Uticaj rastvora amonijum-nitrata i natrijum-sulfata na mehanička svojstva i strukturu geopolimera na bazi elektrofilterskog pepela termoelektrana

Abstract

Geopolimeri na bazi tri različita uzorka elektrofilterskog pepela izloženi su dejstvu koncentrovanog rastvora NH4NO3 (480 g/dm3) i rastvora Na2SO4 (50 g/dm3) u periodu od 18 meseci. Promene mehaničkih svojstava geopolimera ispitivane su nakon 5 termina: 28, 90, 180, 365 i 540 dana. Promene u strukturi geopolimera ispitivane su metodama rendgenske strukturne analize, elektronske mikroskopije, infracrvene spektroskopije i nuklearne magnetne rezonance čvrstog stanja. Sastav sveže pripremljenih rastvora NH4NO3 i Na2SO4 i sastav rastvora nakon izlaganja uzoraka geopolimera njihovom dejstvu u određenom vremenskom periodu analiziran je optičkim emisionim spektrometrom sa indukovanom spregnutom plazmom. Utvrđeno je da je uticaj rastvora NH4NO3 na mehanička svojstva geopolimera pripremljenih od tri različita uzorka elektrofilterskog pepela bio je vrlo sličan. Do najvećeg smanjenja čvrstoće na pritisak (14‒23 %) došlo nakon prvih 28 dana ispitivanja, nakon čega je uočena stagnacija ili blago poboljšanje čvrstoće maltera geopolimera. Analize rastvora NH4NO3 pokazale su da je tokom izlaganja geopolimera dejstvu rastvora došlo do izluživanja alkalnih (Na, K), zemnoalkalnih (Ca, Mg) elemenata i sumpora iz strukture geopolimera i porasta pH vrednosti rastvora NH4NO3 na oko 8. Na osnovu detaljne karakterizacije strukture geopolimera, zaključeno je da je izlaganje uzoraka geopolimera dejstvu rastvora NH4NO3 dovelo do raskidanja Si‒O‒Al veza u strukturi alumosilikatnog gela geopolimera. Popunjavanje defekata u strukturi, nastalih raskidanjem Si‒O‒Al veza, silicijumom i formiranje strukture bogatije silicijumom, zaslužni su za uočenu stagnaciju ili blago povećanje čvrstoće geopolimera tokom testiranja u dužem vremenskom periodu. Izlaganje geopolimera dejstvu rastvora Na2SO4 dovelo je do malog smanjenja čvrstoće na pritisak geopolimera (oko 10 %). Do najvećeg smanjenja čvrstoće na pritisak poroznijih uzoraka geopolimera došlo je već posle prvih 28 dana ispitivanja, dok je u daljem toku testiranja uočeno povećanje čvrstoće. U slučaju maltera geopolimera niže poroznosti do značajnijeg smanjenja čvrstoće došlo je tek posle 365 dana ispitivanja. Ispitivanje promena u sastavu rastvora Na2SO4 pokazale su da je tokom ispitivanja uzoraka geopolimera došlo do difuzije sulfatnih jona u strukturu geopolimera i izluživanja natrijuma, kalijuma i silicijuma iz strukture u rastvor, kao i do porasta pH vrednosti rastvora na oko 12. Na osnovu rezultata ispitivanja strukture geopolimera metodama infracrvene spektroskopije i nuklearne magnetne rezonance utvrđeno je da je smanjenje čvrstoće geopolimera tokom izlaganja dejstvu rastvora Na2SO4 posledica formiranja defekata u strukturi nastalih raskidanjem Si‒O‒Si veza i izluživanjem Si.

Geopolymers based on three different fly ash samples were exposed to concentrated (480 g/dm3) NH4NO3 solution and Na2SO4 solution (50 g/dm3) over a period of 540 days. Mechanical properties of the geopolymer samples were tested after five terms: 28, 90, 180, 365 and 540 days. Changes in the geopolymer structure were investigated using X-ray powder diffraction analysis, scanning electron microscopy, infrared spectrometry and nuclear magnetic resonance (27Al MAS NMR and 29Si MAS NMR). Compositions of NH4NO3 and Na2SO4 solutions were analyzed by optical emission spectroscopy. It was found that the effects of the concentrated NH4NO3 solution on the mechanical properties of the geopolymer samples based on three different fly ash samples were similar. The greatest decrease in compressive strength of the geopolymer samples was observed after the first 28 days of testing (14‒23 %). Analyses of the NH4NO3 solution showed that alkali (Na, K), alkaline earth (Ca, Mg) elements and sulfur were leached from the geopolymer structure, while the pH value of the solution increased to 8. Based on detailed structural characterization of geopolymer samples, it was concluded that exposure of the geopolymer samples to the concentrated NH4NO3 solution caused breaking of Si‒O‒Al bonds in aluminosilicate geopolymer gel. Structural defects formed by breaking of Si‒O‒Al bonds were reoccupied by silicon, forming more siliceous structure, which resulted in the subsequent fluctuation in compressive strength. Treatment with the Na2SO4 solution caused small decrease in compressive strength of the geopolymer samples (about 10 %). The strength of the more porous geopolymer samples decreased after the first 28 days of testing, while subsequent exposure to the sulfate solution caused an increase in strength of the samples. On the other hand, decline in strength of the less porous geopolymer samples was observed after 365 days of testing. Analyses of the sulfate solution showed that, in addition to diffusion of sulfate ions in the geopolymer structure and leaching of Na and K from the structure, treatment of the geopolymer samples with the Na2SO4 solution caused leaching of Si. The pH value of the sulfate solution reached the value of about 12 already after 28 days of testing. Investigation of the geopolymers structure by means of infrared spectroscopy and 29Si MAS NMR showed that exposure of the geopolymer samples to the Na2SO4 solution caused breaking of Si‒O‒Si bonds in aluminosilicate geopolymer gel structure and leaching of Si. Breaking of Si‒O‒Si bonds and formation of structural defects led to the observed strength decrease of the geopolymer samples.