Showing papers by "Francisca Puertas published in 2022"

Influence of Accelerating Admixtures on the Reactivity of Synthetic Aluminosilicate Glasses

Abstract: This research aims at gaining a further understanding of the impact of accelerating admixtures on the reactivity of supplementary cementitious materials (SCMs), which are widely used as a clinker replacement in blended cements. This was done on synthetic glasses with controlled composition and structure that mimic two types of real SCMs (slag and calcium-rich fly ash). The effects of DEIPA, TIPA, NaSCN and Na2S2O3 on the glass dissolution, hydration kinetics and reaction products were investigated. The obtained results concluded that the pH of the NaOH solution and the composition of the synthetic glass play a key role on the effect of the admixtures. In 0.1 M NaOH (pH = 13.0), all the studied admixtures inhibited the dissolution of slag-like glasses while they enhanced the dissolution of Ca-rich fly ash-like glasses, being Na2S2O3 the admixture that led to the highest increase of the dissolution rate of the Ca-rich fly ash-type glasses. In 1 M NaOH solutions (pH = 13.8), only the alkali admixtures (NaSCN and Na2S2O3) enhanced the degree of reaction of both glasses. In slag-type glasses pastes mixed with 1 M NaOH, the addition of 2% Na2S2O3 induced the highest increase of their reactivity as inferred by the total heat release and the amount of bound water. This is related to the formation of a high amount of S(II)-AFm, in addition to C-A-S-H, that would increase the aluminium undersaturation of the pore solution and consequently the further dissolution of the glass.

Hybrid Cements: Mechanical Properties, Microstructure and Radiological Behavior

Abstract: The use of more eco-efficient cements in concretes is one of the keys to ensuring construction industry sustainability. Such eco-efficient binders often contain large but variable proportions of industrial waste or by-products in their composition, many of which may be naturally occurring radioactive materials (NORMs). This study explored the application of a new gamma spectrometric method for measuring radionuclide activity in hybrid alkali-activated cements from solid 5 cm cubic specimens rather than powder samples. The research involved assessing the effect of significant variables such as the nature of the alkaline activator, reaction time and curing conditions to relate the microstructures identified to the radiological behavior observed. The findings showed that varying the inputs generated pastes with similar reaction products (C-S-H, C-A-S-H and (N,C)-A-S-H) but different microstructures. The new gamma spectrometric method for measuring radioactivity in solid 5 cm cubic specimens in alkaline pastes was found to be valid. The variables involved in hybrid cement activation were shown to have no impact on specimen radioactive content. The powder samples, however, emanated 222Rn (a descendent of 226Ra), possibly due to the deformation taking place in fly ash structure during alkaline activation. Further research would be required to explain that finding.

Role of calcium hydroxide in the alkaline activation of coal fly ash

Abstract: Aiming at reducing the environmental effects generated by the manufacturing of Portland cement, the construction sector perceives significant potential in alkali-activated cements. The use of calcium hydroxide becomes an alternative to modify the properties of these types of cements in a controlled manner. This research work examines the influence of calcium hydroxide at different concentrations on the alkaline activation of a coal fly ash with sodium hydroxide. The cements pastes were evaluated according to their compressive strength at 3 and 28 days, and once the curing process had finished at the initial conditions (3 days, >95% RH at 45 °C), they were characterized using the XRD, FTIR, TG, MAS NMR and BSEM/EDX techniques. The presence of calcium hydroxide increased the compressive strength of the pastes at 28 days from 4.5 MPa to 25.65 MPa. Likewise, the maximum resistance growth rate was 41.03 %. The highest percentages of calcium hydroxide fixation were 74.82%, which modified the type and quantity of cementing gels. The calcium hydroxide forms a layer on the fly ash particles and these, when in contact with the activator, produce absorption and desorption processes of network modifying elements (Na and Ca) and network formers (Al and Si). Finally, the confluence of the chemical constituents (Na, Ca, Al and Si) initiates the formation of cementing gels type C−(A)−S–H and (N,C)−A−S–H.