Keywords

1 Introduction

An increasingly important priority worldwide is to transform waste into a valuable resource for other applications, supporting the circular economy. The cement and concrete industry is one of the major sectors that is actively contributing toward a closed loop circular economy by incorporating a range of industrial byproducts and waste materials [1,2,3], such as, fly ash (FA) [4, 5], slag [6, 7], waste tire rubber [8,9,10,11,12], recycled concrete aggregate [13, 14], organic waste [15] and medical waste [16,17,18,19]. However, the production of cement accounts for around 5–7% of global greenhouse gas emissions [20, 21]. Considering the estimated 4 billion tonnes of annual cement production by 2030, a remarkable volume of CO2 will be generated worldwide [22, 23]. In this regard, extensive research is currently being undertaken to reduce CO2 emissions by using industrial byproducts such as FA [24,25,26,27], slag [28, 29], silica fume (SF) [30, 31], and metakaolin [32] as supplementary cementitious materials. FA is a byproduct of coal-fired thermal power plants, and is considered an environmental pollutant and requires considerable financial and environmental input for its safe disposal. However, due to its pozzolanic properties, it can be used to substitute cement in concrete production, which not only improves the mechanical and durability properties of the concrete composite but also contributes to cutting down its carbon footprint.

Most of the worldwide FA production is categorized as ASTM Class F low calcium FA [33, 34], which has low reactivity [35]. Researchers have explored various methods of improving the pozzolanic reactivity of the FA, such as reducing the particle size of FA, adding hydrated lime (HL), sodium hydroxide, lime water, sodium silicate [12], a set accelerator (SA) [13], nano-alumina, nanosilica (NS), nanocalcium carbonate, nanoferrous oxide, nanozinc oxide and nanotitanium oxide. Although an alkaline activator is always required for the pozzolanic reaction of FA, out of all other additives, NS [36] could be the highest performing additive for significant enhancement of the mechanical properties of high-volume FA (HVFA) cement composites. The high reactivity of NS is attributed to its high surface area [37] and highly amorphous structure, which reacts with available lime to produce additional strength-forming calcium–silicate–hydrate gel counterbalancing the low early reactivity of FA. However, the key challenge in using NS in HVFA cement composites is its high cost, which hinders its application in large-scale commercial projects. An alternative to NS is SF, though with relatively lower reactivity because of its micro-sized particles compared with the nano-sized particles of NS, which delivers a considerable difference in their respective reactive surface areas and their corresponding reactivities. Therefore, we carried out a comparative analysis of the use of different micro- and nano-sized additives in HVFA cement composites at 80% replacement of the cement content.

2 Methods

The materials adopted in this experimental program were ordinary Portland cement (OPC), FA, SF, NS, HL, SA, superplasticizer (SP) and potable water. The sand-to-cement ratio was kept at 2.4. The chemical compositions of the different materials are shown in Table 1, and the mineralogical compositions are presented in Fig. 1. The mix designs studied in this experimental program are provided in Table 2. Three replicates of 50 mm cube mortar samples were prepared for each mix design for 7- and 28 day compressive strength tests. The samples were cured as per AS1012.8.1:2014 until the time of testing.

Table 1 Chemical composition of ordinary Portland cement, fly ash, silica fume, nanosilica, and hydrated lime
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Fig. 1
5 frequency graphs, O P C, F A, silica fumes, N S, and H L plot intensity versus 2 theta. In the O P C, F A, and H L graphs, heavy fluctuation is recorded. In silica fumes and N S graphs, the frequency line rises at the start, reaches a peak, then falls and stays linear till the end.

