Resumo de A Study On Mortars Produced With Cdw Recycled Aggregates Containing Cypsum Plaster As Impurity

A study on mortars produced with CDW recycled aggregates containing cypsum plaster as impurity.This is an abstract of a Master of Science project carried out atEPUSP, Universidade de São Paulo, Brazil, aimed at investigating theeffects of the use of recycled aggregates contaminated by smallcontents of gypsum plaster on masonry mortars. In the presence ofmoisture and by the action of gypsum minerals or other more complexones that may occur, there is the risk of the formation of cracks,efflorescence, and progressive deterioration of the masonry walls. Anexperimental program was thus developed for the rapid assessment ofthese processes by assessing the chemical, physical and mechanicalproperties of twenty mortar mixes. Local materials used for masonry andconcrete in Socorro-SP, Brazil, were selected, as in that locationthere exists a recycling unit producing recycled sand for mortars witha high content of fines. Four recycled aggregates of very fine grainsize and similar to each other were artificially produced by controlledconstruction, demolition and recycling processes. Thus a recycled,non-contaminated aggregate (A1, with SO3 content equal to 0,2 %) andthree other identical ones were studied, with expected progressivecontamination (A2, A3 and A4, that result with a total SO3 contentabout 0,5 % to 0,6 %). Four groups of mortars were thus produced withvarying cement content (200, 150 and 100 kg/m3), cement type (BrazilianCP II and CP III) and also with and without the use of a comercialhydrated lime, Brazilian type CH III (of a high carbonate fillercontent). The anhydrous recycled aggregate content in the mortarsvaried between 1300 and 1200 kg/m3. In each group a blended mortar wasalso produced, containing very pure quartzous sand, of grain size andmix proportions closer to standard masonry mortars. The approachfollowed in the experimental program was accelerated aging test ofmortars, starting at 63 days of age and at 7 day cycles in the wetchamber, alternated by 7 days in the ventilated oven at 40 oC. Thiscycling was adopted after critical bibliographical research and under aholistic process of deterioration set up by Collepardi (2003), also forgypsum-contaminated aggregates. From 63 days of age, the followingproperties were regularly characterized for the mortars: flexuralmodulus (tensile strength in bending), compressive strength, and staticand dynamic elasticity modulus. During cycling, complementary testswere done of thermogravimetrical analysis and quantitative chemicalanalysis of total soluble sulphate in mortars. A number of the mortartest specimens was left in the dry chamber and tested at 175 days,including tendency to efflorescence. In between 63 and 175 days, 8weathering cycles were done, and it was concluded that the mainmechanical property affected during cycling was tensile strength inbending (indirect tensile strength) of mortars, and this occurred from91 days, in the four groups of mortars containing recycled aggregates,and in between 63 and 91 days a significant evolution was measured ofcarbonation by thermogravimetry. The groups of mixed mortars withcement and hydrated lime showed better performance in this study thanthe plain cement and fine sand mortars, also as regards efflorescence.The results here obtained are to be seen as restricted to this casestudy, yet they allow us to conclude that the use of recycledaggregates containing even low levels of gypsum plaster as impurity inthe production of mortars was sufficient to produce significantalterations in the mechanical performance, as well as the degree ofsoluble sulphate of mortars, that got altered in a way coherent withthe cycling process, and it is possible to interpret this based on morerecent theories by experts in the subject. Last, this work confirmedthat it is advisable to look for a maximum soluble sulphate limitranging from 0,1 % to 0,2 % by weight of recycled aggregates formortars. Such levels was set up by a German institution specializing inmasonry durability. It was also concluded that the requirements of DIN4226-100 (< 600 mg/L of SO4) can be used as appropriate in the rangementioned.

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