Hydrophobicity and efflorescence of lightweight fly ash geopolymer incorporated with calcium stearate

Abstract

Lightweight concrete is a porous material that is growing in demand because of its attractive properties. However, because of the high porosity of this lightweight material, moisture can fill pores causing cracks and facilitating algal and fungal growth. Calcium stearate (CS) is hydrophobic and could provide a water-repellent layer on the walls of capillary pores. A high dosage of CS increases air content and reduces the density of the concrete, and so this research proposed using CS to create a lightweight fly ash geopolymer. CS was used as a pore generator and water-repellent material in a geopolymer system, replacing fly ash at dosages of 0, 1, 2.5, 5, and 10% by weight. The physical and chemical properties of geopolymers were investigated. Given the high content of soluble alkali metal in the geopolymer system, efflorescence was also studied. The results show that the incorporation of CS increases the hydrophobicity of the geopolymer surface and reduces efflorescence. Calcium silicate hydrate forms in the pores and matrix of the geopolymer, as indicated by the detection of calcium silicate hydrate in the XRD pattern and the Ca–O–Si peak in the FTIR spectra. The use of CS in fly ash geopolymer also results in lower thermal conductivity of the material compared with the control. The K value was 0.13–0.62 W/m·K, depending on the CS content and density. Compressive strengths varied from 18 to 55 ksc. Efflorescence was not observed for mixes with 5 and 10% CS content. In addition, a hydrophobic surface was achieved with a mix with 5% CS, as indicated by the high contact angle of 96.67°. The test results thus confirm that a hydrophobic lightweight fly ash geopolymer without efflorescence can be fabricated with the incorporation of CS.