Greener horizons: Revolutionizing construction materials with waste-based innovations – An experimental study

The demand for concrete has surged as it is the second most consumed substance worldwide, after water. The production of cement; the main binding agent of concrete has resulted in significant fluctuations in weather during the last decade due to excessive CO₂ emissions. Based on available data, the global cement production has reached 4.1 billion tonnes in 2022, indicating a notable increase over the previous thirty years. In 1995, the production amount stood at approximately 1.39 billion tons, making it evident that within these three decades, it has escalated almost by three times [1]. Regrettably, this flying growth of cement production has led to adverse impacts on the environment with the emissions of CO₂.

The International Energy Agency reported that in 2022, the construction industry accounted for 13 % of worldwide CO₂ emissions, with cement production attributing for approximately 7–8% of those emissions [2,3]. Moreover, energy-related fossil fuel production has also expanded proportionately to satisfy the rising demand for cement manufacturing [4]. The total global CO₂ emissions from the energy sector was recorded as 33 Gt in 2021 [5].

Alkaline-activated cement (AAC) is a recently developed sustainable and innovative solution to the expanding need for conventional Portland cement. Even though AAC has attracted a lot of attention since it uses less energy and emits less CO₂ during the process compared to Portland cement, the usage of commercial activators, such as sodium hydroxide (NaOH) and sodium silicate or water glass (Na₂SiO₃), has been a very concerning issue [6]. A life cycle evaluation of NaOH was carried out, and the results showed that the manufacturing of 1 kg of NaOH requires 3.5 MJ of fossil fuel energy, which is equivalent to 0.6329 kg of CO₂ emissions [7]. Consequently, the use of commercial activators makes the production of geopolymer cement and alkali activated cement (AAC) less sustainable.

To find sustainable solutions, researchers have been actively developing different eco-friendly processes and products. One crucial area of focus is green cement production, which involves developing cement from green precursors, activators, and various additives. This approach utilises waste materials, industrial by-products, and natural resources as raw materials, offering a promising solution to the pressing issue of waste landfilling. A Global Snapshot of Solid Waste Management to 2050 which was designed by the World Bank highlights that global waste is expected to grow up to 3.40 billion tons by 2050, more than double population growth over the same period [8]. Although the three R’s rule, which stands for Reduce, Reuse and Recycle, is the globally recognised approach to waste management, Australia predominantly relies on recycling and landfilling as its primary waste disposal methods. In 2020/2021, Australia’s annual per capita waste generation was 2.95 tonnes, resulting in 75.8 million tonnes of total waste generation. The national waste report of Australia indicated that between 2016 and 2019, an annual average of around 20.5 million tonnes of waste was landfilled each year [9,10]. However, due to the economic importance of land, health issues on living organisms, air pollution, leaching heavy metals into the groundwater aquifers and destruction of surface soil, landfilling would not be an enduring solution. Public awareness and knowledge regarding waste sorting remain crucial for sustainable waste management, as inadequate waste sorting may lead to large volumes of waste occupying valuable landfill space, including categories requiring special waste treatment.

Industrial waste is a prominent category in Australia because of the expansive growth of different industries on a large scale. One of Australia’s major waste problems is coal ash, accounting for approximately one-fifth of the country’s total waste stream. Among the coal ashes, fly ash is a leading precursor of green cement, which generates as a by-product of coal combustion in power stations. Blast furnace slag is another industrial waste in Australia generated from smelting iron ore, coke, and fluxes while extracting iron from the iron ore. Both fly ash and blast furnace slag are valuable aluminosilicate precursors for green cement production, with fly ash contributing high levels of Silicon dioxide (SiO₂) [11,12], while blast furnace slag consists of a more excellent composition of Calcium Oxide (CaO). By developing green cement from industrial waste, this research seeks to address critical waste management challenges while offering eco-friendly and resource-efficient solutions for both general and high-strength construction applications. By developing green cement from industrial waste, this research seeks to address critical waste management challenges while offering eco-friendly and resource-efficient solutions for both general and high-strength construction applications. These types secondary or supplementary cementitious materials (SCMs) can be classified under two main categories including Portland cement-based composites (General blended cement) and aluminosilicate materials. Aluminosilicate materials were recently innovated, enhancing the geopolymer chemistry and they are in four different types based on the amount of calcium in the precursor environment and alkalinity: alkaline activated cement binders (high Ca, moderate to high alkalinity), geopolymers (low Ca, low alkalinity), inorganic alkaline polymers (low Ca, highly alkaline media) and blended cement (both FA and Slag, with commercial activators) [13].

