THE INFLUENCE OF CEMENT PASTE VOLUME IN MORTAR ON THE RHEOLOGICAL EFFECTS OF THE ADDITION OF SUPERPLASTICIZER

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1 Gołaszewski J.: The influence of cement paste volume in mortar on the rheological effects of the addition of superplasticizer. Proceeding of 8 th International Conference Brittle Matrix Composites. Ed. A.M. Brandt, Warszawa, Poland, 6 THE INFLUENCE OF CEMENT PASTE VOLUME IN MORTAR ON THE RHEOLOGICAL EFFECTS OF THE ADDITION OF SUPERPLASTICIZER Jacek GOŁASZEWSKI Silesian University of Technology, Faculty of Civil Engineering Akademicka 5, 44-1 Gliwice, Poland, jacek.golaszewski@polsl.pl ABSTRACT The results of investigation into the influence of polyether type superplasticizer addition on rheological parameters and its changes with time of different in cement paste volume fresh mortars are presented and discussed in the paper. Rheology results have been evaluated according to the Bingham model, which describes the rheology with the parameters: yield value and plastic viscosity. Rheological parameters were measured using Two-Point Workability Test. Because of the similar nature of rheological behaviour of fresh mortar and concrete, presented in paper relationships for mortars can be also used to state the rheological properties of fresh concrete. The obtained results show that the nature and range of superplasticizer influence on rheological parameters of mortar is strongly influenced by cement paste volume. Volume of cement paste is also a significant factor affecting range and direction of changes of the rheological parameters of superplasticized mortars with time. The results show, that the nature of influence of superplasticizer on rheological parameters of fresh cement paste and on fresh mortar may significantly differ each from other. These differences increase with decreasing volume of cement paste in mortar. On the ground of obtained results the empirical relations joining the rheological parameters of mortars with superplasticizer content were established. Analysis of results covers also comparison between influence of cement paste volume on rheology of mortars with and without superplasticizer. It is concluded that effects of superplasticizer addition on rheology of fresh mortar and concrete should be also considered in respect to cement paste volume. It is difficult to unequivocally predict effects of superplasticizer content on rheology of mortar or fresh concrete basing only on tests made on cement paste, and not making allowance on degree of aggregate filling by cement paste in this mortar or concrete. Keywords Rheology, Workability, High range water reducers, Mortar INTRODUCTION When using superplasticizers it is possible to produce flowing and self compacting mortars and concretes of low w/c and, in effect, to produce mortars and concretes of high strength and durability. More effective use of superplasticizers requires a creation of the systematised data base concerning their influence on rheological parameters of fresh mortars and concrete depending of composition of these materials and of their components properties [1-3]. It is well documented that fresh mortar and concrete exhibit the bingham viscoplastic behaviour according to the formula: τ = τ + ηpl γ (1) o.

2 where τ (Pa) is the shear stress at shear rate γ (1/s) and τ (Pa) and η pl (Pa.s) are the yield value and plastic viscosity respectively [4-1]. The physical interpretation of yield value is that of the stress needed to be applied to a material in order to start flowing. When the shear stress is higher then yield value the mix flows and its flow resistance depends on plastic viscosity. In a recent years, a lot of studies concerning the influence of superplasticizers on fresh cement paste, mortar and concrete rheology were done. It was stated, that the effects of superplasticizer addition on rheology of these materials depend mostly on cement and superplasticizer physicochemical properties, superplasticizer content, time of superplasticizer addition, and on w/c [1-4,6,9,11]. However, obtained relationships of superplasticizers influence on rheological properties of cement paste, mortar and fresh concrete not always are unequivocal. Superplasticizers reduce both the yield value and the plastic viscosity of cement paste [2]. They also reduce the yield value of mortar and fresh concrete, but either reduce, not change or increase plastic viscosity of these materials [6,9,11]. Analysis of so far executed studies indicates, that different effects of superplasticizer addition on rheological properties of mortars and fresh concrete may be related to the cement paste volume content in these materials. The methodology and the results of investigation into the influence of superplasticizer content on the rheological parameters and its changes with time of different in cement paste volume fresh mortars are presented and discussed in the paper. It is worth to notice, that it was stated in [4,5,1,11] that the nature of rheological behaviour of mortar and concrete is similar, and that mortars can be considered to be a model concrete. Thus, the relationship presented in the paper relate both to mortars and to fresh concrete. EXPERIMENTAL Testing program In the research the influence of cement paste volume in mortar and superplasticizer content on rheological properties of mortars was investigated. Cement paste volume in mortar was expressed in the terms of factor of uncompacted sand filling by cement paste - ϕ m/p. Methods of computing the factor ϕ m/p are presented in details in existing literature [6,12,13]. Factors taken into consideration and its s are shown in Table 1. Cement type, sand type and grading, and w/c ratio ( =.35) were kept constant. For comparison between influence of cement paste volume on rheology of mortars with and without superplasticizer, the influence of cement paste volume and w/c on rheological properties of mortars without superplasticizer were also investigated. The factors taken into consideration in this part of research and its s are shown in Table 2; cement type, sand type and grading were kept constant. Water/cement ratios were selected so as to obtain similar value of g for mortars of certain sand/cement ratio both with and without superplasticizer. Rheological parameters of cement pastes filling mortars were not determined because of its high gravitational and centrifugal separation. Measurements of rheological parameters of fresh mortars Rheological parameters of mortars were measured according to the two-point workability test methodology using Viskomat PC rheometer. The origin and principles of the two-point workability test and are detailed presented in [4]. Viskomat PC and its measuring element are presented in Fig 1; it is in detail described in [6,9]. In general, in the two-point workability test rheological parameters of fresh mortar are measured by applying a given shear rate and measuring the resulting shear stress. Because of the bingham nature of rheological behaviour Table 1. Program of testing of cement paste volume and superplasticizer content influence on rheological properties of mortars

