RHEOLOGY OF PASTES AND MORTARS WITH FINES RESULTING FROM ORNAMENTAL ROCK WASTE

Transcription

1 RHEOLOGY OF PASTES AND MORTARS WITH FINES RESULTING FROM ORNAMENTAL ROCK WASTE Manuel Vieira and António Bettencourt Concrete Division LNEC, Lisbon, Portugal Abstract Self compacting concrete (SCC) contains a higher paste volume than ordinary concrete. This makes it possible to reduce friction among aggregates so that the SCC will flow only due to its one weight. The increase in paste volume is not just done by increasing the cement content, because this would not only augment the concrete price but also would lead to problems associated to high shrinkage and high hydration heat. Therefore, the use of mineral additions is a common practice in the manufacturing of SCC. Various types of additions are mentioned in the literature, such as: fly ash, slag, silica fume and limestone filler. The ornamental rock industry produces a significant amount of waste due to the cut activities. In view of the characteristics of that waste, a study has been implemented so as to evaluate the effectiveness of its use in producing SCC. This paper presents the results of the study about the rheology of pastes and mortars with the addition of fines deriving from that industry, by comparing them with the use of limestone filler currently employed in the manufacturing of SCC. 1. INTRODUCTION As is common knowledge, the self-compacting concrete (SCC) must have a composition presenting some particularities, when compared to current concrete, in order to fulfil the requirements related with: filling of moulds, covering of reinforcement and maintenance of its homogeneity, during placement. One of the differences is the fact that it contains a higher paste volume. This increase is not achieved just by augmenting the cement content, due to the problems associated with higher shrinkage and higher hydration heat. Therefore, the use of mineral additions is a common practice in the manufacturing of SCC. Various types of additions are mentioned in the literature, such as: fly ash, slag, silica fume and limestone filler. In Portugal, limestone filler is one of the additions most commonly used, not only due to its lower cost, but also because it makes possible to achieve a good fluidity and to reduce concrete segregation [1]. An alternative to that addition could be the use of waste deriving from the cut activities of the ornamental rock industry. The most commonly used type of rock 279

2 is marble. During cut and polishing of rocks, tons of mud wastes are rejected. That mud consists of fine rock particles and water, the latter being used for the cooling of blades during cut. Subsequently, that mud is deposited in fills, because it is not fully re-used for other purposes. After evaporation of water, the remaining product is a powder material with some agglomeration. Therefore, this paper presents a few results that make it possible to assess the influence of fines, deriving from an industry producing that type of waste, on the characteristics of pastes and mortars. 2. MATERIALS AND TESTING 2.1. Materials and mixture proportion The materials used in the manufacturing of pastes were as follows: cement CEM I 42.5R (designation in compliance with EN 197-1); limestone filler (FC100); rock powder (PP), deriving from the studied wastes and obtained after sieving in a 500 μm sieve; polycarboxylic-based superplasticizer; and water. Table 1 presents a few characteristics of powder materials. Figure 1 also shows the size distribution of powders, obtained by laser diffraction. In the manufacturing of mortars, two siliceous sands were used, a coarser one and a finer one. Table 2 depicts the characteristics of those fine aggregates. Table 1 Characteristics of powders Characteristic CEM FC100 PP Specific weight (kg/m 3 ) Fineness-Blaine (cm 2 /g) Water retaining ratio, βp [2] 1,21 0,92 1,19 N. Particles, % PP CEM FC100 Cumulative nbr. particles, % PP CEM FC , Particles dimension, μm 0 0, Particle dimension, μm Table 2 Characteristics of fine aggregates Figure 1 Size distribution of powders Characteristic Coarser sand Finer sand Specific weight (g/m 2 ) Fineness modulus Water absorption, %

