Additive manufacturing (AM) facilitates the production of complex structures. It is often combined with structural design by topology optimization to create lightweight structures with minimal material and maximum stiffness. In this paper, we consider the design of lightweight structures to be created by casting in formwork that is produced by additive manufacturing. This problem, arising from the building industry, relates to but differs from prior work on topology optimization for structures that are directly produced by additive manufacturing. Specifically, formwork is not permitted to contain extra supports in casting space since otherwise it results in casting blockages. Moreover, topological structures with cavities cannot be produced through casting. This work presents a topology optimization method for designing structures to be cast in AM-produced formwork. This approach addresses these two key challenges: (i) ensuring the formwork is self-supporting during printing to eliminate the need for additional supports, and (ii) designing the structure to be free of internal enclosed cavities, which would otherwise lead to disconnected or floating parts in the formwork. The effectiveness of the proposed method was demonstrated through several numerical examples and experimental evaluations. Results show that the formwork can be printed without extra supports, and internal enclosed cavities in optimized structures can be fully eliminated. The findings provide a new strategy to produce the lightweight structure and corresponding structural formwork.

High-strength engineered cementitious composites (ECC) typically require higher cement content, which is negative from the sustainability point of view. To alleviate this problem, herein a low-cost and eco-friendly high-strength ECC (with a compressive strength of over 100 MPa) was developed, and diatomite was used to replace a small amount of cement. An appropriate amount of diatomite was found to improve the compressive strength, tensile strength and first cracking strength of ECC, but at the expense of part of the strain capacity (still all higher than 2.9%). Furthermore, the high pozzolanic activity and specific surface area of diatomite also increased the autogenous shrinkage, but reduced the drying shrinkage of ECC due to its internal curing effect. The incorporation of diatomite improved the pore structure of ECC, consumed more Ca(OH)_2, and enhanced the hydration degree of the mixture. In the end, the economic and environmental benefits of diatomite-modified ECC were also evaluated, and the cost, non-renewable energy demand, and global warming potential of ECC with 3% diatomite were reduced compared to plain ECC by 12.9, 15.1, and 13.3%, respectively. The developed high-strength ECC is therefore a low-cost and eco-friendly alternative to the traditional one.