3 Factor ϕ m/p Superplasticizer content , 2.5, 3., , 1.5, 2., , 1., 1.5, 2., , 1., 1.5, 2., Water/cement.35 Sand/cement 3/1 2/1 1.5/1 1/1 Table 2. Program of testing of cement paste volume and w/c ratio influence on rheological properties of mortars without superplasticizer Factor ϕ m/p Water/cement Sand/cement ,.47,.5,.53 3/1.44,.47,.5,.53 2/ ,.5,.53, /1.5,.53,.56 1/1 of fresh mortars, the measurements should be taken at no less than two considerably different shear rates. The rheological parameters are determined by regression analysis according to the relation: T = g + N h (2) where T is the shear resistance of sample measured at rotation rate N and g (N.mm) and h (N.mm.s) are constants corresponding respectively to yield value τ o and plastic viscosity η pl. By suitable calibration of rheometer it is possible to express g and h in fundamental units. According to [5] in the Viskomat PC rheometer τ o = 7.9 g and η pl =.78 h, but all results are given below in terms of g and h. Materials and mixes CEM I 32.5 cement and polyether based superplasticizer were used for the investigations. Their main properties are given in Tables 3 and 4. The sand used was PN EN 196:1996 CEN model sand (2 mm max.). The mix proportions of tested mortars are presented in Tables 1 and 2. The superplasticizer dosage refer to the complete product. Mortar mixing and testing procedures The mixing procedure reminded in accordance with EN 196-1:1994; superplasticizer was added with water. Rheological parameters of mortars were measured at 1, 3 and 6 min after the end of mixing according to the procedure shown in Fig 1. Table 3. Properties of cement CEM I 32.5 Cement ingredients [%] SiO 2 CaO Al 2O 3 Fe 2O 3 MgO Na 2O e SO 3 C 3S C 2S C 3A C 4AF Specific surface [m 2 /kg] Table 4. Properties of superplasticizer Density Concentration Major constituent [g/cm 3 ] [%] polycarboxylate acid 1.6 Start of measurement - N = 1 rev/min held constant for 3 min.

