•Compare different adding porous materials effect on solar pond temperature.•Two styles of outdoor salt gradient solar ponds were used to experimental study.•One dimensional model was used to estimate the solar pond temperature development.•Coal cinder shows better performance than other involved bottom treatments.•Adding coal cinder on bottom can obviously increase solar pond temperature. A higher Low Convective Zone (LCZ) temperature is important for the Salt Gradient Solar Pond (SGSP) thermal engineering application, which is bound to bring a more extensive application. For this purpose, we studied on adding a layer of coal cinder at bottom of LCZ of solar pond, here coal cinder is the burning residues of coal. Temperature development of adding coal cinder at bottom of SGSP has been experimentally and theoretically studied. One dimensional transient numerical model was used to estimate the temperature development in SGSP. The outdoor experiments were used to qualitatively study the temperature evolution of different bottom treatments and porous materials influence, and also used to validate the mathematics model present in this paper. The results show that adding coal cinder at bottom of SGSP leads to a higher LCZ temperature than the traditional bottom treatment. Good consistency has been achieved in the simulation results and the experimental results. Finally, in order to estimate the effect of coal cinder used in large-scale solar pond, a numerical simulation was given and compared to the similar research. The results of this paper show that it can obviously increase LCZ temperature by adding coal cinder at bottom of LCZ, and as a cheap material with perfect thermal performance, it is suitable to be applied in practical SGSP.