Phosphoric acid activation of volcanic ashes: Influence of the molar ratio R = (MgO + CaO) / P2O5 on reactivity of volcanic ash and strength of obtained cementitious material

Generally, the phosphoric acid-activated systems use aluminosilicates as raw material (e.g., metakaolin); and the addition of alkali metals (e.g., Mg or Ca) contributes to obtain best properties. Volcanic ash (Va) is an aluminosilicate globally available and contain these metal oxides, which makes it a suitable raw material to produce acid-activated cements. The research on the production of this type of cements using Va is relatively recent. The knowledge on some parameters that influence the activation process under acid conditions of Va can be of great value as a viable alternative to its industrial application in the foreseeable future. This study examines the reactivity of four samples of volcanic ashes with different chemical composition in phosphoric acid at ambient temperature (25 °C). Two concentrations of phosphoric acid were used (41.1% and 51.2% P2O5 by mass). The main parameter used in this study is the molar ratio R expressed as the relation (MgO + CaO)/P2O5. Properties such as setting time, paste temperature in fresh state and compressive strength at 28 days of curing were evaluated. The final products obtained via acid activation of the volcanic ashes were studied using the XRD and FTIR techniques. The results showed that the initial and final setting time and the compressive strength increases with the increase of the molar ratio R of the mix when the proportion of P2O5 is constant. This coincides with the decrease in the temperature of the pastes. The highest compressive strength at 28 days of curing (81.3 MPa) was obtained with a molar ratio R = 4.2 and 41.1% P2O5 using a volcanic ash with high calcium and magnesium content (13.1% and 9.9% respectively). This volcanic ash when is activated with phosphoric acid of 51.2% P2O5 (R = 3.0) reported lower strength (40%) and a significant increase in the setting time. The results obtained showed that the CaO and MgO content in the volcanic ash and the acid concentration used as activator are parameters that affect the final properties of the acid-activated cements. Also, the presence of amorphous phase hindered the reactivity. Based on the results, the volcanic ashes may be considered precursors in the production of phosphoric acid-activated cements for construction purposes.