Single and combined effects of nano-SiO₂, nano-Al₂O₃ and nano-Fe₂O₃ powders on compressive strength and capillary permeability of cement mortar containing silica fume

Abstract

In this article, the single, binary and ternary effects of three nano-sized powders of the main oxides of cement (nano-SiO₂, nano-Al₂O₃ and nano-Fe₂O₃) on the compressive strength and capillary permeability of cement mortars containing silica fume were investigated. The powder amounts were chosen at proportions corresponding to 0.5%, 1.25% and 2.5% of the binder amount. Compressive strength was determined for early age (3- and 7-day), standard age (28-day), and late age (56- and 180-day) mortars, while capillary permeability was determined for 180-day-old mortars only.

It was concluded from the experimental results that the type and amount of nano the powders, and mortar production methods had a significant effect on the fresh and hardened properties of cement mortars. The nano-powders used singly or in combination increased the 28-day compressive strength of silica fume-containing mortars by up to 27%, with the exception of nano-SiO₂ powder used at a proportion of 2.5%. However, the compressive strength values fluctuated at early and later ages. The best results for compressive strength and capillary permeability at the end of day 180 were obtained with 1.25% nano-Al₂O₃ powder in single uses, 0.5% nano-SiO₂ + nano-Al₂O₃ powders in binary combinations, and 0.5% in ternary combination. However, it was determined that the interaction between the powders used in binary and ternary combinations led to negative effects on the physical–mechanical properties of the mortars. For this reason, nano-Al₂O₃ powder and single use were primarily recommended in cases where an increase in the performance of cement-based composites is desired. The findings of the experiments suggested the conclusion that the improvement in the mechanical and physical properties of mortars was caused by the rise in pozzolanic activity induced by the favorable influence of the powders rather than the filler effect.

Introduction

Studies investigating the use of certain nano-size powders in cement composites started in the early 2000s. While studies [1], [2] were carried out examining how these powders affected the properties of pure cement on the one hand, the effects of the powders on behavior of the mortar and concrete were investigated on the other hand. In general, nano-sized SiO₂ (NS), Al₂O₃ (NA), and Fe₂O₃ (NF) powders were used in the studies. However, though rarely, some studies examining the use of other nano powders (such as nano-ZnO₂ and nano-TiO₂) also exist in the literature.

In mortar and concrete specimens in which NS powder was added, a decrease in the amount of silica fume and improvement in concrete performance was observed [3], in addition to an increased level of fly ash activity which was initially low [4]. This powder also reduced the water permeability of the concrete, effectively giving the concrete a much more compact and uniform structure [4], [5]. Jo et al. [6] reported that amorphous NS-containing concrete had a higher compressive strength than silica fume-containing concrete of the same age, and NS powder did not only serve as a filler material which is used to strengthen microstructure, but also served as an activator helping to increase pozzolanic reaction. Quing et al. [7] compared silica-containing and NS-containing concretes. As a result, they found that NS-containing concrete exhibited a better consistency, a faster hydration, and a higher compressive strength and adherence. In this study, it was especially demonstrated that the adherence improvement was higher than the compressive strength improvement at early ages particularly when the NS amount was increased. Other researchers [8], [9], [10], [11] also reported that NS powder caused pronounced improvements in the mechanical properties of mortars and concretes, most notably in compressive strength. Senff et al. [11] found that fluidity, spreading and hardening time of the cement paste and mortar were reduced as the proportion of NS powder was increased. Givi et al. [12] found that fine powders increased early compressive strength, whereas coarse powders increased late compressive strength.

In the studies performed using NA, it was found that the powder increased compactness of the aggregate-cement paste interface, and also improved the modulus of elasticity and early age compressive strength of mortars up to a certain level [13]. Campillo et al. [14] reported that the more environmentally friendly belite cements possessing a lower initial compressive strength had limited usage, but after blending them with colloidal NA and NA powder belite cements also exhibited an enhanced level of initial compressive strength. However, the effect of this powder on compressive strength varies depending on the proportion of powder and the curing conditions [15].

Mortars containing NF powder demonstrated improved mechanical properties when the powder was used at proportions not exceeding 10% [16]. There is also a researcher [17] who reported that the ability to identify tensions that occurred in NF powder-containing mortars increased with NF content, and therefore, NF-containing materials could be smart materials sensitive to tensions. Li et al. [17] also stated that NF- and NS-containing mortars displayed increased levels of compressive and flexural strength when compared with mortars containing the same ratio of water/binder but not the additives.

Aside from these, nano-ZnO₂ and nano-TiO₂ powders that partially replaced the cement at proportions not exceeding 3–4% improved the mechanical and physical properties of the concrete [18], [19], [20].

In addition to nano-sized powders, studies examining the effect of carbon nano tubes on cement mortars were launched as well. In one of these studies [21], it was found that the compressive strength of mixtures containing 2% carbon nano-tube and 6% nano clay was 29% higher than that of the control groups.

In all but two of the previous studies on the subject, a single type of nano powder was used. Furthermore, no study using three different types of nano powders within the same mixture was encountered. Therefore, the fact that the potential combined effects which are likely to emerge with the use of different types of powders were not examined points to a gap in the literature which needs to be filled. On the other hand, the majority of the previous studies were performed on mortar or concrete specimens that did not contain any mineral additives such as silica fume or fly ash. To what extent it is beneficial, in terms of the compactness which determines the physical–mechanical properties of cement composites, to add nano-size powders without adding micron-sized particles like mineral additives is disputable. Besides, the studies carried out to date generally examined compressive strength on days 7 and 28, and did not, with the exception of a few cases, examine the variations in strength at later ages. In addition, in most of the published studies, the specimen production processes were not described clearly, making it difficult to compare the results derived from the experiments. For example, while some authors (e.g., Ref. [6]) could use NS addition rates up to 10% of the cement weight, some authors (e.g., Refs. [4], [7]) stated that adverse results could be obtained when the powder was used at proportions in excess of 5%, due to resulting in agglomeration.

In this study, the aim was to examine the single, binary and ternary effects of three nano-sized powders of the main oxides of cement (NS, NA and NF) on the physical–mechanical properties of cement mortars containing silica fume. All steps of the specimen production process were clearly described in the study, and the hardened mortar experiments were performed on early and later age cement mortars.