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Effect of mixing on the early hydration of alite and OPC systems
Cement and Concrete Research 42 (2012) 1175–1188
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Effect of mixing on the early hydration of alite and OPC systems Patrick Juilland a,⁎, Aditya Kumar b, Emmanuel Gallucci a, Robert J. Flatt c, Karen L. Scrivener b a b c
Sika Technology AG, Corporate Research and Analytics, Tüffenwies 16, Zürich CH-8048, Switzerland EPFL-STI-IMX, Laboratoire des Matériaux de Construction, Station 15, Lausanne CH-1015, Switzerland ETHZ, D-BAUG, Physical Chemistry of Building Materials, Schafmattstrasse 6, Zürich CH-8093, Switzerland
a r t i c l e
i n f o
Article history: Received 15 October 2010 Accepted 24 June 2011 Keywords: Hydration (A) Mixing (A) Calorimetry (A)
a b s t r a c t The kinetics of hydration of cementitious materials is sensitive to the mixing procedure. High shear mixing conditions lead to an increase in the kinetics of hydration at early age compared to low shearing conditions such as hand mixing. In this study the effect of mixing speed and procedure was studied on alite and Portland cement in the presence or not of aggregates. The kinetics of hydration was monitored using isothermal calorimetry at 20 °C. The early reactivity was enhanced both with an increase in the speed of mixing and the shearing conditions. The principal features are a shortening of the induction period; a higher rate of hydrate precipitation during the acceleration period as well as an increase in the height of the main heat evolution peak. Analysis of the results in terms of dissolution theory, coupled with quantitative simulation with the μic modelling platform indicate different effects of mixing prior to and after the end of the induction period. Before the end of the induction period mixing has an impact on the rate of dissolution in the fast dissolution regime and high undersaturation, which appears to be (at least partially) controlled by the rate of transport of ions away from the alite surface. After the end of the induction period the main effect of mixing appears to be the production of more C-S-H nuclei, due to the possible detachment of the primary C-S-H (metastable) by mechanical action. This higher nucleation density leads to a denser microstructure for systems mixed at high intensities. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Mixing is a crucial step for any concrete application. Poor mixing will result in large inhomogeneities [1] and poor packing of the cement particles resulting in a larger amount of porosity, which can lower mechanical properties and reduce durability. Nowadays the control of viscosity by the addition of specific organic molecules has led to an engineering revolution in cementitious materials, with enhancement of mechanical properties through the reduction of water to cement ratios. Although many studies exist on the rheology of cementitious materials, the focus has mainly been on how hydration affects workability rather than the effect of mixing on the kinetics of hydration. Dollimore and Mangabhai [2] reported that increasing mixing time accelerated hydration. However, no attempt was made to explain the mechanisms responsible for this acceleration. Thomas and Jennings [3] found that rapid hand mixing decreased both the time to reach the heat evolution peak for C3S hydration and the magnitude of this peak as compared to pastes which underwent only minimal mixing. The mechanisms by which the hydration kinetics is affected are still not understood. Therefore the aim of this paper is to present a systematic study on the influence of the mixing procedure as well
⁎ Corresponding author. Tel.: + 41 58 436 42 83. E-mail address: [email protected] (P. Juilland). 0008-8846/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconres.2011.06.011
as of the additions of aggregates on the kinetics of hydration of alite and Portland cement systems. The results obtained are then discussed from the perspective of the dissolution theory which postulates that a decrease of the solution undersaturation will limit the rate of dissolution of the anhydrous phase, switching from a fast dissolution process by etch pit opening at high undersaturation to a slow dissolution process taking place only by step retreat at a critical undersaturation [4] and the mechanisms implemented in the μic modelling platform [5,6]. 2. Experimental procedure 2.1. Materials and methods Portland cement (CEM 1 42.5) and alite synthesised in our laboratory were used in this study. Alite was obtained by mixing calcium carbonate (precipitated GR for analysis, Merck), silica (highly dispersed extra pure, Merck), magnesium oxide (GR for analysis, Merck) and alumina (anhydrous γ-alumina, Merck) and deionised water in a ball mill for 24 h and firing the dry product at 1600 °C for 10 h. Magnesium oxide and alumina were added in order to favour grain growth and to stabilise the monoclinic structure of alite (MIII polymorph), which is similar to the alite found in Portland cement. The alite clinker was quenched in air and ground in an annular grinder for 1 min. Costoya [7] showed that monoclinic alite, ground in this way; has a particle size distribution similar to a CEMI 42.5 ordinary Portland cement. The chemical
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Table 1 XRF and XRD-Rietveld of the cement and alite used in this study (wt.%).
Table 3 Summary of mixing procedures.
XRF
CEMI 42.5
XRD
Normo 4
Alite
LOI CaO SiO2 Al2O3 MgO Na2O SO3 P2O5 K2O TiO2 Fe2O3
2.21 64 19.6 4.52 2 0.15 3.02 0.2 1.3 0.27 2.75
C3S C2S C4AF C3A C M CaSO4-2H2O CaSO4-0.5H2O CaSO4 S CH CaCO3 K2SO4
65.9 8.6 8.4 6 0.7 1.4 0 1.2 1.8 0 0 4.3 1.7
99.6 – – – 0.4 – – – – – – – –
compositions as well as the quantitative Rietveld refinement for both starting materials are reported in Table 1. For mortar samples, silica beads of different narrow particle size distributions (Table 2) were used as model aggregates (Type S, Sigmund Lindner GmbH) in a cement to aggregate weight ratio of 0.66. The water to cement ratio, w/c, was set at 0.4 for all mixes and deionised water was used for every test except when mentioned. Hand mixing was done with a common plastic spatula. For mechanical mixing a steel paddle with six blades was used at different speeds from 200, to 2000 rpm. Two mixing procedures were applied:
Material
w/c
Speeds
Aggregates
Mixing method
1. Alite/OPC
0.4
–
3. Alite/OPC
0.4
Hand mixing, 200, 500, 915, 1370, 1600, 2000 rpm Hand mixing, 500, 1200 rpm
1 and 2 step mixing 1 step mixing
Silica beads
7 days. Hydration was stopped by freeze drying. As with all methods for stopping hydration, some changes to the microstructure may occur. Nevertheless, this is one of the least damaging techniques according to Korpa and Trettin [8]. The samples were then impregnated with an epoxy resin and polished with progressively smaller grades of diamond powder until a mirror surface was obtained. The polished sections were then qualitatively observed with a SEM (Quanta 200) at an accelerating voltage of 15 kV. 3. Results 3.1. Energy input during mixing Fig. 1 reports the temperature measurement of alite and cement pastes before and after each mixing step. It can be seen that it
- One step mixing: 2 min at a given speed, - Two step mixing: 2 min at a given speed followed by a delay of 13 min before a second mix of 1 min at the same speed. Alite and cement pastes were studied with both one and two step mixing at 7 different mixing speeds. The effect on the size of the silica beads was then investigated at three speeds (hand mixing, 500 rpm and 1200 rpm) using the one step mixing procedure. All procedures are summarized in Table 3. The mixed quantities were kept at 46 g for pastes and 96 g for mortars in order to avoid variations due to mass change. Temperature measurements were performed before and after each mixing step in order to assess the energy input induced during the mixing. 2.2. Isothermal calorimetry and SEM Isothermal calorimetry measurements were carried out in a TAM Air microcalorimeter (Thermometrics) at 20 °C. Samples were prepared by external mixing according to the methods mentioned above. The quantity of sample put into the calorimeter cell was 10 g in the case of the pastes and 20 g for mortars. Every sample was balanced with a reference cell containing an amount of water having the same heat capacity as the paste or mortar. The time constant of the calorimeter is 4 min and corrected for, although it is hardly relevant in the time scale of the results presented here. Scanning electron microscopy (SEM) observations were made on alite pastes hand mixed and mechanically mixed at 915 and 1600 rpm. The samples were hydrated until the end of the acceleration period (maximum heat flow of silicate hydration) and until Table 2 Characteristics of the silica beads. Diameter (mm) 0.1–0.2 0.2–0.3 0.3–0.4 0.4–0.6 0.5–0.75
Bulk density (g·cm− 3)
Mean specific surface area (g·cm− 2)
1.52 1.51 1.51 1.50 1.49
6.3·10− 3 8.8·10− 3 1.25·10− 2 1.5·10− 2 2.2·10− 2
Fig. 1. Temperature evolution before and after mixing for paste of (a) alite and (b) PC.
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Fig. 2. Effect of different initial temperatures on OPC system for the same mixing procedure.
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increases almost linearly with the speed and the first mixing step heats the system more than the second one according to the time of mixing. This input of energy converted into heat due to friction [9] as well as an increase of alite dissolution may enhance the kinetics of hydration at early age since it is known that temperature can enhance markedly the kinetics of hydration [10]. However after 13 min, all systems have returned to the same temperature and the temperature would be even more rapidly stabilised in the isothermal calorimeter. Temperature rise should therefore only affect the first minutes of hydration. To check the influence of temperature during the mixing itself and its effect on the first minutes of hydration, two OPC samples were mixed at the same speed but with different initial temperatures. In order to reach these conditions, two stainless steel mixing paddles were kept at two different temperatures: one at room temperature and the other one in a furnace at 110 °C. After one step of mixing at 500 rpm for 2 min, the temperatures of the pastes were respectively 24 °C and 29.5 °C. This difference in temperature is almost equal to the largest temperature difference recorded for the least and most intensely mixed samples. The
Fig. 3. Evolution of the kinetics of hydration for (a) an alite paste and (b) a cement paste (w/c = 0.4) mixed 2 min at various speeds of mixing. (c) and (d) are zoomed area of (a) and (b) respectively.
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results in Fig. 2 show that this temperature difference has no effect on the length of the induction period and that the rate of heat released during the acceleration period is almost identical for both samples. There is a small difference in the height of the heat evolution peaks as well as the level of heat during the induction period. Nevertheless, it is clear that the temperature during the first few minutes, for samples, which as here, quickly returned to 20 °C in the isothermal calorimeter, has very little impact on the hydration process compared to the results obtained with different speeds of mixing presented in the following sections. Therefore we assume that the heating due to mixing can be neglected as having any significant impact on the hydration kinetics. It should be noted, that if heating effects persist, as in the field, they would be likely to have a more significant impact.
