Improvement mechanism of sodium carbonate on traditional composite filler

Certain issues may be encountered when traditional composite fillers are mixed with cement and bentonite (CF-CB) and applied to cavities in the ground. These issues include the bleeding and separating of water, cement and bentonite, and the long time needed for the strength development. In a series of studies, it has been shown that the suppression of bleeding, earlier gelling and earlier development of strength can be expected by mixing a suitable amount of sodium carbonate in the CF-CB, that is, creating a CF-CBS. The mechanisms of the various properties of CF-CBS and CF-CB were investigated by physical analyses. The following points were clarified from the results of this work. Earlier development of strength and gelling are brought about by the earlier formation of ettringite accompanying the promotion of the cement hydration reaction. The suppression of bleeding can be accomplished by earlier gelling due to the bentonite taking the Na⁺ ionised from sodium carbonate as an interlayer cation. The increase in strength is attributed to the calcium carbonate produced by CO₃²⁻ ionising from the sodium carbonate bonding with the Ca²⁺ dissolved from the interlayer cations of the cement or bentonite.