Performance of HVFA concrete incorporating nano silica – a detailed study

<p>High-volume fly ash (HVFA) concrete is an emerging sustainable construction material with the potential to be used as a replacement for Portland cement (PC) concrete. Using HVFA concrete in construction could reduce the cement production, which is estimated to be responsible for 4-8% of global anthropogenic CO2 emissions. Among various alternative cementitious materials available to replace cement in concrete such as silica fume, hydrated lime, blast furnace slag, rice husk etc., fly ash has proven to be one of the most suitable material due to its abundant availability and due to the improvements in the performance of concrete by the addition of the fly ash. The utilization of fly ash in concrete contributes towards sustainability not only by reducing cement production, but also by minimizing environmental impacts from fly ash waste dump sites. Fly ash has been used as a partial cement replacement material for over two decades. Replacement levels up to 30%have been shown to improve the workability, pumpability and flowability of fresh concrete as well as to improve the durability properties of concrete including permeability and sulphate/acid resistance. More recently cement replacement using fly ash content above 50% has been studied, which has led to the development of concrete classified as HVFA (Fly ash content above 50%). Research has shown that a major drawback of HVFA concrete mixes with fly ash percentages over 60% is the low early age strength due to the slow pozzolanic reaction of the fly ash. To address this issue a range of material additions have been used to modify HVFA mixes to enhance the early age strength. </p> <p>These include silica fume, slag, hydrated lime and nano material such as nano silica and nano calcium carbonate etc. Each of these materials has improved the properties of HVFA concrete, with nano silica proving to be one of the most effective modifications in significantly enhancing the early age strength. The literature review illustrates that there is a clear gap of research in the area of the long-term performance of HVFA concrete incorporating nano silica. Furthermore, there are only a few studies which have extended beyond 70% cement replacement using fly ash and nano additives. There is a lack of data available on the durability properties of HVFA concrete mixes with nano silica over the long term, and there are no studies on the long term acid and sulphate resistance of HVFA concrete mixes incorporating nano silica.</p> <p>Thus, it is evident that further studies are required to understand the effectiveness of nano silica on the mechanical and durability performance of HVFA concrete mixes with every high cement replacements (above 70%). Additionally, no scaled-up structural experiments have been conducted using nano silica modified HVFA concrete mixes to date, which should be the ultimate goal of developing the HVFA concrete mixes with nano silica. Therefore, this research will provide an in-depth analysis of the mechanical and durability properties profile for two HVFA concrete mixes upto 80% cement replacement across the initial 450-day period of the concrete mixes. This will assist significantly in the assessment of the short- and long-term performance of the concrete and also identify potential methodologies for improving the HVFA concrete performance. The research will also undertake a scaled-up investigation of the nano silica modified HVFA concrete in reinforced concrete beams. The overall research program was developed to fulfill gaps in the current knowledge of HVFA incorporating nano silica.</p> <p>The first phase of this presented research investigates the performance of two HVFA concrete mixes, one with 65% cement replacement (HVFA-65) and one with 80% cement replacement (HVFA-80) incorporating fly ash, hydrated lime and nano silica. The research studies a range of mechanical and durability properties, the microstructure and pore structure development and the hydration mechanism over a period of 450 days. The mechanical properties tested include compressive strength, flexural strength, splitting tensile strength, elastic modulus, and density. The long-term creep and drying shrinkage and permeation characteristics including air/water permeability, water absorption, AVPV, carbonation and chloride permeability are investigated to ascertain the durability properties of the two HVFA concrete mixes. Microstructural analysis is undertaken using Scanning Electron Microscopy and pore structure analysis using micro-CT (Computer Tomography). A detailed chemistry analysis of paste samples is conducted simultaneously to provide an understanding of the progression of the hydration mechanism with time. The chemistry techniques include X-ray Fluorescence, X-ray diffraction, Thermo Gravimetry Analysis, Nuclear Magnetic Resonance spectroscopy and Fourier Transform Infrared spectroscopy. Furthermore, a detailed experimental program was conducted to investigate the resistance of the two HVFA concrete mixes when exposed to a 1% H2SO4, 3% H2SO4, 5% MgSO4 and a 5% Na2SO4 solution over a period of 24 months. The mass and length change of the exposed specimens were monitored continuously over the 24 months period along with their compressive strength at 12, 18 and 24 months. The microscopy and chemistry of the specimens at 12- and 24- months were analysed to elucidate the mechanisms underlying sulphate and acid attack of the HVFA concrete containing nano silica. In addition, scaled-up reinforce concrete beams were tested under flexural and shear failure, using 3 concrete mixes for comparison. A HVFA-65 concrete mix with nano silica, HVFA-65 concrete without nano silica (with hydrated lime only) and a 100% PC. The beams were designed to fail under shear, by providing no shear reinforcements, and to fail under flexure, with shear reinforcement. The load carrying capacities of the beams, the deflection of the beams at 5 locations along the beam span, the crack propagation and depths were monitored with applied load. </p> <p>Based on the results of the first phase, HVFA-65 concrete showed higher compressive strength and improved mechanical properties compared to HVFA-80 concrete at all ages. However, the HVFA-80 concrete achieved similar strength to HVFA-65 at 450 days indicating continued long term pozzolanic hydration of the fly ash. The permeation characteristics of HVFA-65 concrete were also better than HVFA-80 concrete over the study period. It was observed that the mechanical and durability properties of both HVFA concrete mixes were improved compared to similar HVFA concrete mixes with high cement replacements (over 60%) reported in literature. When exposed to sulphate and acid attack, HVFA-80 concrete showed greater resistance against acid attack while, HVFA-65 performed better against sulphate attack. The second phase of the research showed that the HVFA concrete mixes have comparable flexural behaviour to that of normal OPC concrete but showed a lower stiffness within the elastic range of the load-deflection curve compared to PC concrete. When the beams were subjected to shear failure, the HVFA concrete mixes showed a ductile behaviour preventing the beam from sudden collapse allowing more deformation under the maximum load unlike the PC concrete. Overall, the HVFA concrete mixes showed promise for structural applications.</p>