Not All Flyashes are the Same

摘要

The paper describes a Trial Mix Programme in which two Portland cements and two lowlime flyashes were studied.The two Portland cements had similar chemical and physical properties and the two flyashes also had similar chemical and physical properties and similar Pozzolanic Activities.During the Trial Mix Programme,it was found that the performance of the two Portland cements was quite similar-there were minor differences but these were not significant-but the performances of the two flyashes were completely different.The retarders investigated during the Programme required similar dosages for all four combinations of cementitious materials-the RCC was being retarded so that the Initial Setting Time was circa 21 hours-but the water demand of the two flyashes was different and the strength developments were completely different.Accelerated curing of all the specimens was used during the Programme to try to simulate the long-term strengths of the RCCs.The total cementitious content of all the mixes was 230 kg/m3.With one flyash and Portland cement contents of 35to 50 kg/m3,the long-term strengths were between 30 and 35 MPa; this is probably amongst the best performance for a low-lime flyash that has ever been reported.However with the second flyash,for the same Portland cement contents,the long-term strength was between 10 and 13 MPa.At an age of91 days,the first flyash with the same Portland cement contents,had cylinder compressive strengths between 25 and 28 MPa while with the second flyash the strengths were between 6 and 9 MPa; a very considerable difference.Further research is needed to try to ascertain why there should be such a significant difference,but it seems the study of the chemical and physical properties and indeed the Pozzolanic Activity,does not allow the relative performance of flyashes in concrete to be predicted.