
a backsca e ed scanning elec on mic oscope (BSEM), a BRUKER X- ay de ec o , QUAN-
TAX 400 mic oanalysis so wa e and i e moni o ed axes. The samples we e acuum-d ied
and me allized wi h cha coal.
The hea elease a e and he o al hea associa ed wi h he hyd a ion eac ions we e
also analyzed using iso he mal conduc ion calo ime y, employing a THERMOMETRIC
TAM AIR de ice ope a ing a 25
◦
C. To his end, pas es wi h he same 0.5 wa e /binde
a io we e p epa ed. Measu es o 10 g o he cemen ing mix u e and 5 g o wa e we e
mixed ex si u o 3 min. A 7.5 g sample o his mix u e was inse ed in he calo ime e . Fo
he e e ence ma e ial, dis illed wa e was used.
5. Conclusions
This pape analyzes he e ec o alkaline sal s (Na
2
SO
4
) and alkali ea h (CaSO
4
) on
he hyd a ion o comme cial calcium sul oalumina e clinke (KCSA) a 1, 3, 28, and 90 days.
The esul s a e compa ed wi h he e e ence clinke and e e ence calcium sul oalumina e
cemen , bo h hyd a ed wi h wa e . The gene al conclusions ha can be d awn om his
pape a e summa ized below.
The sul a e sal ype (CaSO
4
o Na
2
SO
4
) and quan i y (5% o 20%) a ec he hyd a ion
kine ics, ype o eac ion p oduc s o med, and de elopmen o mechanical s eng h.
Low sul a e concen a ions (~5%) accele a e p ima y AF o ma ion bu slow seconda y
AF o ma ion. Howe e , highe quan i ies (20%) sligh ly delay he o ma ion o p ima y
AF and accele a e he o ma ion o seconda y AF . These phenomena a e complex and
equi e mo e de ailed s udy. Ne e heless, i is belie ed ha hey may be ela ed o he
ype and mo phology o he AF ini ially o med.
Hyd a ion o he KCSA in he p esence o calcium sul a e leads o he o ma ion
o e ingi e as he main hyd a ion p oduc , along wi h o he phases such as C
4
A
c
H
11
,
AH
3
, CAH
10
, and calci e. Hyd a ion o KCSA con aining sodium sul a e also induces he
o ma ion o e ingi e, alongside o he p oduc s such as AH
3
and calci e. Inc easing he
hyd a ion ime as a unc ion o added Na
2
SO
4
con en causes pa o he AF o b eak down
o o m hena di e and calci e.
The inco po a ion o calcium sul a e sal s has a posi i e e ec on he de elopmen
o mechanical s eng h in KCSA. Adding sodium sul a e, howe e , has less posi i e o
e en ad e se e ec s, especially a ad anced ages. This beha io is la gely associa ed wi h
he ac ha he p esence o Na
2
SO
4
inc eases he pH alue, which pa ly des abilizes he
AF o med, a o ing ca bona ion and he p ecipi a ion o hena di e, which can cause he
specimens o c ack and b eak.
Molecules 2023,28, 7651 19 o 20
Supplemen a y Ma e ials:
The ollowing suppo ing in o ma ion can be downloaded a : h ps://
www.mdpi.com/a icle/10.3390/molecules28227651/s1, Figu e S1: Con inuous XRD di ac og ams
o he KCSA pas es wi h each o he sal s; Figu e S2: KCSA pa icle size.
Au ho Con ibu ions:
Concep ualiza ion, A.F.-J. and A.P.; me hodology, P.P.-E. and A.F.-J.; alida-
ion, A.P. and A.F.-J.; o mal analysis, P.P.-E.; in es iga ion, P.P.-E. and A.F.-J.; esou ces, A.F.-J.; da a
cu a ion, P.P.-E.; w i ing—o iginal d a p epa a ion, P.P.-E.; w i ing— e iew and edi ing, A.F.-J.,
P.P.-E. and A.P.; p ojec adminis a ion, A.F.-J.; unding acquisi ion, A.F.-J. All au ho s ha e ead and
ag eed o he published e sion o he manusc ip .
Funding:
Thanks o he suppo o he Minis y o Science and Inno a ion and Uni e si ies and
FEDER unds ha subsidize he P ojec BIA2016-76466-R and he aid BES-2017-082022. Also hanks
o he P ojec PID 2019-111464RB-I00/AEI/10.13039/501100011033.
Ins i u ional Re iew Boa d S a emen : No applicable.
In o med Consen S a emen : No applicable.
Da a A ailabili y S a emen : Da a a e con ained wi hin he a icle and supplemen a y ma e ials.
Con lic s o In e es : The au ho s decla e no con lic o in e es .
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Conc . Res. 1996,26, 27–33. [C ossRe ]
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