The growing demand for sustainable construction practices has driven research into self-sensing materials incorporating recycled waste for smart SHM (Structural Health Monitoring) systems. However, previous works did not investigate the influence of rheological behavior and piezoresistive properties of sustainable cementitious sensors containing red mud (RM) on the strain monitoring of concrete beams. To address this gap, this study presents an experimental analysis of the rheological, mechanical, and self-sensing performance of mortars incorporating carbon black nanoparticles (CBN) and varying levels of RM (25–100% sand replacement by volume), followed by their application in monitoring strain in a reinforced concrete beam under dynamic loading. The results showed that increasing RM content led to higher viscosity and yield stress, with a 60% reduction in consistency index. Compressive strength increased by up to 80%, while mortars with RM content higher than 50% showed high electrical conductivity and reversible resistivity changes under load cycles. Mortars containing 50–100% RM demonstrated improved piezoresistive response, with a 23% increase in gauge factor, and the best-performing sensor embedded in a concrete beam exhibited stable and reversible fractional changes in resistivity, closely matching strain gauge data during dynamic loading conditions. These findings highlight the potential of RM-based smart mortars to enhance sustainability and performance in SHM applications.

Lightweight aggregates self-compacting concrete (LWASCC) have been studied for structural and non-structural purposes in slender pieces with high reinforcement rates to reduce weight and obtain a more fluid mixture [¹ 1 J. A. Rossignolo, Concreto Leve Estrutural: Produção, Propriedades, Microestrutura e Aplicações. São Paulo, Brazil: PINI, 2009. ]-[³ 3 B. Vakhshouri and S. Nejadi, “Mix design of light-weight self-compacting concrete,” Case Stud. Constr. Mater., vol. 4, pp. 1-14, Jun. 2016, http://dx.doi.org/10.1016/j.cscm.2015.10.002. http://dx.doi.org/10.1016/j.cscm.2015.10... ]. Researches involving lightweight aggregates (LWA), natural or artificial, have been carried out to understand their behavior in cement matrices, reducing density from 2400 kg/m³ to values below 2000 kg/m³ [⁴ 4 A. F. Angelin, R. C. C. Lintz, W. R. Osório, and L. A. Gachet, “Evaluation of efficiency factor of a self-compacting lightweight concrete with rubber and expanded clay contents,” Constr. Build. Mater., vol. 257, pp. 119573, Oct. 2020, http://dx.doi.org/10.1016/j.conbuildmat.2020.119573. http://dx.doi.org/10.1016/j.conbuildmat.... ]-[⁹ 9 S. Yang, X. Yue, X. Liu, and Y. Tong, “Properties of self-compacting lightweight concrete containing recycled plastic particles,” Constr. Build. Mater., vol. 84, pp. 444-453, Jun. 2015, http://dx.doi.org/10.1016/j.conbuildmat.2015.03.038. http://dx.doi.org/10.1016/j.conbuildmat.... ].

Conventional red ceramic and concrete bricks and blocks are some of the main products used in civil construction. However, reducing the exploitation of natural resources and CO ₂ emissions by replacing conventional materials with new waste-based alternatives is necessary. Alkali-activated cement (AAC) has been studied in recent years and has been identified as a possible alternative to Portland cement (PC). The present study proposes the development of bricks/blocks composed of rice husk ash (RHA), eggshell lime (ESL), and sodium hydroxide, without the use of conventional PC or fired ceramics. This study aims to evaluate the effect of molding pressure and, more importantly, how the resulting porosity influences the unconfined compressive strength (UCS), initial modulus of elasticity (E ₀ ), and water absorption (WA) of alkali-activated lightweight bricks. Specimens were molded with dry unit weights (γ _d ) of 8, 9, 10, 11, and 12 kN/m ³ to obtain different porosities, thermally cured at 60 °C for 24 h, and tested at 7 days of age. The molding pressure, porosity, UCS, ultrasonic pulse velocities (UPV), E ₀ , and WA were determined for each specimen. Results showed that porosity controlled the mechanical response (UCS and E ₀ ) and the WA of the cemented matrix. Specimens with porosity lower than 56% (γ _d ≥ 10 kN/m ³ ) meet strength requirements for use as structural blocks. Similarly, porosity lower than 55% (γ _d ≥ 10 kN/m ³ ) is required to meet water absorption criteria, indicating that this is the minimum molding density to be used. Moreover, relationships between γ _d , porosity, and molding pressure were established for practical applications. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) results confirmed the polymerization reactions of the raw materials, leading to the formation of a C(N)–S–H gel in the cemented matrix.