3.2. Effect of speed of mixing on alite and cement pastes The evolution of the heat released during the hydration of alite and cement pastes mixed with deionised water (w/c = 0.4) at different speeds for 2 min is presented in Fig. 3. In Fig. 3, it is seen that the different speeds of mixing have an impact on both alite and cement pastes. Since there is no major change
Fig. 4. Comparison between the one (dashed lines) and two step mixing procedures (solid lines) on a) alite and b) cement pastes.
in the shape of the curves it is assumed that the underlying mechanisms of hydration remain the same. The hydration is accelerated: • The induction period is reduced as the speed of mixing increases; • The slope of the heat flow curve during the acceleration period increases with the speed of mixing. Mixing has a larger impact on the reduction of the induction period of alite compared to PC, this is discussed further in Section 5 below. The definition of the induction period is still a subject of controversy; here we consider the “apparent” induction period to end when the heat flow curve starts to increase at the beginning of the acceleration period. The two step procedure shows similar trends with somewhat more acceleration compared to the one step mixing procedure as shown in Fig. 4. There seems to be some effect on the minimum heat evolution during the induction period, which is most marked between the hand mixed systems and the mechanically mixed ones. In this case, it is probable that this is due to failure to completely break up the agglomerations of particles in the hand mixed systems, which limits the surface of the cement available for dissolution. Fig. 5 shows the end of the induction period and the slope of the heat flow increase during the acceleration period as a function of the mixing speed for the paste systems. (The end of the induction period was arbitrarily defined by taking the intersection between the
Fig. 5. Plot of a) the time at the end of induction period and b) the slope of heat released during the acceleration period as function of the speed of mixing.
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horizontal tangent at the minimum value during the induction period and the one for the acceleration period.) The effect on the slope of the heat flow increase seems to be similar for both systems and there is an almost linear relation between mixing speed and the slope of acceleration. This suggests a mechanistic link between the two.
Fig. 7 a) and b) shows the time to reach the maximum heat flow during the acceleration period as a function of the diameter of the silica beads at various speeds of mixing. The values are almost constant for different bead sizes for each speed. It appears that the size of silica beads has no effect within the range of sizes studied.
3.3. Effect of aggregates
3.4. Observation of alite microstructure for pastes mixed at different speeds
In these systems, silica beads with narrow particle size distributions were added. The evolution of the heat flow for the mortar containing beads of size 0.3–0.4 mm for alite and PC systems is compared with the reference pastes without aggregates in Fig. 6. It can be seen that as the speed increases, the difference between the curves of mortars compared to pastes becomes larger. The presence of the silica beads seems to accentuate the shearing conditions during mixing and further increase the kinetics of hydration. A small peak appearing before the acceleration period is observed for mixing at 500 rpm and 1200 rpm for alite systems. This feature was already observed by Costoya [7] on C3S as well as by Makar and Chan [11] on PC. This feature might result from an endothermic peak associated with the precipitation of hydrates such as Portlandite [12] or a delay between the precipitation of Portlandite and the onset of rapid growth of C-S-H as suggested by Kumar and Scrivener [6].
To study the influence of mixing speed on the microstructural development of alite systems the hydration was stopped at the maximum heat flow of the acceleration period by freeze drying. Fig. 8 shows that as the shear forces increase during mixing, the resulting matrix is much denser at the end of the acceleration period. There are clear differences between samples mixed mechanically and those which are hand mixed. In the hand mixed paste capillary porosity can be easily identified. Small crystals of CH embedded in the C-S-H matrix are present in larger amount for pastes mixed at 915 rpm and 1600 rpm compared to the hand mixed system. The microstructure appears even finer at mixing speeds of 1600 rpm. Fig. 9 shows the microstructures of the alite systems at 7 days of hydration. It can be seen that in the hand mixed specimen some large capillary pores are still present whereas the two other mixing conditions
Fig. 6. Heat of hydration for pastes (dashed lines) and mortars (solid lines) with the size of beads from 0.3 to 0.4 mm for a) the alite and b) the OPC systems at different mixing speeds.
Fig. 7. Time to reach the maximum peak of heat flow during the acceleration period in function of the average diameter of aggregates at three different speeds of mixing for a) alite systems and b) PC systems.
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Fig. 8. Micrographs of polished section of alite samples at the maximum heat flow mixed twice by (a) hand mixing (16 h00 of hydration), (b) at 915 rpm (13 h30 of hydration) and (c) at 1600 rpm (12 h30 of hydration).
Fig. 9. Micrographs of polished section of alite samples at 7 days of hydration mixed twice by (a) hand mixing, (b) at 915 rpm and (c) at 1600 rpm.
lead to very similar dense microstructures at these later ages of hydration. We are aware that these comparisons remain qualitative since the degrees of hydration are slightly different for each system. Nevertheless, these images show that there is an effect of the mixing on the morphology of the precipitating hydrates which is not just due to a difference in the degree of hydration.
saturation of the mixing water influences the effects of mixing on hydration, alite was mixed again at different speeds but starting with a solution already saturated with lime. The results are presented in Fig. 10. It can be observed that the rate of mixing does not affect the length of the induction period when the initial solution is already saturated with lime. There is a slight delay for the hand mixed system which may be due to poor deagglomeration of the alite particles. This would lead to a lower amount of anhydrous surface in contact with water and a slightly longer induction period. However it should be noted that, even in this case, the induction period is much shorter than in Fig. 5. This is not due to an accelerating effect of the saturated lime solution but to a different particle size distribution since it is impossible to obtain identical particle size distributions for different
3.5. Effect of mixing solution In a recent publication [4] we argued that the slowdown in the initial rate of reaction and the low rate of reaction during the induction period are caused by the direct impact of the undersaturation of the solution on the dissolution of alite. In order to investigate how the state of
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earlier in this paper. Following this mechanism, the dissolution of alite is described by Eq. (1). dmalite ðt Þ ¼ ½−kdiss ⋅aSSA ⋅ðβmax −βalite ðt ÞÞ−aSSA ⋅cstep dt
retreat
ð1aÞ
for βalite(t) b = βmax, and dmalite ðt Þ ¼ −aSSA ⋅cstep retreat dt
Fig. 10. a) Evolution of the kinetics of hydration for an alite paste (w/c = 0.4) mixed 2 min at various mixing speeds starting with a saturated lime solution and b) an enlargement of the induction period.
batches of alite ground separately. Therefore absolute comparisons cannot be made; however, since the same batch was used within each set of experiments relative comparisons are valid.
4. Modelling the effect of mixing on alite hydration 4.1. Overview of the simulation method The 3D microstructure modelling platform μic [5] was used in this study. The model has been used to study the hydration of alite describing the effect of particle size of the alite in a recent study [6]. Two different mechanisms are used in the simulation: one – solution controlled dissolution (SCD) – during the dissolution and induction periods and another – nucleation with densifying growth (NDG) – during the acceleration and deceleration periods of the main heat evolution peak. During the SCD period thermodynamics are also integrated in the model in order to capture the influence of the pore solution chemistry on the kinetics. The SCD mechanism of alite has been integrated in the model based upon the hypothesis proposed by Juilland et al. [4] described
ð1bÞ
for βalite(t) > βmaxwhere, βmax is the critical degree of undersaturation of alite, kdiss (μmol.kg.m − 4.s − 1) is a dissolution rate constant, aSSA (m 2.kg − 1) is the specific surface area of particles, and cstep retreat (μmol.kg.m − 4.s − 1) is the step retreat constant. In the simulations, βalite(t) is calculated at each step based on the amount of alite dissolved, the amount of water present in the representative volume and the precipitation of phases. The dissolution of alite follows 2 regimes (1a) undersaturation greater than critical undersaturation, βmax, (1b) step retreat regime beyond critical undersaturation. Eq. (1a) contains three parameters that needed calibration: kdiss, cstep retreat and βmax. These parameters were fitted to the calorimetry curves (see Table 4). The parameter βmax was independent of the mixing rate but the parameters kdiss and cstep retreat change as the mixing rate changes and this will be discussed in the later sections. In the simulations, a product is allowed to precipitate when supersaturated with respect to the pore-solution. Thermodynamic calculations were implemented to compute the time of precipitation of C-S-H and Portlandite using the Truesdell Jones form of the Extended Debye Hückel equation [13]. A detailed description of the calculations can be found in [6]. For the precipitation of C-S-H, the evolution of the Ca/Si ratio, CaOH + and Ca 2 + ions were taken into account. The ion activity product equation used for C-S-H is described by Eq. (2), where y represents the ratio of concentration of CaOH + ions in the solution to the sum of the Ca 2 + and CaOH + concentration. The widely published CaO-SiO2 solubility relationship [14,15] and Ca/Si versus Ca 2 + relationship [10,16–18] are used to compute the equilibrium solubility product of C-S-H as a function of Ca/Si ratio. The saturation index of C-S-H was calculated at each step from these data. Precipitation was assumed to occur at the first step where SIC-S-H > 0. Subsequently calcium and silicate ions were removed from the solution to keep SIC-S-H = 0. A detailed description of the aforementioned steps can be found in [6]. n o Ca y n o n oy 2þ ð Si −2Þ − 2⋅Ca−y−2:0Þ 2− 1:0 þ ð2Þ IAP CSH ¼ Ca ⋅ CaOH ⋅ H 2 SiO4 ⋅fOH gð Si 2 ð2Þ The time at which Portlandite first precipitates is computed using the thermodynamic calculations at a supersaturation of 0.4. The SCD part of the simulation is terminated at this point. Subsequently the main heat evolution peak is simulated by nucleation and densifying growth (NDG) [5], this fits the main heat evolution peak and so there is a virtual starting point for the NDG regime at t0. The transition between the two regimes leaves a gap, which is discussed in more detail in [6]. Once switched to NDG mechanism,
Table 4 Calibrated values of dissolution parameters for the temperature of 293.15 K. Dissolution parameters
Values
kdiss βmax cstep retreat
24.0 (10− 6 mol.kg.m− 4.s− 1) 10− 22 (Unitless) 4.6 (10− 29 mol.kg. m− 4.s− 1)
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the amount of alite reacted with respect to time is governed by the equation: −δmalite ¼
1 ρðt r Þ dV extended;CSH ðt þ δt Þ−dV extended;CSH ðt Þ ð1−V real Þ ð3Þ k ρ0 ρðt r þ δt r Þ−ρðt r Þ þ V real;CSH ρ0
where k is the ratio of mass of alite reacting to the mass of C-S-H produced. The equation now takes into account both the space occupied by other phases and the change of density of C-S-H which is already present in the system with the extended volume: Gout ⋅t V extended ¼ ∫ aBV ⋅ 1− exp −Af dy
ð4Þ
and the nucleation density with the square of the parallel growth 2 2 rate (IrateGpar and IdensityGpar ). The dependence of the density of C-S-H on time can be written as: −kden ⋅t r ρðt r Þ ¼ ρ max −ðρ max −ρ min Þ⋅ exp ρ max −ρ min
ð6Þ
where ρmax (g.cm − 3) is the final density of outer C-S-H, ρmin (g.cm − 3) is the initial density of outer C-S-H, kden (g.cm − 3.h − 1) is the rate of densification of outer C-S-H and aBV is the total surface area of alite powder per unit volume (μm − 1). In order to compute the values of aBV, the total specific surface area of the particles aSSA (cm 2.g − 1), as measured from experimental techniques, were used as inputs and the dimensions were transformed to (μm − 1) using the density value of alite of 3.15.
