EFFECT OF PIGMENTS ON THE RHEOLOGICAL PROPERTIES OF MORTARS FOR SELF-COMPACTING CONCRETE

Transcription

1 EFFECT OF PIGMENTS ON THE RHEOLOGICAL PROPERTIES OF MORTARS FOR SELF-COMPACTING CONCRETE Anahi López (1), Juan M. Tobes (2), María C. Torrijos (2), Bryan E. Barragán (4), Graciela Giaccio (3) and Raúl Zerbino (2) (1) GINTEMAC, National Technological University-FRC, Córdoba, Argentina (2) LEMIT, National University of La Plata, CONICET, La Plata, Argentina (3) LEMIT, National University of La Plata, CIC, La Plata, Argentina (4) Universitat Politècnica de Catalunya, Barcelona, Spain Abstract The development of self-compacting concrete appears as a promising alternative for new applications of coloured concrete. This paper presents studies on mortars performed with the aim of analyzing the effects of pigments on the fluidity and filling ability of cement pastes and mortars, respectively. Grey and white portland cements, calcareous filler and polycarboxylic ether based superplasticizers were used. Three iron oxide pigments, colour yellow, red and black, were employed in different doses, as a replacement of calcareous filler by volume. Mini-slump and Marsh cone tests were performed on cement pastes to obtain the saturation dosage of the superplasticizer. At mortar level, engineering test such as slump flow and V-Funnel were carried out, and fundamental rheological parameters measured by means of a BML viscometer. It has been observed that the type and proportion of pigment modifies the rheological behaviour, especially the cohesion, and varies the demand of superplasticizer. The changes in flowability also depend on the type and amount of pigment incorporated as well as on the cement used, with greater losses along time for the case of white cement. 1. INTRODUCTION The development of self-compacting concrete (SCC) appears as a promising alternative for new applications of coloured concrete, for example, in the case of challenging architectural demands. The addition of fine particles as pigments may modify the flowability, the cohesion and general rheological properties of concretes, making necessary to study their effect when self compactability is required. In fundamental terms, concrete workability can be characterized in terms of its rheological parameters; usually the Bingham model is used to characterize its behaviour. This model is based on the relationship τ = τ + μγ, where γ is the shear rate, τ the yield value (representing the resistance to flow), and μ the plastic viscosity (a measure of the resistance of the material to an increase in the rate of flow). In this study, the rheological parameters of coloured self compacting mortars and its time dependence are analysed together with the results of engineering tests such as slump-flow, V-Funnel and Marsh cone tests. 39

2 2. EXPERIMENTAL RESEARCH This study is a result of a joint research project between LEMIT-CIC (La Plata, Argentina) and the Department of Construction Engineering-UPC (Barcelona, Spain). The first part includes evaluations on pastes and mortars incorporating different pigments. The compatibility between the component materials, fluidity and superplasticizer requirement was studied on cement pastes by means of mini-slump and Marsh cone tests 1-3. Slump-flow and the V-funnel tests 3-5 were performed on mortars with the aim of evaluating the flowability and the segregation resistance. The second part of the study analyses the rheological properties of coloured mortars using a BML Viscometer 3 together with slump-flow and Marsh cone tests after the saturation dosage of the superplasticizer was determined on pastes without pigments. The mortar-based design method for SCC presented in 5 was implemented. 2.1 Materials and mixtures Three inorganic pigments, yellow (y), red (r) and black (b) were used. All of them were iron oxides with different shape and fineness (Table 1). Ordinary grey portland cement (Ga), white cement (Wa), calcareous filler, an ether policarboxylated based superplasticizer (solid content 35.5 %) and natural siliceous sand (fineness modulus 2.39, density 2.6, absorption.6 %) were used in the first part of these experiences. Seven Series of pastes were prepared with filler/cement, f/c =.7 and w/c =.5, varying the dosage of superplasticizer (SP). In coloured pastes, the pigment dosage was equal to 5% of the cement weight, substituting the same volume of filler. The pastes are identified in accordance with the cement and pigment used (PGa, PGa-y, PGa-r, PGa-b and PWa, PWa-y, and PWa-r). Seven Series of mortars with 54% by volume of paste were carried out (MGa, MGa-y, MGa-r, MGa-b and MWa, MWa-y, and MWa-r). In the second part, ordinary portland cement (Gb), white cement (Wb), calcareous filler, an ether policarboxylated based superplasticizer (solid content 28 %) and crushed sand (density 2.74, absorption 1.52 %) were used. Two Series of pastes without pigment (PGb and PWb) with f/c =.3 and w/c =.5 were elaborated, varying the dosage of superplasticizer. Finally, eight Series of mortars with 47% by volume of paste were evaluated, using in this case a 4% of pigment. These are identified as MGb, MGb-y, MGb-r, MGb-b and MWb, MWb-y, MWb-r, and MWb-b. Table 1. Characteristics of the pigments. Pigment Shape Specific gravity Ph Soluble salts (%) Ret. # 325 (%) Yellow (y) elongated Red (r) cubical Black (b) spinal , Experimental details Pastes and mortars were made in a Hobart-type mixer with two speeds, following the same mixing procedures. Tests were performed at 6 and 45 minutes to evaluate the variation of properties with time. The saturation dosage of the superplasticizer was obtained on cement pastes by mini-slump and Marsh cone tests. In the mortar Series of the first part, the slumpflow and V-Funnel tests were done; the relative flow area () and the relative flow time 31

