5th International RILEM Symposium on Self-Compacting Concrete 3-5 September 2007, Ghent, Belgium

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1 A STUDY ON THE DISPERSION/FLOCCULATION STATE AND FRICTION OF CEMENT PARTICLES IN MORTAR BASED ON THE RULES OF DEFORMABILITY AND AMOUNT OF WATER BLED BY CENTRIFUGAL FORCE Hidenori Nagamine (1) and Toshiharu Kishi (2) (1) Development Center, BASF Pozzolith Co., Ltd. (2) Human Society Division, Institute of Industrial Service, University of Tokyo, Japan Abstract The authors determined the amount of water in mortar bled by centrifugal force to defined the percentage of this amount to powder volume as WcsP. We found that it is an index of the space between two particles or an index of the dispersion / coagulation states of the particles derived from the relationship between WcsP and the deformability of mortar Γm. This study clarified the follows: (1)A focus exists among the linear relationships between WcsP and Γm of mortars having various dosages of superplasticizers. (2) It indicates that in mortar on this focus, the water acting upon the fluidity is zero and the difference in WcsP from the focal point can be regarded as a new index WfP of free water for fluidity. By using the index WfP that also indicates the space between two particles, the categories of coagulation states of the particles are clearly classified based on the relationships between WfP and deformability of mortar and between WfP. This study clarified the follows: (1) The coagulation states are loosened as the dosage of superplasticizer(sp) increases. (2) The coagulated body have two types of friction; one is friction between the surfaces of the bodies, which influences on the deformability of mortar, and the other is friction inside of the body, which influences on the viscosity of mortar. 1. INTRODUCTION As seen in the diagram of Figure 1, the control mechanism of fresh mortar is considerably to be complicated. This study aims to study the fluidity from the microscopic viewpoint of contact and friction between solid particles, to elucidate the dispersion and coagulation states of particles and the essential mechanism of fluidity. As an effective means, the authors define the centrifugally separated water/powder volume ratio (WcsP,%) based on the volume of centrifugally separated water when centrifugal force is applied to mortar. 341

2 Condition Microscopic state State value /Dynamics Rheological property Materials W/C SP Mixing flocculation Contact frequency Friction Viscosity Deformability Temperature dispersion Fig.1: The diagram of fluidity mechanism in fresh mortar The authors analyze in detail the physical meaning of WcsP based on the relationship between WcsP and fluidity. The authors define water that contributes to fluidity as an index for the inner state volume of fresh mortar, to elicit a new free-water ratio ( WfP). This enables the authors to study in detail the effects of the mixing proportion on free-water, and the effects of free-water and superplasticizer (SP) on fluidity. Furthermore it enables the authors to discuss the mechanism of particle friction that controls dispersion and the state of coagulation of the cement particles in fresh mortar. In this study, the authors discuss the difference in coagulation state and the mechanism of friction between particles using a free-water index WfP and fluidity index Γm. 2. OUTLINE OF THE EXPERIMENT Mixing water (W) Cement (C) Fine aggregate (S) Superplast icizer (SP) Table 1:Materials used Tap water Ordinary portland cement (Density=3.15 g/cm 3, Blaine ratio of surface area = 3,33 cm 2 /g) River land sand (Surface-dry density = 2.6 g/cm 3, Water absorption = 1.83%, FM = 2.67) Polycarboxylateether-based compound Mortar with a Vw/Vp = 8 to 15%,at an SP dosage of. to 3.9% and using ordinary portland cement were tested. The authors conducted flow and funnel tests and measured the amounts of centrifugally separated bleeding water. (1) Materials used Table 1 shows the materials used. (2) Mixing Mortar using ordinary portland cement is placed into a ASTM mortar mixer for mixing by the following scheme.(s: fine aggregate, C: cement) S+C-(1sec.) +W 1 -(12sec.) +W 2 +SP-(6sec.) Scrape off-(6 sec.) Discharge The mixing water was divided and added in two portions: 1) the first amount of water corresponding to W/C = 2% (W 1 ), 2) the second amount of water (W 2 ) and SP were added. (3) Flow test Figure 2 shows the outline of the flow test. The mortar was filled in the flow-cone specified in JIS R 521:1997 Strength test of cement to determine the spread of flow due to self 342