4 Measurement of M at decreasing N = rev/min. Total test time - 4 min s. Fig. 1. Viskomat PC, its measuring element, and measuring procedure RESULTS AND DISCUSSION Obtained relations of influence of superplasticizer content on rheological parameters of fresh mortars different in cement paste volume (in factor ϕ m/p ) are presented in Fig. 2. The analysis of variance ANOVA of influence of cement paste volume (factor ϕ m/p ) and superplasticizer content on rheological parameters of mortars are presented in Tables 5 and 6. It can be seen, as one can expect, that cement paste volume in mortar (factor ϕ m/p ), superplasticizer content, and interaction of these factors significantly influences parameters g and h of mortars and its changes with time. Increasing content of superplasticizer, likewise increasing cement paste volume (increasing factor ϕ m/p ) causes a non-linear decrease in g of mortars. Initially increasing superplasticizer content causes fast decrease of parameter g until a certain minimum. Further increasing of superplasticizer content no longer changes parameter g. Such nature of changes in g does not depend on cement paste volume in mortar (on factor ϕ m/p ). Nevertheless, cement paste volume in mortar (factor ϕ m/p ) influences strongly range of changes in value of g caused by increasing superplasticizer content. As may be seen from obtained results, for mortars with small cement paste volume changes of g when superplasticizers quantity increases are great and reduce as cement paste volume in mortar increases. Simultaneously, the lesser is cement paste volume in mortar (factor ϕ m/p ), the higher superplasticizer addition is necessary to obtain mortar of given value of g. It is also worth to notice, that if cement paste volume in mortar is small, obtainment of mortar of similar value of g as for mortar with high content of paste is difficult, even if quantity of added superplasticizer is very large. Value of g of all tested mortars, independently on cement paste volume (factor ϕ m/p ) and superplasticizer content, increases with time. The range of increase of g with time is lesser when superplasticizer content and/or cement paste volume in mortar (factor ϕ m/p ) are larger. For example, increase in value of g of mortars of ϕ m/p = 1.89 with 1., 1.5, 2., 2.5, and 3.% addition of superplasticizer within 5 minutes is correspondingly of 31.4, 15.3, 1.7, 7.4, and 5.8 N.mm. It means that it increases in comparison to initial value of g by correspondingly of 96, 86, 79, 69 and 61%. For mortars of ϕ m/p = 1.89, 2.52, and 3.77 with 1% addition of superplasticizer increase in value of g within 5 min is correspondingly of 31.4, 5.8 and 1.9 N.mm. It means that it increases in comparison to initial value of g by correspondingly of 96, 54 and 32 %. Nature of the influence of superplasticizer content on h of mortars, in opposition to nature of influence on g, depends on cement paste volume in mortar (on factor ϕ m/p ).

5 1 after 1 min 6 after 1 min 1 5 g, N mm h, N mm s after 3 min 6 after 3 min 1 5 g, N mm h, N mm s after 6 min 6 after 6 min 1 5 g, N mm h, N mm s Fig. 2. Influence of superplasticizer content on rheological properties of mortars different in cement paste volume (factor of uncompacted sand filling by cement paste ϕ m/p) after 1, 3 and 6 min

6 Table 5. Analysis of variance ANOVA of influence of cement paste volume in mortar and superplasticizer content on g of mortars after 1, 3 and 6 min after end of mixing g after 1 min g after 3 min g after 6 min Source of variation A: Cement paste volume B: SP content AB Table 6. Analysis of variance ANOVA of influence of cement paste volume in mortar and superplasticizer content on h of mortars after 1, 3 and 6 min after end of mixing h after 1 min h after 3 min h after 6 min Source of variation A: Cement paste volume B: SP content AB In the case of mortars of larger cement paste volume (ϕ m/p = 2.52 and 3.77) increase of superplasticizer content initially slightly increases value of h and next, after reaching some maximum value, value of h decreases. Range of such changes increases as cement paste volume in mortar increases, but generally is not too great and only in limited extent influences rheological properties of mortars. In the case of mortars of lower cement paste volume (ϕ m/p = 1.24 and 1.89) increase of superplasticizer content causes large (larger if cement paste volume in mortar is lower) increase in the value of h. Maximum value of h is reached when superplasticizer content is very high - further addition of superplasticizer may develop only small decrease in value of h. Maximum value of h and superplasticizer content when such value is reached, depends on cement paste volume in mortar. The lower the cement paste volume in mortar, the higher maximum value of h reached, and it occurs at higher superplasticizer content. It should be mentioned that h of mortars with a given superplasticizer content in general increases when cement paste volume in mortars is decreased. Use of smaller cement paste volumes facilitates obtainment of mortars of higher value of h. It is essential in case of self compacting mortars [12]. Direction of changes of h of mortars with time depends first of all on volume of cement paste in mortar and, in the case of mortars with low cement paste volume, also on quantity of superplasticizer. In the case of mortars with larger cement paste volume (of ϕ m/p = 2.52 and 3.77), the value of h increases as a time passes. It should be mentioned that scope of such increase to a lesser extend depends on superplasticizer content. For example, increase in the value of h within 5 min for mortars of ϕ m/p = 1.89 with 1., 1.5, 2., 2.5, 3.% superplasticizer content is correspondingly of 4.2, 3.9, 3.8, 3.4 and 2.8 N.mm.s, what means that its increase is in comparison to initial value of h correspondingly of 29, 28, 22, and 18%. In the case of mortars with lower cement paste volume (of ϕ m/p = 1.24 and 1.89) direction of h changes with time depends on superplasticizer content. Initially, when superplasticizer content is low, value of h decreases with time. Range of such decrease is lower when superplasticizer content increases. After exceeding some specific superplasticizer content (the higher, the lower cement paste volume in mortar) value of h of mortars shows increase with time. For example, increase of the value of h within 5 min for mortars of ϕ m/p = 2.52 with

