Freeze-Thaw Resistance of Concrete: Insight from Microstructural Properties

Composite cements offer low carbon alternatives to conventional CEM I. These binders also generally tend to perform better than CEM I in aggressive chemical environments. However, their freeze-thaw resistance, evident through surface scaling and internal damage is usually impaired. Postulated theories on freeze-thaw induced damage do not fully explain the origin of this weakness in composite cement concretes. This paper systematically presents the phase assemblage changes associated with the freeze-thaw of concrete specimen made from composite cements with and without limestone. The freeze-thaw test was performed on concrete according to CIF method based on CEN/TR 15177 and the corresponding cement pastes characterized by X-ray powder diffraction (XRD) and thermogravimetric analysis (TGA). In all investigated composite cements, portlandite was already depleted after the 7d capillary suction. The implications of this and other modified assemblages during the conditioning and the freeze-thaw test are consequently discussed

Circular economy, data analytics, and low carbon concreting: A case for managing recycled powder from end-of-life concrete

Reuse and recycling are vital practices in the circular economy. Despite progress in recycling aggregates from end-of-life concrete, the potential for reusing recycled concrete powder (RCP) as supplementary cementitious material (SCM) is now attracting research and commercial interest. Herein, a meta-analysis of concrete in which RCP was used as an SCM was conducted, and a multivariate regression model was developed for predicting strength from mix composition. The carbon footprint of alternative beneficiation strategies, including milling only, milling plus thermal treatment and milling plus CO2 injection, were quantified and used with the regression model to investigate RCP-containing concrete's embodied carbon (eCO2). The comparison was made with conventional SCMs and end-of-life scenarios of concrete from different cement types. The meta-analysis and regression model showed that 15% cement replacement by the non-beneficiated RCP caused a 40% reduction in 28-day compressive strength, and at 50% replacement, the strength reduction was 70%. Above 30% cement replacement, the RCP beneficiated through CO2 injection reduced the concrete's eCO2 per unit strength by 10–25%, while the thermally treated RCP had greater eCO2 than conventional SCMs. Thus, circularising end-of-life concrete does not guarantee low carbon concrete production. Instead, treating RCP with waste CO2 leads to a carbon-negative SCM, presenting the most promising route for low-carbon concrete in the circular economy

Influence of limestone addition on sodium sulphate activated blast furnace slag cements

The effect of limestone replacement in the reaction and phase assemblage evolution of two sodium sulfate activated slag cements was investigated. The slag composition and its reactivity influenced the reaction kinetics of these materials. Paste with limestone addition exhibited an acceleration in reaction kinetics, particularly for slowly reacting slags. Ettringite and an aluminium substituted calcium silicate hydrate were identified as main reaction products in these cements, independently of the slag type or limestone replacement level. No significant changes in phase assemblages were observed with limestone addition for over 365 days of testing; however, these composite cements exhibited an increased compressive strength, consistent with a refined pore structure. These results indicate that it is possible to partially replace slag by limestone in sulfate activated slag cements without changing the type of reaction products forming in these systems, while also increasing compressive strength and achieving a similar refined pore structure

Early age reaction of slag in composite cement: Impact of sulphates and calcite

Ground granulated blast furnace slag (GGBS) is an important supplementary cementitious material (SCM) for producing low carbon and durable concrete. There are however questions around the early age reactivity of GGBS and the factors that influence this. To elucidate the fundamental mechanisms controlling the early age reactivity and particularly the influence of anionic species, simplified systems comprising GGBS and calcium hydroxide were examined in the presence of limestone, anhydrite, or both at 4:1 SCM-to-activator ratio. Limestone and GGBS were considered as SCMs, but calcium hydroxide and anhydrite were considered as activators. Multiple techniques, including isothermal calorimetry, thermogravimetry, X-ray diffraction, electron microscopy, mass balance calculation and mercury intrusion porosimetry were used to study hydration and microstructure. The results show that GGBS hydration commences immediately in the alkaline media provided by calcium hydroxide. Sulphates and limestone influence hydration through reactions with aluminates to form ettringite and carboaluminates, but prevalence of macro-capillary pores in sulphate containing binders sustains diffusion-controlled hydration. Consequently, optimization of the alumina to sulphate and carbonate ratios is essential for exploiting the pore solution and space filling effects in composite cements

Toward performance improvement of supersulfated cement by nano silica: Asynchronous regulation on the hydration kinetics of silicate and aluminate

Supersulfated cement (SSC) is a traditional low-carbon cement, but its slow hydration and strength development has limited its practical applications. Nano silica (NS) was used to activate the hydration of SSC by taking advantage of its ability to regulate silicate and aluminate reactions. The mechanical performance of various mixes was determined, as a function of sulfation degree and NS addition, as pore structure, phase assemblage, hydration degree, and microstructure. Results showed that NS improves the hydration degree of slag, densifies the microstructure, and significantly increases both early- and late-age compressive strength. The enhancement was attributed to its effects on the hydration of slag in SSC: delaying ettringite formation, but promoting C-(A)-S-H precipitation, reducing microporosity. This study reveals the critical role of the regulation of hydration kinetics of silicate and aluminate in controlling the performance of SSC as NS does