0
4.2. Effect of speed mixing in DI-water
where Af can be written as: " Af ¼ π Ndensity ⋅
2 tr −
y2 G2out
!
2y3 t 3 y2 t þ r− 2r þ Nrate ⋅ 3 3Gout 3 Gout
!# :
ð5Þ
where, Gout (μm.h − 1) is the outward growth rate of outer C-S-H, tr (h) is the age of the product, Nrate (h − 3) is the rate coefficient of nucleation per unit area of the untransformed boundary and Ndensity (h − 2) is the nucleation density coefficient of the product. In Eq. (5), Nrate and Ndensity are respectively the products of the nucleation rate
Simulations for alite hydration in DI-water under different mixing conditions were implemented. Fig. 11 ((a) for early ages and (b) for the full plots) shows the comparison of the results obtained from the simulations with the measured rate of heat evolution. To capture the effects of mixing it was necessary to vary the values of the SCD parameters kdiss and cstep retreat. Amongst the NDG parameters, the nucleation density coefficient (Ndensity) was varied in the widest range with small variations in the ρmin and the t0 parameters. The SCD and NDG parameters used are listed in Table 5. The variation
Fig. 11. (a) and (b): Measured and simulated early age rates of heat evolution for alite systems in DI-water at different mixing rates. The mixing rates are indicated on top of each plot. The w/c ratio for all systems is 0.40.
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Fig. 11 (continued).
of these parameters with respect to the mixing rate is discussed in Section 5.
were needed (shown in Table 6). The variations of these parameters with respect to the mixing rate are discussed in Section 5.2. 5. Discussion
4.3. Effect of speed mixing in saturated lime solution 5.1. Impact of mixing up to the end of the induction period Simulations for the alite hydration in saturated lime solution at different mixing conditions were implemented and the comparison with experimental results is shown in Fig. 12 ((a) for early ages and (b) for the full plots). The difference in PSD of the two alites was taken into account in the simulations. To simulate the initial conditions of saturated lime, the values of initial calcium concentration of 22 mmol.L − 1 and pH of 12.45 were used as inputs. Similar to simulations described in Section 4.2, for alite hydrating in saturated lime solution, variations in SCD and NDG parameters Table 5 SCD and NDG parameters used for the simulations of alite hydrating in DI-water at different mixing conditions. The units of parameters are the same as described in Section 4.1. The other input parameters used for these simulations were w/c ratio and aSSA equal to 0.4 and 1300 cm2/g respectively. NDG parameters Mixing speed Hand 500 915 1350 1600 2000
ρmin 0.255 0.255 0.300 0.310 0.340 0.360
SCD parameters Nrate 0.006 0.006 0.006 0.006 0.006 0.006
Ndensity 0.0110 0.0115 0.0170 0.0210 0.0240 0.0270
Gout 0.215 0.215 0.215 0.215 0.215 0.215
kden 0.0025 0.0025 0.0025 0.0025 0.0025 0.0025
t0 0.7 0.7 0.6 0.4 0.4 0.4
kdiss 8.0 10.0 14.0 20.0 24.0 26.0
cstep retreat 3.2 3.3 3.5 3.8 4.6 4.8
βmax 10− 22 10− 22 10− 22 10− 22 10− 22 10− 22
As presented in a previous paper [4] and supported by simulations of hydration [6], we propose that up to the onset of the acceleration period, the hydration of alite is dominated by dissolution processes. There are basically two processes involved in dissolution: 1. Detachment of atoms from the surface 2. Transport of atoms through the solvent phase (diffusion).
Table 6 SCD and NDG parameters used for the simulations of alite hydrating in saturated lime solution at different mixing conditions. The units of the parameters are the same as described in Section 4.1. The other input parameters used for these simulations were w/c ratio, aSSA, initial calcium concentration and initial pH equal to 0.4, 1900 cm2/g, 22 mmol.L− 1 and 12.45 respectively. NDG parameters
SCD parameters
Mixing ρmin speed
Nrate
Ndensity Gout
kden
t0
kdiss
cstep retreat βmax
Hand 500 800 1600
0.006 0.006 0.006 0.006
0.014 0.015 0.017 0.022
0.0025 0.0025 0.0025 0.0025
− 1.0 − 1.3 − 1.3 − 0.7
24.0 24.0 24.0 24.0
3.80 4.25 4.45 4.60
0.167 0.170 0.175 0.208
0.237 0.237 0.237 0.237
10− 22 10− 22 10− 22 10− 22
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The first process may itself consist of several sub processes, which are not discussed further here, but basically one of these two processes will be rate controlling. Experimentally, if the rate of dissolution is controlled by surface reactions, there should be nearly no concentration gradient between the surrounding medium and the dissolving phase. It is however difficult to measure concentration gradients on this scale in aqueous solutions. In this case, a crude method to determine if surface reactions control the dissolution reaction is to vary the mixing rate [19,20]. Changing the mixing rate should vary the thickness of the diffuse electrical boundary layer through which the molecules must diffuse from the dissolving surface to the homogeneous bulk solution [15] (Fig. 13). If dissolution rate is independent of the stirring rate, it is then likely that the reaction rate is surface controlled. A rule of thumb proposed by Berner [21] states that for aqueous solutions, minerals with low solubility dissolve by surface control whereas high solubility minerals dissolve by transport control. His results are summarized in Table 7. The significant impact of mixing on the hydration kinetics of alite and cement (Fig. 4) indicates that, at least to a certain extent, the overall rate of dissolution is transport controlled. This is not surprising as we already know that alite is extremely reactive and its solubility constant, KSP, is thought to be between 3 10 0 [22] and 1.14 10 −-4 [23]. The upper value would place alite beside Na2CO3.10 H2O and MgSO4.7 H2O (transport controlled) in Table 4 while the lower value would place alite in a mixed controlled regime.
The surface reactions are therefore faster than the transport reactions when the solution is highly undersaturated immediately after the addition of water and the reaction will tend to be transport controlled. This indicates that during the first dissolution stage of hydration, the surface of alite is surrounded by a gradient of ionic concentrations as already proposed by Tadros [24], and that the mixing process, which only lasts for a few minutes can have a profound effect on this gradient as shown schematically in Fig. 13. As the undersaturation of the solution is reduced, surface reactions progressively become the limiting process [4,25]. The simulations indicate that the parameter affecting this regime in lime saturated systems (cstep retreat) does not need to be changed to capture the heat evolution of the different curves (except for the hand mixed sample but we think that in this case poor mixing could have resulted in incomplete wetting of the alite particles). So the impact of mixing really only affects the first minutes of hydration. Nevertheless the faster dissolution in this period, leads to significantly more ions entering solution and so to the earlier end of the induction period. Two processes may determine the end of the induction period: The precipitation of calcium hydroxide or the onset of rapid growth of C-S-H, or a combination of the two. In any case, both experiments [4,7] and simulations [6] indicate that this point occurs after the build up of definite quantity of ions in solution. In the experiments where alite and cement were mixed with saturated lime solution the evolution of the solution concentration is different, as shown from the simulations in Fig. 16. The system spends
Fig. 12. (a) and (b): Measured and simulated early age rates of heat evolution for alite systems in saturated lime solution at different mixing rates. The mixing rates are indicated on top of each plot. The w/c ratio for all systems is 0.40.
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Fig. 12 (continued).
only a very short time in the fast dissolution regime and also, because the concentration of calcium in the solution is already high, no large concentration gradients should exist. This results in an induction period which is almost insensitive to the rate of mixing as seen in the results (Fig. 10). Fig. 14 shows the variation of the dissolution parameters, kdiss and cstep retreat used in the simulations against the mixing rates. For the simulations in DI-water it was observed that the value of both these parameters increase with mixing speed. The increase in the kdiss parameter captures the increase in dissolution rate. This increase corresponds to the effect of the removal of the ions from the surface by the mixing process. With a higher value of kdiss, faster dissolution kinetics are observed (the dissolution equation as implemented in the model
Fig. 13. Schematic representation of the velocity profile and boundary layers close to the surface of material for a) low shearing conditions and b) high shearing conditions. The dark grey area represents alite and the medium and light greys represent schematically the Stern and diffuse electrical layers respectively. The diffuse electrical layer becomes thinner for higher shearing conditions according to Sangwal [15].
does not explicitly represent the process of removal of ions but provides a simple mathematical description of the dissolution kinetics). The increase in the cstep retreat with the mixing rate for the systems mixed with deionised water implies a higher density of steps at higher mixing rates. It could be hypothesized that at higher mixing rates, more etch pits open up during the undersaturation regime. These pits later evolve as a source for steps to retreat. Table 7 Limiting process for various substances arranged in order of solubilities in pure water [17]. Substance Surface reaction control KAlSi3O8 NaAlSi3O8 BaSO4 SrCO3 CaCO3 Ag2CrO4 SrSO4 Opaline SiO2 Mixed Control PbSO4 Transport control AgCl Ba(IO3)2 CaSO4.2 H2O NaSO4.10 H2O MgSO4.7 H2O Na2CO3.10 H2O KCl NaCl MgCl2.6 H2O
Solubility 3.10− 7 6.10− 7 1.10− 5 3.10− 5 6.10− 5 1.10− 4 9.10− 4 2.10− 3 1.10− 4 1.10− 5 8.10− 4 5.10− 3 2.10− 1 3.100 3.100 4.100 5.100 5.100
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Fig. 14. Variation in the SCD parameter (a) kdiss and (b) cstep retreat. The round and rectangular symbols represent the values used for alite systems in DI-water and saturated lime solution respectively. The variation in the NDG parameters is described in Section 5.2.
