Hydration Activation of Alite-Belite-Ye’elimite Cements by Doping with Boron

Abstract

The hydration behavior of two alite-belite-ye’elimite (ABY) cements has been studied. The production of these materials releases into atmosphere ∼17% less CO2 compared to Portland cement. The ABY cement contains alite, β-C2S, and ye’elimite as main phases, while dABY (activated by adding borax) contains not only these three phases but also α′H-C2S. The role of boron in these systems is 2-fold: (i) acting as a retarder at early hydration ages since the precipitation of AH3 gel has been delayed from over 4 h in ABY to over 24 h in dABY; and (ii) as an activator at late hydration ages by stabilizing highly reactive β- and α′H-belites. The degree of hydration of β-C2S in ABY is 42% at 28 days, while that of β-C2S and α′H-C2S in dABY at the same age is 80 and 88%, respectively. Moreover, the main hydration products in both systems are ettringite and C−S−H gel. The local structure of these hydrated products has been studied by 27Al and 29Si magic angle spinning nuclear magnetic resonance (MAS−NMR) and found to be similar in both cements. However, the amount of C−S−H gel is 63% of all silicon-bearing phases in ABY after 28 days while it is 76% (determined by 29Si MAS−NMR) in dABY, which justifies the higher mechanical strengths of dABY mortars.