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5,494,513 56 Mixtures containing 30-80 wt. % non-treated zeolites, 20-70wt.% Portlandcementand0—5wt.% limerequire large amounts of water to satisfy workability requirements. The water/solid ratio can be greater than 0.7. High water/ solid ratio gives the products a lightweight characteristic. Zeolites have high pozzolanic activity and react with cal cium hydroxide produced during cement hydration or with added lime to contribute to strength. The density and com pressivestrengthofzeolite~basedlightweightconcreteprod uctscontaining50-80wt.% naturalzeolite,20-50wt.% Portland cement and 0—5% lime are in the range of 1000-1300kg/m3and10-30MParespectivelydepending on zeolite source after 3-6 hours autoclave curing at l70°-l80° C., or after 8-14 hours moist-curing at 75°~85° C.,orafter28daysmoist-curingat23°C. and 20wt. % Portland cement after 3-6 hours autoclave curingatl70°-l80°C.,orafter8-14hoursmoist-curingat 75°—85° C., or after 28 days moist-curing at 23° C. Zeolite plays two important roles in the production of aerated concrete products of the invention: air generation and strengthcontribution. Itisapparentthattheparticlesize of zeolite affects the density and strength of zeolite-based lightweight concrete. The water requirement increases lin early with the increase of ?ne particles of zeolite due to greater surface area. Satisfactory values of density and strength can be obtained for aerated concrete containing zeolitehavingabout40-60wt.% particleslessthan0.175 mm indiameter,andatleast85wt.% havingparticlesize less than about 0.295 mm. Optimum ?neness of zeolite for 15 non-aeratedlightweightconcreteisintherangeof75-85wt. % (atleast75wt.%)particlespassing0.175mm sieve.The ?nescontributemostlytothestrengthbecauseofpozzolanic activity.Thelargeparticles,0.2-1.0mm contributetoair generation. Strength decreases when the amount of ?ne Heattreatmentintherangeof400°-600°C.willactivate the surface of zeolites and result in high surface energy. Heat-treatedzeolitesreleaselargeamountsofairfromtheir surfaceafterre-wettingaswaterisreadsorbedonthezeolite surface.Heatgenerationduringairdesorptionandwater20 particlesexceedsacertainthresholdvalueasthereisa adsorption increases air bubble formation in the mixture. Heat-treated zeolites in this temperature range can be used as both air-generation agents and pozzolanic binding mate rials to produce aerated-lightweight concrete products. The densityandstrengthoftheproductscontaining50-80wt.% 25 cedures for masonry products, steam-cured or autoclave ofheat-treatedzeolite,20-50wt.% Portlandcementand 05% limeareintherange400-1000kg/m3and2~10MPa respectively, depending on zeolite type, after 3-6 hours autoclave curing at l70°—l80° C. Heat treatment at temperatures greater than 800° C. will decompose the structure of zeolites. Zeolites lose their air-generation characteristics after this heat-treatment. A mixturecontainingsuchzeoliteshassatisfactoryworkability with lower water/solid ratio. The hardness of zeolite par ticlesafterhightemperaturecalcinationappearstoincrease. Zeolite-based lightweight concrete products containing 50-80 wt. % of zeolites treated at high temperature (800°-1000°C.),20-50wt.% Portlandcementand0-5wt. % limehavecompressivestrengthgreaterthan20MPa after 3-6hoursautoclavecuringatl70°-l80°C.,orafter8-14 40 ing.Lowtemperaturecuredproductsshouldbemoist-cured hours moist-curing at 75°-85° C., or after 28 days moist curing at 23° C. The effect of calcination on the characteristics of zeolites at20°- 38° C. for28 days toachieve 50-80% oftheultimate strength. The ultimate strength for such concrete can be developed after another 3-6 months’ room temperature curing. can be demonstrated by thermogravimetric analysis (TGA, FIG.1).Theweightlossofzeolite,indicatingthedegreeof 45 dehydration, is about 17% (83% left) at 500° C. and about 3% (97%left)at800°C.Itisbelievedthatactivatedsurface area increases correspondingly to the release of adsorbed water from the surface and interlayers of the zeolite. The higherweightlossat500°C.indicatesthatthezeolitemay 50 thehydrationsystem.Strengththereforeisnotproportional have been activated to a higher level of surface energy. The ?rstpeakinTGA derivativecurveofthezeoliteoccursat around 80° C. This is attributed to the loss of capillary water. The second and thirdpeaks atabout 180° and 550° C. may beduetothelossofinterlayerwater.Thefourthpeakat650° 55 requiredfortheconcretemixtureincreaseswithadecrease C. and further peaks after 800° C. are believed to result from thedehydrationofthezeolitestructuralwater.Calcinationof zeolite at the temperatures lower than 600° C. does not appear to change its mineral composition as indicated by X-raydiffraction(XRD).Zeolitecalcinedat550°C.hasthe60 sameXRD spectrumasthenaturalzeolite.Themineral decomposes when calcined at 800° C. as indicated by the disappearance of the characteristic peaks of clinoptilolite. ZeolitecanreactwithOPC (OrdinaryPortlandCement)and limeandproducehighstrengthproductsafterautoclaving.A 65 weak clinoptilolitepeak ornone atallisfound inthe sample ofzeolite-basedaeratedconcretecontaining80wt.% zeolite of Portland cement content in the concrete and correspond ingly the dry density and compressive strength of the concrete products decreases. For example, a typical dry density value is at 1000 kg/m3 for the concrete containing 50 wt.% zeoliteand50wt.% Portlandcement.Itisreduced about 100 kg/m3 when Zeolite content increases to about 80 wt.% andPortlandcementcontentdecreasestoabout20wt. %. The effects of lime content on dry density and compres sivestrengthofaeratedconcretedependoneffectivesilica (SiO2) content. An optimum lime content will provide just su?icient calcium to react with silica to form calcium silicate higher water requirement for workability. The mixing and curing procedures for non- or low-aerated zeolite-based concrete products using natural or calcined zeolite (above 800° C.) are the same as conventional pro cured.Specialmixingandpre-storageproceduresshouldbe employedinproductionofzeolite-basedaeratedconcrete products. The mixing time should be sufficiently short i.e. within 2-5 minutes as the air bubbles are mostly generated in about lO-25 minutes. The mixture should be pre-stored for30-60minutesforair-generationandvolumeexpansion oftheconcreteuntilthemixturedevelopssu?icienthardness to be shaped. The mixture can then be autoclaved. Auto claved products should be cured at l70°—l80° C. for 3-6 hours to obtain ultimate strength. High temperature cured products should be moist-cured at 75°—85° C. for 8-14 hours toobtainsufficientdemouldingstrength,about50-80%of theultimatestrength.Theultimatestrengthforsuchconcrete can be developed after 3-6 months room temperature cur Cost considerations would suggest the use of a low content of Portland cement. A certain minimum amount of OPC intheproductisnecessarytoachievesu?icientcom pressivestrength.Strengthdevelopmentofautoclavedcon crete products depends primarily on the CaO/SiO2 ratio of to the amount of Portland cement used. Test results indicate thattheoptimum contentofPortlandcementinzeolite based concrete products should be 20-50%. The optimum cement content will change with zeolite source. The waterPDF Image | ZEOLITE-BASED LIGHTWEIGHT CONCRETE PRODUCTS
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