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With the growing global concerns regarding sustainable development in the building and construction industries, concentration only on the engineering properties of building materials can no longer meet the requirements. Although some studies have been implemented based on the lifecycle assessment of lightweight cement-based materials, very few attempts have been made pertaining to multi-criteria optimization, especially when fly ash cenospheres are used as lightweight aggregates and nano additives are incorporated as modifying admixtures. This investigation utilized cenospheres as fine aggregates to produce green, sustainable, lightweight cement mortar. Multi-walled carbon nanotubes at 0.05, 0.15, and 0.45% were binarily added, together with 0.2, 0.6, and 1.0% of nano silica to improve the mechanical performance. Strength tests were conducted to measure the flexural and compressive behaviors, combined with a cradle-to-gate lifecycle assessment and direct cost analysis to assess the environmental and economic viability. Integrated indexes and the TOPSIS method were adopted to systematically evaluate the mortar mixes and determine the optimal mix. The outcomes show that nano additives worked synergically to enhance the mechanical properties of the mortars. The utilization of cenospheres effectively reduced environmental impacts and improved economic feasibility. Nano additives significantly affected the sustainability and economic viability; in particular, the utilization of multi-walled carbon nanotubes increased the material costs. To minimize the impact of the price of multi-walled carbon nanotubes, it is proposed to binarily use less expensive nano silica. In the multi-parameter optimization, the mix with 0.05% multi-walled carbon nanotubes and 0.02% nano silica was recommended to be the optimal mix.

The utilisation of Portland cement has aroused tremendous concerns owing to its production exerting a lot of pressure on the environment. Alternative eco-binders have been developed to replace it, among which alkali-activated materials (AAMs) have drawn great attention, especially due to the possibility of encompassing industrial and agricultural waste, which significantly improves the sustainability and cost-efficiency of the material. Biomass wood ash (BWA) is a by-product generated from power plants and, along with the advocation for biomass fuel as a renewable energy resource, there have been increasing applications of BWA in building and construction materials. This review examines the use of BWA as a precursor source in AAMs. Due to its low chemical and hydraulic reactivity, more active binary precursors are usually introduced to guarantee mechanical properties. Whereas the increment of BWA content can have a negative influence on material strength development, it is still a promising and feasible material, and new approaches should be developed to improve the effectiveness of its utilisation. Currently, study of BWA-based AAMs is still in the beginning stages and more research is needed to investigate the effects of BWA characteristics on the property evolution of AAMs, focusing on the durability and analysis of eco-efficiency. Overall, this review provides a comprehensive overview of the characterisation of BWA and its potential applications in AAMs, and meanwhile, based on the analysis of present research trends, proposes some prospective directions for future research.

With the development of nanotechnology, nanomaterials have been introduced to improve the engineering properties of cement-based building materials. An abundant number of studies have been carried out on normal-weight concrete using multi-walled carbon nanotubes (MWCNTs) or nano-silica (NS) and have proven their effectiveness. Nevertheless, still very few investigations are available in terms of lightweight cement-based materials, especially when MWCNTs and NS are binarily incorporated. Thus, in this study, fly ash cenospheres (FACs) according to cement weight were applied as lightweight fine aggregates to produce lightweight mortar (LWM). MWCNTs at 0.05, 0.15, and 0.45% and NS at 0.2 and 1.0% were binarily added as modifiers. Compressive and flexural strengths were tested to investigate mechanical behaviors. A water absorption test was conducted, together with scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP), to identify the impacts of the nano-additives on the pore structure of LWM. The following results were obtained: MWCNTs and NS demonstrated synergic effects on enhancing the mechanical properties of LWM. MWCNTs exerted positive impacts on reducing the porosity and improving the pore distribution at low dosages of 0.05 and 0.15%. The hybrid addition of NS further transformed large voids into small ones and introduced closed pores.

Owing to their convenient manufacture, transportation, low energy consumption, and environmental impacts, lightweight cement composites have been applied as building and construction materials. However, its decreased density is associated with a reduction in mechanical strength. In most existing investigations, attempts have been made to improve mechanical behaviours via supplementary cementitious or fibre materials, whereas limited studies have been implemented on the effects of nano additives, especially their synergic influence. In this study, industrial waste fly ash cenosphere (FAC) has been utilized as lightweight aggregate by 73.3% cement weight to fabricate sustainable lightweight cement mortar (LWCM). Carbon nanotubes (CNTs) at a dosage of 0.05%, 0.15%, and 0.45% and nano silica (NS) with the content of 0.2%, 0.6%, and 1.0% by cement weight have been applied as modifying additives. Experiments were carried out to test flexural strength, compressive strength, and water absorption. SEM, TG, and XRD analyses were conducted to evaluate microstructure and hydration characteristics. Based on the outcomes, the inclusion of CNTs and NS can effectively increase flexural and compressive strength and reduce absorbed water weight. The analysis of SEM, TG, and XRD reveals that the binary usage of CNTs and NS can improve pore structure and facilitate hydration reaction.