Carbon nanotube reinforced alkali-activated slag mortars
Introduction
The development of infrastructure leads to an increase in the demand for natural resources, which are limited, and the building industry in particular consumes a huge amount of raw materials for the production of building materials. Ordinary Portland Cement (OPC) will remain a key player in the future, although its production is energy demanding and contributes to the ongoing increase in global CO2 emissions. There are two possible ways of reducing the negative impact of the building industry. One way is to utilize secondary raw materials as supplementary cementing materials, among which blast furnace slag is the most effective at reducing CO2 footprint [1]. The other way is to utilize alkali-activated concrete. This type of material is even more effective in reducing CO2 emissions and energy consumption. Different sources show that the global warming potential of alkali-activated concrete is approximately 40–70% lower than that of OPC concrete [2], [3], [4].
One of the most common and intensively studied alkali-activated materials is alkali-activated slag (AAS). It is composed of finely ground granulated blast furnace slag and alkaline activator. Alkaline hydroxides, carbonates and especially silicates (water glass) are known to be the most effective activator for this type of material [5], [6]. The mixture sets to form a very stable product and its properties depend on a number of factors such as the chemical and mineralogical composition of the slag, the type, composition and dosage of alkali activator, curing conditions, etc. The mechanical properties and application possibilities of AAS are very similar to those of OPC concrete. However, in contrast to OPC-based binders, AAS offers superior properties such as higher corrosion resistance against acid or sulphate attack [7], [8], [9], [10], [11] and also higher resistance to elevated temperatures and fire [12], [13], [14], [15], [16]. Its major disadvantage is increased shrinkage. This effect is caused by both autogenous and drying shrinkage and finally results in volume contraction, micro-cracking and the deterioration of tensile and bending properties [5].
Autogenous shrinkage is a basic property of CSH gel, which is a predominant binding phase; however, due to its different character, the shrinkage that affects AAS is more severe than in the case of OPC. Autogenous shrinkage increases with the increasing amount of Na2O in the activator and it becomes more evident in the case of water glass activated materials as opposed to NaOH or Na2CO3 activated ones [17]. Drying shrinkage is even more severe because it acts unevenly and causes cracking especially in the surface layer, which is then responsible for the deterioration of the mechanical properties and decreasing serviceability of the structure. It depends not only on the nature of the material itself, but also on exterior conditions such as curing temperature, relative humidity, drying rate, etc. Collins and Sanjayan [18] used a crack-detection microscope to examine the cracking of AAS concrete. When the concrete specimens were cured in a water bath they did not notice any visible surface cracks. However, specimens cured under 50% relative humidity conditions had lots of cracks within one day. The average crack width is three times greater than for OPC concrete [19]. One possible way to reduce shrinkage is via the application of shrinkage reducing admixtures (SRA), which are commonly used for OPC concrete. The effect of SRA on alkali-activated slag was studied by Palacios and Puertas [20]. They observed that polypropylenglycol-based admixture reduces autogenous shrinkage by 85% and drying shrinkage by 50% in waterglass-activated slag mortars, but the effect strongly depends on curing conditions. However, the SRA retards the alkali activation of slag, with longer delays at higher dosages of admixture.
The main aim of this work is to apply multi-walled carbon nanotubes (MWCNTs) as a shrinkage reducing admixture for AAS-based mortars. Regarding the properties of MWCNTs, they have a great potential to reduce the cracking tendency of silicate-based materials caused by autogenous and drying shrinkage, which is one of the essential problems arising during the practical application of materials in the building industry [21], [22].
Carbon nanotubes exhibit extraordinary mechanical properties, with the Young’s modulus of an individual nanotube being around 1 TPa and tensile stresses being in the range of 65–93 GPa [23]. MWCNTs are thus the most promising nanomaterials for enhancing the mechanical fracture properties of building materials, and their resistance to crack propagation. Some problems have appeared connected with the aggregation of MWCNTs, which reduces the efficiency of single nanotubes. Nevertheless, effective dispersion can be achieved by applying ultrasonic or high shear rate mechanical dispersion with the use of a surfactant [24].
Since microcracks have a strong negative effect on mechanical performance, the efficiency of MWCNTs as a potential nanoscale reinforcement and shrinkage reducing agent can be monitored by the fracture behaviour of the composite material and by acoustic emission methods. In this study, fracture testing and acoustic methods were applied to determine the performance of MWCNTs in AAS mortars.
Section snippets
Materials
The alkali-activated slag mortars used in the tests were composed of granulated blast furnace slag and water glass. Slag supplied by Kotouč, s.r.o. (CZ) was ground to a fineness of about 380 m2 kg−1 (Blaine). The average grain size of the slag obtained by laser granulometry was d50 = 15.5 μm and d90 = 38.3 μm, indicating that 50% or 90%, respectively, of all grains is smaller than a given value. The slag was neutral with a basicity coefficient Mb = (CaO + MgO)/(SiO2 + Al2O3) equal to 1.08, and its chemical
Mechanical fracture parameters
The mean values (obtained from 3 independent measurements) and standard deviations (presented as error bars) of the compressive strengths of the tested AAS composites are depicted in Fig. 3. The compressive strength value increased with the addition of 0.05, 0.1, 0.15, 0.2, and 0.5% of MWCNTs by 3, 27, 17, 19 and 10%, respectively, whereas for the specimens with 1.0% of MWCNTs the compressive strength decreased by 13% in comparison with the reference mixture.
The mean values and standard
Discussion
The addition of a small amount of MWCNTs has been proven to have a positive effect on the mechanical properties of cement paste [34]. Kosta-Gdoutos et al. [22] suggested that MWCNTs strongly reinforce the cement paste matrix by increasing the amount of high stiffness C-S-H. In this paper, the same presumption was adopted for the improvement of the mechanical fracture properties of alkali-activated slag. The results of compressive strength and modulus of elasticity measurements show that the
Conclusions
The fracture properties of alkali-activated slag composites with various amounts of MWCNTs as reinforcement have been investigated. It has been shown that the addition of MWCNTs in the range of 0.05–0.2% of the mass of slag improves the mechanical fracture properties of alkali-activated slag. Compressive strength, modulus of elasticity, and effective fracture toughness were improved for all these dosages; however, the results achieved as regards the stated parameters suggest that the optimum
Acknowledgement
This outcome has been achieved with the financial support of the Czech Science Foundation, project No. 13-09518S (NANOFRAM), and the Ministry of Education, Youth and Sports of the Czech Republic under the “National Sustainability Programme I”, project No. LO1408 (AdMaS UP).
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