Cementless binder consisting of high-calcium fly ash, silica fume and magnesium chloride

Abstract

This work aims to study the effect of MgCl2 additive on the strength, heat of hydration, and phase composition of hydration products of the binder consisting of high-calcium fly ash and silica fume. Fly ash from Berezovskaya thermal power plant, containing a lot of CaOfree, in combination with silica fume does not expand and exhibits the properties of a binder. However, the strength of this binder is low. The MgCl2 additive significantly increases the hardening rate and the final strength of the mix. The compressive strength of 32×32×32 mm specimens from the paste at the age of 7 days is 15.2 MPa and 3.7 MPa with and without the MgCl2 additive, respectively. Exothermic data show that silica fume inhibits CaO hydration from the first minutes of the reaction to 2 days, after which the process accelerates and proceeds evenly. The total value of the thermal effect for 10 days is 500 kJ per 1 kg of binder. The MgCl2 additive does not increase this final value, however, it accelerates the release of heat in the initial periods, excluding the indicated stagnation period. The results of XRD and DTA showed that in the presence of MgCl2, calcium hydrochloraluminate (Friedel's salt) is formed, which was not previously observed in the composition of binder hydration products.

Keywords

• fly ash
• microsilica
• silica fume
• early strength agent
• mgcl2
• x-ray diffraction analysis
• strength
• heat release
• differential thermal analysis
• heat of hydration
• phase composition