3 (Rm) were calculated as Rm = 1/FT and = (SF/) 2 1, where FT is the flow time expressed in seconds and SF is the spread diameter expressed in mm. In the mortars of the second part, a BML Viscometer 3 was used to evaluate the rheological properties; the yield stress (τ o ) and the plastic viscosity (µ) were calculated assuming that mortars behave like a Bingham fluid. In addition, engineering test measures such as the relative flow area from the slump-flow test were obtained. 3. TEST RESULTS AND ANALYSIS Figures 1 and 2 show the variation of the Marsh cone flow time and mini-slump with SP on pastes prepared with cements Ga and Wa. Continuous and dashed lines correspond to measures at 6 and 45 minutes, respectively. Based on these curves, a saturation dosage was obtained (Table 2). It is defined as the dosage beyond which the Marsh cone flow time does not decrease significantly or the mini-slump does not increase notably; the latter correlates to a final spread of about 1 mm 1-3. There were no great differences in saturation dosages in coloured pastes, being slightly higher when black, red and yellow pigments were incorporated. When Wa was used, the values were rather greater, but keeping the same order of magnitude. The effect of pigments on the flow time and mini-slump is particularly noted when yellow pigment was used, i.e. the plots of yellow pastes are placed at the right of Figs. 1 and 2. This effect was more important in PGa-y than in PWa-y, and it can be justified considering the elongated shape of the yellow pigment particles. Pastes prepared with Wa had higher losses of fluidity with time than those with Ga. Log [TM] PGa PGa-y PGa-r PGa-b D [mm] PGa PGa-y PGa-r PGa-b Fig. 1: Pastes with Ga: Variation of Marsh Flow time (left) and final diameter (right) with SP. Log [TM] 2, 1,8 1,6 PWa PWa-y PWa-r D [mm] PWa PWa-y PWa-r 1,4 1,2,13,18,23,28,33 Fig. 2: Pastes with Wa: Variation of Marsh Flow time (left) and final diameter (right) with SP.,13,18,23,28,33 311

4 Figure 3 shows the curves of Marsh cone flow time and mini-slump versus SP of pastes corresponding to the second part of the test program. Again, the white cement (Wb) has higher saturation dosage than the ordinary cement (Gb), as shown in Table 2. The applied mortar-based design method for SCC 5 requires finding adequate values of Rm and. Figure 4.a and 4.b presents the results from the series of mortars, including the variation of Rm and with SP. Continuous and dashed lines correspond to measures at 6 and 45 min. As expected, as SP increases both Rm and increase. Table 2: Saturation dosages. Marsh cone test. Pastes Series PGa PGa-y PGa-r PGa-b PWa PWa-y PWa-r PGb PWb Saturation point (%) Log [TM] Fig. 3: Pastes with Gb and Wb: Marsh Flow time (left) and final diameter (right) vs. SP. Rm Fig. 4.a: Rm vs. SP (left) and vs. SP (right) obtained on mortars with cement Ga. Rm PGb PWb MWa MWa-y MWa-r MGa MGa-r MGa-b MGa-y Fig. 4.b: Rm vs. SP (left) and vs. SP (right) obtained on mortars with cement Wa. D [mm] PGb PWb MGa MGa-y 4 MGa-r 2 MGa-b MWa MWa-y MWa-r 312

