The effect of fly ash and limestone fillers on the viscosity and compressive strength of self-compacting repair mortars
Introduction
Self-compacting repair mortars, as new technology products, are especially preferred for the rehabilitation and repair of reinforced concrete structures [1]. The water/powder (cement, fly ash, limestone filler, silica fume, etc.) ratio of mortar and the type of chemical admixtures should be determined, in order to place the fresh mortar without any external compaction and at the same time without causing any segregation. In other words, the paste phase rheology of repair mortar should possess suitable properties from the viewpoint of flowability and segregation [2], [3], [4], [5]. The self-compactability of repair mortars may bring considerable advantages at narrow mould systems such as coating [6]. With the development of new-generation plasticizers, to obtain high filling rates is possible even for complex molding systems.
The fresh rheological characteristics, strength and durability of repair mortars can be enhanced by the addition of powders which can be collected in two groups as inert or pozzolanic [7]. The selection of amount and type of cementitious or inert powders depends on the physical and physico-chemical properties of these powders which are affecting the performance of fresh paste such as particle shape, surface texture, surface porosity and rate of superplasticizer adsorption, surface energy (zeta potential), finest fraction content, Blaine fineness and particle size distribution. There is no universally accepted agreement on the effect of these factors due to the complex influence of the combination of these factors [8].
In general, the increase in fine-grounded materials content in cements brings about the modification of rheological properties of pastes and consequently influences the workability of mortars and concrete mixtures. The observed changes can be advantageous or not. This is because of many factors influencing the rheology of cement pastes [9]. It is usually expected that, if the volume concentration of a solid is held constant, for a specific workability, the replacement of cement with a fine powder will increase the water demand due to the increase in surface area. This is valid for silica fume [10]. However, in some cases, the above-mentioned conclusion is not appropriate. Lange et al. [11] concluded that for a specific workability, the inclusion of specified amount of fly ash reduced the water content and improved the workability. The workability enhancement is explained by the spherical shape of fly ash which causes the particle to easily roll over one another, reducing the interparticle friction [12]. The spherical shape also minimizes the particle's surface-to-volume ratio, resulting in low fluid demands.
Another factor influencing the rheology is the fineness of the powder used. Collins and Sanjayan [10] reported that, in concrete containing alkali-activated ground granulated slag as the binder, the workability was improved by replacing part of the binder with ultrafine materials. Yijin et al. [13] found that the addition of ultrafine fly ash (UFA) to cement paste, mortar and concrete can improve their fluidity, but some coarse fly ash cannot reduce water. Baoju et al. [14] reported similar findings. In their study, the addition of ultrafine fly ash with a Blaine specific surface area of 740 m2/kg improved the fluidity and reduced the water demand for normal consistency. It should be noted that the method of detection of fineness is also important to characterize the effect of powders. According to Grzeszczyk and Lipowski [9], the grinding of the high-calcium fly ash brings about the rheological properties improvement (increase of fluidity) as compared with the paste containing coarse fly ash. However, the cement pastes containing fly ashes of similar grain size distribution in the range below 24 μm reveal similar rheological properties, although they show different specific surface area values. Grzeszczyk and Lipowski [9] have concluded that the finest fractions content is a better parameter to characterize the rheological properties of cement pastes than the fly ash Blaine specific surface.
Ferraris et al. [8] studied on the effect of addition of fine grounded materials with a comprehensive literature survey. They have concluded that the selection of a fine mineral admixture for improved concrete workability is not a trivial problem. At present, this selection cannot be predicted from the physical or chemical characteristics of the powders and can only be determined using the properly designed tests.
For improving strength and durability properties; limestone fillers produce a more compact structure by pore-filling effect. In the case of fly ash, it also reacts with cement by binding Ca(OH)2 with free silica by a pozzolanic reaction forming a non-soluble CSH structure [15], [16].
In this study, the effect of fly ash and two types of limestone fillers on the fresh properties of paste phase of repair mortars was studied. The replacement ratios were by masses of 20%, 40% and 60% of cement, respectively. Additionally, compressive strength developments of mixes were determined.
Section snippets
Experimental studies
The experimental studies consist in two stages. In the first stage, the fresh paste flow diameter, V-cone flow time and viscosity measurements were conducted; in the second stage, compressive strengths of the specimens prepared from the paste mixtures were determined after 1, 7 and 28 days of standard curing.
Results and discussion
Mixtures incorporating EM, OK type limestone powders showed higher V-funnel flow times when compared with control paste (Table 2). In other words, both of these two powders reduced the flowability when replacing with cement. However, the increase in replacement ratio reduced this effect; moreover, at 60% replacement of OK type limestone powder, the same flow time was measured with the control mix. The V-funnel flow times of mixtures with fly ash could not be determined due to the high cohesive
Conclusions
Based on the test results presented in this paper, the following conclusions can be drawn:
- 1.
The behavior of all paste mixtures with or without any mineral admixtures or inert fillers can be classified as pseudoplastic. In other words, the viscous behavior is evident for low rotational speeds of mixing, while at higher speeds, flowable behavior becomes dominant. While mixing, the shear thinning effect at high rotation speeds breaks down the formation of the high viscous behavior of the mix at
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