Mineralogical composition of ordinary Portland cement, fly ash, silica fume, nanosilica, and hydrated lime

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Table 2 Mix designs
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3 Results and Discussion

Figure 2 shows the compressive strength values of different mix designs at 7 and 28 days of curing. It can be seen that by replacing 80% OPC with 65% FA and 15% SF in mix M1, there was a considerable reduction in compressive strength. FA has low reactivity that negatively influences strength development [38], thereby significantly reducing the 7- and 28 day compressive strength results of mix M1. Moreover, class F FA has negligible lime content, and the calcium hydroxide released by the hydration reaction of 20% OPC content was insufficient for accelerating the pozzolanic reaction. In mix M2, an additional 5% HL was added to the mix design as a percentage of the total cementitious material (CM). The increase in the alkaline activator accelerated the pozzolanic reaction, thereby bringing about 76.4% and 108.5% rise in the 7- and 28 day compressive strength results, respectively, compared with M1.

Fig. 2
A grouped error bar graph of compressive strength versus different mix designs at 7 and 28 days of curing. They depict a gradually increasing trend from mix 1 to mix 9. The grouped bars under mix 9 hold the highest values of 52 and 62 for bars 7 and 28 days. Values are estimated.

Compressive strength results

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In mix M3, sodium thiocyanate and calcium nitrate-based SA was used at 12.5 mL/kg of CM. The addition of SA to mix M3 brought about 68% and 10% improvement in 7- and 28 day compressive strength, respectively, compared with M2. In mix M4, the quantity of the SA was doubled to 25 mL/kg of CM, which led to a considerable increase in the pozzolanic reaction, thereby increasing the 7- and 28 day compressive strength results by 11.7% and 14.1%, respectively, compared with M3. In mix M5, the water/cement ratio of the mix design was reduced from 0.3 to 0.25. Because the water–cement ratio has an inverse relationship with compressive strength, the 7- and 28 day strengths of M5 were increased by 21.4% and 7.8%, respectively.

In mix M6, SF was replaced by NS, which has a considerably higher surface area than SF and accelerates the pozzolanic reaction of NS-modified mix designs. The NS content was 4%, and the FA content was increased to 71% to keep the total cement replacement level at 80%, with the remaining 5% being the HL content. Because NS has a high surface area, replacing SF with NS significantly increased the SP requirement. Mix M6 showed a significant increase (i.e., 54.8%) in its 7 day compressive strength. At 28 days, though relatively small, it showed a 6% improvement in its compressive strength compared with M5. On increasing the NS content to 5% in mix M7, the 7- and 28 day compressive strength showed a further increase of 9.4% and 5.3%, respectively, compared with M6. In mix M8, the NS content was increased to 6%, which brought about 19.3% and 2.7% increase in the respective 7- and 28 day compressive strength results, which can be attributed to the increase in pozzolanic activity due to the addition of highly reactive NS. Mix M9 increased the NS content to 8% and reduced the FA content to 67%. The SP content was considerably increased to keep similar workability. The 7- and 28 day compressive strength increased by 19% and 9.2%, respectively, reflecting the positive effect of nano-sized highly reactive amorphous silica on the pozzolanic activity of blended cement composites.

4 Conclusions and Recommendations for Future Work

  1. (C1)

    The cementitious blend of HVFA and SF, replacing 80% of OPC content, required the inclusion of HL to increase the pozzolanic reaction of both the SF and FA.

  2. (C2)

    The addition of a SA increases the pozzolanic reaction of FA and SF, which increases with increasing SA content. This increase in the pozzolanic reaction is also reflected in the increase in compressive strength.

  3. (C3)

    The replacement of micro-sized SF with nano-sized highly amorphous NS significantly increases the pozzolanic reaction of the mix, resulting in significant increases in the 7- and 28 day compressive strength results as the NS content increased. However, the SP requirement increased significantly to maintain workability.

  4. (C4)

    NS-blended very HVFA cement composites replacing 80% of the cement content that can give 28 day compressive strength results on par with those of a control mix containing 100% cement content.

  5. (R1)

    Mix 9 had the highest results for improving the 7- and 28 day compressive strengths. It should be explored further to look for any potential of further enhancement of its mechanical properties.

  6. (R2)

    Further work needs to be carried out on workability (slump), setting time, durability and techno-economic analysis to carry this work forward.