strength levels durability is improved with a large unexpected decrease in permeability at relatively low doses for a material at this particle size dosage rates are under 10 cement replacement whereas fly ash or ground granulated blast furnace slags need to be used from 15 and typically 25 or more for fly ash and 40 or more for slag to achieve permeabilities comparable to what is achieved with the invention as will be shown in the examples that follow unlike fly ash or slag there is not an early strength trade off and the product has one day strengths equivalent to better than silica fume and maintains these strength advantages to at least 90 days of curing the invention is a method of reducing the portland cement content in a mortar or concrete without reducing initial strength and increasing long term strength and reducing long term and early permeability by use of the addition of ground expanded perlite as a cement replacement centerstar according to one preferred example the size characteristics are 100 of the particles less than 200 u03bcm 90 below 80 u03bcm with a volume weighted mean of approximately 38 u03bcm it will be appreciated that the mean particle size can be varied over some range without departing from the spirit of the invention this range may vary with particular intended applications as well as with the particular grinding processes and specifications of perlite manufacturers and may range from 10 to 100 u03bcm this.