Quaternary blended composite cement-a pathway to sustainable and circular construction

The depletion of natural resources and the substantial carbon emissions associated with ordinary Portland cement (OPC) production pose critical environmental challenges to the construction sector. This study investigates the development of a sustainable quaternary blended cement system incorporating steel and iron industry by-products, i.e. Linz-Donawitz slag (LDS), ladle furnace slag (LFS), and ground granulated blast furnace slag (GGBFS), in combination with OPC. Binder pastes were formulated with varying proportions of LDS and LFS, while maintaining OPC at 50% and GGBFS at 10% by mass. The blended systems were comprehensively evaluated with respect to mechanical performance, durability in aggressive environments, phase assemblage, microstructural characteristics, and thermogravimetric behaviour. The optimised composition, consisting of OPC (50%), GGBFS (10%), LDS (20%), LFS (20%), and 10% chemical activator, achieved a 28-day compressive strength of 62.9 MPa, significantly surpassing the OPC control (53.4 MPa). Microstructural and mineralogical analysis confirmed the formation of a dense and homogeneous calcium silicate hydrate (C-S-H) gel network, consistent with thermogravimetric findings indicating enhanced hydration and reduced portlandite content. Durability assessment of the optimised mix demonstrated superior resistance to exposure to MgSO₄ and HCl. The control mix showed weight changes of 0.70% and 0.64% in the presence of MgSO4 and HCl, respectively, while the optimised mix exhibited the lowest gains at 0.45% and 0.49%, respectively. Furthermore, exposure to sulphate and acid solutions reduces strength. The control mix exhibited strength losses of 6.37% and 9.55% under sulphate and acid exposure, whereas the optimised mix demonstrated comparatively lower reductions of 1.74% and 5.08%, respectively. The ingress of sulphate and chloride ions from the MgSO₄ and HCl solution into the pore structure of the binder matrix initiates chemical reactions with the calcium hydroxide and monosulphate phases, leading to the formation of expansive products, such as gypsum and ettringite. Although a marginal increase in drying shrinkage of the optimised mix (0.038%) compared with the control (0.034%) was observed, the overall durability performance remained superior to that of conventional OPC systems. The optimised blends met the requirements of Indian and ASTM standards for OPC 43-grade cement, confirming their technical viability for practical implementation. Beyond performance improvements, the study highlights significant sustainability and economic implications. Partial substitution of clinker with steel and iron slags reduces embodied CO₂ emissions, diverts industrial by-products from landfills, and advances circular economy principles within the construction industry. While initial production costs are comparable to conventional OPC due to supplementary cementitious material processing, lifecycle benefits, including reduced clinker demand, improved durability, extended service life, and potential environmental incentives, enhance overall cost-effectiveness. In conclusion, this research establishes steel and iron slags as viable constituents in high-performance quaternary blended cements. By demonstrating enhanced strength, improved durability, regulatory compliance, and environmental benefits, the study provides a practical pathway for lowering the carbon footprint of cementitious materials.