Nishant Garg

The reaction kinetics and structure of alkali-activated binders are known to be affected by their calcium (Ca) contents. However, the intrinsic difference between high Ca and low Ca alkali-activated systems, especially at the atomic scale, has not been fully investigated. Here, we compare local atomic ordering and reaction kinetics of blast furnace slag and metakaolin as representatives of high and low Ca systems, respectively. In situ X-ray pair distribution function (PDF) analysis and isothermal calorimetry were utilized in parallel to investigate the evolution of local structure in these systems. Isothermal calorimetry results showed contrasting reaction kinetics of high Ca (alkali-activated slag) and low Ca (alkali-activated metakaolin) systems, where the latter shows no induction period. The evolution of specific PDF peaks is related to the main binder gels in each respective system and can be used to track their reaction kinetics, exhibiting excellent correlation with the cumulative heat data ( R ² > 0.97). Finally, and most importantly, the PDF results revealed that the atomic ordering of N-A-S-(H) gel is limited up to ∼10 Å, whereas C-(N)-A-S-H gel grew up to ∼40 Å, which suggests that there is an intrinsic difference in the formation mechanism and local structures of dominant gels formed in high and low Ca systems.

Belite‐rich cements are a lower‐energy and lower‐CO ₂ alternative to Portland cements, but the low hydraulic activity of belite remains a major limitation. This study explores the impact of aluminum and sulfur co‐doping on the structure and hydration of synthetic dicalcium silicate (C ₂ S). Al/S‐doped C ₂ S powders were synthesized and characterized using X‐ray diffraction (XRD), Raman spectroscopy, and solid‐state nuclear magnetic resonance (NMR). XRD and Raman confirmed stabilization of a disordered form of β‐C ₂ S. Specifically, Rietveld refinement showed unit cell expansion (from 344.966 to 346.084 Å ³ ), and NMR showed structural distortions in the doped C ₂ S. Consequently, isothermal calorimetry and thermogravimetry demonstrated a significant reactivity enhancement, with the 3% Al/S‐doped C ₂ S sample achieving ∼4× the cumulative heat @ 72 h versus control (∼82 J/g vs. ∼21 J/g) and ∼2.5× increase in Ca(OH) ₂ formation (7.4% vs. 3.0%). These results introduce Al/S co‐doping as a potential pathway toward enhancing C ₂ S reactivity in belite‐rich sustainable cements.

This study investigated the mechanical strength and freeze-thaw durability of carbonate-based aggregate quarry by-products (QBs) stabilized with cement alone and in combination with calcined clay (metakaolin, MK) for potential use in pavement foundation layers. The primary objective was to evaluate the chemical and physical characteristics of dolomitic and limestone QBs and determine how these factors influence strength development and durability under different stabilization conditions. Four QBs with varying carbonate compositions, ranging from highly dolomitic aggregates to nearly pure limestone, were examined. QBs were stabilized with 3% cement and tested under short- and long-term curing conditions. Short-term testing indicated the curing period was insufficient for the mineralogical properties of carbonate aggregates to fully manifest; instead, performance was governed primarily by physical characteristics, particularly particle packing. In contrast, long-term testing showed the effects of carbonate mineralogy became more pronounced, as the mineral components began contributing more noticeably to overall performance. Dolomitic QBs demonstrated superior strength and stiffness development along with higher retention of these properties after 10 freeze-thaw cycles due to the formation of hydrotalcite, a magnesium-aluminum layered double hydroxide phase. When cement was partially replaced with MK at the optimum MK-to-cement ratio of 1:4, both early-age and long-term performance improved relative to cement-only specimens. Chemical analyses showed that MK participated in pozzolanic and carboaluminate reactions, producing additional calcium-alumino-silicate-hydrate (C-A-S-H) gels and carboaluminate phases. Since MK rapidly consumed available calcium hydroxide to form C-A-S-H gel and carboaluminate phases, hydrotalcite formation was suppressed. This mechanism resulted in comparable long-term performance for all QBs, irrespective of carbonate types. The findings demonstrate that carbonate-based QBs can be transformed into high-performing, durable, and sustainable pavement foundation materials when their physical and mineralogical properties are considered properly and supplementary cementitious materials such as MK are incorporated effectively.

Assessing water absorption, or 'sorptivity,' in cement-based materials is a crucial method for evaluating durability. The ASTM C1585 standard provides a straightforward approach for measuring sorptivity, yet it is often labor-intensive and time-consuming. This study presents two innovative strategies to accelerate and automate sorptivity measurements through computer vision. The first method involves a droplet test, demonstrating that the initial dynamics of a water droplet's contact angle, characterized by CNNs, strongly predict the initial sorptivity of paste samples. By analyzing data from 63 different paste systems, this method estimates the 6-hour initial sorptivity in mere seconds. The second method utilizes a vision-based algorithm to monitor the advancing waterfront in a sample, leveraging the wetted area ratio to predict sorptivity and streamline the measurement process. By training the algorithm on a dataset of over 6,000 images and 1,400 data points, the system achieves real-time predictions of initial and secondary sorptivities with an R² exceeding 0.9, closely aligning with ASTM standards. These cost-effective solutions, using cameras priced at around $30, provide laboratories worldwide with an accessible and automated method to enhance sorptivity testing, especially for assessing the durability of new low-carbon cementitious materials.