A NEW METHOD FOR DETERMINING SOLUBLE SILICA IN HARDENED CONCRETE

To date, the best procedures obtained for determining cement content in hardened concrete were ASTM D-1084 and BS 1881-124:1988, both, with a variation of results in the cement content of 71 and 50 to 60 kg/m³ respectively. This deviation is too large with respect to a truly operational procedure for the determination of the water/cement ratio, for which reason they have not been applied to other standards. The author was able to demonstrate for the first time, in the field of analytical chemistry, the underlying causes of the passage of silica into the filtrate which are due, on the one hand, to the incomplete process polymerisation of polysilicic acids produced in the usual insolubilisation, in which case remaining colloidal silica. On the other hand, there is a double depolymerisation effect of the temperature (>5°C) and the dilute acids solutions on the polysilicic acids get in the usual extraction, with a subsequent passage of the silica to filtrates, of extraction solution and washing, as a true solution. The author has been able to produce analytic conditions that reduce this passage of the silica to the filtrate to a constant and minimum value of 1 mg. The conditions for the new method are given. The standard deviation is reduced by three times and it provides maximum deviations obtained over the amount of cement content, of 10 to 15 kg/m³. The author demonstrated that the effects of the polyethylene oxides, gelatine and others on the colloidal silica do not prevent the passage of silica into filtrates and that this effect is only apparent. These errors are pointed out for the first time. Under these conditions, it can be employed to determine the water/cement ratio for current concrete technology with true operational validity. Finally, the application of the so-called Soluble Fractions Method for hardened concrete, from the same author, is indicated and referenced which makes it possible to determine the amount of silica by these additions and distinguish it from cement.

• INTRODUCTION

• EXPERIMENTAL RESULTS AND DISCUSSION

• CONCLUSIONS

• REFERENCES