Effect of Multi-Walled Carbon Nanotubes on the Mechanical Properties of Fly Ash-Modified Foam Concrete: Experimental Investigation and Numerical Beam Analysis

Abstract

Foam concrete is valued as a lightweight, thermally insulating material, but its low compressive, tensile and flexural strengths restrict its structural use. This study examines the effect of multi-walled carbon nanotubes (MWCNTs) on the mechanical properties of foam concrete of 1500 kg/m³ target density. The MWCNTs were dispersed by ultrasonication in a polycarboxylate-ether superplasticizer solution and replaced 0.1–0.5% of the cement by mass, while Class F fly ash replaced 0–80% of the fine aggregate (water–cement ratio 0.3; sand–cement ratio 2.5). Compressive strength, split tensile strength, modulus of elasticity and flexural strength were evaluated at 28 days. All four properties improved with MWCNT addition up to an optimum dosage of 0.3% (with 40% fly ash replacement), beyond which agglomeration of the nanotubes reduced performance. At the optimum, the compressive strength was 19.61 MPa, about 40% higher than that of conventional foam concrete (14.01 MPa) and about 31% higher than that of M15 normal concrete (15 MPa). The load–deflection response of three simply supported beams (150 × 200 × 1800 mm) made of normal concrete (B1), conventional foam concrete (B2) and MWCNT foam concrete (B3) was then simulated in SAP2000. The results show that MWCNTs are a promising nano-additive for strengthening foam concrete, provided that effective dispersion is achieved.