A Review of the Mechanical Properties and Durability of Ecological Concretes in a Cold Climate in Comparison to Standard Ordinary Portland Cement-Based Concrete

Most of the currently used concretes are based on ordinary Portland cement (OPC) which results in a high carbon dioxide footprint and thus has a negative environmental impact. Replacing OPCs, partially or fully by ecological binders, i.e., supplementary cementitious materials (SCMs) or alternative binders, aims to decrease the carbon dioxide footprint. Both solutions introduced a number of technological problems, including their performance, when exposed to low, subfreezing temperatures during casting operations and the hardening stage. This review indicates that the present knowledge enables the production of OPC-based concretes at temperatures as low as −10 °C, without the need of any additional measures such as, e.g., heating. Conversely, composite cements containing SCMs or alkali-activated binders (AACs) showed mixed performances, ranging from inferior to superior in comparison with OPC. Most concretes based on composite cements require pre/post heat curing or only a short exposure to sub-zero temperatures. At the same time, certain alkali-activated systems performed very well even at −20 °C without the need for additional curing. Chemical admixtures developed for OPC do not always perform well in other binder systems. This review showed that there is only a limited knowledge on how chemical admixtures work in ecological concretes at low temperatures and how to accelerate the hydration rate of composite cements containing high amounts of SCMs or AACs, when these are cured at subfreezing temperatures.

Waterglass impregnation of municipal solid waste incineration bottom ash applied as sand replacement in mortars

Incineration has been recognized as one of the most applied strategies for the processing of the municipal solid waste (MSW). The primary output from the incineration of the MSW is Bottom Ash (BA), whose particles are highly porous and contaminated with heavy metals, chlorides, and sulphates, limiting its application in concrete. For improving the applicability as aggregates, many porous materials are impregnated with a pozzolanic solution, easing their use as building materials. However, this treatment has never been applied to by-products like BA, and therefore the influence of a coating on the leaching behaviour of the by-products has never been investigated. This study analyses the effect of an impregnation treatment based on different sodium silicate amounts on BA particles between 1 and 2 mm. The application of the coating lowers the pore volume of BA by 2.5 times, allowing a sand replacement up to 100% and improving the rheological behaviour of mortars till 38%, compared to uncoated samples. Replacing 50 vol.% of coated BA achieves 22% higher flexural and comparable compressive strength than the uncoated BA samples, thanks to the reduction of the Ca/Si ratio in the reaction products. Finally, in the presence of the coating, the leaching of the following contaminants Cl, Ba, Cu, Zn is reduced by 88, 98, 94 and 97%, respectively, compared to the uncoated BA application. Therefore, the impregnation treatment not only favours the application of higher amounts of BA, but it also improves the final performances of the product both mechanically and environmentally.