Impact of Water Reducers and Superplasticizers on the Hydration of Portland Cement

Transcription

1 Ara A. Jeknavorian, Ph.D. Research Fellow W.R. Grace Conn. Cambridge, MA Impact of Water Reducers and Superplasticizers on the Hydration of Portland Cement July 27 29, 2009

2 Outline General Considerations in Understanding the Impact of WRAs on Cement Hydration Common Normal Range and High Range Water Reducing Chemistries WRA/HRWR performance as a function of the balance between aluminate reactivity-sulfate availability. Modeling Challenge: Examples of Cement-Admixture Interactions Gaps and Opportunities 2

3 The Impact of WRAs on Cement Hydration: General Considerations Normal and high range (superplasticizing) chemical admixtures almost invariably retard the rate of the cement hydration. For a given cement, this affect can vary significantly depending on: Cement/SCM fineness, chemistry, and degree of pre-hydration Chemistry of dispersing admixture, especially when formulated with multiple components. Admixture addition rate, the time of addition, amount adsorbed by the cement and amount of admixture in the pore water as a function of time. Selective and complex cement-admixture interactions. Mixture rheology, composition and temperature 3

4 Fate of Chemical Admixture added to Hydrating Cementitious System What happens to WRAs during the initial minutes of cement hydration has major consequence on subsequent hydration rate. Balance between aluminate reactivity and sulfate availability as a function time control the fate of essentially all chemical admixtures. Aluminate hydrates have a strong, irreversible adsorption for organic compounds, and WRAs are no exception. The workability and impact on setting are far less influenced when WRAs are adsorbed on aluminate hydrates. WRAs not adsorbed and intercalated in/on aluminate hydrates, are available to adsorb on hydrating C3S and ettringite increased cement dispersion and increased set retardation. 4

5 Cement Dispersing Action of Superplasticizers Increased water demand Higher particle surface area Less than optimum 5 increased strength strength development

6 Common Normal Range Water Reducing Admixtures Hydroxycarboxylic acid salts Corn Syrup Hydroxylated Polymer lignin O lignin SO 3 - Ca 2+ Sodium Gluconate Sodium Glucoheptonate O OH Calcium/Sodium lignosulfonate lignin 6

7 Common High Range Water Reducing Admixtures 3 SO - Na + O NSFC (SNF) n MSFC(SMF) Ca/Na Salts of Naphthalene and Melamine Sulfonate Formaldehyde Condensate CH 3 O CH 2 R CH 2 CH 2 ( O CH 2 CH ) N n CH 2 PEG amino di-methylphosphonate ( CH CH CH 2 CH ) n O C C O N O OH NH SO 3 H 2 PO 3 H 2 PO 3 H 2 Sulfanilic acid grafted poly-co-alt-(maleic anhydride-vinylpyrrolidone) CH2 CH CH2 CH CH2 CH CH2 CH CH2 CH a b c d Polycarboxylate-Polyether Comb Polymer 7 C OH O PC H3C H3C C O NH3 CH CH2 O CH2 CH2 x O CH O CH2 y OCH3 H3C H3C C NH CH CH2 O CH2 CH2 x O CH O CH2 y OCH3 O C H3C H3C N CH CH2 O CH2 CH2 x O CH CH2 y OCH3 C O

8 Teeth length Polycarboxylate-Polyether Technology: Unlimited Possibilities Polymer Variables: - Backbone/teeth length and chemistry - Teeth density and chemistry Teeth density Polycarboxylates are to concrete as designer drugs are to medicine 8 Backbone length

9 Mortar Flow As a Function of WRA Chemistry Dosage Flow (mm) ADVSP NSFC (PC) (PC) 220 LIGNIN Dosage to Cement (w t%) Various WRAs can alter the flow of cementitious mixtures over different dosage ranges, which can strongly impact rate of cement hydration. 9

