The Influence of Alkali Sulphates on the Fluidity of ...K2SO4-versus Na2SO4-addition 150 180 210 240 270 300 0 20 40 60 80 100 120 Time (min) T o r qu e (N mm) C_0,29M Na2SO4 C_0,18M

Brno University of Technology (CZ) Bauhaus-University Weimar (Ger)

The Influence of Alkali Sulphates on the Fluidity of Superplasticizer Containing Cement Pastes

Institute of Building Materials and Components

Hana KučerováF.A. Finger-Institute

for Building Materials Science

Christiane Rößler


Aim of investigation

Influence of syngenite crystallisation on the rheology of cement paste Influence of superplasticizers on the morphology / quantity of

syngenite formed

Comparison of the influence of K2SO4 and Na2SO4 on fluidity of superplasticizer containing cement pastes


Introduction

Effects of soluble alkalis (Na2SO4, K2SO4) on early cement hydration :

Increase pH of pore solution (activation of C3A hydration and dissolution of set regulator)

Lead to formation of syngenite (K2SO4·CaSO4 H2O) – possibility of false setting (Locher et al. ZKG 1980)

Lower adsorption of superplasticizers (competitive adsorption)NSF-type superplasticizers: a positive effect on the fluidityPC-type superplasticizers: a negative effect on the fluidity

(Yamada et al. CCR 1998)


Methods

Viskomat NT (Schleibinger, Germany): determination of cement paste fluidity

TOC-Analysis of the pore solution of cement pastes (LiquiTOC, Foss Hereaus, Germany)

Adsorption characteristics of superplasticizers

Environmental SEM (ESEM, Philips, Netherlands)Characterisation of cement paste microstructure

X-Ray Powder diffraction (Siemens D5000, Germany) and quantification by Rietveld-Analysis (software BGMS, Germany)

quantification of phase composition of cements


Materials:

Cement (CEM I 52,5 R)

0,761,180,29 MC_0,29M0,51,050,18 MC_0,18M

0,350,850,12 MC_0,12M0,060,40 MC_0M

Na2Oaq[%]

K2Oaq[%]

K2SO4 / Na2SO4 in the mixing water Cement paste

Alkali sulphate content of cement pastes (water/cement = 0,39)


Materials: Superplasticizers

Ready - mixPrecastReady - mixApplication

--highlowMolecular weight of side chains

--83,5acid / esther

NSFPCPCType

NSFPC 2PC 1Superplasticizer

acid (COO-)

Esther (MPEG)

acid / esther = highacid / esther = low


Results: Adsorption of Superplasticizers (TOC-Analysis) - Influence of K2SO4-concentration

0

10

20

30

40

50

60

70

80

90

100

0 0,12 0,18 0,29 0 0,12 0,18 0,29 0 0,12 0,18 0,29

[M] K2SO4

Ads

orpt

ion

%

PC 1_1,5% NSF_1,5%PC 2_1,5%


Preliminary fluidity tests: Determination of superplasticizer dosage by fluidity measurements (Viskomat NT)

0

20

40

60

80

100

120

140

160

180

200

220

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2superplasticizer dosage in %

Torq

ue a

fter 3

0 m

in (N

mm

)

PC1_0 M K2SO4

PC2_0 M K2SO4

NSF_0M K2SO4

K2SO4

Na2SO4

Fluidity of cement pastes (Viskomat NT)

K2SO4-versus

Na2SO4-addition

150

180

210

240

270

300

0 20 40 60 80 100 120Time (min)

Torq

ue (N

mm

)C_0,29M Na2SO4C_0,18M Na2SO4C_0,12M Na2SO4C_0M


C_0MC_0,12M

C_0,18M

150

180

210

240

270

300

0 20 40 60 80 100 120Time (min)

Torq

ue (N

mm

)

NSF_0M

NSF_0,18M

NSF_0,12M

NSF_0,29M

0

30

60

90

120

150

0 20 40 60 80 100 120

Time (min)

Torq

ue (N

mm

)

Na2SO4

K2SO4


addition of

NSF-type SP (1.5 %)


NSF_0M

NSF_0,12M

NSF_0,18M

NSF_0,29M

0

30

60

90

120

150

0 20 40 60 80 100 120Time (min)

Torq

ue (N

mm

)

PC1_0M

PC1_0,18MPC1_0,12M

PC1_0,29M

0

30

60

90

120

150

0 20 40 60 80 100 120

Time (min)

Torq

ue (N

mm

)

Na2SO4

K2SO4


addition of

PC1 (0.4%)


PC1_0M

PC1_0,18M

PC1_0,12M

PC1_0,29M

0

30

60

90

120

150

0 20 40 60 80 100 120

Time (min)

Torq

ue (N

mm

)

PC2_0M

PC2_0,18M PC2_0,12M

PC2_0,29M

0

30

60

90

120

150

0 20 40 60 80 100 120

Time (min)

Torq

ue (N

mm

)

Na2SO4

K2SO4Brno University of Technology (CZ) Bauhaus-University Weimar (Ger)


addition of

PC2 (0.4%)

PC2_0M

PC2_0,12M

PC2_0,18M

PC2_0,29M

0

30

60

90

120

150

0 20 40 60 80 100 120

Time (min)

Torq

ue (N

mm

)


Microstructure of cement pastes after 10min hydration (ESEM-FEG)

syngenite & ettringite

ettringite

only

without addition of K2SO4

0.18 M K2SO4

XRD-Rietveld- quantification: syngenite formation (10 min hydration)

0,0

0,5

1,0

1,5

2,0

2,5

unhydrated 0M K2SO4 0,12M K2SO4 0,18M K2SO4

Am

ount

of S

ynge

nite

(%) unhydrated

without SP

PC1-1,5%

PC2-1,5%

NSF-1,5%



Model for cement paste microstructure in the presence of long tabular / prismatic crystals

Unsheared cement paste:random orienation of crystals.

Shearing of cement paste:orienation of crystals in the shear direction improves the fluididty (decreases yield strength according to Kurdowski et al. 1997).


ConclusionsSuperplasticizer Adsorption (TOC):

addition of K2SO4 decreases SP-adsorption

The rheological behaviour of cement pastes is controlled by:quantity of syngenite formed adsorption behaviour of superplasticizers reactivity and amount of C3A

Microstructure and Phase composition of cement pastes (ESEM):0.85 % K2Oaq results in significant quantities of syngenitesyngenite content > 1%: significant influence on the fluidity of cement pastessuperplasticizer influences the syngenite formation specifically (PC-type superplasticizers (especially PC1) decrease the quantity of syngenite). changes in the morphology of syngenite and ettringite due to SPs addition could not be detectedaddition of K2SO4 did not cause a significant change in C3A consumption.

The study was financed by the project GA ČR Nr. 103/04/0668 and by funds of F.A. Finger Institute of Building Materials Science of Bauhaus-University Weimar

Documents

The Influence of Alkali Sulphates on the Fluidity of ...K2SO4-versus Na2SO4-addition 150 180 210 240 270 300 0 20 40 60 80 100 120 Time (min) T o r qu e (N mm) C_0,29M Na2SO4 C_0,18M