In the simulations with saturated lime solution, the SCD parameter kdiss was kept constant (Fig. 14). As can be seen in Fig. 15, in the presence of additional calcium ions and a higher initial pH value, the initial undersaturation of alite is much lower as compared to DI-water. As a result of this, dissolution starts in the surface reaction
Fig. 16. Simulated evolution of Ca2 + concentration in (a) DI-water (b) saturated lime solution. Once portlandite precipitates, the concentrations decrease (indicated schematically by arrows) from portlandite supersaturation value (shown by the thin grey line) towards portlandite solubility equilibrium (shown as the thick dotted black line).
controlled regime. This is reflected in these simulations, which are relatively insensitive to the value of kdiss chosen, as the system spends so little time in this regime. However, as the system transitions to the step retreat regime, the mixing rate imparts a small influence on the
Fig. 15. Simulated alite dissolution rate against alite undersaturation for the DI-water and saturated lime solution systems. In this figure, the dotted line marks the split between the two dissolution regimes: undersaturation and STEP-retreat regime. For the alite systems with saturated lime solution, the dissolution regime begins very close to the step-retreat regime. Once portlandite precipitates, the concentrations decrease causing an increase in alite undersaturation and alite dissolution rate (shown schematically by dotted arrow) until portlandite solubility equilibrium is reached (shown by circle symbol).
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dissolution rate. This is reflected in the cstep retreat parameter, which was observed to increase slightly with the mixing rate. This variation was also found for hydration of alite in DI-water. It therefore seems that the dissolution facilitated by step retreats is slightly faster at higher mixing rates. The simulated evolution of calcium ions in solution in the two cases is shown in Fig. 16. For the DI-water systems the first part of the curve is steep, corresponding to the fast dissolution regime controlled by diffusion/transport. This part of the curve is very sensitive to the speed of mixing; higher slope implying faster dissolution at higher mixing rates. Then there is a transition to a much less steep slope, corresponding to the surface reaction controlled regime. For the systems mixed with saturated calcium hydroxide solution, the first steep part is short and the systems enter very quickly into the slow regime. Beyond this transition point, the small variation in the step retreat parameter results only in a minor variation in the dissolution rate and, consequently, a small variation in the evolution of the calcium concentration. From these experiments and simulations, it seems that the overall rate of dissolution up to the end of the induction period is limited by two different processes. At the start, when mixed with pure water the anhydrous phase quickly dissolves with the formation of etch pits at high undersaturation, the concentration at the interface quickly approaches the critical concentration at which the surface reactions slow down. In this regime mixing homogenises the concentration in solution so the dissolution continues at a higher rate longer. However, when the initial solution is already saturated with lime, the systems enter quickly into the slow regime controlled by surface reactions and so are not significantly affected by mixing (see Figs. 15 and 16). Another interpretation of the effect of the speed of mixing on the early hydration based on the protective membrane theory would postulate that higher shearing forces from mixing promotes the breakup of the layer and exposes further the anhydrous surface to the solution. This would lead as well to an increased amount of anhydrous phase dissolving as the speed of mixing is more intense. However, this interpretation fails to explain the experiments performed in saturated lime solutions where no impact was seen on the length of the induction period. Since from the point of view of this theory the saturated lime condition is not rate limiting for the dissolution of the anhydrous phases, the effect of a change in the speed of mixing should lead to the same features as for the systems starting with deionised water. This is not what is seen experimentally which leads us to favour the hypothesis of solution controlled dissolution described previously. 5.2. Impact of mixing after the end of the induction period From the end of the induction period the hydration kinetics are controlled by nucleation and densifying growth of C-S-H. To capture the effect of mixing speed in the simulations, it was necessary to change the parameter pertaining to the number of nuclei of C-S-H per unit surface area, Ndensity, along with minor variations in the ρmin and t0 parameters. The variation in the ρmin parameter and its correlation to a physical process is not well understood at this stage. The t0 parameter for the DI-water systems decreased with increasing mixing rates suggesting an earlier start of the nucleation and growth regime of C-S-H in mixes prepared at higher mixing rates. This effect can also be seen in the simulated heat-evolution profiles (Fig. 11) and simulated calcium concentration profiles (Fig. 16). For the saturated lime solutions, negative values of t0 were used, indicating a very rapid transition to the regime of rapid C-S-H growth Fig. 17 shows the variation of the Ndensity parameter for the systems mixed with DI-water and those mixed with saturated lime solutions. In contrast to the regime prior to the induction period, the effects of mixing speed are the same in the two systems and it is clear that mixing speed significantly increases the number of nuclei of hydrates in the system. The increase in nucleation density for both
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Fig. 17. Variation of Ndensity for the simulations. The round and rectangular symbols represent the values used for alite systems in DI-water and saturated lime solution respectively. As can be interpreted, the density of C-S-H nuclei per unit surface area increases with the rate of mixing.
DI-water and saturated lime solutions is proportional and almost linear with respect to the mixing rate. The increase in nucleation density of hydrates is likely to be a purely mechanical effect. C-S-H precipitates (mainly on the surface of the alite grains), very quickly after the addition of water. During mixing these “nuclei” are dislodged from the surface forming extra nucleation sites away from the surfaces of the alite particles. The microstructures of the different systems (Figs. 8 and 9) support this hypothesis as it is clear that there is much more growth of C-S-H between the cement grains when the mixing speed is increased. This hypothesis of mechanical detachment of primary C-S-H, causing the acceleration of the main hydration peak is also consistent with the study of Thomas and co-workers [26], who showed that a small addition of pure C-S-H to pure tricalcium silicate causes a significant acceleration of the early hydration kinetics. They proposed that the C-S-H seeds increase the amount of early hydration by causing product to form in the capillary pore space away from the alite surfaces. It was shown that the model can reproduce the influence of the mixing conditions on hydration kinetics by varying the simulation parameters (a) kdiss and cstep retreat, which are related to the ease at which species detach from the dissolving surface of alite, and (d) Ndensity, which is related to the number of C-S-H nuclei per unit surface area of the particles. Experimental results confirm the increase in the nucleation density of hydrates as a higher packing density of C-S-H in the microstructural matrix is observed for samples prepared at higher mixing rates. 5.3. Differences between alite and cement In Fig. 3 it was seen that mixing had a much larger impact on the induction period of the alite systems than the cement systems. Although there are many differences between alite and cement: particle size, presence of other clinker phases, etc.; the most important difference is probably the presence of rapidly soluble alkali metal salts. As with saturated calcium hydroxide solution, the presence of the alkalis changes the solution composition of the mixing solution at the very early stages. Consequently the system should spend less time in the fast dissolution regime, which is most susceptible to the effects of mixing. 6. Conclusions This study contributes to our understanding of the mechanisms linking the mixing intensity (speed, duration and presence of aggregates) to the hydration process.
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• Prior to the end of the induction period, experiments in different mixing solutions coupled with simulations, indicate that when the solution is very undersaturated with respect to alite and dissolution is fast, the rate of dissolution is controlled by the rate of transport of ions from the surface into the bulk. Mixing leads to a decrease in thickness of the diffuse electrical layer and so there is an increase in dissolution rate in the first minutes with a reduced length of the induction period as a consequence. • When the mixing solution is close to the critical undersaturation of alite marking the onset of the step retreat regime, the rate of dissolution seems to be mainly controlled by surface reactions and the length of the induction period is not much affected by mixing. • After the end of the induction period, mixing increases the acceleration of the rate of reaction and decreases the time to the maximum heat peak. These effects are independent of the initial composition of the mixing solution. Simulations indicate that these effects are due to an increase in the density of C-S-H nuclei, most likely due to the mechanical action of mixing. • The presence of more C-S-H nuclei between the cement particles leads to a denser and more homogeneous microstructure, which is evident even after long hydration times. • The agreement between the experimental results and the simulations based on the two mechanisms of solution controlled dissolution prior to the end of the induction period and then nucleation with densifying growth, during the main heat evolution peak; supports the validity of these mechanism and demonstrates the advantages of being able to look at their consequence quantitatively with the modelling platform μic. Acknowledgements Patrick Juilland is grateful for financial support from Sika Technology AG for his PhD study, from which this work results. He also wants to thank Dr. D. Lootens who initiated this work at Sika Technology AG. Aditya Kumar is grateful for the support of the Swiss National Science Foundation for support of his work on the development of the μic modelling platform. References [1] D.A. Williams, A.W. Saak, H.M. Jennings, The influence of mixing on the rheology of fresh cement paste, Cem. Concr. Res. 29 (1999) 1491–1496.
