Effect of porosity on the strength and stiffness of eggshell lime/rice husk ash alkali-activated lightweight bricks

Abstract 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.