Understanding the binder chemistry, microstructure, and physical properties of volcanic ash phosphate geopolymer binder

Abstract

This work aims to assess the influence of the chemical composition of the binder resulting from the reaction of phosphoric acid and volcanic ash on its final characteristics. Six initial compositions of volcanic ash phosphate geopolymer with molar ratios Fe/P of 0.27, 0.5, 0.54, 0.81, 1, and 1.5 were designed by adding various dosages of phosphoric acid to volcanic ash. The results show that the hardening time increases with the decrease of molar ratios Fe/P. An excess of phosphoric acid leads to an unstable binder that is partially destroyed with the aging of the binder. The volcanic ash phosphate geopolymer with molar ratios of Fe/P = 0.5-0.54 has the optimum compressive strength (49-53 MPa at 90 days), the lowest water absorption (8.8-9.5 wt.%) as well as porosity (18-19.6 vol.%). The main binder is a porous phase of ferro-silico-aluminophosphate. Secondary phases were also identified in some mixes including ferro-aluminophosphate and magnesium phosphate.

The role of curing temperature and reactive aluminum species on characteristics of phosphate geopolymer

“…That main band is characteristic of the stretching vibration of the P–O–(Fe), P–O–(Al) and P–O–(Si) bonds of the reaction products (binder). 35 That band is accompanied by the one appearing as a small shoulder at 910 cm −1 , also characteristic of vibration of P–O of the phosphate binder. The band at 1640 cm −1 is characteristic of the bending vibration of H–O bonds of water.…”

“…That is because the limited dissolution of the ferro/aluminosilicate was recorded due to the lowering of the acidity attributed to the acid–base reaction that occurred at the beginning of the reaction. 26,35 As a general rule, the dissolution–enhancement–precipitation–condensation is the reaction mechanism dominating the phosphoric acid activation of ferro/aluminosilicate at room or mild temperatures. Whereas, at high temperatures or in presence of any setting accelerators, the dissolution–inhibition–precipitation–condensation is the dominant reaction mechanism.…”

“…12 The peak at 686-697 °C is related to the partial crystallization of the amorphous phase of the phosphate binder in aluminum phosphate minerals. 35,37 These two peaks are well visible in the DSC curves of all phosphoric acid-activated volcanic ash, while it is only visible on the curve of acid aluminum phosphate geopolymer cured at room temperature. That observation can be attributed to the extent of the reaction and the amount of reacted products formed.…”

“…These were also observed in previous work and reported to be the main reacted product resulting from the acid phosphate activation of volcanic ash. 35 It thus conrms that though silicon solubility is weak in an acid phosphate solution, the part dissolved takes part in the reaction and modies the chemistry of the binder. Elsewhere, the presence of soluble aluminum species in the activating solution and the curing temperature did not affect the phase formed.…”

Phosphate-activated geopolymers: advantages and application

“…They also discovered that aer soaking SAP geopolymer samples in water, the binder began to breakdown, resulting in white powder on the surface of the binder. 95 97 investigated the water resistance capabilities of produced binders using FA as the raw material for SAP geopolymer samples. Samples were soaked for 7, 28, 180, and 270 days.…”