Sumit Srivastava

This paper presents a microstructural, mineralogical, and mechanical study of low-carbon autoclaved concrete (AC), achieved by partially or fully replacing ordinary Portland cement (OPC) with ground-granulated blast furnace slag (BFS) and substituting lime with calcium carbide slag (CCS). Fourteen mixes were produced and evaluated in the green state and after autoclaving. Quantitative X-ray diffraction (XRD) using the Rietveld method, density, compressive strength, and life cycle assessment (LCA) were conducted. Results show that mixes containing BFS achieve green strengths equal to or higher than the OPC reference, ensuring integrity during autoclaving. Using BFS with an adequate calcium supply promotes the formation of pre-autoclave portlandite, which in turn favors tobermorite development and yields post-autoclave strengths comparable to the OPC reference. Partial lime replacement with CCS (50%) maintains mineralogy and strength, whereas excessive CCS may reduce available portlandite and lower strength. Life-cycle assessment indicates that raw material supply dominates emissions and that removing OPC cuts total CO₂ by 44% without compromising mechanical performance. These findings demonstrate the feasibility of OPC-lean/OPC-free, lime-optimized autoclaved concretes with substantially lower embodied impacts.

The environmental impact of traditional construction materials has led to increasing interest in developing more sustainable alternatives. This study addresses the development of low-carbon autoclaved aerated concrete (AAC) through the complete replacement of ordinary Portland cement (OPC) with ground granulated blast furnace slag (BFS), activated with lime and, in some formulations, supplemented with calcium carbide slag (CCS). Five different AAC mixtures were prepared and evaluated in terms of workability, foaming behavior, compressive strength, phase composition, density, thermal conductivity, and life cycle assessment (LCA). The BFS-based mixtures activated with lime exhibited good workability and foaming stability. After pre-curing, the addition of CCS significantly improved the formation of tobermorite during autoclaving. As a result, the BFS–CCS formulations achieved compressive strengths comparable to the reference OPC-based mix while maintaining low densities (420–441 kg/m³) and thermal conductivities in the range of 0.111–0.119 W/(m·K). These results confirm the technical feasibility of producing structural-grade AAC with a lower environmental footprint.

Medium and highly sulfidic tailings are high-volume wastes that can lead to severe environmental damage if not properly managed. Due to the high content of sulfide minerals, these tailings can undergo weathering if put in contact with oxygen and water, generating acid mine drainage (AMD). The moderate-to-high sulfide content is also an important technical limitation for their implementation in the production of construction materials. This paper reviews the use of sulfidic tailings as raw material in construction products, with a focus on cement, concrete, and ceramics. When used as aggregates in concrete, this can lead to concrete degradation by internal sulfate attack. In building ceramics, their implementation without prior treatment is undesirable due to the formation of black reduction core, efflorescence, SO_x emissions, and their associated costs. Moreover, their intrinsic low reactivity represents a barrier for their use as supplementary cementitious materials (SCMs) and as precursors for alkali-activated materials (AAMs). Nevertheless, the production of calcium sulfoaluminate (CSA) cement can be a suitable path for the valorization of medium and highly sulfidic tailings. Otherwise difficult to upcycle, sulfidic tailings could be used in the clinker raw meal as an alternative raw material. Not only the SO₃ and SiO₂-rich bulk material is incorporated into reactive clinker phases, but also some minor constituents in the tailings may contribute to the production of such low-CO₂ cements at lower temperatures. Nevertheless, this valorization route remains poorly explored and demands further research.

FeCO3 scaling is utilised in a system of closely packed wet Fe-particles (10 μm) to prepare blocks composed of unreacted Fe-cores connected by a ‘cementing matrix of FeCO3 scales’. Within four hours of carbonation at different temperatures (30 °C – 90 °C) and CO2-pressures (1 barg–20 barg), a compressive strength of up to 160 MPa, and a siderite (FeCO3) content of up to 28 % is observed. Typical rhombohedral and atypical needle-shaped FeCO3 crystal morphologies are observed. The dependence of phase composition/morphologies on reaction conditions/processes is discussed. This study is proposed as a prelude towards utilisation of Fe-rich industrial wastes for cementation.

The paper reports the observation that siderite (FeCO3 mineral) cementation occurs when compacted Fe-powder (10 µm in this study) is subjected to aqueous carbonation. It is further shown that the siderite contents of the reaction products vary based on the temperature and the CO2-pressure conditions under which they are carbonated. Using SEM-SE observations, two different types of morphologies of siderite crystals are observed based on the temperature are which the aqueous carbonation of iron is performed. The XRD-Rietveld observations and the SEM-SE observations are combined to infer that dendritic Fe-oxide crystals are formed away from the Fe surfaces, and that the siderite crystals grow as scales at the carbonating Fe surfaces. The concurrence of the variation in the siderite content and compressive strength is observed based on XRD-Rietveld analysis. The results of XRD-Rietveld analysis and MIP are combined to show that pore filling and refinement may not be a suitable indicator of compressive strength in such systems.

We report the synthesis and characterization of two‐dimensional homodimetallic {Co³⁺–Co²⁺} and heterodimetallic {Fe³⁺–Co²⁺} coordination networks with Co³⁺‐ and Fe³⁺‐based metalloligands. The crystal structures reveal 2D networks in which the metalloligands are connected through secondary Co²⁺ ions. The kinetic lability of the secondary Co²⁺ ions, their Lewis acidity, and the presence of dissociable coordinated water molecules have been utilized in Knoevenagel condensation and cyanation reactions of assorted aldehydes. These reusable networks catalyze these reactions heterogeneously and efficiently.