10 Adsorption of Water Reducing Admixtures as a Function of Dosage ICCC

11 Normalizing Set Time Response of WRAs as a function of Slump MIX DESIGN Cement Factor 300 kg/m 3 Water 160 kg/m 3 Fine Aggregate 740 kg/m 3 Coarse Aggregate 1040 kg/m 3 PC NSFC Lignin Corn Syrup % s/s % s/s % s/s % s/s 11

12 Slump, in Slump vs Set Response for various WRAs 10 Slump vs Set: Water Reducing Performance in Concrete PC Lignin Corn syrup NSFC Set Time, hr.min Common normal and high range (superplasticizing) chemical admixtures almost invariably retard the rate of the cement hydration process. 12

13 Understanding the Complexity of Cement-Admixture Interactions V.S. Ramachandran, V.M. Malhotra, C. Jolicoeur, and N. Spiratos, Superplasticizers: Properties and applications in Concrete, p 178. Other Factors Affecting Admixture Adsorption and subsequently Cement Hydration Admixture addition rate Time of addition Formulation Temperature Cement Pre-hydration 13

14 Understanding Impact of Aluminate- Sulfate Balance on Cement Hydration in Presence of Admixtures 14

15 Possible Outcomes: Aluminate-Sulfate Balance Low C 3 A activity Plaster FALSE SET OK Plaster High C 3 A activity OK Gypsum Medium C 3 A activity FLASH SET FLASH SET Gypsum Anhydrite High C 3 A activity High C 3 A activity 15

16 Adsorption of NSFC by Various Hydrating Cement Minerals V.S. Ramachandran, V.M. Malhotra, C. Jolicoeur, and N. Spiratos, Superplasticizers: Properties and applications in Concrete, p

17 Adsorption of NSFC as a Function of Addition Mode: Mix Water vs. Delayed Mix Water Addition Delayed Addition Far less NSFC adsorbed with delayed addition resulting in improved dose-slump response and greater impact on cement hydration. V.S. Ramachandran, V.M. Malhotra, C. Jolicoeur, and N. Spiratos, Superplasticizers: Properties and applications in Concrete, p

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19 Set Time Variations with OPC with and without Admixtures Chemical Sulfate depletion Water/cement ratio 0.5, 23 C, water only 5 Cement Samples From Same Plant - with reported setting time problems - Tested Without Admixtures Water/cement ratio 0.5, 23 C,0.35% water reducing admixture by weight of cement in all samples With Admixtures 0.15% WRA [0.10% CS +0.05% TEA] 0.20% MRWR [0.10% PC % SG % Ca(NO 3 ) 2 ] 19

20 ASTM C359 Early Stiffening of Mortar Test Detecting Cement Admixture Incompatibilities 50 mm 600g cement 600 g sand w/c = 0.30* * Revised procedure calls for variable w/c 20

21 Modified C 359 Procedure Admixture Addition Modes Step Procedure w/o Admixture Time Add sand and cement, mix sec 1 sec slow speed 2 Add water sec 3 Mix medium speed 15 sec - 3:15 4 Stop and scrape, measure 3:15-4:00 temp 5 Mix medium speed 4:00-4:15 Stop, fill ISC vial, place in ISC 4:15-4:45 6 Stop, fill container 4:45-5:15 7 Initial penetration 5:15 8 Penetration readings 8, 11, and 14 minutes 9 Remix Sop, fill container 15:00-15:45 10 Penetration reading 15:45 11 Penetration reading 18:45 12 Penetration reading 21:45 Mix Wat 1 min 2 min Ad n del. Del. 10 sec 1:15 2:15 21

22 Cement Initial 3 / / / / / / Minutes 1 / / / / / / Minutes 1 / / / / / / Minutes 1 / / / / / / 24.0 Remix ASTM C 359 Early Stiffening Results w/ Fixed Water Content (W/C = 0.30) Penetration in mm / ºC 13 Minutes 2 / / / / / / Minutes 2 / / / / / /

23 Penetration, mm Impact of Delayed Chemical Admixture Addition on ASTM C359 Mortars Cement W64 = LS/CS/TEA, 3 oz/cwt (195 ml/100 kg); 0.10% s/s) (mm) (mm) W64,m (mm) W64, 1d (mm) W64, 2d Minutes 23

