Thermal and mechanical study of chemically bonded phosphate cements formulated with magnesium by-product incorporating phase changing materials.

Abstract

Magnesium Phosphate Cements (KMgPO4·6H2O; k-struvite), that are known as MPC, part of the family of Chemically Bonded Phosphate Ceramics (CBPC), widely used in the field of biomaterials. These cements are obtained from the acid-base reaction on an aqueous medium between pure MgO and mono-potassium phosphate. It is a spontaneous and highly exothermic reaction that leads to a very fast setting of the material. The main disadvantage of these cements compared with others such as Portland is the high cost of raw materials that are necessary to elaborate the MPC. In the present study we use a by-product called LG-MgO (low magnesium oxide), supplied by the company Magnesitas Navarras, S.A., with the aim to reduce the final cost of MPC and promote aspects such as sustainability and green environment as a consequence of the reduction of the pure MgO mining activity. The research presented here consists in the exhaustive characterization of different dosages of MPC elaborated with magnesium by-product that incorporate air entraining additive and Phase Changing Materials (PCM) to improve the thermal behavior of material when there are thermal oscillations, and thus reduce the use of cooling and heating systems helping to the decrease of CO2 emissions and increasing energy efficiency on the buildings. Moreover, mechanical properties such as elastic modulus, compressive strength and flexural strength are analyzed to test the feasibility of the use of these cements as a passive cooling and heating system. Finally, the degradation of MPC when subjected to thermal cycles is analyzed (thermal durability). Results show that k-struvite is the major product formed in the MPC, although there are also inert phases of magnesium by-product that act as reinforcement. It is verified that the increase of PCM and additive in the content reduces the mechanical properties due to their contribution to increase the porosity and decrease the density. Moreover, we demonstrated that MPC and PCM do not suffer degradation after 750 thermal cycles equivalent to a year.