Effects of CSF on strength, rheology and cohesiveness of cement paste
Highlights
► Addition of CSF up to an optimum content would increase strength. ► Addition of CSF would significantly increase packing density. ► Addition of CSF could improve flowability and cohesiveness at the same time. ► Effect of adding CSF on water film thickness is dependent on W/CM ratio. ► Water film thickness is the key factor governing rheology of cement paste.
Introduction
Self-consolidating concrete (SCC) is, in recent years, becoming more and more popular because of its superior performance [1], [2], [3]. This kind of concrete is able to flow readily under its own weight, pass through narrow gaps without loss of homogeneity, fill far-reaching corners in the mould and achieve full consolidation without vibration. To attain such self-consolidating capabilities, the SCC should have high flowability, passing ability and segregation stability [4], [5]. With these superior capabilities of SCC over conventional concrete, the concreting operation would be much easier, quicker and quieter than before. Hence, the successful development of SCC is one of the greatest advancement in concrete technology.
However, the mix proportions of SCC are not easy to design. For the SCC to perform as required, the cement paste portion must have appropriate rheology [6], [7], [8]. More specifically, the cement paste has to have both high flowability and high cohesiveness. The high flowability of the cement paste is to let the concrete mix flow readily under its own weight while the high cohesiveness of the cement paste is to avoid bleeding of the cement paste itself and to avoid segregation when the concrete mix flows and passes through narrow gaps. Somehow, most measures for increasing the flowability, such as increasing the water/cementitious materials (W/CM) ratio and adding more superplasticizer (SP), would decrease the cohesiveness whereas most measures for increasing the cohesiveness, such as decreasing the W/CM ratio and adding less SP, would decrease the flowability. Therefore, in general, it is not easy to achieve high flowability and high cohesiveness simultaneously. Measures that would have the combined effects of increasing the flowability and cohesiveness at the same time are very much desired [9], [10].
Early in 1960s, Powers [11] pointed out that apart from the water content, the particle size distribution and packing density of the cementitious materials also play important roles in the rheology of cement paste. Recently, Kwan and Wong [12] proved that it is the excess water content (the water content in excess of that needed to fill the voids in the cementitious materials) rather than the water content that governs the flowability of cement paste. Hence, an increase in the packing density of the cementitious materials should decrease the volume of voids and increase the excess water content to improve the flowability of the cement paste. In this regard, Huang and Feldman [13] and Feldman and Huang [14] found that the addition of condensed silica fume (CSF), an ultrafine supplementary cementitious material, can densify the cement paste matrix, and Nehdi et al. [15] and Park et al. [16] found that the addition of ultrafine particles can improve the flowability of cement paste and concrete.
The addition of CSF may have other benefits as well. For instance, it can significantly increase the cohesiveness of the cement paste and thus also that of the concrete mix. This is probably because of the dramatic increase in surface area, which increases the affinity of the solid particles, and the dramatic reduction in pore size, which increases the viscosity of the water–solid mixture [17]. In this regard, Khayat [9] suggested that the incorporation of powder finer than 80 μm can effectively improve the cohesiveness of the concrete mix while Park et al. [16] pointed out that the addition of CSF can increase the stability of the concrete mix. Furthermore, Arias et al. [18] recently showed that the adhesion of cement paste could be substantially enhanced by adding CSF to the cement paste.
However, despite the various benefits as well as the high pozzolanic activity which increases the strength and durability of the hardened concrete [19], the addition of CSF may also have certain demerits. According to ACI 238.1R-08 [20], there is a threshold value of CSF replacement level below which the addition of CSF produces little change in yield stress but reduces plastic viscosity and above which the addition of CSF increases both yield stress and plastic viscosity. Mork [21], Faroug et al. [22] and Shi et al. [23] observed that the addition of CSF as cement replacement could have beneficial or adverse effects on the rheology of concrete and mortar, depending on the replacement level. On the other hand, Claisse et al. [24] and Ji and Cahyadi [25] found that because of the large specific surface area of CSF, the addition of CSF could impair the flowability of cement paste and concrete.
Although extensive research on the effects of adding CSF to the cement paste [19] has been carried out, no general conclusions can yet be made because different results have been obtained by different researchers. Particularly, there is no consensus as to whether the addition of CSF could really improve both the flowability and cohesiveness of the cement paste. This is due partly to the difficulty of overcoming the agglomeration of CSF [26] and partly to the lack of suitable test methods for measuring the packing density of cementitious materials and the cohesiveness of cement paste.
In a recent study, the authors’ research group [27] has developed a new wet packing test for measuring the packing density of cementitious materials under the wet condition (the real condition because the cementitious materials are actually wet in a cement paste) so as to investigate how the particle size distribution of the cementitious materials and the addition of supplementary cementitious materials would affect the packing density. In this study, the authors are proposing to use a mini version of the sieve segregation test for measuring the cohesiveness of cement paste. With the adoption of these two tests, the packing density and cohesiveness as well as the strength and rheological properties of cement paste containing CSF have been investigated through a comprehensive experimental program, in which a total of 34 cement paste samples were tested, as reported herein.
Section snippets
Materials
Two types of cementitious materials, namely, ordinary Portland cement (OPC) and condensed silica fume (CSF), were used in the experiments. The OPC was obtained locally and had been tested to verify its compliance with BS 12: 1996, whereas the CSF was imported from Norway and according to the supplier it complied with ASTM C 1240-03. The solid densities of the OPC and CSF had been measured in accordance with BS 812: Part 2: 1995 as 3112 kg/m3 and 2196 kg/m3, respectively. Their particle size
Experimental program
The experimental program consisted of two parts. The first part was to measure the strength, flow spread, flow rate, yield stress, apparent viscosity and cohesiveness (the cohesiveness was measured in terms of two sieve segregation indices as will be explained later) of cement paste samples containing different amounts of CSF and at different water/cementitious materials (W/CM) ratios. The second part was to measure the wet packing densities of the cementitious materials containing different
Strength
The 28 day cube strength results are plotted against the W/CM ratios and CSF contents in Fig. 2. Each cube strength result presented is the average of the three cubes cast of the same cement paste. From the variation of the cube strength with the W/CM ratio, it can be seen that at a W/CM ratio of 0.8 or higher, the cube strength increased as the W/CM ratio decreased but at a W/CM ratio of between 0.8 and 0.6, the cube strength remained more or less the same as the W/CM ratio varied. From the
Role of packing density
The packing density results of the six mixes of cementitious materials with different CSF contents are tabulated in the fifth column of Table 1 and plotted against the CSF contents in Fig. 7. These results show that blending of OPC with CSF can significantly improve the packing density of the solid particles. With just 5% CSF added, the packing density was already increased from 0.654 to 0.681. With even more CSF added, the packing density kept on increasing to 0.736 at a CSF content of 25%.
Conclusions
A systematic study on the strength, rheological properties, cohesiveness and packing density of cement paste containing CSF has been carried out. In the study, the rheological properties were measured using commonly adopted tests with a mini slump cone, a Marsh cone and a rheometer while the cohesiveness and packing density were measured using a mini version of the sieve segregation test and a recently developed wet packing test. The results revealed that the addition of CSF would increase the
Acknowledgment
The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. 713309).
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