3 weight without vibration. The relative flow area ratio (Γm) that is obtained from the equation (1) based on flow value F(mm) is set as an index of deformation. Γm=(F )/1 2 (1) (4) Measurement of the amount of bleeding water separated by centrifugal force. Figure 3 shows the diagram of the centrifugal separating method. By using a centrifugal separator (Himac CR2 made by HITACHI, Angled rotor: R19A), the amount of bleeding water from mortar due to centrifugal separation was measured. WcsP is calculated by equation (2) based on the volume of centrifuged bleeding water (Vwcs) and the powder volume (Vp). WcsP= Vwcs/VpX1(%) (2) The authors applied centrifugal separation time for 1 minutes, when is enough to become a steady value. And, since the flow measured immediately after mixing changed in the pilot test, the authors decided to measure the flow value after 15 minutes when it stabilized. To meet the timing with the measurement of the flow water, centrifugal force is applied during the period from 1 to 2 minutes immediately after mixing. Rmax θ Rmax 7mm 1mm F Fig.2: Flow test Bottle 6mm Mortar Fig.3: An illustration of centrifugal separation h 3. TEST RESULTS Figure 4 shows the relationship between the WcsP and Γm when Vw/Vp is varied with SP dosages in terms of mixing conditions of Vw/Vp = 8 to 15%,SP/C=. to 3.9%. The relationship between WcsP and Γm high linearity (R 2 =.95 or more) in terms of SP dosages. Hereafter this line is referred as WcsP-Γm line. It can be seen that the gradient of the WcsP-Γm line is regularly varied with SP dosage and that these lines focus in the negative region of Γm. The authors supposed that the WcsP-Γm line group should conjugate in a focal point to implement the following treatment to clarify the regularity. First, the authors represent the relationship between a): the gradient of WcsP-Γm line and SP dosage (SP/C), and Y-axis intercept and SP dosage (SP/C). The equations of (3) and (4) are obtained. Γm Plain 1.3%.5% 8.8% 1.% 6 1.5% 1.8% 4 2.1% 2.4% 2 2.7% 3.% 3.3% % WcsP(%) -4 Focus(.88,-2.1) Fig.4:Relationship between WcsP and Γm a=.119sp/c SP/C+.71 (3) b=-.99 SP/C SP/C-2.56 (4) 343

4 Next, when Γm is formulated with WcsP and SP/C by using equation (3) and (4), the equation of (5) is conduced. Γm=(.119SP/C SP/C+.71)WcsP -(.99SP/C SP/C+2.56) (5) In this study, the authors conducted experiments on mortars of 13 dosages of SP, including those without SP. The WcsP-Γm line in terms of SP dosage was obtained by using the equation of (5) to obtain the intersection points (x i,y i)of all 13 lines, the coordinates are concentrated in a very narrow area (x i =.858~.933, y i =-2.37 ~ ) and the average coordinate (x,y ) is.879, Measured Γm R 2 = Γ m Calculated by eq.(6) Fig.5: Relationship between the calculated Γm and actual measured Γm As mentioned above, it is considered to be reasonable to assume that the WcsP-Γm lines conjugate at a focal point. Furthermore based on this assumption of conjugation at a focal point, the authors regarded the average coordinates (.879, -2.6) as the focal point, and reformulated Γm in the equation (5). It can be simply represented as the equation (6). Γm=(.119SP/C SP/C+.71)X(WcsP-.879)-2.6 (6) Γm that is calculated by equation (7) is compared with actual measurement values (see fig.5). It is proved that there is a high correlationship (R 2 =.967) between both parties and this formulation is reasonable. Here, to discuss the physical meaning of Γm on the focal point, the concept of a negative Γm is assumed. The focal point of the relationship of WcsP-Γm is also hypothetical and the mortar to be seems hard to create. However, there is a specific point where if the dosage of SP is varied, deformation does not vary. Therefore, it can be considered that the conjugation of the focal point indicates that free-water contributing to deformation of mortar at this focal point is in the null state. It can be considered that the value (WcsP-.879), where.879 is subtracted from WcsP, is the index of free-water WfP. 4. CONSIDERATIONS Figure 6 shows the relationship between SP/C and WfP in terms of different Vw/Vp. The remarkable characteristic is the fact that WfP decreases when SP/C increases in the low SP/C zone. On the other hand, WfP hardly changes even if SP/C increases in the high SP/C zone. Based on these results, it is considered that the coagulation state due to application of SP changes near the boundary area of the SP dosage from 1.% to 1.5%. Figure 7 shows the relationship between Vw/Vp with several markers in terms of SP/C. The characteristic to note is that the relationship between Vw/Vp and WfP indicates constant linearity as follows in the wide range of SP dosage from.5% to 3.9%, regardless of SP/C. WfP=.286Vw/Vp (7) In case of plain mortar and mortar containing SP at a small dosage of.3%, the relationship between Vw/Vp and WfP greatly moves to the high WfP region from the linear relationship based on equation (7). Based on these results, the authors consider that mixing conditions in particular SP dosage affects WfP at the boundary dosage of SP from.3% to.5%. 344

