Novel Plastering Mortar Incorporating Cenospheres for Autoclaved Aerated Concrete Based on Magnesium Phosphate Cement

Abstract

Autoclaved aerated concrete (AAC) is an energy-saving and environmentally friendly building material. However, ordinary portland cement (OPC) mortar is prone to cracking and spall when it is used for surface plastering of AAC. To resolve this situation, this research proposes a novel plastering mortar appropriate for AAC based on magnesium phosphate cement (MPC) incorporating fly ash (FA) and cenospheres. Two series of mortar mixtures were formulated with different FA and cenosphere dosages. Firstly, MPC was partially replaced with FA by weight. Then, in the case of fixed FA content, sand was replaced with an equal volume of cenospheres at different percentages. The mechanical strength, water retention, drying shrinkage, and thermal conductivity of the MPC mortar was investigated. The hydration products and microscopic properties of the MPC mortar were examined by scanning electron microscope (SEM) and X-ray diffraction (XRD). The results show that FA greatly improves the water retention of MPC mortar. The water retention of MPC mortar is 97.5% at FA replacement of 40%. Although cenospheres replacing sand slightly reduced mechanical strength of MPC mortar, they can significantly decrease the thermal conductivity and drying shrinkage of mortar. When sand was completely replaced with cenospheres, the bond strength, drying shrinkage rate, and thermal conductivity were 0.6 MPa, 0.08 mm/m, and 0.26 W/m·K, respectively. Finally, XRD and SEM tests of MPC mortar also indicated that cenospheres inhibited the hydration of MPC and reduced the strength of MPC mortar. In conclusion, the developed MPC mortar has excellent bond strength as well as low drying shrinkage rate and thermal conductivity, which can effectively deal with the problem of cracking and spall in traditional plaster mortar and is also an eco-friendly building material.