3 The effect of replacing the cement by mineral additions was assessed, in volume, on 25%, 50% and 100% percentages. The mixture proportions of pastes and mortars are presented in Table 3 and Table 4, respectively, which also indicate the different superplasticizer dosages studied. Furthermore, the water/fine ratio was kept equal to 1.25, in volume. The superplasticizer dosages used ranged from a dosage close to the saturation (1% of the cement mass for the reference mixture proportion) to the double of that dosage. The sand content in mortars was kept constant and only the paste components were modified. Table 3 Mixture proportion of pastes (kg/m 3 ) Material P REF P 25 P 50 P 100 Cement Mineral addition Superplasticizer 14.3; 21.4; ; 21.4; ; 21.4; ; 21.4; 28.6 Effective water Table 4 Mixture proportion of mortars (kg/m 3 ) Material M REF M 25 M 50 M 100 Coarse sand Fine sand Cement Mineral addition Superplasticizer 7.7; ; ; ; 15.5 Effective water Testing The rheologic parameters of pastes viscosity and yield stress were determined using a concentric cylinders viscometer Haake RV2. The test procedure in the viscometer consisted of subjecting the paste sample to an increasing and decreasing velocity cycle, ranging from 4 s -1 to 126 s -1, at a geometric progression with a common ratio of 2, in 30s steps. By assuming the paste behaviour as the Bingham behaviour, the viscosity is determined by the slope in the trend straight line of the decreasing branch, which was obtained by linear regression in the graph plotting the stresses and the shear rate, whereas the yield stress results from the value obtained from the intersection of that straight line with the ordinate axis. Both the V-funnel and flow spread were determined on mortars [3]. 3. RESULTS AND DISCUSSION 3.1 Paste results Figure 2 depicts the results of the determinations done with the viscometer. The figure on the left shows that the replacement of cement by increasingly higher addition amounts makes it possible to achieve a decrease in the viscosity of the paste. For low replacement rates, the influence of the type of addition is not very significant. Nevertheless, the introduction of a percentage of rock powder starting at 50% does not affect the variation in the viscosity, since the difference between the results of the paste with a 50% percentage of rock powder and the results of the paste with just rock powder (100%) are fairly small. By comparing the viscosity 281

4 results with the fineness of additions, it is not possible to observe a higher fluidity of the pastes containing finer particles, the PP pastes, as initially expected. Nevertheless, the results are in agreement as refers to the higher effect of the 100% powder on viscosity, with the limestone filler paste, FC100, presenting a lower viscosity. The influence of the admixture dosage is of little importance for viscosity, since the latter is in a saturation level. Viscosity, Pa.s. 0,30 0,25 0,20 0,15 0,10 0,05 0,00 FC100 SP1,5 PP SP1,5 Yield stress, Pa 16,0 14,0 12,0 10,0 8,0 6,0 4,0 2,0 0,0 Figure 2 Rheological parameters of the paste FC100 SP1,5 PP SP1,5 As regards the yield stress, the variation of that property with the addition dosage follows the same trend as the one observed in viscosity. Nevertheless, the paste having just rock powder has shown a fairly high yielding stress, even higher than the paste containing 50% of that addition. This indicates that despite the higher fineness of rock powder particles, their grain size exhibits a certain packing level, which obstructs the flow. From the observation of the graph on the left side of figure 1, it is possible to deduce that whereas the cement and the limestone filler exhibit peaks in the percentage of particles, as a function of their diameter, the rock powder presents an almost rectangular configuration. As is known, a single size granulometry presents a larger space between particles than a continuous one. This likely packing effect makes it possible to justify the higher viscosity and the higher yield stress of mixtures containing rock powder. Another interesting aspect is the fineness of particles. The finer the particles, the lesser the distance between them, which increases the paste cohesion, namely in pastes with a 100% mineral addition, in which the van der waals attraction forces may be more significantly felt due to a likely deficiency in the repulsion energy [4]. This fact is corroborated with the observations of the flow spread of pastes, in which after the test, as regards the pastes with limestone filler, it was possible to notice segregation signs, which consisted of a liquid halo around the flow area. Those signs were not observed in the pastes containing rock powder. Those observations are in agreement with the water retaining ratio of powders. In fact, for an equal water volume, the higher the water retaining ratio of paste powders the higher the cohesion of those very pastes. 3.2 Mortar results Figure 3 shows the results of the determinations done on mortars. Generally, it is observed, on the one hand, that the flow spread results of mortars are in agreement with the yield stress 282

