Investigating the suitability of locally available materials to develop low-carbon cementitious materials for 3D printing applications

Abstract

The environmental impact of traditional OPC has driven the search for sustainable alternatives like alkali-activated materials, calcined clay blends, and recycled aggregates. This doctoral research explores low-carbon cementitious materials using locally sourced construction waste. The work is divided into two parts: developing one-part AAMs with end-of-life brick and LC3 using excavated London clay and recycled aggregates, suitable for cast and 3D-printed applications. One-part AAMs provide significant environmental benefits due to the elimination of OPC, which can be a potentially suitable alternative to the traditional cement mixture. This part of the thesis investigates the feasibility of using locally available waste material, i.e. brick waste, as part of a one-part AAM binder as an alternative to fly ash (FA). The mortar binder is composed of 40% GGBS and 60% FA, activated using 12% solid sodium silicate to the weight of the binder. Hence, bricks in powder (BP) form were incorporated to replace FA with increments of 10% until full elimination of FA, and bricks in particle form were used to replace natural aggregates by 30%, 50% and 70%. It was demonstrated that BP-based AAMs achieved comparable mechanical performance to FA-based controls. Despite BP’s higher crystallinity (70% vs. FA’s 50%), pore refinement and accelerated geopolymerisation kinetics enabled the development of comparable strength. Moreover, BP-based AAM exhibited comparable water absorption, freeze-thaw resistivity, and around 65% higher resistivity to high temperature. Replacing natural aggregates with up to 70% brick aggregates (BA) in brick-based one-part AAM showed a maximum improvement of around 17% and 27% for the flexural and compressive strength performances, respectively, and showed better resistivity to freeze-thaw and high-temperature, while increasing the water absorption of the mixtures. Adding 0.1% nano graphite decreased the water absorption by 18%. On the other hand, incorporating BA improved the early-age strength and enhanced the mixtures' shape stability, improving their buildability compared to the mix with natural sand. Moreover, BA incorporation significantly enhanced the compressive strength of the 3D printed samples. From an environmental perspective, one-part geopolymer shows the potential as an alternative low-carbon concrete for conventional concrete, allowing the reduction of CO2 emissions by around 40%. Moreover, the carbon footprint from replacing fly ash with brick powder or natural sand with brick aggregates did not show observable changes. On the other hand, LC3 presents another promising low-carbon cementitious mixture that can reduce high amounts of OPC in the binder. This part of the thesis investigates the feasibility of using excavated London clay as calcined clay to develop a 3D printable LC3 mixture composed of 50% OPC, 30% calcined clay, 15% limestone and 5% gypsum. The results revealed the feasibility of using excavated low-grade London clay to develop LC3 mixture, having the highest strength performance when calcined at 800 ֯C in the LC3 system and showing good pozzolanic reactivity. Incorporating and optimising admixtures (i.e., 0.5, 1 and 1.5% superplasticiser ‘SP’ and 0.4, 0.6 and 0.8% of viscosity-modifying agent ‘VMA’) was compulsory to modify and adjust the fresh behaviour of the mixture to obtain a 3D printable dosage. Adding 1% SP significantly improved the mixture's strength performance, achieving around 44 MPa. Nevertheless, adding superplasticiser produced a too flowable fresh mixture. Incorporating 0.6% VMA was found to balance the workability of the mixture, producing a mixture with low slump, which is suitable for extrudability and buildability. On the other hand, incorporating BA to replace natural aggregates up to 100% in LC3 was found to be beneficial for both the engineering and printing properties of the mixtures. Replacing natural aggregates with BA fully significantly increased the compressive strength from around 34 MPa to 54 MPa. Moreover, the mixtures with BA showed a similar water absorption level with better freeze-thaw resistance while significantly improving the printed samples' mechanical strength. Incorporating BA could have served as a curing agent, allowing LC3 binder hydration to continue through absorbing part of the water from the mixture at the initial stage and releasing it during ageing. This reveals the potential of downcycling BA in LC3 mixtures to develop more sustainable mix formulation. Replacing 50% OPC with limestone-calcined clay and gypsum reduced carbon dioxide by around 40%, whereas replacing natural sand with BA did not show noticeable changes in the carbon footprint. Overall, this research lays the foundations for the feasibility of using end-of-life brick and excavated London clay after recycling to develop low-carbon cementitious mixtures.