Packing density of cementitious materials: measurement and modelling H. H. C. Wong and A. K. H. Kwan The University of Hong Kong Packing density has great effect on the performance of a concrete mix. However, little research has been carried out on the packing density of cementitious materials owing to the lack of an established measurement method. Herein, a new method, called the wet packing method, is presented. With this method, the packing densities of blended cementitious materials, consisting of ordinary Portland cement (OPC), pulverised fuel ash (PFA) and condensed silica fume (CSF), were measured. The results verified the theory that the packing density could be significantly increased by blending two or even three cementitious materials together. Comparison between the measured results and the predicted values by three existing packing models, together with some additional tests, revealed that in the presence of a third-generation superplasticiser, the packing density of CSF is dependent on the lime content. When the lime-containing OPC and PFA contents are low, the CSF particles would flocculate and pack rather loosely, but at higher OPC and PFA contents, or with lime added, the CSF would pack to a higher density. With the effect of lime accounted for, very good agreement between the measured results and the predictions by the packing models was achieved. Introduction The packing density of particles, which is defined as the ratio of the solid volume of the particles to the bulk volume occupied by the particles, is a fundamental parameter governing the properties of many materials made from particles, such as ceramics. 1 In the field of powder technology, the packing density of particles has been a major theme of research. 2 As a concrete mix is also composed largely of particles, its properties are greatly influenced by the packing density of the parti- cles and hence research on the packing density of the constitutive materials can help to improve understand- ing of the behaviour of concrete. Early in the 1960s, Powers 3 studied the effects of the packing density of aggregate on the properties of fresh concrete. He argued for the excess paste theory, which states that concrete may be conceived as a mixture of aggregate and cement paste and it is the cement paste in excess of the amount needed to fill up the voids between the aggregate particles that disperses the parti- cles and lubricates the concrete mix. Basically, a higher packing density of the aggregate would lead to a re- duced paste demand (the amount needed to fill up the voids) and a larger amount of excess paste (the amount in excess of that needed to fill up the voids) for lubri- cating and improving the workability of the concrete mix. Apart from improving the workability, the higher packing density of the aggregate may also be utilised to reduce the paste volume for higher dimensional stabi- lity or to reduce the water/cementitious materials (w/cm) ratio for higher strength. As the overall performance of a concrete could be enhanced by increasing the packing density of the aggregate, packing density optimisation of the aggre- gate has been a key issue in concrete mix design. Standard and fairly simple methods for measuring the packing density of aggregate are available in the exist- ing codes. 4,5 However, the packing density optimisation of aggregate is still largely conducted by means of a lengthy trial-and-error process. To reduce the labour and time required, some packing models developed in the field of powder technology have been successfully applied to predict theoretically the packing density of aggregate for packing density optimisation. 6 Extending the excess paste theory and following the geometric similarity principle, it may be postulated that the packing density of the cementitious materials Department of Civil Engineering, The University of Hong Kong, Hong Kong, China (MACR-D-07-00004) Paper received 19 December 2006; last revised 19 June 2007; accepted 21 November 2007 Magazine of Concrete Research, 2008, 60, No. 3, April, 165–175 doi: 10.1680/macr.2007.00004 165 www.concrete-research.com 1751-763X (Online) 0024-9831 (Print) # 2008 Thomas Telford Ltd