5 of pastes, and, on the other hand, the V-funnel time is well related with the viscosity of pastes. Those observations agree with the results of other works, in which there is a good relation between the viscosity and the V-funnel time and between the yield stress and the flow spread, which is due to the different shear rates introduced by each test [5, 6, 7] ,0 Mortar flow spread, mm Mortar V funnel time, s 3,0 2,0 1,0 0,0 Figure 3 Mortar flow spread and V-funnel time The effect of the superplasticizer dosage is rather significant in the flow spread of mortars with a low cement replacement dosage. That observation is partly due to the presence of a larger solid surface area in mortars and therefore the saturation dosage in the paste does not correspond to the saturation dosage in the mortar, which is generally higher. The V-funnel time decreases with the cement replacement percentage and does not depend on the superplasticizer dosage level In the V-funnel test, the most influent parameter on the flow time is the mineral addition percentage, with the addition nature or the superplasticizer dosage having less influence. Since the volume of fines is constant, the cement replacement by addition not only changes the size of fine particles but also their inter-action energy. Those variations are obvious in paste tests and differ as a function of the type of fines. Nevertheless, the V-funnel test results demonstrate well the influence of the cement replacement by addition, but seem to be irresponsive to the type of addition. In fact, by comparing the viscosity results indicated in Figure 2, and unlike what was actually observed, a larger V-funnel time in mortars containing rock powder was expectable. From these results, it can be deduced that the influence of additions in mortar flow is not sufficiently characterised by the two properties analysed in the paste. The influence of pastes on the friction between sand particles conditions their V-funnel flow time and may not be just represented by the yield stress and by viscosity. An additional effect of the direct contact between solids of paste and sand may have such significance that it covers up the differences found in the characteristics of both pastes. In this case, the size of the solid particles of the paste could be the conditioning aspect, namely their fineness, which would better agree with the V-funnel test results of Figure 3, since both the filler and the rock powder have finer particles than the cement. That hypothesis might contradict the fact that the Blaine fineness of the rock powder is fairly higher than the limestone filler fineness. 283

6 Nevertheless, in view of the higher viscosity of the paste with rock powder, the opposed effects would annul each other, being thus possible to obtain mortars with similar behaviours, as was subsequently observed. On the other hand, the V-funnel test has a restraint that forces the contact between particles, namely aggregate particles, which minimises the paste influence on such test. The lubrication effect of the PP finer particles might also lead to an easier flow. 4. CONCLUDING REMARKS The following conclusions can be drawn from the results presented in this paper: The replacement of cement by the mineral additions used makes it possible to reduce the paste viscosity. The size of the mineral addition influences the yield stress of pastes and the effect of the superplasticizer dosage on that rheologic parameter. The flow spread of mortars depends on the characteristics of the paste, whereas the results of V-funnel test are more dependent on the lubricant effect of particles. The use of rock powder, deriving from wastes from ornamental rock industry can be a competitive alternative to the use of limestone filler, namely as regards manufacturing of self compacting concrete. This is due to the fact that it also makes it possible to increase fluidity, with the advantage of providing better results in terms of segregation resistance. REFERENCES [1] Ribeiro, A. B., Gonçalves, A., A low cost self compacting concrete, in Proceedings of the 2nd International Symposium on SCC, Tokyo, Oct. 2001, (Ed. K. Ozawa & M. Ouchi, 2001), [2] Okamura, H., Ozawa, K., Mix design for self-compacting concrete, Concrete Library of JSCE, 25 (1995) [3] Okamura, H., Maekawa, K., Ozawa, K., High Performance Concrete, (Gihodo Shuppan, Tokyo, 1993) [4] Tattersall, G.H., Banfill, P.F.G., The rheology of fresh concrete, (Pitman Books Ltd, London, 1983) [5] Roussel, N., Le Roy, R., The Marsh cone: a test or a rheological apparatus?, Cement and Concrete Research, 35 (5) (2005) [6] Wallevik, J.E., Relationship between the Bingham parameters and slump, Cement and Concrete Research, 36 (6) (2006) [7] Noor, M.A., Uomoto, T., Rheology of high flowing mortar and concrete, Materials and Structures, 37 (8) (2004)