Effect of silica fume fineness on the improvement of Portland cement strength performance
Introduction
Silica fume (SF) composed of submicron particles of silicon dioxide is produced by an electric arc furnace as a by-product of the smelting process in the production of metallic silicon or ferrosilicon in the alloys industry. The reduction of high-purity quartz to silicon at temperatures up to 2000 °C produces SiO2 vapours which oxidise and condense in the low-temperature zone to tiny particles consisting of 85–99% amorphous silica [2]. Then, SF is composed of submicron particles of silicon dioxide which occur as almost-perfect spheres with diameters ranging from 20 to 500 nm [1]. It is estimated that current global output of SF is, at most, between one and 1.5 million tonnes per year [58].
Portland cement (PC) is unquestionably the primary cementitious material now used in construction. However, SF has been used as a high pozzolanic reactive cementitious material to make high-performance concrete (HPC). Moreover, it may be said that SF is the key ingredient for producing such high-performance concrete. This addition in PCs, such as CEM II/A–D according to the European Standard EN 197-1:2011 [18] has been shown to give rise to physical and chemical effects on the microstructure of hardened pastes, leading to improved properties well recognised in concrete technology, such as higher strength [4], [10], [27] and lower permeability [63]. Therefore, HPC has increasingly been used in civil engineering work because it allows reduction of the size of structural elements, which is essential in high-rise building. In addition to this, such HPC use is now growing in conditions of a severe nature [48], [5]. In summary, the benefits obtained when utilising SF include substantial increases in compressive strength and increased durability of hardened concrete when added in optimum amounts [36], [50], [55], [35], [60], [10]. Particularly, the main advantages of using SF are higher mechanical strength (high early compressive strength, high tensile strength, high flexural strength and modulus of elasticity, increased toughness and high bond strength) [10], [61] and enhanced durability (sulphate resistance [61], [65], higher electrical resistivity, low permeability to chloride and increased resistance to chemical attack, seawater resistance [28], freeze–thaw resistance [64] abrasion resistance [49], controlling expansion due to alkali–silica reaction [12] and good fire resistance [47]).
Therefore, SF is used in construction to make HPC for highway bridges, marine structures, parking decks, and bridge deck overlays. Furthermore, SF is utilised to manufacture shotcrete for use in rock stabilisation, mine tunnel linings, and rehabilitation of deteriorating bridge and marine columns and piles. Furthermore, SF is applied for oil well grouting and in a wide variety of cementitious repair products, among other applications.
In the case of some pozzolanic materials such as fly ash, a correlation between pozzolanic properties determined by using the compressive strength of mortars and the fineness and the soluble silica was identified [62]; the effect of the soluble silica content was found to be more significant than the effect of their fineness at later ages [56].
The purpose of this study is to evaluate the influence of SF reactive SiO2 and grain size on SF reactivity. The pozzolanicity test method was complemented by another widely accepted method, the strength activity index for strength performance quantification, with the aim of clarifying the pozzolanic reaction and effects of SF on the hydration process of SF-containing composite cement system. Hence, the study focused on two sections: the effects of SF fineness on the evolution of pozzolanic reaction and strength performance quantification.
Section snippets
Materials
Materials used in this study consisted of PC type CEM I 52.5 N-SR 3 according to the European Standard EN 197-1:2011 [18], fine aggregates (sand), silica fume and distiled water at 20 °C. Two brands of SF, namely SF I and SF II, in powder form were used. Both of them conformed to the mandatory requirements of the European Standard EN 13263-1:2005+A1:2009 [15] issued by the European Committee for Standardisation (Comité Européen de Normalisation, CEN). High-quality standardised sand was used as
Chemical composition of materials
The oxide analyses for cement CEM I 52.5 N-SR 3 and SF samples SF I and SF II are presented in Table 1. The fraction of SiO2 which is active for pozzolanic reactions is also given (active silica). The Table 1 shows that SF has a significantly high content of amorphous silicon dioxide, with small amounts of iron, magnesium, alumina, calcium and alkali oxides being found too (Table 1).
ASTM C1240-12 [7] and EN 13263-1:2005+A1:2009 [15] issued by ASTM and the CEN, respectively, cover SF
Conclusions
The following conclusions have hence been drawn:
- 1.
The replacement of Portland cement with 25% of silica fume (45 μm sieve residue between 0.98%, for SF I (C) and 4.13%, for SF I (B)) produces high-strength mortar and such fineness gives the highest compressive strength. However, the coarser silica fume named SF I (A) does not exhibit a good performance when added in high amounts of 25% (45 μm sieve residue of 32.11%).
- 2.
Mortar made of silica fume that has a fineness of between 0.98% and 4.13%
Acknowledgement
The authors gratefully acknowledge the financial support provided by Ministry of Economy and Competitiveness of Spain by means of the Research Fund Project DPI 2011-24876.
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