[2] D. Dollimore, R.J. Mangabhai, Effect of mixing time on heat evolution pattern of cement pastes, Thermochim. Acta 85 (1985) 223–226. [3] J.J. Thomas, H.M. Jennings, Effect of D2O and mixing on the early hydration kinetics of tricalcium silicate, Chem. Mater. 11 (1999) 1907–1914. [4] P. Juilland, E. Gallucci, R.J. Flatt, K.L. Scrivener, Dissolution theory applied to the induction period in alite hydration, Cem. Concr. Res. 40 (2010) 831–844. [5] S. Bishnoi, K.L. Scrivener, μic: a new platform for modelling the hydration of cements, Cem. Concr. Res. 39 (2009) 266–274. [6] A. Kumar, K.L. Scrivener, Modelling early age hydration kinetics of alite, Cem. Concr. Res. 42 (2012) 903–918. [7] M.M. Costoya, Effect of particle size distribution on the hydration kinetics and microstructural development of tricalcium silicate, EPFL PhD Thesis, 2008. [8] A. Korpa, R. Trettin, The influence of different drying methods on cement paste microstructures as reflected by gas adsorption: comparison between freeze-drying (F-drying), D-drying, P-drying and oven-drying methods, Cem. Concr. Res. 36 (2006) 634–649. [9] M.L. Brown, H.M. Jennings, W.B. Ledbetter, On the generation of heat during the mixing of cement pastes, Cem. Concr. Res. 20 (1990) 471–474. [10] H.W. Taylor, Cement Chemistry, 2nd edition Thomas Telford Publishing, 1997. [11] J.M. Makar, G.W. Chan, End of the induction period in ordinary Portland cement as examined by high-resolution scanning electron microscopy, J. Am. Ceram. Soc. 91 (2008) 1292–1299. [12] D. Damidot, Etude de l'hydratation du silicate tricalcique en suspension diluées par microcalorimétrie isotherme, Université de Bourgogne, PhD Thesis, 1990. [13] A.H. Truesdell, B.F. Jones, WATEQ, a computer program for calculating chemical equilibria of natural waters, J. Res. U.S. Geol. Surv. 2 (1974) 233–274. [14] H.M. Jennings, Aqueous solubility relationships for two types of calcium silicate hydrate, J. Am. Ceram. Soc. 69 (1986) 614–618. [15] P.W. Brown, E. Franz, G. Frohnsdorff, H.F.W. Taylor, Analyses of the aqueous phase during early C3S hydration, Cem. Concr. Res. 14 (1984) 257–262. [16] S.A. Greenberg, T.N. Chang, Investigations of the colloidal hydrated calcium silicates II. Solubility relationships in the calcium oxide–silica–water system at 25 °C, J. Phys. Chem. 69 (1965) 182–188. [17] K. Fuji, W. Kondo, Heterogeneous equilibrium of calcium silicate hydrate in water at 30 °C, J. Chem. Soc., Dalton Trans. 2 (1981) 645–651. [18] A. Atkinson, J.A. Hearne, C.F. Knights, Aqueous Chemistry and Thermodynamic Modelling of CaO–SiO2–H2O Gels, AERE R 12548, UKAEA, 1987. [19] K. Sangwal, Etching of Crystals, vol. 15, North Holland, 1987. [20] A.C. Lasaga, Atomic treatment of mineral–water surface reactions, Rev. Mineral. 23 (1990) 17–85. [21] R.A. Berner, Rate control of mineral dissolution under earth surface conditions, Am. J. Sci. 278 (1978) 1235–1252. [22] H.N. Stein, Thermodynamic considerations on the hydration mechanisms of Ca3SiO5 and Ca3Al2O6, Cem. Concr. Res. 2 (1972) 167–177. [23] F. Bellmann, D. Damidot, B. Möser, J. Skibsted, Improved evidence for the existence of an intermediate phase during hydration of tricalcium silicate, Cem. Concr. Res. 40 (2010) 875–884. [24] M.E. Tadros, J. Skalny, R.S. Kalyoncu, Early hydration of tricalcium silicate, J. Am. Ceram. Soc. 59 (1976) 344–347. [25] R.S. Arvidson, A. Luttge, Mineral dissolution kinetics as a function of distance from equilibrium — new experimental results, Chem. Geol. 269 (2010) 79–88. [26] J.J. Thomas, H.M. Jennings, J.J. Chen, Influence of nucleation seeding on the hydration mechanisms of tricalcium silicate and cement, J. Phys. Chem. C 113 (2009) 4327–4334.
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Cement and Concrete Research journal homepage: http://ees.elsevier.com/CEMCON/default.asp
Effect of mixing on the early hydration of alite and OPC systems Patrick Juilland a,⁎, Aditya Kumar b, Emmanuel Gallucci a, Robert J. Flatt c, Karen L. Scrivener b a b c
Sika Technology AG, Corporate Research and Analytics, Tüffenwies 16, Zürich CH-8048, Switzerland EPFL-STI-IMX, Laboratoire des Matériaux de Construction, Station 15, Lausanne CH-1015, Switzerland ETHZ, D-BAUG, Physical Chemistry of Building Materials, Schafmattstrasse 6, Zürich CH-8093, Switzerland
a r t i c l e
i n f o
Article history: Received 15 October 2010 Accepted 24 June 2011 Keywords: Hydration (A) Mixing (A) Calorimetry (A)
a b s t r a c t The kinetics of hydration of cementitious materials is sensitive to the mixing procedure. High shear mixing conditions lead to an increase in the kinetics of hydration at early age compared to low shearing conditions such as hand mixing. In this study the effect of mixing speed and procedure was studied on alite and Portland cement in the presence or not of aggregates. The kinetics of hydration was monitored using isothermal calorimetry at 20 °C. The early reactivity was enhanced both with an increase in the speed of mixing and the shearing conditions. The principal features are a shortening of the induction period; a higher rate of hydrate precipitation during the acceleration period as well as an increase in the height of the main heat evolution peak. Analysis of the results in terms of dissolution theory, coupled with quantitative simulation with the μic modelling platform indicate different effects of mixing prior to and after the end of the induction period. Before the end of the induction period mixing has an impact on the rate of dissolution in the fast dissolution regime and high undersaturation, which appears to be (at least partially) controlled by the rate of transport of ions away from the alite surface. After the end of the induction period the main effect of mixing appears to be the production of more C-S-H nuclei, due to the possible detachment of the primary C-S-H (metastable) by mechanical action. This higher nucleation density leads to a denser microstructure for systems mixed at high intensities. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Mixing is a crucial step for any concrete application. Poor mixing will result in large inhomogeneities [1] and poor packing of the cement particles resulting in a larger amount of porosity, which can lower mechanical properties and reduce durability. Nowadays the control of viscosity by the addition of specific organic molecules has led to an engineering revolution in cementitious materials, with enhancement of mechanical properties through the reduction of water to cement ratios. Although many studies exist on the rheology of cementitious materials, the focus has mainly been on how hydration affects workability rather than the effect of mixing on the kinetics of hydration. Dollimore and Mangabhai [2] reported that increasing mixing time accelerated hydration. However, no attempt was made to explain the mechanisms responsible for this acceleration. Thomas and Jennings [3] found that rapid hand mixing decreased both the time to reach the heat evolution peak for C3S hydration and the magnitude of this peak as compared to pastes which underwent only minimal mixing. The mechanisms by which the hydration kinetics is affected are still not understood. Therefore the aim of this paper is to present a systematic study on the influence of the mixing procedure as well
⁎ Corresponding author. Tel.: + 41 58 436 42 83. E-mail address: [email protected] (P. Juilland). 0008-8846/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconres.2011.06.011
as of the additions of aggregates on the kinetics of hydration of alite and Portland cement systems. The results obtained are then discussed from the perspective of the dissolution theory which postulates that a decrease of the solution undersaturation will limit the rate of dissolution of the anhydrous phase, switching from a fast dissolution process by etch pit opening at high undersaturation to a slow dissolution process taking place only by step retreat at a critical undersaturation [4] and the mechanisms implemented in the μic modelling platform [5,6]. 2. Experimental procedure 2.1. Materials and methods Portland cement (CEM 1 42.5) and alite synthesised in our laboratory were used in this study. Alite was obtained by mixing calcium carbonate (precipitated GR for analysis, Merck), silica (highly dispersed extra pure, Merck), magnesium oxide (GR for analysis, Merck) and alumina (anhydrous γ-alumina, Merck) and deionised water in a ball mill for 24 h and firing the dry product at 1600 °C for 10 h. Magnesium oxide and alumina were added in order to favour grain growth and to stabilise the monoclinic structure of alite (MIII polymorph), which is similar to the alite found in Portland cement. The alite clinker was quenched in air and ground in an annular grinder for 1 min. Costoya [7] showed that monoclinic alite, ground in this way; has a particle size distribution similar to a CEMI 42.5 ordinary Portland cement. The chemical
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Table 1 XRF and XRD-Rietveld of the cement and alite used in this study (wt.%).
Table 3 Summary of mixing procedures.
XRF
CEMI 42.5
XRD
Normo 4
Alite
LOI CaO SiO2 Al2O3 MgO Na2O SO3 P2O5 K2O TiO2 Fe2O3
2.21 64 19.6 4.52 2 0.15 3.02 0.2 1.3 0.27 2.75
C3S C2S C4AF C3A C M CaSO4-2H2O CaSO4-0.5H2O CaSO4 S CH CaCO3 K2SO4
65.9 8.6 8.4 6 0.7 1.4 0 1.2 1.8 0 0 4.3 1.7
99.6 – – – 0.4 – – – – – – – –
compositions as well as the quantitative Rietveld refinement for both starting materials are reported in Table 1. For mortar samples, silica beads of different narrow particle size distributions (Table 2) were used as model aggregates (Type S, Sigmund Lindner GmbH) in a cement to aggregate weight ratio of 0.66. The water to cement ratio, w/c, was set at 0.4 for all mixes and deionised water was used for every test except when mentioned. Hand mixing was done with a common plastic spatula. For mechanical mixing a steel paddle with six blades was used at different speeds from 200, to 2000 rpm. Two mixing procedures were applied:
Material
w/c
Speeds
Aggregates
Mixing method
1. Alite/OPC
0.4
–
3. Alite/OPC
0.4
Hand mixing, 200, 500, 915, 1370, 1600, 2000 rpm Hand mixing, 500, 1200 rpm
1 and 2 step mixing 1 step mixing
Silica beads
7 days. Hydration was stopped by freeze drying. As with all methods for stopping hydration, some changes to the microstructure may occur. Nevertheless, this is one of the least damaging techniques according to Korpa and Trettin [8]. The samples were then impregnated with an epoxy resin and polished with progressively smaller grades of diamond powder until a mirror surface was obtained. The polished sections were then qualitatively observed with a SEM (Quanta 200) at an accelerating voltage of 15 kV. 3. Results 3.1. Energy input during mixing Fig. 1 reports the temperature measurement of alite and cement pastes before and after each mixing step. It can be seen that it
- One step mixing: 2 min at a given speed, - Two step mixing: 2 min at a given speed followed by a delay of 13 min before a second mix of 1 min at the same speed. Alite and cement pastes were studied with both one and two step mixing at 7 different mixing speeds. The effect on the size of the silica beads was then investigated at three speeds (hand mixing, 500 rpm and 1200 rpm) using the one step mixing procedure. All procedures are summarized in Table 3. The mixed quantities were kept at 46 g for pastes and 96 g for mortars in order to avoid variations due to mass change. Temperature measurements were performed before and after each mixing step in order to assess the energy input induced during the mixing. 2.2. Isothermal calorimetry and SEM Isothermal calorimetry measurements were carried out in a TAM Air microcalorimeter (Thermometrics) at 20 °C. Samples were prepared by external mixing according to the methods mentioned above. The quantity of sample put into the calorimeter cell was 10 g in the case of the pastes and 20 g for mortars. Every sample was balanced with a reference cell containing an amount of water having the same heat capacity as the paste or mortar. The time constant of the calorimeter is 4 min and corrected for, although it is hardly relevant in the time scale of the results presented here. Scanning electron microscopy (SEM) observations were made on alite pastes hand mixed and mechanically mixed at 915 and 1600 rpm. The samples were hydrated until the end of the acceleration period (maximum heat flow of silicate hydration) and until Table 2 Characteristics of the silica beads. Diameter (mm) 0.1–0.2 0.2–0.3 0.3–0.4 0.4–0.6 0.5–0.75
Bulk density (g·cm− 3)
Mean specific surface area (g·cm− 2)
1.52 1.51 1.51 1.50 1.49
6.3·10− 3 8.8·10− 3 1.25·10− 2 1.5·10− 2 2.2·10− 2
Fig. 1. Temperature evolution before and after mixing for paste of (a) alite and (b) PC.