5 Fig. 4.a presents the results with cement Ga. Regarding the effect of pigments, an increase of Rm appears in MGa-r, while remains similar to MGa. MGa-b shows similar Rm and near 18% decrease of with respect to the mix without pigments. Finally, in accordance with the behaviour observed on cement pastes, both Rm and decrease in MGa-y (15 and 47 %, respectively). Although the yellow pigment produced reductions in flowability, the obtained values are in self-compacting ranges and would make possible the fabrication of SCC 6. When mortars prepared with Wa were analyzed (Fig. 4.b), similar values of Rm and were found, existing small reductions in the case of MWa-y. The comparison of MGa and MWa show that mortars prepared with white cement demand more SP to achieve the same flowability; the reductions in Rm and produced in MWa for SP =.3 % were 18% and 44%, respectively. Regarding the measurements at 6 and 45 minutes, in both cases Rm and decreased, but losses in mortars with Wa along time were much more important. Rm decreased near 35 % and near 5 % after 45 minutes (sometimes tests were impossible to perform). This is consistent with the behaviour of cement pastes. Torque (N.m) Gb Gb-y Gb-r Gb-b Torque (N.m) Wb Wb-y Wb-r Wb-b Yield stress (Pa) Gb Gb-y Gb-r Gb-b Wb Wb-y Wb-r Wb-b Rotation speed (rps) Rotation speed (rps) Fig. 5. Rheological tests on mortars. Table 3. Test results obtained on mortars prepared with cements Gb and Wb. τ Pigment Time Cement μ μ Cement (Pa) (Pa.s) (Pa) (Pa.s) none , ,4 yellow , ,8 Initial red , ,8 black , ,2 Gb Wb none , yellow After , red 45 min , black , Figure 5 and Table 3 present the rheological parameters and the correlation with engineering test results on mortars corresponding to the second part of the study. The torque increases almost linearly with the rotation speed (Fig. 5). For a given speed the torque τ 313

6 increase is less for MGb, followed by MGb-b and MGb-r, with MGb-y presenting the largest increment. The torque values were higher when white cement was used, and with less influence of the pigment material. Comparing the torque vs. rotation speed responses at 6 and 45 minutes, there were no important variation when using cement Gb. However it was impossible to carry out the engineering tests at 45 minutes when using Wb. It is interesting to note that all mortars had similar plastic viscosity (Table 3); however the use of pigments produced an increase of τ, with the highest values for yellow mortars. Again Wb showed higher τ than Gb cement. Finally, it was also observed that the decreases in correspond to higher values of yield stress (Fig. 5c). 4. CONCLUSIONS This paper presents rheological studies on pastes and mortars carried out as a previous step to developing coloured self-compacting concrete. The incorporation of pigments modifies the fresh state behaviour of pastes and mortars, and consequently of SCC. Studies on pastes are useful to predict those effects and are consistent with the behaviour of the mortars. Rheology measurements confirmed the tendencies found by means of engineering tests, with good correlation between them. Pigments increased the cohesion of mortars. In the case of yellow pigment, the increase in yield stress might produce a greater resistance to flow. The use of pastes and mortars appears as a practical and useful step to improve mixture design of Coloured-SCC. ACKNOWLEDGEMENTS Funding from CONICET, CIC-LEMIT, the Spanish Ministry of Education and Science (grants MAT23-553, BIA C2-1; PSS 11-25, PSE : HABITAT 23) and the Programme Alβan, (European Union Programme of High Level Scholarships for Latin America, scholarship Nº E4E47473AR), is greatly appreciated. We thank also to Meranol S.A., Cementos Avellaneda S.A., Cerro Blanco S.A. and Basf S.A. for the provision of materials. REFERENCES [1] Giaccio, G. and Zerbino, R. 'Optimum superplasticizer dosage for systems with different cementitious material', The Indian Concrete Journal, 76, (9) (22) [2] Roncero, J., 'Effect of superplasticizers on the behavior of concrete in the fresh and hardened states: implications for high performance concretes'. Doctoral Thesis Universidad Politéctnica de Catalunya (2) [3] Gomes P. C. C., 'Optimization and characterization of high strength self-compacting concrete', Doctoral Thesis, Universidad Politéctnica de Catalunya (22) 23-45, [4] Okamura, H.; Ouchi, M., Self-compacting concrete, Journal of Advanced Concrete Technology, 1 (1) (23) [5] Tobes, J. M., Giaccio, G., Barragán, B. and Zerbino, R. Mortar-based design for self compacting concrete, fib Symposium: Concrete Structures-Stimulators of Development, Dubrovnik, Croatia, 27, in press. [6] López, A., Tobes, J. M., Giaccio, G., Positieri, M., Oshiro, A., Zerbino, R. and Barragán B. Design and characterization of coloured self-compacting concrete, Proceedings fib Symposium: Concrete Structures-Stimulators of Development, Dubrovnik, Croatia, (27). 314