Johanna M. Mejía

Bio: Johanna M. Mejía is an academic researcher from University of Valle. The author has contributed to research in topics: Compressive strength & Fly ash. The author has an hindex of 7, co-authored 9 publications receiving 224 citations.

Rice husk ash as a source of silica in alkali-activated fly ash and granulated blast furnace slag systems

Abstract: This study assesses the viability of using an agro-industrial by-product, rice husk ash (RHA) from a Colombian rice company’s combustion facility, as a total replacement for the commercial sodium silicate ordinarily used in alkaliactivated binders. Fly ash (FA), granulated blast furnace slag (GBFS) and binary 50FA:50GBFS blended pastes were activated with a mix of sodium hydroxide and either sodium silicate or one of two types of RHA. The pastes were characterised for strength, mineralogy and microstructure. The findings showed that the agro-industrial by-product can be used to yield alkali-activated materials with 7-day mechanical strengths on the order of 42 MPa. The study confirmed that both amorphous silica and part of the crystalline silica present in RHA participate in the alkaline activation process, providing the alkalinity is suitably adjusted.

Decalcification of alkali-activated slag pastes. Effect of the chemical composition of the slag

Abstract: Portland cement decalcification and its effects on paste microstructure and mechanical strength have been widely studied. Decalcification in alkali activated slag (AAS) pastes is still not fully understood, however. The present study therefore explored the process in AAS cement pastes, accelerated by submerging specimens in concentrated ammonium nitrate solutions (NH4NO3) for 3–21 days to induce leaching. Two AAS pastes were prepared with slag of different origins (Spanish and Colombian) and chemical compositions. OPC pastes were used as a reference. The findings showed that decalcification has a more adverse impact on OPC than AAS pastes strength. BSEM/EDX and 29Si MAS NMR data nonetheless confirmed that Ca leaches out of C–A–S–H gels (formed in AAS pastes) to an extent that depends on the nature of the prime material. OPC pastes were shown to generate more silica gel with a very low Ca content (Q3 and Q4 units). Moreover, the higher the percentage of such units, the lower was mechanical strength. Decalcification in slag with lower MgO and higher Al2O3 contents leads to the formation of smaller amounts of silica gel. The resulting gel was more compact and stable due to more intense chain cross-linking a possible tri-dimensional structure.

A review on alkaline activation: new analytical perspectives

Abstract: For many years now the idea of including alkalis in a Portland cement matrix has been regarded as a daft or inexcusably erroneous proposition: despite its absurdity, that opinion has been widely accepted as a basic premise by the scientific and technical community working in the area of the chemistry of cement In 1957 Glukhovsky proposed a working hypothesis in which he established a close relationship between alkalis and cementitious materials That hypothesis has become consolidated and has served as a basis for developing a new type of binders, initially called “alkaline cements” The present paper reviews the most significant theoretical interpretations of the role played by alkalis in the formation of the “stony” structure of cement It ends with a broad overview of the versatility of this type of materials for industrial applications and a discussion of the possibility of building on the existing legislation to meet the need for the future regulation of alkaline cement and concrete manufacture