Ready-mix concretes meet specific formulation criteria that require using sands presenting a certain number of characteristics in order to be used in this application. Among these characteristics there is one that is essential, this is the cleanliness of the sands. The cleanliness is related to the amount of noxious impurities for a given application. The latter may be of various types but the most noxious impurities are clays.
Indeed, the nature and level of clays in the sands influences the workability characteristics of the concretes. Now, on worksites, workers generally work at a constant workability level. Consequently, the presence of clays in the sands requires modifications of the formulation. The correction is possible by adding admixtures or water. Nonetheless, the addition of admixtures represents an important cost. Furthermore, if the admixture is a retarder, the correction with the admixture is only obtained to a certain degree. As for the addition of water, it is problematic because it induces a drop of the hardened concrete's mechanical strengths, cracking problems and secondary effects in fluid concretes such as bleeding or segregation of the gravel.
Standards related to the cleanliness of the aggregates for concrete applications therefore exist.
The first standard is a test known as the <<Equivalent Sand: SE>> test (NF EN 933-8 standard). According to this standard a test is done consisting of flocculating the fines of the tested sand in a test tube and comparing the volume of the floes with the total volume of the sediments. The more flocculated fines there are, the more the sand equivalent value decreases and the more the sand is considered to be dirty. This test is nevertheless not sensitive enough to predict future problems in the concrete.
The second test is called the <<Methylene Blue Value: MBV>> (NF EN 933-9 standard). It comprises determining the maximum amount of methylene blue that would adsorb itself on the sand's impurities. Indeed, this dye has a very great affinity for clays, this affinity directly depends on the level of clay in the sand and the nature of the given clays. This test may be simply done by adding <<drop by drop >> (progressive additions of given volumes of the blue solution) the methylene blue in a solution containing sand and making stains with this solution at different time periods on filter paper. The measurement is done on the sand's 0-2 mm fraction, and the current standard imposes one measurement per week at the quarry on sand to be used in concrete applications.
An apparatus called the <<valise de Bleu >> is also known to measure the noxiousness of sand fines (see the NF 18-595 standard). This compact equipment comprises a beaker with an agitator, a peristaltic pump and a spectrophotometric measuring system comprising a measuring cell. The measurement (called turbidimetric) is done on a suspension of sand fines, that is, particles smaller than 80 μm. in size. The principle consists of progressively adding, in the beaker containing this suspension, a solution of methylene blue, then sending it to the cell (by the pump) to measure the absorption, at the 660 nm wave length (corresponding to maximum absorption of the methylene blue), of a light beam crossing the latter. The measurement is done while the solution of methylene blue is added to the suspension. At the beginning of the dosage, the methylene blue is consumed by the clays, therefore it does not reduce the light signal. Then, when the sand's maximum adsorption level is reached, residual blue remains in the solution and the intensity of the spectrophotometric signal drops. The blue value has been reached.
Nonetheless, the above-mentioned methods provide an insufficient control of sand cleanliness, notably due to possible variations of this cleanliness when working at the quarry. Furthermore they have long response times.
A real need to improve the control of sand cleanliness exists as well as to improve the cleanliness of the produced sands.