Effect of Calcium Additions on N-A-S-H Cementitious Gels

This paper reports a research on the properties of synthetic sodium aluminosilicate hydrate gels as models for the gels arising in alkali-activated aluminosilicate systems. The interaction between these N-A-S-H gels and aqueous calcium was studied to explore a potential consequence of mixing conventional Portland cements and alkali-activated systems. The properties of two N-A-S-H gels (with Si/Al=1 and 2) synthesized in highly alkaline media, in the presence of calcium are reported. N-A-S-H gels were synthesized using aluminum nitrate, sodium hydroxide, and a sodium silicate. FTIR, NMR, and TEM/EDX techniques were used to characterize N-A-S-H gels before and after adding calcium. The results obtained show that calcium is uptaken in N-A-S-H gels resulting in a modification of the chemical composition due to the substitution of sodium per calcium.