The fineness and particle size distribution of Portland cement The final step in the production of cement is to grind the clinker pellets (and added gypsum) into a fine powder. The resulting individual cement particles have a variety of angular shapes, and a wide range of sizes. Most of the cement particles will consist of regions of each of the four main cement minerals. : SM micrograph of a !ortland cement showing the wide range of particle sizes and shapes. The size of a cement particle has an important effect on the rate at which it will hydrate when e"posed to water. #s it reacts, a layer of hydration product forms around the outside of the particle, separating the un reacted core of the particle particle from the surrounding water. #s this layer grows thicker, the rate of hydration slows down. Therefore, a small particle will react much more $uickly than a large particle. particle. # particle that has a diameter of % &m will react completely in a'out % day, whereas a particle with a diameter of % &m will react completely completely in a'out % month. month . !articles larger than a'out &m will pro'a'ly never 'ecome fully reacted , even if there is a sufficient source of water. *learly, the particle size distri'ution is critical for controlling the rate at which cement sets and gains strength. There must 'e a certain amount of small particles to ensure that the cement sets in a reasona'le reasona'le amount of time, time, 'ut if there are too many small particles the cement will set too $uickly, leaving no time for mi"ing and placing. +ortunately, the grinding process has a natural tendency to produce a wide range of particle sizes, so this is not a pro'lem. #'out % wt of the cement is made of particles larger than &m, and only a few wt is particles larger than - &m. n the fine end, less than % of the cement is particles smaller than / &m. Such a distri'ution is traditionally measured 'y a sedimentation process, in which the size of the particles is determined from the rate at which they sink in a column of li$uid. # faster and more precise method is laser particle analysis. # small amount of powder is dispersed in a solvent such as methanol and then pumped through a clear tu'e in front of a laser. The particle size distri'ution is calculated from the diffraction pattern generated 'y the scattering of the laser light off of the particles. 0owever, this is only one aspect of the particle size distri'ution, the other 'eing its 'readth. 1f two cements have the same average particle size 'ut different distri'utions, the cement with the 'roader distri'ution will have more fine particles and thus will e"hi'it a faster rate of early hydration. # 'etter parameter for descri'ing the fineness of the cement (at least in terms of knowing how reactive it will 'e at early times) is the specific surface area, 'ecause most of the surface area comes from the smallest particles. The most common method for charact characteri erizin zing g the surface surface area area of cement cement is the 2laine 2laine air permea' permea'ili ility ty test, test, which which is descri'ed 'y an #STM standard 3%45. This test is 'ased on the fact that the rate at which air can pass through a porous 'ed of particles under a given pressure gradient is a function of the surface area of the powder. # cham'er of known cross sectional area and volume is filled with a known mass of cement, and then the time re$uired to pass a known volume of air through the powder is measured. 6hile the surface area can in theory 'e calculated e"plicitly from this data, in practice the surface area is determined through an empirical e$uation developed 'y measuring powders of a known surface area using the same instrument. The resulting value, called the 2laine fineness, is today e"pressed in units of m/7kg, although in previous times it was e"pressed in cm/7g. The 2laine fineness of !* usually ranges from
4 – m/7kg (4 – cm/7g). 1t should 'e noted that the specific surface area is a notoriously difficult parameter to pin down 'ecause most materials have features at many different length scales, so that the more sensitive the measurement, the more surface area will 'e found. #s a final note, the specific surface area of a cement powder tells little a'out the size of the largest particles or the 'readth of the particle size distri'ution. These parameters affect the worka'ility of the cement at a given water7cement ratio and the degree of hydration and strength development over long periods of time, #mong other factors. 8o single parameter can su'stitute for a full particle size distri'ution.