The objectives of this research study are listed below.

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  Development of a Sustainable Green Cement Mix: To formulate a green cement blend entirely composed of industrial by-products, mining waste, and agro-industrial waste, thereby promoting the use of waste materials and reducing environmental impact.

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  Optimization of Ordinary Portland Cement (OPC) Replacement: To determine the optimal replacement percentage of Ordinary Portland Cement (OPC) with the waste-based green cement (WGC), ensuring a balance between performance and sustainability.

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  Optimization of Binder Composition: To optimize the initial molar ratios of SiO₂/Al₂O₃, Na₂O/Al₂O₃, H₂O/Na₂O, and the liquid/solid ratio for the waste-based green cement binder, to enhance its chemical reactivity and performance.

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  Evaluation of Mechanical Properties: To assess the impact of mineralogical and microstructural variations on the compressive strength of WGC-OPC composites, considering different curing durations to understand their long-term durability.

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  Reusability Investigation: To explore the potential for reusing the waste-based green cement after demolition, evaluating its performance and sustainability in a circular economy framework.

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  Estimation of CO₂ Emission Reduction: To estimate the reduction in CO₂ emissions associated with the production of waste-based green cement compared to traditional Ordinary Portland Cement (OPC), contributing to the understanding of its environmental benefits.

The main aluminosilicate precursors used to develop waste-based green cement (WGC) composites were Class F- fly ash (FA) and ground granulated blast furnace slag (GGBFS) supplied by Independent Cement and Lime, Australia. FA is a siliceous by-product from coal combustion. At the same time, GGBFS has calcareous properties generated from iron refining. Commercial alkaline activators, sodium silicate (Na₂SiO₃) and sodium hydroxide (NaOH) were replaced by alternative waste materials, rice husk ash (RHA) and mine tailings (MT), respectively. To evaluate the performance of adding ordinary Portland cement (OPC) into WGC, 42.5-grade general-purpose cement was employed. Coal combustion residuals (CCRs) are the ashes from coal-fired power plants that have increased dramatically in response to the increased demand for electricity. Fly ash, bottom ash, boiler slag, flue gas desulphurization material, fluidised bed combustion ash, cenospheres, and scrubber residues are just a few examples of the several coal-burning by-products that are included in CCRs [14]. Fly ash (FA) is a commonly used industrial by-product that is divided into two classes; Class F and Class C, according to its chemical compositions. Class F is for fly ash with a high silicate and low calcium content, and Class C is for fly ash with a low silicate and high calcium content [15]. Because of its pozzolanic reactivity, FA can be used in the concrete industry to replace cement partially, depending on the polymerisation mechanism. Therefore, in this research study Class -F FA was utilized as a siliceous precursor to develop the main mix.

The names given to slags vary according on the furnaces in which they are produced. Steel-furnace slag and blast-furnace slag are the two primary forms of furnace slags that are produced as by-products of the iron and steel manufacturing sectors, respectively. The remnant left over from removing the molten iron during the iron-making process is referred to as blast furnace slag, and it can be gathered in the following forms: air-cooled blast furnace slag, expanded blast furnace slag, pelletised blast furnace slag, or granulated blast furnace slag [14]. In this experimental research, ground granulated blast furnace slag (GGBFS) was used as a calcareous precursor to achieve the main mix. GGBFS is a low-hydraulic cement that can be activated by alkaline chemicals like sodium hydroxide; it is not a pozzolan. Thermal curing produced the best strength results for activated alkali cement (AAC) based on blast furnace slag (the specimens were kept at the same temperature for 7 and 28 days until testing) with 10 % Na₂O [16].

Given the explosive growth of biomass energy production, agro-industrial waste is a major waste category globally. Regrettably, due of landfilling, this category of waste has grown to be a serious environmental problem. A common type of agricultural waste is rice husk ash (RHA), which has a higher silica content compared to other ashes like palm oil fuel ash, sugar cane bagasse ash, etc [14]. The presence of unburnt carbon in the RHA was discovered to have the potential to decrease the workability and durability of blended concrete, despite its high potential as a cementitious ingredient [17]. In geopolymer chemistry, raw RHA was used after treatment (treated with HCL to remove the metallic ingredients and calcined at 600 °C to produce white colour RHA) to develop synthesis metakaolin-based geopolymers [18]. In this project, RHA (not treated) was engaged as an alkaline activator because it has a great potential to replace commercial sodium silicate (water glass). Compared to the commercial manufacturing of sodium silicate, usage of RHA is more environmentally friendly and it has a major impact on reducing GHGs emissions.

