This investigation focuses mainly on highlighting the contribution of the IR spectrum of the mineral matrix in co-composts after removing the organic phase by slow pyrolysis. The mineral matrix is still confusion subject in the interpretation of FTIR spectra of co-composts. The XRF and XRD analysis of the low pyrolysis substrates of sewage sludge and date palm waste during six months of co-composting, show that the mineral fraction is homogeneous in the samples and that the inorganic matrix consists mainly of silica and carbonates including a residual portion remaining after pyrolysis.

The comparison of FTIR spectra of heated samples at 105°C and 650°C shows that the 3500-3280 cm^-1 region characterises essentially the hydroxyl groups OH (ν_(OH)) of water molecules. The observed bands in this region provide information on the co-compost hydrophilic nature but can only provide limited information about the humic substances composition. The band at 1035 cm^-1, assigned commonly to organic material, which persists after the thermal treatment, should be assigned for the Si-O vibration of silica.