Extensive rockfill cylinder testing was performed to determine the effect that aggregate size, aggregate moisture, Portland cement, fly ash and cement dispersant addition has on compressive strength of a cemented backfill. In this paper `two-level factorial' and `central composite' designs were used to determine the single and interaction effects that each factor have on the overall strength. The results from the design- work indicated that the per cent of aggregate fines had the greatest influence on the compressive strength of the rockfill cylinders. From the factorial design results, a compressive strength estimation equation was derived, which enables the strength to be estimated, given varying input Values.