Understanding the setting and hardening process of wollastonite-based brushite cement. Part 1: Influence of the Ca/P ratio and H3PO4 concentration of the mixing solution

Abstract

Wollastonite-based brushite cement are prepared by mixing wollastonite with a phosphoric acid solution containing metallic cations and boric acid. This work investigates simplified systems comprising wollastonite and H3PO4 solutions only, in order to clarify the influence of the H3PO4 concentration, Ca/P and l/s ratios (this latter parameter being correlated to the two others) on the setting and hardening process. At constant H3PO4 concentration, increasing the Ca/P ratio, and decreasing the l/s ratio, accelerates the early stages of hydration but limits its progress at 7 d. At high Ca/P ratio, more basic calcium orthophosphates form in addition to dicalcium phosphate. At constant l/s ratio, increasing the H3PO4 concentration in the range 7-10 mol.L-1 retards cement hydration, and inhibits the setting at concentrations  12 mol.L-1. A good compromise is obtained for H3PO4 concentrations between 9 and 10 mol.L-1 : hydration is not too fast and yields the maximum amount of dicalcium phosphate.

Understanding the setting and hardening process of wollastonite-based brushite cement. Part 2: Influence of the boron and aluminum concentrations in the mixing solution

“…The main role of boron was to delay the hydration and reduce the maximum heat flow. The X-Ray diffraction results showed that boron mainly influenced the conversion of MCPM to brushite which occurred later as compared to the MCPM-monetite transition evidenced in the boron-free reference paste [17]. The retarding effect of boron could thus result from a delay in MCPM dissolution and/or brushite precipitation.…”

“…(1)). In a previous work, it was shown that decreasing the acidity (or, equivalently, increasing the acidity function H 0 within the range [− 2.13, − 1.28]) decreased the period of low thermal activity and cumulative heat but increased the maximal heat flow [17]. Consequently, the reduction in the maximal heat flow observed with increasing [Al 3+ ] could not be attributed to an acidity effect, but to the presence of metallic cations.…”

“…Even if the hydration process was slowed down, wollastonite was almost totally consumed at 48 h. Brushite (CaHPO 4 .2H 2 O) precipitated instead of monetite (CaHPO 4 ), the product observed for the boron-free reference [17]. This result could be explained by the lower selfheating of the boron-containing material which tended to promote the precipitation of metastable brushite instead of thermodynamically stable monetite [22].…”

“…As for the variation of the cumulative heat and duration of the period of low thermal activity, both parameters (acidity and [Al 3+ ]) could be involved. The evolution of the cumulative heat was plotted versus the acidity function and compared to the curve obtained for solutions containing phosphoric acid only [17] (Fig. 6).…”

“…However, data about the influence of the mixing solution composition on the properties of the resulting material are scarce in the literature. A previous work [17] was focussed on the impact of the phosphoric acid concentration, Ca/P molar ratio and liquid-to-solid (l/s) ratio. Increasing the Ca/P ratio (at constant H 3 PO 4 concentration) and decreasing the l/s ratio were shown to accelerate the early stages of hydration but limited its progress at 7 d. Moreover, basic calcium orthophosphates formed in addition to brushite.…”