The effect of superplasticiser, borax and the water-to-cement ratio on BYF hydration and
mechanical strengths has been studied. Two laboratory-scale BYF cements—st-BYF (with -C2S
and orthorhombic C4A3 S) and borax-activated B-BYF (with a’H-C2S and pseudo-cubic C4A3s)
have been used, and both show similar particle size distribution. The addition of superplasticiser
and externally added borax to BYF pastes has been optimised through rheological measurements.
Optimised superplasticiser contents (0.3, 0.4 and 0.1 wt%for st-BYF, B-BYF and st-BYF with externally
added 0.25 wt % B2O3, respectively) result in low viscosities yielding homogeneous mortars. The
calorimetric study revealed that st-BYF is more reactive than B-BYF, as the values of heat released are
300–370 J/g and 190–210 J/g, respectively, after 7 days of hydration; this fact is independent of the
water-to-cement ratio. These findings agree with the higher degree of hydration at 28 days of b -C2S
in st-BYF (from 45 to 60%) than a’H-C2S in B-BYF (~20 to 30%). The phase assemblage evolution has
been determined by LXRPD coupled with the Rietveld method and MAS-NMR. The formation of
stratlingite is favoured by increasing the w/c ratio in both systems. Finally, the optimisation of fresh
BYF pastes jointly with the reduction of water-to-cement ratio to 0.40 have allowed the achieving
of mortars with compressive strengths over 40 MPa at 7 days in all systems. Moreover, the st-BYF
mortar, where borax was externally added, achieved more than 70 MPa after 28 days. The main
conclusion of this work does not support Lafarge’s approach of adding boron/borax to the raw meal
of BYF cements. This procedure stabilises the alpha belite polymorph, but its reactivity, in these
systems, is lower and the associated mechanical strengths poorer.