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Fig. 2. Effect of different initial temperatures on OPC system for the same mixing procedure.
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increases almost linearly with the speed and the first mixing step heats the system more than the second one according to the time of mixing. This input of energy converted into heat due to friction [9] as well as an increase of alite dissolution may enhance the kinetics of hydration at early age since it is known that temperature can enhance markedly the kinetics of hydration [10]. However after 13 min, all systems have returned to the same temperature and the temperature would be even more rapidly stabilised in the isothermal calorimeter. Temperature rise should therefore only affect the first minutes of hydration. To check the influence of temperature during the mixing itself and its effect on the first minutes of hydration, two OPC samples were mixed at the same speed but with different initial temperatures. In order to reach these conditions, two stainless steel mixing paddles were kept at two different temperatures: one at room temperature and the other one in a furnace at 110 °C. After one step of mixing at 500 rpm for 2 min, the temperatures of the pastes were respectively 24 °C and 29.5 °C. This difference in temperature is almost equal to the largest temperature difference recorded for the least and most intensely mixed samples. The
Fig. 3. Evolution of the kinetics of hydration for (a) an alite paste and (b) a cement paste (w/c = 0.4) mixed 2 min at various speeds of mixing. (c) and (d) are zoomed area of (a) and (b) respectively.
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results in Fig. 2 show that this temperature difference has no effect on the length of the induction period and that the rate of heat released during the acceleration period is almost identical for both samples. There is a small difference in the height of the heat evolution peaks as well as the level of heat during the induction period. Nevertheless, it is clear that the temperature during the first few minutes, for samples, which as here, quickly returned to 20 °C in the isothermal calorimeter, has very little impact on the hydration process compared to the results obtained with different speeds of mixing presented in the following sections. Therefore we assume that the heating due to mixing can be neglected as having any significant impact on the hydration kinetics. It should be noted, that if heating effects persist, as in the field, they would be likely to have a more significant impact.
3.2. Effect of speed of mixing on alite and cement pastes The evolution of the heat released during the hydration of alite and cement pastes mixed with deionised water (w/c = 0.4) at different speeds for 2 min is presented in Fig. 3. In Fig. 3, it is seen that the different speeds of mixing have an impact on both alite and cement pastes. Since there is no major change
Fig. 4. Comparison between the one (dashed lines) and two step mixing procedures (solid lines) on a) alite and b) cement pastes.
in the shape of the curves it is assumed that the underlying mechanisms of hydration remain the same. The hydration is accelerated: • The induction period is reduced as the speed of mixing increases; • The slope of the heat flow curve during the acceleration period increases with the speed of mixing. Mixing has a larger impact on the reduction of the induction period of alite compared to PC, this is discussed further in Section 5 below. The definition of the induction period is still a subject of controversy; here we consider the “apparent” induction period to end when the heat flow curve starts to increase at the beginning of the acceleration period. The two step procedure shows similar trends with somewhat more acceleration compared to the one step mixing procedure as shown in Fig. 4. There seems to be some effect on the minimum heat evolution during the induction period, which is most marked between the hand mixed systems and the mechanically mixed ones. In this case, it is probable that this is due to failure to completely break up the agglomerations of particles in the hand mixed systems, which limits the surface of the cement available for dissolution. Fig. 5 shows the end of the induction period and the slope of the heat flow increase during the acceleration period as a function of the mixing speed for the paste systems. (The end of the induction period was arbitrarily defined by taking the intersection between the
Fig. 5. Plot of a) the time at the end of induction period and b) the slope of heat released during the acceleration period as function of the speed of mixing.
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horizontal tangent at the minimum value during the induction period and the one for the acceleration period.) The effect on the slope of the heat flow increase seems to be similar for both systems and there is an almost linear relation between mixing speed and the slope of acceleration. This suggests a mechanistic link between the two.
Fig. 7 a) and b) shows the time to reach the maximum heat flow during the acceleration period as a function of the diameter of the silica beads at various speeds of mixing. The values are almost constant for different bead sizes for each speed. It appears that the size of silica beads has no effect within the range of sizes studied.
3.3. Effect of aggregates
3.4. Observation of alite microstructure for pastes mixed at different speeds
In these systems, silica beads with narrow particle size distributions were added. The evolution of the heat flow for the mortar containing beads of size 0.3–0.4 mm for alite and PC systems is compared with the reference pastes without aggregates in Fig. 6. It can be seen that as the speed increases, the difference between the curves of mortars compared to pastes becomes larger. The presence of the silica beads seems to accentuate the shearing conditions during mixing and further increase the kinetics of hydration. A small peak appearing before the acceleration period is observed for mixing at 500 rpm and 1200 rpm for alite systems. This feature was already observed by Costoya [7] on C3S as well as by Makar and Chan [11] on PC. This feature might result from an endothermic peak associated with the precipitation of hydrates such as Portlandite [12] or a delay between the precipitation of Portlandite and the onset of rapid growth of C-S-H as suggested by Kumar and Scrivener [6].
To study the influence of mixing speed on the microstructural development of alite systems the hydration was stopped at the maximum heat flow of the acceleration period by freeze drying. Fig. 8 shows that as the shear forces increase during mixing, the resulting matrix is much denser at the end of the acceleration period. There are clear differences between samples mixed mechanically and those which are hand mixed. In the hand mixed paste capillary porosity can be easily identified. Small crystals of CH embedded in the C-S-H matrix are present in larger amount for pastes mixed at 915 rpm and 1600 rpm compared to the hand mixed system. The microstructure appears even finer at mixing speeds of 1600 rpm. Fig. 9 shows the microstructures of the alite systems at 7 days of hydration. It can be seen that in the hand mixed specimen some large capillary pores are still present whereas the two other mixing conditions
Fig. 6. Heat of hydration for pastes (dashed lines) and mortars (solid lines) with the size of beads from 0.3 to 0.4 mm for a) the alite and b) the OPC systems at different mixing speeds.
Fig. 7. Time to reach the maximum peak of heat flow during the acceleration period in function of the average diameter of aggregates at three different speeds of mixing for a) alite systems and b) PC systems.
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Fig. 8. Micrographs of polished section of alite samples at the maximum heat flow mixed twice by (a) hand mixing (16 h00 of hydration), (b) at 915 rpm (13 h30 of hydration) and (c) at 1600 rpm (12 h30 of hydration).
Fig. 9. Micrographs of polished section of alite samples at 7 days of hydration mixed twice by (a) hand mixing, (b) at 915 rpm and (c) at 1600 rpm.
lead to very similar dense microstructures at these later ages of hydration. We are aware that these comparisons remain qualitative since the degrees of hydration are slightly different for each system. Nevertheless, these images show that there is an effect of the mixing on the morphology of the precipitating hydrates which is not just due to a difference in the degree of hydration.
saturation of the mixing water influences the effects of mixing on hydration, alite was mixed again at different speeds but starting with a solution already saturated with lime. The results are presented in Fig. 10. It can be observed that the rate of mixing does not affect the length of the induction period when the initial solution is already saturated with lime. There is a slight delay for the hand mixed system which may be due to poor deagglomeration of the alite particles. This would lead to a lower amount of anhydrous surface in contact with water and a slightly longer induction period. However it should be noted that, even in this case, the induction period is much shorter than in Fig. 5. This is not due to an accelerating effect of the saturated lime solution but to a different particle size distribution since it is impossible to obtain identical particle size distributions for different
3.5. Effect of mixing solution In a recent publication [4] we argued that the slowdown in the initial rate of reaction and the low rate of reaction during the induction period are caused by the direct impact of the undersaturation of the solution on the dissolution of alite. In order to investigate how the state of
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earlier in this paper. Following this mechanism, the dissolution of alite is described by Eq. (1). dmalite ðt Þ ¼ ½−kdiss ⋅aSSA ⋅ðβmax −βalite ðt ÞÞ−aSSA ⋅cstep dt
retreat
ð1aÞ
for βalite(t) b = βmax, and dmalite ðt Þ ¼ −aSSA ⋅cstep retreat dt
Fig. 10. a) Evolution of the kinetics of hydration for an alite paste (w/c = 0.4) mixed 2 min at various mixing speeds starting with a saturated lime solution and b) an enlargement of the induction period.
batches of alite ground separately. Therefore absolute comparisons cannot be made; however, since the same batch was used within each set of experiments relative comparisons are valid.
