Influence of Portland cement characteristics on air-entrainment in fly ash concrete

A study examining air-entrainment in fly ash concretes combined with different Portland cements (PCs) is described. Tests were carried out (using five PCs, ten fly ashes and a standard chemical reagent (to entrain air)) on paste suspensions (foam index), mortar and concrete to quantify the material effects. Preliminary tests indicated that the foam index increased with the fineness (specific surface area (SSA) (by nitrogen adsorption Brunauer-Emmett-Teller (BET) method)) of the PC used (varied by grinding) with fly ash. Reductions in the property were found with increasing alkali content in the paste suspensions (by sodium hydroxide addition), which tended to be slightly greater with higher SSA/lower alkali content fly ash. Tests on the wider range of PCs and fly ashes gave a strong correlation between their combined SSA and foam index, with their combined alkali content having less effect on the latter. The influence of fineness was again apparent in the mortar tests, which also showed that when PC and fly ash were of comparable SSA, a change in either material had a similar effect on air-entrainment. However, fly ash had an increasing influence as the difference in this between materials became greater. The results also suggest that air losses after mixing tend to increase with the SSA of PC + fly ash (and hence admixture dose). Similar effects were generally noted in the tests made on concrete. A possible approach to controlling air-entrainment in fly ash concrete may therefore be to ensure that its SSA is similar to that of the PC with which it is used. A test method to enable this to be evaluated is suggested.