Effect of Polycarboxylate–Ether Admixtures on Calcium Aluminate Cement Pastes. Part 2: Hydration Studies

Durability, thermo-physical characteristics, and mechanical strength prediction of green Portland cement matrix incorporating recycled soda-lime glass and lead glass

The current study is concerned with acid and calcination durability, thermal and thermo-physical properties, and mechanical strength prediction of mortars containing soda-lime glass (PVS) and lead glass (PVP). It demonstrates that up to 30% of PVP (PVP30) and PVS (PVS30) enhancements lessen the consequences of acid attack. In both cases, 20% additions show the best acid resistance at 2 days, but mortars with 10% addition resist better at 28 days. Furthermore, sulfuric acid damages the formed mortars more aggressively than hydrochloric acid. According to the thermal study, the loss of mass owing to calcination is reduced with increasing glass addition. It falls from 22% to -19.5% for PVS30 and -18% for PVP30. The flexural strengths of the calcined mortars significantly drop after firing, although the compressive strengths are higher at 400 °C than at ambient temperature. However, at 600 °C, a 20% glass addition retains the mortar's fire resistance. However, around 800 °C, all formulations mechanically deteriorate. PVP20 has the best fire behavior with relative variations of 48.6% at 400 °C, 18.5% at 600 °C, and -45.8% at 800 °C, while PVS20 has 45.4% at 400 °C, 24.8% at 600 °C, and -33.1% at 800 °C. The hydrates found in the calcined mortars emphasize autoclave reactions that improve mechanical characteristics between 400 and 600 °C, whereas at 800 °C, advanced dehydration of the matrix results in a generalized decrease in resistance. Furthermore, the gradual inclusion of glass reduces the thermal conductivity of mortars correspondingly. The inclusion of 30% PVS results in a reduction of -38.99%, while 30% PVP results in a reduction of -49.95%. The other thermophysical parameters are calculated as a function of these values. The models developed in the area of mechanical strength prediction using the Multilayer Perceptron (MLP) method of Artificial Neural Network (ANN) allow for R2 correlation coefficients of 0.86-0.92 during training with the database and 0.77 to 0.90 during validation, with values of MAE ≤ 2.12 and RMSE ≤ 2.67 in all situations.

New Approach to Determine the Activity Concentration Index in Cements, Fly Ashes, and Slags on the Basis of Their Chemical Composition

The manufacture of Portland cement entails high energy and environmental costs, and various solutions have been implemented in recent years to mitigate this negative impact. These solutions include improvements in the manufacture of cement clinker or the use of supplementary cementitious materials (SCMs), such as fly ash (FA) or slag as a replacement for a portion of the clinker in cement. The incorporation of these SCMs in cement may increase its radiological content as they are naturally occurring radioactive materials (NORMs). The Activity Concentration Index (ACI) is a screening tool established in the European EURATOM Directive 2013/59 to determine the radiation protection suitability of a final construction material. The ACI is determined by the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K, usually determined by gamma spectrometry. The methodology of gamma spectrometry is accurate and appropriate, but this technique is not available in all laboratories. For this reason, and taking into account that there is a relationship between the chemical and radiological composition of these building materials, a new approach is proposed to determine the radiological content of these materials from a chemical analysis such as X-ray fluorescence (XRF). In this paper, principal component analysis (PCA) is used to establish the relationships between the chemical composition and radiological content of cements, FAs, and slags of different natures. Through PCA it was possible to group the cements based on two variables: CaO content and Fe₂O_3-Al₂O_3-TiO₂ content. A lower correlation was observed for the FAs and slags, as the sample scores were centered around the origin of the coordinates and showed greater dispersion than the cements. The clusters obtained in the HJ-Biplots allowed the determination, using multiple regression, of models relating the activity concentration of ²²⁶Ra, ²³²Th (²¹²Pb), and ⁴⁰K to the oxide percentages obtained for the three matrices studied. The models were validated using five cements, one FA and one slag with relative percentage deviations (RSD(%)) equal to or less than 30% for 89% of the activity concentrations and 100% of the ACI determined.

Utilizing optical spectroscopy and 2',7'-difluorofluorescein to characterize the early stages of cement hydration

The increasingly sophisticated nature of modern, more environmentally friendly cementitious binders requires a better understanding and control particularly of the complex, dynamic processes involved in the early phase of cement hydration. In-situ monitoring of properties of a constantly changing system over a defined period of time calls for simple, sensitive, fast, and preferably also non-invasive methods like optical spectroscopy. Here, we exploit the time-dependent changes in the absorbance and fluorescence features of the negatively charged optical probe 2',7'-difluorofluorescein (DFFL) for the study of the hydration processes in pastes of white cement (WC), cubic tricalcium aluminate (C₃A), and tricalcium silicate (C₃S), the main phases of cement, and in pastes of quartz (Q) over 24 h after addition of the dye solution. For comparison, also conventional techniques like isothermal heat flow calorimetry were applied. Based upon the time-dependent changes in the spectroscopic properties of DFFL, that seem to originate mainly from dye aggregation and dye-surface interactions and considerably vary between the different pastes, molecular pictures of the hydration processes in the cement pastes are derived. Our results clearly demonstrate the potential of optical spectroscopy, i.e., diffuse reflectance, steady state and time-resolved fluorometry in conjunction with suitable optical reporters, to probe specific hydration processes and to contribute to a better understanding of the early hydration processes of cement at the molecular scale.