4. Modelling the effect of mixing on alite hydration 4.1. Overview of the simulation method The 3D microstructure modelling platform μic [5] was used in this study. The model has been used to study the hydration of alite describing the effect of particle size of the alite in a recent study [6]. Two different mechanisms are used in the simulation: one – solution controlled dissolution (SCD) – during the dissolution and induction periods and another – nucleation with densifying growth (NDG) – during the acceleration and deceleration periods of the main heat evolution peak. During the SCD period thermodynamics are also integrated in the model in order to capture the influence of the pore solution chemistry on the kinetics. The SCD mechanism of alite has been integrated in the model based upon the hypothesis proposed by Juilland et al. [4] described
ð1bÞ
for βalite(t) > βmaxwhere, βmax is the critical degree of undersaturation of alite, kdiss (μmol.kg.m − 4.s − 1) is a dissolution rate constant, aSSA (m 2.kg − 1) is the specific surface area of particles, and cstep retreat (μmol.kg.m − 4.s − 1) is the step retreat constant. In the simulations, βalite(t) is calculated at each step based on the amount of alite dissolved, the amount of water present in the representative volume and the precipitation of phases. The dissolution of alite follows 2 regimes (1a) undersaturation greater than critical undersaturation, βmax, (1b) step retreat regime beyond critical undersaturation. Eq. (1a) contains three parameters that needed calibration: kdiss, cstep retreat and βmax. These parameters were fitted to the calorimetry curves (see Table 4). The parameter βmax was independent of the mixing rate but the parameters kdiss and cstep retreat change as the mixing rate changes and this will be discussed in the later sections. In the simulations, a product is allowed to precipitate when supersaturated with respect to the pore-solution. Thermodynamic calculations were implemented to compute the time of precipitation of C-S-H and Portlandite using the Truesdell Jones form of the Extended Debye Hückel equation [13]. A detailed description of the calculations can be found in [6]. For the precipitation of C-S-H, the evolution of the Ca/Si ratio, CaOH + and Ca 2 + ions were taken into account. The ion activity product equation used for C-S-H is described by Eq. (2), where y represents the ratio of concentration of CaOH + ions in the solution to the sum of the Ca 2 + and CaOH + concentration. The widely published CaO-SiO2 solubility relationship [14,15] and Ca/Si versus Ca 2 + relationship [10,16–18] are used to compute the equilibrium solubility product of C-S-H as a function of Ca/Si ratio. The saturation index of C-S-H was calculated at each step from these data. Precipitation was assumed to occur at the first step where SIC-S-H > 0. Subsequently calcium and silicate ions were removed from the solution to keep SIC-S-H = 0. A detailed description of the aforementioned steps can be found in [6]. n o Ca y n o n oy 2þ ð Si −2Þ − 2⋅Ca−y−2:0Þ 2− 1:0 þ ð2Þ IAP CSH ¼ Ca ⋅ CaOH ⋅ H 2 SiO4 ⋅fOH gð Si 2 ð2Þ The time at which Portlandite first precipitates is computed using the thermodynamic calculations at a supersaturation of 0.4. The SCD part of the simulation is terminated at this point. Subsequently the main heat evolution peak is simulated by nucleation and densifying growth (NDG) [5], this fits the main heat evolution peak and so there is a virtual starting point for the NDG regime at t0. The transition between the two regimes leaves a gap, which is discussed in more detail in [6]. Once switched to NDG mechanism,
Table 4 Calibrated values of dissolution parameters for the temperature of 293.15 K. Dissolution parameters
Values
kdiss βmax cstep retreat
24.0 (10− 6 mol.kg.m− 4.s− 1) 10− 22 (Unitless) 4.6 (10− 29 mol.kg. m− 4.s− 1)
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the amount of alite reacted with respect to time is governed by the equation: −δmalite ¼
1 ρðt r Þ dV extended;CSH ðt þ δt Þ−dV extended;CSH ðt Þ ð1−V real Þ ð3Þ k ρ0 ρðt r þ δt r Þ−ρðt r Þ þ V real;CSH ρ0
where k is the ratio of mass of alite reacting to the mass of C-S-H produced. The equation now takes into account both the space occupied by other phases and the change of density of C-S-H which is already present in the system with the extended volume: Gout ⋅t V extended ¼ ∫ aBV ⋅ 1− exp −Af dy
ð4Þ
and the nucleation density with the square of the parallel growth 2 2 rate (IrateGpar and IdensityGpar ). The dependence of the density of C-S-H on time can be written as: −kden ⋅t r ρðt r Þ ¼ ρ max −ðρ max −ρ min Þ⋅ exp ρ max −ρ min
ð6Þ
where ρmax (g.cm − 3) is the final density of outer C-S-H, ρmin (g.cm − 3) is the initial density of outer C-S-H, kden (g.cm − 3.h − 1) is the rate of densification of outer C-S-H and aBV is the total surface area of alite powder per unit volume (μm − 1). In order to compute the values of aBV, the total specific surface area of the particles aSSA (cm 2.g − 1), as measured from experimental techniques, were used as inputs and the dimensions were transformed to (μm − 1) using the density value of alite of 3.15.
0
4.2. Effect of speed mixing in DI-water
where Af can be written as: " Af ¼ π Ndensity ⋅
2 tr −
y2 G2out
!
2y3 t 3 y2 t þ r− 2r þ Nrate ⋅ 3 3Gout 3 Gout
!# :
ð5Þ
where, Gout (μm.h − 1) is the outward growth rate of outer C-S-H, tr (h) is the age of the product, Nrate (h − 3) is the rate coefficient of nucleation per unit area of the untransformed boundary and Ndensity (h − 2) is the nucleation density coefficient of the product. In Eq. (5), Nrate and Ndensity are respectively the products of the nucleation rate
Simulations for alite hydration in DI-water under different mixing conditions were implemented. Fig. 11 ((a) for early ages and (b) for the full plots) shows the comparison of the results obtained from the simulations with the measured rate of heat evolution. To capture the effects of mixing it was necessary to vary the values of the SCD parameters kdiss and cstep retreat. Amongst the NDG parameters, the nucleation density coefficient (Ndensity) was varied in the widest range with small variations in the ρmin and the t0 parameters. The SCD and NDG parameters used are listed in Table 5. The variation
Fig. 11. (a) and (b): Measured and simulated early age rates of heat evolution for alite systems in DI-water at different mixing rates. The mixing rates are indicated on top of each plot. The w/c ratio for all systems is 0.40.
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Fig. 11 (continued).
of these parameters with respect to the mixing rate is discussed in Section 5.
were needed (shown in Table 6). The variations of these parameters with respect to the mixing rate are discussed in Section 5.2. 5. Discussion
4.3. Effect of speed mixing in saturated lime solution 5.1. Impact of mixing up to the end of the induction period Simulations for the alite hydration in saturated lime solution at different mixing conditions were implemented and the comparison with experimental results is shown in Fig. 12 ((a) for early ages and (b) for the full plots). The difference in PSD of the two alites was taken into account in the simulations. To simulate the initial conditions of saturated lime, the values of initial calcium concentration of 22 mmol.L − 1 and pH of 12.45 were used as inputs. Similar to simulations described in Section 4.2, for alite hydrating in saturated lime solution, variations in SCD and NDG parameters Table 5 SCD and NDG parameters used for the simulations of alite hydrating in DI-water at different mixing conditions. The units of parameters are the same as described in Section 4.1. The other input parameters used for these simulations were w/c ratio and aSSA equal to 0.4 and 1300 cm2/g respectively. NDG parameters Mixing speed Hand 500 915 1350 1600 2000
ρmin 0.255 0.255 0.300 0.310 0.340 0.360
SCD parameters Nrate 0.006 0.006 0.006 0.006 0.006 0.006
Ndensity 0.0110 0.0115 0.0170 0.0210 0.0240 0.0270
Gout 0.215 0.215 0.215 0.215 0.215 0.215
kden 0.0025 0.0025 0.0025 0.0025 0.0025 0.0025
t0 0.7 0.7 0.6 0.4 0.4 0.4
kdiss 8.0 10.0 14.0 20.0 24.0 26.0
cstep retreat 3.2 3.3 3.5 3.8 4.6 4.8
βmax 10− 22 10− 22 10− 22 10− 22 10− 22 10− 22
As presented in a previous paper [4] and supported by simulations of hydration [6], we propose that up to the onset of the acceleration period, the hydration of alite is dominated by dissolution processes. There are basically two processes involved in dissolution: 1. Detachment of atoms from the surface 2. Transport of atoms through the solvent phase (diffusion).
Table 6 SCD and NDG parameters used for the simulations of alite hydrating in saturated lime solution at different mixing conditions. The units of the parameters are the same as described in Section 4.1. The other input parameters used for these simulations were w/c ratio, aSSA, initial calcium concentration and initial pH equal to 0.4, 1900 cm2/g, 22 mmol.L− 1 and 12.45 respectively. NDG parameters
SCD parameters
Mixing ρmin speed
Nrate
Ndensity Gout
kden
t0
kdiss
cstep retreat βmax
Hand 500 800 1600
0.006 0.006 0.006 0.006
0.014 0.015 0.017 0.022
0.0025 0.0025 0.0025 0.0025
− 1.0 − 1.3 − 1.3 − 0.7
24.0 24.0 24.0 24.0
3.80 4.25 4.45 4.60
0.167 0.170 0.175 0.208
0.237 0.237 0.237 0.237
10− 22 10− 22 10− 22 10− 22
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The first process may itself consist of several sub processes, which are not discussed further here, but basically one of these two processes will be rate controlling. Experimentally, if the rate of dissolution is controlled by surface reactions, there should be nearly no concentration gradient between the surrounding medium and the dissolving phase. It is however difficult to measure concentration gradients on this scale in aqueous solutions. In this case, a crude method to determine if surface reactions control the dissolution reaction is to vary the mixing rate [19,20]. Changing the mixing rate should vary the thickness of the diffuse electrical boundary layer through which the molecules must diffuse from the dissolving surface to the homogeneous bulk solution [15] (Fig. 13). If dissolution rate is independent of the stirring rate, it is then likely that the reaction rate is surface controlled. A rule of thumb proposed by Berner [21] states that for aqueous solutions, minerals with low solubility dissolve by surface control whereas high solubility minerals dissolve by transport control. His results are summarized in Table 7. The significant impact of mixing on the hydration kinetics of alite and cement (Fig. 4) indicates that, at least to a certain extent, the overall rate of dissolution is transport controlled. This is not surprising as we already know that alite is extremely reactive and its solubility constant, KSP, is thought to be between 3 10 0 [22] and 1.14 10 −-4 [23]. The upper value would place alite beside Na2CO3.10 H2O and MgSO4.7 H2O (transport controlled) in Table 4 while the lower value would place alite in a mixed controlled regime.
The surface reactions are therefore faster than the transport reactions when the solution is highly undersaturated immediately after the addition of water and the reaction will tend to be transport controlled. This indicates that during the first dissolution stage of hydration, the surface of alite is surrounded by a gradient of ionic concentrations as already proposed by Tadros [24], and that the mixing process, which only lasts for a few minutes can have a profound effect on this gradient as shown schematically in Fig. 13. As the undersaturation of the solution is reduced, surface reactions progressively become the limiting process [4,25]. The simulations indicate that the parameter affecting this regime in lime saturated systems (cstep retreat) does not need to be changed to capture the heat evolution of the different curves (except for the hand mixed sample but we think that in this case poor mixing could have resulted in incomplete wetting of the alite particles). So the impact of mixing really only affects the first minutes of hydration. Nevertheless the faster dissolution in this period, leads to significantly more ions entering solution and so to the earlier end of the induction period. Two processes may determine the end of the induction period: The precipitation of calcium hydroxide or the onset of rapid growth of C-S-H, or a combination of the two. In any case, both experiments [4,7] and simulations [6] indicate that this point occurs after the build up of definite quantity of ions in solution. In the experiments where alite and cement were mixed with saturated lime solution the evolution of the solution concentration is different, as shown from the simulations in Fig. 16. The system spends
Fig. 12. (a) and (b): Measured and simulated early age rates of heat evolution for alite systems in saturated lime solution at different mixing rates. The mixing rates are indicated on top of each plot. The w/c ratio for all systems is 0.40.
