Sustainable cementitious composites reinforced with metakaolin and halloysite nanotubes for construction and building applications

Abstract The purpose of this study was to assess the performance, durability and mechanical strength of the following three nanofillers: Metakaolin (MK), calcined halloysite (CHAL) and non-calcined halloysite nanotubes (Hal) as Supplementary Cementitious Materials (SCM) when used as a partial substitute for Ordinary Portland Cement (OPC) in lightweight cementitious composites. Naturally occurring kaolinitic and halloysitic clays are abundant throughout the world with substantial deposits in the USA, Brazil, China, Turkey, Australia, UK, Germany, Czech Republic, Indonesia and Malaysia. Pozzolanic materials containing high proportions of silica and alumina are known as SCMs and include fly ash, granulated ground blast furnace slag (GGBFS), silica fume, pumice, MK, etc. Production cost for MK is cheaper and more sustainable through energy saving compared to cement production. According to many studies, the substitution of cement by MK provides a substantial reduction in CO2 emission. Greenhouse-gas emissions for MK are 55% lower than Portland cement, mainly because cement requires calcining at 1450 °C while MK requires typically only 700 °C to 800 °C to be fabricated. In this study, compressive and flexural strength tests were carried out for three different curing times at 3, 7 and 28 days while field emission scanning electron microscopy and Xray diffraction analysis was performed to observe the interaction between nanofillers and the cement matrix and to analyze the crystalline structure of the nanofillers and the final composites, respectively. From the result, partial replacement of Portland cement with MK can increase flexural strength while reducing the compression strength. The influence of MK with Portland cement was more apparent than the use of Hal or calcined Hal in terms of mechanical strength. It was essential to observe how CHAL and MK can improve the flexural strength of the cementitious composites due to the bridging and filling effect of Hal within the cement microstructure. A sustainability analysis was completed, which indicates that MK and calcined Hal should be considered as suitable materials (SCM's) for the partial replacement of OPC used in the construction industry, especially when a self-healing concrete structure is needed in the future.