5 WfP(%) SP/C( %) Vw/Vp 78% 8% 84% 86% 88% 92% 96% 1% 14% 11% 12% 13% 14% Fig.6: Relationship between SP/C and WfP WfP(%) [y] Dispersion region 14 Plain.3%.5%.8% 1.% 1.5% 1.8% 2.1% 2.4% 2.7% 3.% 3.3% 3.9% Strong flocculation region Transient region 11% 114% 12% y =.286x R 2 = Vw/Vp(%) [x] Fig.7: Relationship between Vw/Vp and WfP As stated above, the authors analyzed the change in WfP in terms of mixing proportion of SP/C and Vw/Vp. A specific point of change in SP/C can be found between.3% to.5%, and between Vw/Vp from 1.% to 1.5%. It is considered that the addition of SP increases the repellent effect of particles and affects the coagulated state of particles. WfP is an index representing the distance between particles and the contact-frequency of particles. It is not easy to consider that the distance between particles is directly spread out due to the increase in reaction of particles. Therefore, it is assumed that the change in WfP as SP/C changes mainly suggests a characteristic change in the coagulation structure. That is, it is considered that the coagulated form of the particles in terms of the change in SP/C is divided into three regions with different characteristics at least with a boundary of two specific points. This paper discusses the three separate regions in terms of SP/C: 1) a strong coagulation region of. to.3%; 2) a transition region of.5 to 1.5%; and 3) a dispersion region of 1.5% or more. i) Strong coagulation region As seen in Figure 7, the relationship between Vw/Vp and WfP of plain mortar and mortar containing SP at Case 1 Case 2 an increase of free water a decrease of free water Fig.8: Conception of disperse by SP added the dosage of.3% is clearly far from linear, which is represented by the formula (7). In addition, it is characteristically found in the higher WfP region. The remarkable high quantity of free-water conversely indicates that there is very little held-water. The characteristics of the plain mortar that has poor dispersibility of particles, and mortar containing a low dose of SP cannot be explained by using the concept of coagulation dissolution containing the heldwater. (See Case 1 in Fig.8) However, the decrease in the volume of free-water as SP/C increases as mentioned above, can be explained if it assumed as follows: Vapor Liquid High tension Rigid Liquid Low tension Flexible Fig.9: The conception of friction inter-particles 345

6 Since the particle dispersion in plain and mortars containing quite low doses of SP are quite insufficient, the surface of particles cannot be fully wetted, and air is involved in the coagulated body such as pendular and/or funicular state. (See Case 2 in Figure 8) Three types of binding and/or retention of water exist: by absorbing onto the surface of particles, by consumption for early hydration, and by filling the voids between particles. Hereafter these types of water are referred as held-water. And it can be considered that the held-water which is not held by the particles remains in the free space in mortar as free-water. This water bleeds on the surface of mortar by centrifugal force. The coagulation strength of such a coagulated body, in which a part of surface does not contact with water, is very strong and exceeds the simple bonding ability on the wetted surface due to capillary forces formed on the boundary face between liquid and gas phases between particles (see Fig. 9). Even if fluidity is generated, the coagulated body hardly changes shape. In addition, it is considered that the dimension is relatively large; therefore, fluidity resistance due to friction in the coagulated body must be significantly high. Therefore we may think tow type of friction for fluidity. One is friction between the surfaces of the coagulated bodies, and the other is friction inside of the body, which is depended on the states of inside ii) Transition region Next, the authors consider the results in the medium region from.5% to 1.% dosage of SP. The change in WfP decreases in this region as SP/C increases as seen in Figure 6. A different result from the region 1.5% or more is seen. In addition, as seen in Figure 7, if the SP dosage is.5% or more, it seems to correspond to the linear relationship represented by equation (7) in this region as well. It is reasonable to consider that this medium region is a transition region from the strong coagulation region. iii) Dispersion region Finally, the authors consider the stable region where WfP is certainly seen above the SP dosage of 1.5% and roughly constant, regardless of the change in SP/C. In this range of SP dosage, WfP is provided only based on Vw/Vp regardless of the SP dosage (see Fig. 6). That is, it is considered that WfP is not affected coagulation in this region. Furthermore, similar results are seen up to a high SP dosage range of 3.9% where particles are considerably dispersed. It is reasonable to consider that this range is the dispersed range of cement particles. 5. CONCLUSION The authors define the centrifugally separated water/powder volume ratio (WcsP, %) based on the volume of centrifugally separated water when centrifugal force is applied to mortar. The authors analyze in detail the physical meaning of WcsP based on the relationship between WcsP and fluidity. As a result, the authors define water that contributes to fluidity as an index for the inner state volume of fresh mortar, to elicit a new free-water ratio (WfP). This enables the authors to study in detail the effects of the mixing proportion on free-water, and the effects of free-water and superplasticizer on fluidity. Furthermore it enables the authors to discuss the mechanism of particle friction that controls dispersion and the state of coagulation of the cement particles in fresh mortar. 346