The heat transfer fluid NaNO3-KNO3 salt was prepared by using analytical-grade NaNO3 and KNO3. Samples were calcinated at 600 degrees C, 750 degrees C and 800 degrees C respectively for various times to investigate the influence of heat treatment on solidus temperature of the salt. The temperature was measured by the analysis of DSC curves, and results show that it decreases with the extension of calcination time at 600 degrees C, 750 degrees C and 800 degrees C. The lowest solidus temperature found in the experiment was about 130 degrees C. The correlation between solidus temperature and calcination time was summarized. Results of phase analysis and infrared analysis indicate that NaNO2 is generated after calcinating at high temperature. Alkali oxide appears at 800 degrees C based on the results of X-ray photoelectron spectroscopy. Durability experiment was conducted to evaluate whether the solidus temperature of a partly decomposed sample changes under operation condition. Results show that the solidus temperature maintains steady after 300 times of heat recycling under 350 degrees C, but it would rise when the sample is heated above 450 degrees C in the air. (C) 2015 Elsevier Ltd. All rights reserved.