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

Transcription

1 The Influence of Alkali Sulphates on the Fluidity of Superplasticizer Containing Cement Pastes Hana Kučerová Institute of Building Materials and Components Christiane Rößler F.A. Finger-Institute for Building Materials Science

2 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 K 2 and Na 2 on fluidity of superplasticizer containing cement pastes

3 Introduction Effects of soluble alkalis (Na 2, K 2 ) on early cement hydration : Increase ph of pore solution (activation of C 3 A hydration and dissolution of set regulator) Lead to formation of syngenite (K 2 Ca H 2 O) possibility of false setting (Locher et al. ZKG 1980) Lower adsorption of superplasticizers (competitive adsorption) NSF-type superplasticizers: a positive effect on the fluidity PC-type superplasticizers: a negative effect on the fluidity (Yamada et al. CCR 1998)

4 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

5 Materials: Cement (CEM I 52,5 R) Alkali sulphate content of cement pastes (water/cement = 0,39) Cement paste K 2 / Na 2 in the mixing water K 2 Oaq [%] Na 2 Oaq [%] C_0M 0 M 0,4 0,06 C_0,12M 0,12 M 0,85 0,35 C_0,18M 0,18 M 1,05 0,5 C_0,29M 0,29 M 1,18 0,76

6 Materials: Superplasticizers Superplasticizer PC 1 PC 2 NSF Type PC PC NSF acid / esther 3, Molecular weight of side chains low high -- Application Ready - mix Precast Ready - mix acid (COO -) acid / esther = low Esther (MPEG) acid / esther = high

7 Results: Adsorption of Superplasticizers (TOC- Analysis) - Influence of K 2 -concentration Adsorption % PC 1_1,5% PC 2_1,5% NSF_1,5% 0 0,12 0,18 0,29 0 0,12 0,18 0,29 0 0,12 0,18 0,29 [M] K 2

8 Preliminary fluidity tests: Determination of superplasticizer dosage by fluidity measurements (Viskomat NT) 220 Torque after 30 min (Nmm PC1_0 M K2SO4 PC2_0 M K2SO4 NSF_0M K2SO4 0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2 superplasticizer dosage in %

9 Torque (Nmm) C_0,29M Na2SO4 C_0,18M Na2SO4 C_0,12M Na2SO4 C_0M Fluidity of cement pastes (Viskomat NT) K 2 -versus Na 2 -addition Time (min) 300 Na 2 Torque (Nmm) C_0,18M K C_0,12M C_0M Time (min)

10 150 Torque (Nmm) NSF_0M NSF_0,29M NSF_0,18M NSF_0,12M Time (min) Fluidity of cement pastes (Viskomat NT) addition of NSF-type SP (1.5 %) Na Torque (Nmm) NSF_0,29M NSF_0,18M NSF_0M 30 NSF_0,12M K Time (min)

11 150 Torque (Nmm) PC1_0,29M PC1_0,12M PC1_0,18M Fluidity of cement pastes (Viskomat NT) addition of PC1 (0.4%) 0 PC1_0M Time (min) Na 2 K 2 Torque (Nmm) PC1_0,29M PC1_0,18M PC1_0,12M PC1_0M Time (min)

12 150 Torque (Nmm) PC2_0,29M PC2_0M Fluidity of cement pastes (Viskomat NT) addition of PC2 (0.4%) 30 PC2_0,18M PC2_0,12M 0 Time (min) Na Torque (Nmm) PC2_0,29M PC2_0,18M PC2_0M PC2_0,12M 30 K 2 0 Time (min)

13 Microstructure of cement pastes after 10min hydration (ESEM-FEG) syngenite & ettringite without addition of K M K 2 ettringite only

14 XRD-Rietveld- quantification: syngenite formation (10 min hydration) Amount of Syngenite (%) 2,5 2,0 1,5 1,0 0,5 unhydrated without SP PC1-1,5% PC2-1,5% NSF-1,5% 0,0 unhydrated 0M K2SO4 0,12M K2SO4 0,18M K2SO4

15 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).

16 Conclusions Superplasticizer Adsorption (TOC): addition of K 2 decreases SP-adsorption The rheological behaviour of cement pastes is controlled by: quantity of syngenite formed adsorption behaviour of superplasticizers reactivity and amount of C 3 A Microstructure and Phase composition of cement pastes (ESEM): 0.85 % K 2 O aq results in significant quantities of syngenite syngenite content > 1%: significant influence on the fluidity of cement pastes superplasticizer 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 detected addition of K 2 did not cause a significant change in C 3 A 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