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This paper reports a study on the mechanical behaviour of recycled aggregate concretes (RACs) and is mainly intended at unveiling how they are fundamentally influenced by the mixture constituents and conditions. To this end, it summarises the results of some experimental tests carried out at the Laboratory of Material testing and Structure (LMS) of the University of Salerno (Italy) on samples of concrete mixtures made with natural aggregates (NAs) and recycled concrete aggregates (RCAs), characterised by two different initial moisture conditions. The compressive strength determined at different curing ages and the time evolution of temperature developed within the concrete samples during setting and hardening are reported for each concrete mixture. By making use of a theoretical heat flow model, temperature measurements could be employed for identifying the hydration process of cement, which take place in such mixtures. These analyses clearly highlighted that the hydration reaction is significantly influenced, not only by the nominal water–cement ratio, but also by the initial moisture conditions of recycled aggregates, whose significantly higher porosity modifies the water content actually available in the mixture for cement hydration. Moreover, linear correlations emerged between the so-called “degree of hydration” and the corresponding compressive strength. In fact, introducing the degree of hydration as a fundamental parameter connects the micro-structural evolution to the development of the macro-scale mechanical properties of RACs. This multi-scale approach is considered as a key step in simulating the behaviour of structures made of concretes with recycled aggregates.

This paper deals with the correlation between the time evolution of the degree of hydration and the compressive strength of Recycled Aggregate Concrete (RAC) for different water to cement ratios and initial moisture conditions of the Recycled Concrete Aggregates (RCAs). Particularly, the influence of such moisture conditions is investigated by monitoring the hydration process and determining the compressive strength development of fully dry or fully saturated recycled aggregates in four RAC mixtures. Hydration processes are monitored via temperature measurements in hardening concrete samples and the time evolution of the degree of hydration is determined through a 1D hydration and heat flow model. The effect of the initial moisture condition of RCAs employed in the considered concrete mixtures clearly emerges from this study. In fact, a novel conceptual method is proposed to predict the compressive strength of RAC-systems, from the initial mixture parameters and the hardening conditions.

There is an increased interest in the reuse of ash waste from biomass combustion, being a sustainable source of energy. This paper investigates the partial replacement of cement and sand in building materials with fly ash waste generated from combustion of woody biomass waste. The results show that the ash widens the particle size distribution of cement and has minerals complementary to portland cement, thus justifying its application as cement replacement, but with a relatively high amount of free CaO and MgO that may exert significant expansion. With ash addition, the rate of hydration, strength, and workability decreased. The optimum dosage showed 15 % of ash, replacing 5 % of cement and 3.33 % of sand, which can still produce a structural grade cementitious material with acceptable workability and mechanical properties. The ash exhibits hydraulic and pozzolanic activity that initially increases and subsequently decreases the amount of Ca(OH) 2 in the hydrated material.

Silica fume (SF) is used as a constituent to formulate cementitious materials for subsurface (oil-well) applications, e.g. by the offshore industry. Using green supplementary pozzolanic materials like silica fume could both improve the material’s performance and the footprint of the sustainable solution. X-ray diffraction and isothermal calorimetry were employed to investigate the evolution of cement paste hydration (water to binder ratio 0.44) after 14 h, 3, 7, and 28 days of curing. Experimental results are presented for both cement pastes made of Class G Portland cement and for a blended paste system where 8% of the Class G cement has been replaced by silica fume. The results indicate beneficial effects of SF on the microstructural development of the slurry.

The secondary pozzolanic reaction mechanism has been modeled explicitly in the Delft hydration model Hymostruc. The model calculates the progress of the hydration process as a function of the particle size distribution, the water cement ratio, the temperature and the cement and pozzolanic chemistry. The consumption of portlandite due to the activation of the pozzolanic materials is shown in detail. The numerical results are validated by an experimental testing plan on G-cement and 8% of silica fume and a water to cementitious ratio of 0.44. The simulated development of portlandite and degree of hydration and the experimental results are in good agreement.

This paper presents a sound theoretical formulation and an effective numerical implementation of a heat-flow and hydration model for concrete hardening. The model is based on the Fourier equation of heat flow with the adiabatic hydration curve employed as a reference for simulating the hydration heat source. The proposed formulations are based on a consistent scheme for the partial differential equation and its boundary and starting conditions. The hydration kinetics is simulated through the Arrhenius approach. Formulations for the compressive strength and the elastic modulus are provided and the maturity function is also considered. A finite difference numerical solution is derived with a forward explicit time integration in the time–space domain. The numerical solution is designed as a stepwise “recipe” specifically conceived to be easily implemented by means of either a high-level programming language or even a spreadsheet tool. Experimental temperature measurements for two different mixtures, under adiabatic and semi-adiabatic conditions, are used for validating the proposed model. The adiabatic and semi-adiabatic temperature simulations show good agreement with the experimental data for both concrete mixtures. The degree of hydration could be simulated and used as the fundamental parameter for scrutinising the evolution of the compressive strength. Particularly, a linear trend between the compressive strength and the degree of hydration and the maturity was figured out.