Mechanical properties and durability performance of fly ash based mortar containing nano- and micro-silica additives

Highlights

• Nano-silica is more promising than micro-silica in improving mechanical properties.

• Optimum strength and workability are obtained at 5% nano-silica.

• Nano-silica has high pozzolanic activity and filling effect.

• Increasing content of fly ash reduces the performance of mortar containing nano-silica.

Abstract

The aim of this paper is to investigate the influence of nano-silica (NS) and micro-silica (MS) on the properties of fly ash based geopolymer mortar. The use of such additives has the potential to provide greener and cheaper concrete with enhanced properties. A screening study was conducted to compare the performance of mortar containing varying percentages of NS and MS (up to 15%). The performance of each specimen was determined through tests on compressive strength, splitting tensile strength, workability, and water absorption. Results indicated that mortar with a dosage of 5% NS shows optimum results in workability and mechanical properties. This is mainly due to the pozzolanic and filling effects of large surface area nanosized particles on the cementitious matrix. Moreover, the effect of 5% NS was investigated for mortars containing different amounts of fly ash (up to 30%) which replaced cement by weight. With the presence of 5% NS, variation in fly ash content has no significant influence on the flowability. Although replacement of cement with fly ash at 10% in the presence of 5% NS decreased the splitting tensile strength, no significant reduction was noticed when compared with control samples (without nanoparticle). Compressive strengths also show significant reduction as the percentage of fly ash increases, however the presence of 5% NS aids in the strength development over 28 days. Observations from this study show the effects of nano-silica and fly ash dosage in the performance of cementitious composites. Thus, the results of this study is beneficial for future applications of nanomaterials in cement-based composites for generation of multifunctional construction materials.

Introduction

Being one of the mostly used materials worldwide, researchers have been looking for ways to enhance the properties of concrete. So far, lot of fillers and additives were investigated which can potentially provide more durable, greener, and cheaper concrete by substituting the ordinary Portland cement (OPC) [1]. Among all studied additives and fillers, particles at nanoscale sizes have potential to modify the cement chemistry [2]. According to Nguyen-Tri et al. [2], the global consumption of nanomaterials is expected to grow to USD 100 billion by 2025 among which 7% will be implemented in construction and building materials. Nanotechnological applications and advances in building and construction materials had been uneven until recently [3], however nanoparticles have received more attention as an additive for the development of construction materials [4]. Nanotechnology in construction materials is generally referred to nanoscience and nanomodification (nano-engineering). Nanoscience is referred to the measurement and characterization of particle structure of cementitious composites which provides an insight on how the structure influences macroscale properties. On the other hand, nanomodification offers the required knowledge and techniques to manipulate the structure of cementitious materials at nanoscale for production of multifunctional cement-based composites [2]. Fabrication and incorporation of nanosized particles provide the opportunity for development of cement additives such as nanoreinforcements, fillers, and plasticizers. As compared to bulk materials, nanoparticles exhibit a number of special properties. A high surface area to volume ratio is one of the unique characteristics of nanosized particles that results in outstanding chemical reactivity and thereby further promoting cement hydration. The presence of nanomaterials in cement-based composites provide additional seeds for nucleation and crystal growth of hydration products and contribute to produce additional C-S-H [3]. In this context, the application of nanomaterials in cement-based matrices can result in production of building materials with an advanced mechanical properties and durability performance such as self-sensing capability [5], self-healing behaviour [6], self-cleaning feature, and low electrical resistivity [7]. Therefore, the modification of materials at the nanostructural level guarantees to deliver the optimization of material behavior as well as performance needed to improve the volume change characteristics, mechanical features, durability performance and sustainability of concrete [8].

Emission of carbon dioxide (CO₂) is the major concern associated with the usage of huge quantities of cement in concrete materials. The cement industry emits approximately one ton of CO₂ per ton of OPC produced which contributes to 8% of the global CO₂ emissions into the atmosphere [9]. Recently, the production of fly ash based geopolymer materials (concrete and mortar) as a greener alternative to OPC has become a prominent topic of research [10], [11]. Global combustion products in the form of fly ash were reported to be 780 Mt tonnes annually around 2011–2012. Approximately 415 Mt of fly ash is effectively utilized which is only 53% of the total production, hence a huge amount of excess is an industrial hazard [12]. Typically, replacement of cement with fly ash has its drawbacks and fails to retain the desired performance. Mortar with fly ash exhibited poor strength at ambient temperature of curing due to slow polymerization process [13], [14]. However, incorporating other materials such as nanoparticles can contribute to enhance a better result regarding strength and durability [15], [16], [17]. So far, various types of nanoparticles have been added to the concrete for additional pozzolanic and filler effects such as nano-oxides [18], [19], [20], carbon nanofiber and nanotubes [21], Cu nanomaterials [22], and graphene [23]. Nanoparticles have been utilized to improve/modify different properties of cement-based composites. For example, it has been reported that the microstructure of cement-based composites containing nanomaterials is more uniform and denser than normal cementitious materials [24]. Furthermore, the effects of nanoparticles on radiation shielding properties of concrete have been investigated in a study by Hassan et al. [25]. The authors have tested the influence of nano PbO and PbTiO₃ addition on γ ray shielding behaviour of concrete.

Among all tested particles in cement-based composites, silica particles (nano and micro size) have drawn more attention because it is a major compound involve in cement hydration and strength development [15]. Conventionally, silica fume is used to provide a better strength and durability performance in ultra-high-performance concrete (UHPC). However, due to relatively high price of silica fume as compared to other concrete constituents and its limited availability, UHPC technology has become less demanding in comparison with high strength concrete [26]. However, with the advances in nanotechnology, silica particle can mimic the action of silica fume and therefore can be used as a substitute to silica fume.

The aim of the present study is to investigate the properties of fly ash based geopolymer mortar in the presence of nano-silica particles (NS) and micro-silica particles (MS). The optimum concentration of particles in a typical mortar mixture cured at ambient temperature is to be obtained by conducting a screening study and evaluating the physical and mechanical properties of mortar. Afterwards, the effects of fly ash addition to mortar containing the optimum amount of NS or MS are to be investigated on its workability, mechanical strength and durability.