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To investigate the variation in the degree of polymerization calcium aluminium silicate hydrate (C-A-S-H) gel and its role in the evolution of the strength of waterglass slag binders, the compressive strength, hydration products, degree of hydration of the slag, and the degree of polymerization of C-A-S-H gels of binders were examined. The experimental results indicate that the pH of the pore solution increased with an increase in the Na 2 O concentration. However, mortar with an optimum compressive strength value of 81.0 MPa at 28 d was obtained when water glass modulus was 1.5. The main hydration product is a C-A-S-H gel for which the quantity and the degree of polymerization depend strongly on the Na 2 O concentration; for a given range, both increase with increasing Na 2 O concentration, thus yielding an enhanced strength. A further increase in the Na 2 O concentration continuously increases the quantity of C-A-S-H gels while drastically reducing the degree of polymerization. The positive effect of the former is counteracted by the adverse effect of the latter, ultimately, leading to a decreased strength. Furthermore, we reveal that the degree of polymerization for C-A-S-H gels may be affected by pH, through a series of complex chemical reactions.