Zakasnela tvorba etringita (Delayed Ettringite Formation - DEF) je kemična reakcija z dokazano škodljivim vplivom na hidratizirani beton. Kristali ki pri tej kemični reakciji nastanejo lahko povzročijo nastanek razpok. Pozitivne spremembe prostornine, ki je posledica nastalih produktov prav tako poveča pritiske na stene razpok in poroznih mestih. Zaradi rasti kristalov etringita ima lahko mlad hidratizirani beton višje trdnosti, kar pa navadno pomeni spremljajoč pojav nastanek razpok in razpad betona. V disertaciji smo raziskali možnosti kako izkoristiti pozitivne volumensko spremembo, ki je posledica DEF za izboljšanje trdnostnih lastnosti drobno zrnatega betona.. Ugotovili smo, da z dodajanjem aeranta in zelo kratkim časom vibriranja betona dosežemo takšno strukturo betona, kjer zračni mehurčki v hidratiziranem betonu predstavljajo nukleacijska mesta. Zakasnela tvorba etringita je z razpršenimi zračnimi mehurčki, ki so nastali od aeranta kontrolirana. Rast kristalov etringita ne povzroči hitrega povečanja trdnosti in razapada betona temveč v fazi stabilizacije strukture doprinese 6% povečanje tlačne trdnosti ob minimalnem znižanju gostote hidratiziranega drobno zrnatega betona.Delayed ettringite formation (DEF) is a chemical reaction with proven damaging effects on hydrated concrete. Ettringite crystals can cause cracks and their widening due to pressure on crack walls caused by the positive volume difference in the reaction. Concrete may show improvements in strength at early ages but further growth of cracks causes widening and spreading of these cracks through the concrete structure. In this study, a potential to utilise the positive volume difference in DEF in order to improve mechanical properties of hydrated fine grained concrete is investigated. Finely dispersed crystallization nuclei achieved by adding air-entraining agent (AEA) and short vibration of specimens is presented as the main prerequisite for such improvements. The study presents the method and mechanism for obtaining the required crystallisation nuclei. Controlling long-term DEF by providing AEA-induced crystallisation nuclei prevented excessive and rapid initial strength improvements, and resulted in a 6% increase of compressive strength of fine grained concrete with only marginally lower density