24 Power,mW/g Isothermal Calorimetry on Mortars with Delayed Admixture Dosages Time vs. Power 6.00E E Blank with 177.0g water 078 Blank with water 078 with Mix Water 078 with 1 Min Delay 078 with 2 Min Delay 4.00E E+00 Control Note decrease in initial exotherm 2.00E+00 W64,MW W64, 1 min del 1.00E+00 W64, 2 min del 0.00E Time,hrs. 24

25 Admixture Mechanisms of Set Retardation Precipitation of calcium salts on cement surface to re-enforce the semi-permeable membrane formed during induction period. Complex formation - Organic molecules complex with Ca 2+ lowering Ca 2+ in solution and reducing growth of Ca(OH)2. However, Ca complexes tend to have low stability constants, and [Ca 2+ ] in pore water is quite high. Nucleation - CH Crystals act as calcium sinks and cause increase in C3S hydration near end of induction period. Organic compounds can delay nucleation and growth of CH, thus causing retardation. However, chemical admixtures are rarely single component formulations. 25

26 Impact of Cement Prehydration on Retarding Effect with WRAs 26

27 Impact of Cement Prehydration on Retarding Effect of WRAs Gluconate/ sugar Setting indicator CS/LS Cement Admixture Dose, oz/cwt 078-I/II, Fresh CS/LS 3 Graph 078-I/II, Fresh 078-I/II, Fresh 078-I/II, Fresh 078-I/II, 50% hr 078-I/II, 50% hr 078-I/II, 50% hr 078-I/II, 50% hr Gluconate/Sugar CS/TEA LS/CS/TEA CS/LS Gluconate/Sugar CS/TEA LS/CS/TEA

28 Effect of Prehydration on Cement-Admixture Interactions Incremental retardation by prehydration, hours Cement was spread out in a ½ inch layer in the shrinkage lab, 72 50% RH We initially found strong effect of 16 hours prehydration on the retardation of 078 cement with certain admixtures. No effect visible by LOI, TG, XRD or optical microscopy, confirming that the extent of prehydration was very mild. Significant prehydration is easily visible by TG, optical microscopy, and is known to alter the hydration of cement in presence of admixture Additional retardation by prehydration of 078 cement with 3 oz/cwt Lignin/Corn Syrup Blend dosed upfront Hours 50% RH 28

29 Time (Hrs.) Effect of PC Type and Slag Content on Set Time Time to Initial Set with Slag PC1 (Short teeth ) PC 500 (Long teeth) % Slag (0.13% s/c) 16% Slag (0.11% s/c) 16% Slag (0.15% s/c) 40% Slag (0.11% s/c) 40% Slag (0.15% s/c) (Avg of Two Proctors) 29

30 Effect of PCE Structure, Dose on Cement Hydration Conduction Calorimetry, PCE Co-polymer MPEG-MA m:n = 1.2, 1.4, 1.6, p=23, 102 Onset of early Cement Hydration can be controlled with selected PCE compositions: smaller the surface footprint, earlier the onset of hydration. ICCC

31 Knowledge Gaps and Opportunities Admixture adsorption by aluminate versus silicate phases Impact of cement pre-hydration and hydration rate associated with chemical admixtures Performance from delayed admixture addition more predictable versus mix water addition? 31

32 Possibilities for Predicting Admixture Performance in Cementitious Systems Considering the multitude of factors, coupled with the variation in those factors, that can effect the interaction of portland cements with water reducing chemical admixtures, modeling cement hydration in the presence of admixtures on the basis of material characterization - would appear to be a very difficult challenge. A model for predicting the impact of chemical admixtures on cement hydration may be possible by measuring the performance of chemical admixtures with several selected, well characterized cements. The resulting performance indexes (i.e. isothermal calorimetry coupled with paste or mortar rheology) could be predictors with a wide range of cements. 32