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Fig. 12 (continued).
only a very short time in the fast dissolution regime and also, because the concentration of calcium in the solution is already high, no large concentration gradients should exist. This results in an induction period which is almost insensitive to the rate of mixing as seen in the results (Fig. 10). Fig. 14 shows the variation of the dissolution parameters, kdiss and cstep retreat used in the simulations against the mixing rates. For the simulations in DI-water it was observed that the value of both these parameters increase with mixing speed. The increase in the kdiss parameter captures the increase in dissolution rate. This increase corresponds to the effect of the removal of the ions from the surface by the mixing process. With a higher value of kdiss, faster dissolution kinetics are observed (the dissolution equation as implemented in the model
Fig. 13. Schematic representation of the velocity profile and boundary layers close to the surface of material for a) low shearing conditions and b) high shearing conditions. The dark grey area represents alite and the medium and light greys represent schematically the Stern and diffuse electrical layers respectively. The diffuse electrical layer becomes thinner for higher shearing conditions according to Sangwal [15].
does not explicitly represent the process of removal of ions but provides a simple mathematical description of the dissolution kinetics). The increase in the cstep retreat with the mixing rate for the systems mixed with deionised water implies a higher density of steps at higher mixing rates. It could be hypothesized that at higher mixing rates, more etch pits open up during the undersaturation regime. These pits later evolve as a source for steps to retreat. Table 7 Limiting process for various substances arranged in order of solubilities in pure water [17]. Substance Surface reaction control KAlSi3O8 NaAlSi3O8 BaSO4 SrCO3 CaCO3 Ag2CrO4 SrSO4 Opaline SiO2 Mixed Control PbSO4 Transport control AgCl Ba(IO3)2 CaSO4.2 H2O NaSO4.10 H2O MgSO4.7 H2O Na2CO3.10 H2O KCl NaCl MgCl2.6 H2O
Solubility 3.10− 7 6.10− 7 1.10− 5 3.10− 5 6.10− 5 1.10− 4 9.10− 4 2.10− 3 1.10− 4 1.10− 5 8.10− 4 5.10− 3 2.10− 1 3.100 3.100 4.100 5.100 5.100
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Fig. 14. Variation in the SCD parameter (a) kdiss and (b) cstep retreat. The round and rectangular symbols represent the values used for alite systems in DI-water and saturated lime solution respectively. The variation in the NDG parameters is described in Section 5.2.
In the simulations with saturated lime solution, the SCD parameter kdiss was kept constant (Fig. 14). As can be seen in Fig. 15, in the presence of additional calcium ions and a higher initial pH value, the initial undersaturation of alite is much lower as compared to DI-water. As a result of this, dissolution starts in the surface reaction
Fig. 16. Simulated evolution of Ca2 + concentration in (a) DI-water (b) saturated lime solution. Once portlandite precipitates, the concentrations decrease (indicated schematically by arrows) from portlandite supersaturation value (shown by the thin grey line) towards portlandite solubility equilibrium (shown as the thick dotted black line).
controlled regime. This is reflected in these simulations, which are relatively insensitive to the value of kdiss chosen, as the system spends so little time in this regime. However, as the system transitions to the step retreat regime, the mixing rate imparts a small influence on the
Fig. 15. Simulated alite dissolution rate against alite undersaturation for the DI-water and saturated lime solution systems. In this figure, the dotted line marks the split between the two dissolution regimes: undersaturation and STEP-retreat regime. For the alite systems with saturated lime solution, the dissolution regime begins very close to the step-retreat regime. Once portlandite precipitates, the concentrations decrease causing an increase in alite undersaturation and alite dissolution rate (shown schematically by dotted arrow) until portlandite solubility equilibrium is reached (shown by circle symbol).
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dissolution rate. This is reflected in the cstep retreat parameter, which was observed to increase slightly with the mixing rate. This variation was also found for hydration of alite in DI-water. It therefore seems that the dissolution facilitated by step retreats is slightly faster at higher mixing rates. The simulated evolution of calcium ions in solution in the two cases is shown in Fig. 16. For the DI-water systems the first part of the curve is steep, corresponding to the fast dissolution regime controlled by diffusion/transport. This part of the curve is very sensitive to the speed of mixing; higher slope implying faster dissolution at higher mixing rates. Then there is a transition to a much less steep slope, corresponding to the surface reaction controlled regime. For the systems mixed with saturated calcium hydroxide solution, the first steep part is short and the systems enter very quickly into the slow regime. Beyond this transition point, the small variation in the step retreat parameter results only in a minor variation in the dissolution rate and, consequently, a small variation in the evolution of the calcium concentration. From these experiments and simulations, it seems that the overall rate of dissolution up to the end of the induction period is limited by two different processes. At the start, when mixed with pure water the anhydrous phase quickly dissolves with the formation of etch pits at high undersaturation, the concentration at the interface quickly approaches the critical concentration at which the surface reactions slow down. In this regime mixing homogenises the concentration in solution so the dissolution continues at a higher rate longer. However, when the initial solution is already saturated with lime, the systems enter quickly into the slow regime controlled by surface reactions and so are not significantly affected by mixing (see Figs. 15 and 16). Another interpretation of the effect of the speed of mixing on the early hydration based on the protective membrane theory would postulate that higher shearing forces from mixing promotes the breakup of the layer and exposes further the anhydrous surface to the solution. This would lead as well to an increased amount of anhydrous phase dissolving as the speed of mixing is more intense. However, this interpretation fails to explain the experiments performed in saturated lime solutions where no impact was seen on the length of the induction period. Since from the point of view of this theory the saturated lime condition is not rate limiting for the dissolution of the anhydrous phases, the effect of a change in the speed of mixing should lead to the same features as for the systems starting with deionised water. This is not what is seen experimentally which leads us to favour the hypothesis of solution controlled dissolution described previously. 5.2. Impact of mixing after the end of the induction period From the end of the induction period the hydration kinetics are controlled by nucleation and densifying growth of C-S-H. To capture the effect of mixing speed in the simulations, it was necessary to change the parameter pertaining to the number of nuclei of C-S-H per unit surface area, Ndensity, along with minor variations in the ρmin and t0 parameters. The variation in the ρmin parameter and its correlation to a physical process is not well understood at this stage. The t0 parameter for the DI-water systems decreased with increasing mixing rates suggesting an earlier start of the nucleation and growth regime of C-S-H in mixes prepared at higher mixing rates. This effect can also be seen in the simulated heat-evolution profiles (Fig. 11) and simulated calcium concentration profiles (Fig. 16). For the saturated lime solutions, negative values of t0 were used, indicating a very rapid transition to the regime of rapid C-S-H growth Fig. 17 shows the variation of the Ndensity parameter for the systems mixed with DI-water and those mixed with saturated lime solutions. In contrast to the regime prior to the induction period, the effects of mixing speed are the same in the two systems and it is clear that mixing speed significantly increases the number of nuclei of hydrates in the system. The increase in nucleation density for both
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Fig. 17. Variation of Ndensity for the simulations. The round and rectangular symbols represent the values used for alite systems in DI-water and saturated lime solution respectively. As can be interpreted, the density of C-S-H nuclei per unit surface area increases with the rate of mixing.
DI-water and saturated lime solutions is proportional and almost linear with respect to the mixing rate. The increase in nucleation density of hydrates is likely to be a purely mechanical effect. C-S-H precipitates (mainly on the surface of the alite grains), very quickly after the addition of water. During mixing these “nuclei” are dislodged from the surface forming extra nucleation sites away from the surfaces of the alite particles. The microstructures of the different systems (Figs. 8 and 9) support this hypothesis as it is clear that there is much more growth of C-S-H between the cement grains when the mixing speed is increased. This hypothesis of mechanical detachment of primary C-S-H, causing the acceleration of the main hydration peak is also consistent with the study of Thomas and co-workers [26], who showed that a small addition of pure C-S-H to pure tricalcium silicate causes a significant acceleration of the early hydration kinetics. They proposed that the C-S-H seeds increase the amount of early hydration by causing product to form in the capillary pore space away from the alite surfaces. It was shown that the model can reproduce the influence of the mixing conditions on hydration kinetics by varying the simulation parameters (a) kdiss and cstep retreat, which are related to the ease at which species detach from the dissolving surface of alite, and (d) Ndensity, which is related to the number of C-S-H nuclei per unit surface area of the particles. Experimental results confirm the increase in the nucleation density of hydrates as a higher packing density of C-S-H in the microstructural matrix is observed for samples prepared at higher mixing rates. 5.3. Differences between alite and cement In Fig. 3 it was seen that mixing had a much larger impact on the induction period of the alite systems than the cement systems. Although there are many differences between alite and cement: particle size, presence of other clinker phases, etc.; the most important difference is probably the presence of rapidly soluble alkali metal salts. As with saturated calcium hydroxide solution, the presence of the alkalis changes the solution composition of the mixing solution at the very early stages. Consequently the system should spend less time in the fast dissolution regime, which is most susceptible to the effects of mixing. 6. Conclusions This study contributes to our understanding of the mechanisms linking the mixing intensity (speed, duration and presence of aggregates) to the hydration process.
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• Prior to the end of the induction period, experiments in different mixing solutions coupled with simulations, indicate that when the solution is very undersaturated with respect to alite and dissolution is fast, the rate of dissolution is controlled by the rate of transport of ions from the surface into the bulk. Mixing leads to a decrease in thickness of the diffuse electrical layer and so there is an increase in dissolution rate in the first minutes with a reduced length of the induction period as a consequence. • When the mixing solution is close to the critical undersaturation of alite marking the onset of the step retreat regime, the rate of dissolution seems to be mainly controlled by surface reactions and the length of the induction period is not much affected by mixing. • After the end of the induction period, mixing increases the acceleration of the rate of reaction and decreases the time to the maximum heat peak. These effects are independent of the initial composition of the mixing solution. Simulations indicate that these effects are due to an increase in the density of C-S-H nuclei, most likely due to the mechanical action of mixing. • The presence of more C-S-H nuclei between the cement particles leads to a denser and more homogeneous microstructure, which is evident even after long hydration times. • The agreement between the experimental results and the simulations based on the two mechanisms of solution controlled dissolution prior to the end of the induction period and then nucleation with densifying growth, during the main heat evolution peak; supports the validity of these mechanism and demonstrates the advantages of being able to look at their consequence quantitatively with the modelling platform μic. Acknowledgements Patrick Juilland is grateful for financial support from Sika Technology AG for his PhD study, from which this work results. He also wants to thank Dr. D. Lootens who initiated this work at Sika Technology AG. Aditya Kumar is grateful for the support of the Swiss National Science Foundation for support of his work on the development of the μic modelling platform. References [1] D.A. Williams, A.W. Saak, H.M. Jennings, The influence of mixing on the rheology of fresh cement paste, Cem. Concr. Res. 29 (1999) 1491–1496.
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