Optimum particle packing is a key for designing a dense, strong, and durable cement-based material. By optimizing cement and aggregate particle size distribution, the voids among the particles can be significantly minimized, thus increasing packing density, reducing the amount of binder required for filling pores, and improving the material strength, impermeability, and volume stability of the resulting products. Dense particle packing is generally formed by particles with varying particle size distributions, where voids can be successively filled up with smaller particles. Various models have been developed for achieving maximum density, or optimal packing, of aggregate particles in concrete, among which is the Andreasen and Andersen (A&A) model. In concrete practice, groups of aggregate particles with a specific particle size distributions (PSD) are often combined in such a way that the PSD of the blended aggregate is getting as close as possible to a modeled PSD curve.

In this study, modified A&A model is used for achieving optimum packing density of mortars made with various cementitious materials (cement, limestone fines, fly ash, and silica fume) and river sand. The influences of the mortar material proportion on the packing density, flow property, and strength of concrete mortar were investigated. According to the A&A model theory, optimum packing can be achieved when the cumulative PSD obeys equation: P(D) = (Dq-Dq min)/(Dq max-Dq min), where D represents the size of the sieve used for analyzing the particles studied. D min and D max are accounting for the minimum and maximum particle size in the mix, respectively. The distribution modulus q is related to the fineness of the aggregate particles. (Generally, a high q value (>0.50) results in a coarse mixture, whereas a small q value (q<0.25) results in a mixture that is rich in fine particles.

The study consists of two parts, as written of two research papers. In paper one, a fixed distribution modulus (q) value was used, and the PSD of a given mixture was modified by different amount of limestone fines (LFs) and river sand addition. The effects of the LF and sand addition on the particle packing as well as on flow property and strength of concrete mortar were examined. The results indicate that enhanced particle packing improves mortar density and increases viscosity, but it had minimal effect on heat of hydration and yield stress of the mortar mixture. In paper two, various distribution modulus values (q = 0.25, 0.35, and 0.45) were used to evaluate the particle packing quality of a given high performance mortar (HPM). Single sized sand was selected and added to the HPM mixture so as to make the PSD of the modified HPM mixture to the A&A model curve. The minimum sum of squares of the residuals (RSS) was used to assess the quality of the PSD modification. Dry density, rheology, and compressive strength tests were performed for the both original mortar mix and sand-modified mixes. The results show that the increasing q value decreased mortar density, viscosity, and early age strength but had little/no effect on 28-day mortar strength.