Effect of aluminum hybrid additive on water absorption of fiber-cement composites

Abstract

Moisture movement and aggressive ions being transferred into a fiber-reinforced cement composite structure have negative effects in terms of mechanical properties and durability. When an aluminum hybrid additive (AHA) is incorporated into a fiber-reinforced cement composite composition, its reaction product provides a water-repellent layer along the capillary pores, resulting in a damp-proofing effect. In this study, the influences of AHA on the hydration reaction kinetics and microstructural features were characterized by isothermal calorimetry and thermogravimetry analysis. The percentages of AHA added to the mixtures varied from 0 to 5% w/w. The samples were formed by a filter-pressing process. After the sample forming process, the green samples were cured in air for 7, 14 and 28 days. The addition of 3% w/w AHA, based on the weight percent of cement, shortened the setting time of cement paste by almost 40%. Microstructural analysis of the cement matrix showed packing efficiency of the crystalline phase was improved when adding 3% w/w AHA to the mixture. This AHA mixture showed integral hydrophobicity, with contact angles greater than 140° on the surface and 16.51% porosity. These results corresponded to a decrease in water absorption of almost 50%, compared to the reference composition. Moreover, the enhancement of the mechanical properties of the samples, including modulus of elasticity, modulus of rupture and impact strength, surpassed the requirements for wallboard applications.