7 Material constants and R 2 g eq (3) h eq (4) Table 7. Regression and correlation coefficients for equations (3), (4) Factor ϕ m/p g & h after 1 min g & h after 3 min g & h after 6 min A g B g R A h B h C h R , 1.5, 2., 2.5, and 3. % superplasticizer content is accordingly of -3., -3.3, -.8, 4., and 6.1 N.mm.s what means change in comparison to the initial value of h by correspondingly of -17, -15, -3, 14 and 22%. It should be mentioned that in case of mortars of low cement paste volume (of ϕ m/p = 1.24), even in case of 3% superplasticizer content, value of h decreases as time passes. Presented above results are unfortunately insufficient for formulation of general relations connecting the values of g and h parameters and superplasticizer content and factor of uncompacted sand filling by cement paste ϕ m/p. However, the regression analysis of obtained test results shows that changes of g and h of mortars with changing superplasticizer content may be described using the following relations: g = A g SP B g (3) h = A h SP 2 + B h SP + C h (4) where: SP - superplasticizer content; A g, B g, A h, B h, C h - material constants depending mainly of cement paste volume in mortar and sand graining (factor ϕ m/p ), of properties of cement and superplasticizer, and of time. Values of material constants and correlation coefficients for tested mortars are presented in Table 7. Obtained relations of influence of w/c on rheological parameters of fresh mortars different in cement paste volume (in factor ϕ m/p ) are presented in Fig 3. The analysis of variance ANOVA of influence of cement paste volume in mortar (factor ϕ m/p ) and w/c on rheological parameters of mortars are presented in Tables 8 and 9. It can be seen, that cement paste volume in mortar (factor ϕ m/p ), w/c, and interaction of these factors significantly influences g and h of mortars and its changes with time. Both rheological parameters of mortars decrease as cement paste volume in mortar (factor ϕ m/p) and w/c increase, and range of such changes is larger when cement paste volume in mortar is lower. As time passes, both g and h increase independently of w/c and cement paste volume. Only mortars with low paste volume (factor ϕ m/p ) and low w/c indicate slight decrease in value of h with time. Range of g changes with time decreases, while range of h changes increases if w/c increases. Comparing relations for mortars with and without superplasticizer, one may say that nature of increasing superplasticizer content influence on value of g is analogical as nature of increasing w/c influence, while nature of superplasticizer content influence on h is other than nature of w/c influence. Increase of w/c causes decrease in the value of h, while increasing superplasticizer content causes initially distinct increase in value of h and then, after exceeding specific superplasticizer content, value of h decrease. At a given value of g, value

8 g, N mm after 1 min h, N mm s after 1 min 1,41,44,47,5,53,56,59,41,44,47,5,53,56,59 g, N mm after 3 min h, N mm s after 3 min 1,41,44,47,5,53,56,59,41,44,47,5,53,56,59 g, N mm after 6 min h, N mm s after 6 min 1,41,44,47,5,53,56,59,41,44,47,5,53,56,59 Fig. 3. Influence of water/cement ratio on rheological properties of mortars different in cement paste volume (factor of uncompacted sand filling by cement paste ϕ m/p) after 1, 3 and 6 min

9 Table 8. Analysis of variance ANOVA of influence of cement paste volume in mortar and w/c ratio on g of mortars after 1, 3 and 6 min after end of mixing g after 1 min g after 3 min g after 6 min Source of variation A: Cement paste volume B: w/c ratio AB Table 9. Analysis of variance ANOVA of influence of cement paste volume in mortar and w/c ratio on h of mortars after 1, 3 and 6 min after end of mixing h after 1 min h after 3 min h after 6 min Source of variation A: Cement paste volume B: w/c ratio AB of h for mortar without superplasticizer is always distinctly lower than the value of h for mortar with lower w/c, but with superplasticizer addition. Nature of influence of increasing w/c on rheological parameters may be explained by simultaneous increase of cement paste fluidisation and increase of aggregate s grains dissipation resulting from cement paste volume increase. Increase of superplasticizer content in mortar of given w/c causes de-flocculation of cement grains and increase of free water volume in mortar, but without essential influence on aggregate s grains dissipation. Larger volume of free water causes both fluidisation of paste and reduction of capillary cohesion in mortar [6]. These effects cause decrease in value of g of mortar. The nature of such changes may be interpreted as apparent increase of cement paste volume in mortar. Decrease of capillary cohesion does not influence considerably h of mortar [6]. Therefore changes in value of h when superplasticizer content is increased depends on changes in cement paste rheological properties and free water volume. Clarification of such changes mechanism requires further studies. Greater aggregate s grains concentration in mortar explains, why for similar value of g mortars with superplasticizer of low w/c are characterised by greater value of h than for mortar without or with lower superplasticizer quantity but of greater w/c. Changes in rheological parameters with time of mortars of given w/c and superplasticizer content depend on cement paste volume in this mortar and are greater when its volume is lower. Basing on [6] is possible to say, that in the same time occur effects of decrease in free water volume in cement paste and of gradual decay of superplasticizer action in result of cement hydration, and of increase of capillary cohesion in mortar. In mortar with lower paste volume, and therefore lower grade of dissipation of aggregate s grains decrease of free water quantity leads to quicker increase of capillary cohesion of mortar, and therefore to quicker increase in the value of g. SUMMARY Range, and in the case of h parameter, also nature of superplasticizer content influence on rheological parameters of mortars depend on cement paste volume in mortar. Increasing superplasticizer content causes fast decrease in value of g of mortars until a certain minimum. The minimum value of g possible to obtain due to increasing superplasticizer content increases with decreasing cement paste volume in mortar. Value of g of superplasticized mortars increases with time and the range of this increase is clearly lesser when cement paste volume in mortar is higher. Increase of superplasticizer content first causes increase in value of h of