According to Fig. 3, FA has spherical morphology compared with the other raw materials. The combustion temperature and cooling rate influence the morphology of FA, and typical fly ash is spherical. With the temperature and cooling rate changes, morphological variation can be seen clearly in FA, including hollow cenospheres, unburned particles, mineral aggregates (quartz), agglomerated particles and irregularly shaped amorphous particles [21]. Most GGBFS grains highlight an angular nature, while OPC and MT have irregular-shaped particles. However, RHA shows a distinct morphology consisting of silicon wafers. The morphology of raw materials is essential when analysing the workability of the mix and the formation of hydrates.

A HOBART 5L and 20L mixers were used for mechanical mixing of the cement binder which consists of fly ash, ground granulated blast furnace slag, rice husk ash, mine tailing and different proportions of ordinary Portland cement. A standard practice for mechanical mixing of hydraulic cement pastes and mortars was followed to ensure a homogeneous mix. After placing the dry paddle and the dry bowl in the mixing position in the mixer, materials were introduced for a batch into the bowl. A dry mixture of fly ash, ground granulated blast furnace slag, rice husk ash, mine tailing and ordinary Portland cement was prepared after 5 min of dry mixing (including 2 min of hand mixing and 3 min of mechanical mixing at slow speed), followed by 5 min of wet mixing (including 5 min of mechanical mixing at medium speed) after water had been poured to the mixture. Slow and medium speed of the mechanical mixing describes 140 ± 5 rpm and 285 ± 10 rpm respectively [24].

After obtaining a homogeneous mix, the samples were compacted in cylindrical PVC moulds which were prepared according to the aspect ratio of 2.0 (38 mm diameter and 76 mm height). To ensure a proper compaction, a vibration table was employed. The samples were compacted by three layers and each layer was manually compacted with 25 number of blows using a tamping rod and kept on the vibration table for 30 s [[25], [26], [27], [28]]. Then the top surface of the samples was covered with polythene to mitigate the dehydration and kept for 24 h at room temperature before demolding. After demolding, the samples were clean wrapped and kept under ambient environment (maintained temperature 21 ± 2 °C and maintained relative humidity 50 ± 10 %) for 7 days, 28 days and 56 days of curing periods. Ambient curing was selected to investigate the optimum strength properties in the critical environment.

After casting, cement composites enhance the strength due to the hydration over time. The time it takes for concrete to obtain 100 % strength is yet unknown. After the first 28 days of casting, the pace at which compressive strength of concrete increases is higher and gradually slows. It has been recorded that the strength gains of concrete after 1 day, 3 days, 7 days, 14 days and 28 days are 16 %, 40 %, 65 %, 90 % and 99 % respectively [32]. Therefore, 28 days strength is considered as the compressive strength of concrete or cement composite in concrete technology. Hence, the experimental findings from the uniaxial compressive strength test are compared especially concerning the 28-day strength results. Waste-based green cement which was cast without adding ordinary Portland cement depicts the 22 MPa of considerable strength after 56 days because of the alkaline activation and hydration of the mixed components. When the presence of silica or lime-rich raw materials such as fly ash and granulated blast furnace slag in the alkaline environment, the following activations which are shown in equations Eq (2), Eq (3), Eq (4)) take place in the system [33].

Eq (2)=Si-O– + R⁺ = Si-O-REq (3)=Si-O-R + OH⁻ = = = Si-O-R-OH^–Eq (4)=Si-O-R-OH^– + Ca²⁺ = = = Si-O-Ca-OH + R⁺

Sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃) are heavily used commercial activators in the alkaline activated cement industries which releases alkaline cations to the system. Through cationic exchange with the Ca²⁺ ions, the alkaline cation (R⁺) just catalyses the early stages of hydration [33].

