Alkaline activation of volcanic ash using Na2SiO3 as activator showed its limits because of their low reactivity, nevertheless, phosphoric acid activation using H3PO4 as activator can offer a great possibility of utilization of the latter aluminosilicate. The present study aims to determine the chemical composition of amorphous phase of volcanic ash and examine through the microstructure of the obtained cement-based phosphoric acid activation the relation between silicon and phosphorus atoms. Chemical composition of amorphous phase was determined by dissolution with NaOH and HCl. Cement fresh paste was subjected to isothermal calorimetry and temperature variation. Meanwhile, the hardened paste was characterized by X-ray diffraction, thermogravimetry analysis, scanning electron microscopy, energy dispersive spectrometry and Fourier transform infrared spectroscopy in order to study the microstructure. Also, the cement product was submitted to compressive strength test. The results obtained showed that, the amorphous phase of volcanic ash contain SiO2, Al2O3, Fe2O3, MgO and CaO oxides. This study allowed to understand the fact that phosphoric acid activation of volcanic ash at ambient temperature is an exothermic reaction and its described as a dissolution/precipitation process. Also, the obtained product was hydrated. Compressive strength of the cement product aged 1 and 28 days was respectively 20.6 and 30.5 MPa. Based on SEM/EDS analysis, silicon are partially replaced by phosphorus atoms and the product is made of –Si–O–Al–O–P–O–phospho-sialate network. Phosphoric acid activation is a promising route of valorisation of volcanic ash.