Fly ash, a byproduct of coal-fired thermal power plants is considered an environmental pollutant if it is to be disposed of and requires considerate financial liabilities on fly ash producers for its safe disposal. But fortunately, because of its physical and chemical properties, it has found applications in many fields; like mineral extraction, soil stabilization, waste treatment and as a supplementary cementitious material. As cement industry is responsible for 5-7% of global greenhouse gas emissions, application of fly ash as a cement replacement material provides the dual benefit of cutting down the greenhouse gas emissions and in increasing the utilization rate of fly ash produced worldwide. The majority of global fly ash production falls under class F low calcium category, which has low reactivity. Therefore to improve the performance of fly ash blended cement composites, the researchers have looked at many ways like; reducing the particle size, making use of hydrated lime, silica fume, and Metakaolin, etc. Recently, the use of nanomaterials has been gaining widespread attention of the research community due to their small particle size and high reactivity that help in improving the properties of the concrete at the nanoscale level. The majority of the past research on low calcium, class F fly ash cement composites, concentrated on 60% or less of fly ash content and there is a great potential for the further improvement in replacement levels. Therefore, our research aimed at developing a cement composite that not only increases the percentage of fly ash content but also considerably increases the cement replacement level having comparable mechanical properties to that of ordinary portland cement (OPC). Based on the review of existing literature, there appeared to be no research available that investigates the effect of micro (silica fume) and nano silica in combination with hydrated lime and set accelerator on high volume fly ash (HVFA) cement composite replacing 80% of cement. Therefore, our first study was undertaken to fill up this knowledge gap. The fly ash used in this study was micronised using Sturtevant jet mill microniser to produce ultra fine fly ash (UFFA). The research findings show that when UFFA is partially replaced with silica fume (SF) and used in combination with both the set accelerator (SA) and hydrated lime (HL), there is a considerable improvement in its pozzolanic activity resulting in large improvement in its 7 and 28 day compressive strength. On the contrary, nano silica (nS) modified HV-UFFA performed better when used without the SA and HL, resulting in significantly improving its 7 and 28 day compressive strength. The strength achieved at 28 days was at par with that of OPC. The addition of SA and HL to nS modified HV-UFFA blend resulted in the development of high early age microcracking, thereby considerably reducing its 28 day strength compared to that of nS, HV-UFFA and OPC blend not containing HL and SA. The second experiment was planned to look at the effect of different concentrations of nS on raw HVFA cement blends with and without SA and HL, replacing 70% of cement. The difference in experiment 2 compared to experiment 1 were (i) raw fly ash was used in experiment 2 in lieu of UFFA as micronising fly ash consumes electricity that adds to the carbon footprint (ii) two concentrations of nano silica were used i.e. 5 and 7.5% to study the effect of increase in nano silica content on HVFA cement composite. The fly ash content was partially replaced with 5 and 7.5% of nS and 5% HL. The results show that the 7 day strength increased with the increasing amount of nano silica. However, at 28 days both the samples showed approximately the similar improvement in strength. With the addition of set accelerator to nano silica modified HVFA cement blends, there was a considerable reduction in 7 and 28 day compressive strength results. Addition of HL to HVFA cement blend, containing 5% nS showed no effect on its further strength development. However, when HL was added to the 7.5% nS modified HVFA cement coposite, a considerable reduction in its 7 and 28 day compressive strength results were observed. This shows that nS is highly effective in improving the strength of HVFA cement composites when used alone without any additives, but when it is used in combination with either HL or SA, shows no or negative effect on the development of compressive strength of raw fly ash blended cement composites. Looking at the significant improvement in the strength results of HVFA cement composites by incorporating nano silica, the next experiment was planned to looking at the effect of other nano materials on HVFA cement composites. As the major elemental oxides present in OPC are SiO2, CaO and Al2O3. This experiment was planned to incorporate the nano sized sources of these elemental oxides. Since the reaction of pure CaO is highly thermal in nature and addition of nano CaO would have been highly exothermic and could have introduced high thermal stresses at early ages of curing, nano CaCO3 was used as a source of CaO. Therefore, the third experiment plan looked at a quantitative comparative study of the effect of 5% and 7.5% of nano silica, 2.5% and 5% of nano alumina (nA) and 2.5% and 5% of nano calcium carbonate (nCC), on the properties of HVFA cement composites, replacing 80% of cement. The results show that the addition of nS significantly improves the compressive strength of HVFA cement composites and considerably increases the formation and thermal stability of silica-rich hydrogarnet phase, which increases with the increase in nS content. The addition of nCC to HVFA cement composite does not show any effect on the pozzolanic reaction at 7 days of curing but at 28 days considerably improves its pozzolanic reaction, which increases with the increase in nCC content. The performance of nCC in improving the mechanical properties is less pronounced than that of nS. The addition of nA though improves the hydration/pozzolanic reaction of HVFA cement composite, resulting in the improvement in compressive strength, but only if added in small quantities (2.5% or less). If it is added in higher amounts, it promotes the formation of Al(OH)3 gel that severely inhibits the hydration/pozzolanic reaction within the cement matrix. Based on the knowledge gained from the previous experiments, we found that the amorphous nano silica holds a great potential for the development of zero cement composite. That motivated us to design our fourth experiment, replacing 100% of cement. In the fourth experiment we replaced OPC with slag, which is another industrial by product. Zero cement mix designs were developed incorporating slag, HVFA and various concentrations of nano silica and hydrated lime. The results show that the optimum content of nano silica in a high volume fly ash, slag and HL blend is 5%. With a further increase in nano silica content although the pozzolanic reaction and the resulting C-S-H/C-A-S-H gel formation increases but it also increase the micro-cracking within the cement matrix, resulting in negatively impacting the strength development. The portlandite added externally in the form of Ca(OH)2 powder, to the nS modified SCM blend, activates the pozzolanic reaction which increases with the increase in portlandite content. The best mix design containing 5% nS, 70% FA and 25% slag in combination with 15% HL as an additive, achieved a 28 day compressive strength of 70% compared to that of OPC. It is to be noted that though this research aimed at minimising the carbon footprint of the cement composites, it was not completely eliminated. The various raw materials that have CO2 emissions associated with their production are listed below: Nano silica – As per the information provided by the supplier of Nano silica, it was produced by pyrogenic method, which is highly energy intensive process. It is produced from the flame hydrolysis of silicon tetrachloride (SiCl4) at ~1800 °C temperature. Calcium hydroxide – Calcium hydroxide is produced commercially by treating lime (CaO) with water (H2O). The CaO used in this process is produced by the de-carbonation of limestone (CaCO3), which releases CO2 into the atmosphere during production. Ultrafine fly ash (UFFA) – Though fly ash is a by-product of thermal power plants using coal as a fuel, micronizing the fly ash (i.e. reducing the particle size of the raw fly ash) is an energy intensive process. The Sturtevant jet mill microniser used to reduce the particle size of the raw fly ash runs on electricity which otherwise is produced primarily by the thermal power plants in Australia.
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