Initially, the mineralogy of raw materials which is illustrated in Fig. 2 shows the obvious variations. The siliceous precursor class F- fly ash mainly consists of 55 % of quartz (SiO₂), 41 % of mullite (3Al₂O3.2SiO₂) and 4 % of hematite (Fe₂O₃). Then the calcareous precursor, granulated blast furnace slag obtained as a by-product from the iron manufacturing has mainly gypsum (CaSO₄.2H₂O). Agricultural waste: rice husk ash is the great replacement for the commercial alkaline activator; sodium silicate which basically consists of silica. When analysing the minerology of rice hush ask using a semi-quantified method; DIFFRAC.EVA v6.1.0.4 software with the 2023 recent database in the International Centre for Diffraction Data (ICDD-PDF4+) silica presented in the waste material in three different phases, including quartz, cristobalite and tridymite. At extremely high temperatures, the mineral polymorph of silica known as cristobalite is created. Although it has a different crystal structure, it has the same chemical formula as quartz, SiO₂. Furthermore, after partially devitrifying to cristobalite, porous fused silica ceramics are weaker at 25 °C following the transformation to alpha-cristobalite, but stronger at 350 °C in the beta-cristobalite stability zone [35]. Tridymite is a high-temperature polymorph of silica that typically appears in the form of tiny tabular, colourless, or white pseudo-hexagonal crystals in the voids of felsic volcanic rocks. Therefore, in the rice husk ash all silica minerals are not in the amorphous phase which would be reactive. Approximately 20 % of rice husk is made up of hydrated amorphous silica [36]. Mine tailings waste is another prominent waste category in the world which is the left-over part from the process of separating an ore’s profitable fraction. This contributes to providing alkaline cations to the system, acting as an alternative for commercial sodium hydroxide. Therefore, in the XRD pattern of the mineralogical analysis of mine tailing represents 38 % of sodium titanium oxide (Na₄Ti₅O₁₂). Apart from that it consists of 8 % anatase (TiO₂), 23 % hematite (Fe₂O₃), 24 % bredigite (Ca₁₄Mg₂(SiO₄)₈) and 7 % almandine (Fe₃Al₂(SiO₄)₃).

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  The optimum compressive strength of WGC-OPC composites was achieved with 50 % WGC replacement, resulting in 22 MPa after 56 days, suitable for general construction.

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  Mine tailings and rice husk ash can replace commercial activators (sodium hydroxide and sodium silicate) for green alkaline activation.

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  WGC exhibits reusability after demolition, with recast samples showing 6 MPa compressive strength after 28 days, unlike OPC without admixtures.

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  In the early curing stages (7 days), the compressive strength of 75 % WGC substitution and the original green cement are similar, with values around 32 MPa for 25 % and 50 % replacements. After 28 days, strength increases significantly, especially for 50 % and 75 % WGC, with an estimated 99 % improvement.

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  A 25 % increase in OPC doubles the development of Portlandite (calcium hydroxide) in the early stages of curing (7 days).

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  The addition of OPC alters crystallization in WGC-OPC composites, with elongated calcite crystals forming in 50 % WGC samples for optimum results.

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  The optimized molar ratios for the WGC mix were SiO₂/Al₂O₃ = 3, Na₂O/Al₂O₃ = 1, and H₂O/Na₂O = 9, with a liquid-to-solid ratio of 0.48.

The production of ultra-high strength cement binders is being investigated through new avenues opened up by this research study. Additionally, it emphasizes how green resources must be used in place of commercial activators like sodium hydroxide, potassium hydroxide, or sodium silicate in order to generate alkaline activated cement or geopolymers. Coal combustion energy would not be a sustainable way to generate energy given the increasing trend in the renewable energy industry. Therefore, it’s imperative to look into other options for flammable ashes such as bottom ash and fly ash. There is one potential to use soda lime glass powder in part instead of fly ash and it improves the compressive strength, tensile strength, and workability of the alkaline activated binder as the amount of replacement of fly ash by crushed glass [31]. On the other hand, there are numerous kinds of environmentally friendly additives that can improve both the mechanical and rheological qualities. For instance, an discarded aluminium foil can employ a high-quality fire retardant [11]. Moreover, this research study is a great pathway to investigate the possibility of developing an ultra-high strength cement binder and a well-cement mix which withstands against the high pressure and high temperature environment using green materials completely found from waste.

The acoustic and thermal properties of new sustainable materials can be greatly improved by recycled microplastic waste. The microplastic powder has a superb open-cell structure (recycle marine waste) and it demonstrates that this substance can be used in place of common insulators like rock wool, polyurethane, etc [43]. Utilising recycled engineering sediment waste is another cutting-edge method for producing sustainable supplemental cementitious materials. This method’s speciality is contrasted with landfilling, the suggested recycling strategy can save 1204.1 MJ/t of energy and 326.1 kg CO₂eq/t of global warming potential [44]. Alkali-activated materials (AAM) have recently been developed through photovoltaic modules. After 28 days of curing, commercial NaOH, slag, and waste solar glass powder demonstrated a higher compressive strength as 75.3 MPa [45]. After a solar panel’s life, this would be a secure way to dispose the waste. Therefore, future researchers have higher potential to reveal different sustainable techniques to develop green cementitious alternatives using completely from various waste types and enhance the properties according to the specific applications.