Interfacial interactions in lightweight aggregate concretes and their influence on the concrete strength

The interactions between sintered fly ash light-weight aggregates and the matrix in portland cement concretes was studied to resolve factors other than aggregate strength which influence the concrete strength. Aggregates of variable properties were produced and concretes of equal effective water/cement ratio were prepared and tested for strength and microstructure. It was found that differences in concrete strength could not always be accounted for by differences in the aggregate strength. These trends could be related to physical and chemical interfacial processes, which have an influence on the overall strength beyond that of the aggregate strength. The physical process identified was densification of the interfacial transition zone due to absorption of the aggregates; this process has considerable influence already at early age. The chemical processes were associated with pozzolanic activity of the aggregate and deposition of CH in the pores in the shell of the aggregate; these processes became effective only at later age, beyond 28 days. The enhancement in strength due to these influences ranged between 20 and 40%. Such influences should be taken into account when predicting the concrete strength or in the design of lightweight aggregate of optimal properties.