10 mortars. In the case of mortars with high cement paste volume such increase is low, its range increases as cement paste volume in mortar decreases and in the case of low cement paste volume mortars it is very high. In the case of mortars with high cement paste volume value of h reaches specific maximum value and then, as superplasticizer content further is increased, its value decreases. Such effect is weak or does not occur in case of mortars with low volume of cement paste. Value of h of mortars with higher cement paste volume increases with time, scope of such changes to a lesser degree depends on superplasticizer content. In the case of mortars with lower cement paste volume direction of changes in value of h with time in such mortars depends on superplasticizer content. If superplasticizer content is low, value of h of mortars decreases with time. After exceeding a particular superplasticizer content, larger when quantity of cement paste in mortar is lower, value of h of mortars increases with time. Nature of influence of increasing superplasticizer content and increasing w/c on the value of g of mortars is similar. In the same time such influence on the value of h may be distinctly different. Due the above, it is not possible to set out such composition of mortars without and with superplasticizer that for similar value of g will be characterised by the same value of h. Effects of superplasticizer addition on rheology properties of fresh mortar and concrete should be considered in respect to cement paste volume. It is difficult to unequivocally predict effects of superplasticizer content on rheology of mortar or fresh concrete and its changes with time basing only on tests made on cement paste, and not making allowance on degree of aggregate filling by cement paste in this mortar or concrete. REFERENCES [1] Aitcin, P-C. High Performance Concrete, EF&N SPON, London, [2] Ramachandran, V S. Concrete Admixtures Handbook. Properties, Science and Technology. 2nd Edn, Noyes Publications, Park Ridge, USA, [3] Neville, A M. Properties of Concrete (in polish), Polski Cement, Kraków,. [4] Tattarsall, G H, Banfill, P F G. The Rheology of Fresh Concrete, Pitman Books Limited, Boston, [5] Banfill, P F G. The rheology of fresh mortar. Magazine of Concrete Research. Vol. 43 (154), pp [6] Szwabowski, J. Rheology of mixes on cement binders (in polish), Wydawnictwo Politechniki Śląskiej, Gliwice, 1999, 239 pp. [7] Ferraris Ch. F.: Measurement of the Rheological Properties of High Performance Concrete: State of Art Report. Journal of Research of the National Institute of Standards and Technology, Vol. 14, No. 5, 1999, [8] Larrard de F.: Concrete Mixture Proportioning. A scientific approach. E&FN SPON, London and New York [9] Gołaszewski, J. Adjusting of fresh concrete workability using superplasticizers, (in polish), Wydawnictwo Politechniki Śląskiej, Budownictwo z. 99, Gliwice, 3, 216. [1]Banfill P.F.G.: The rheology of fresh cement and concrete - a review. Proceeding of 11th International Cement Chemistry Congress, Durban, South Africa 3, [11]Gołaszewski, J. Rheology of mortars and rheology of fresh concrete. Cement Wapno Beton, No. 1, 6, [12]Szwabowski, J. Rheology of self compacting fresh concrete (in polish). IV Sympozjum Naukowo - Techniczne Reologia w Technologii Betonu, Gliwice, Poland 2, [13]Kuczyński, W. Technology of concrete. Part 2. Concrete proportioning (in polish). Arkady, Warszawa 1972.