No abstract

No abstract

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.

No abstract

Representative elementary volumes (REV) are of major importance for modeling the transport properties of multi-scale porous materials. REVs can be used to schematize heterogeneous microstructures and form the basis of a numerical analysis. In this paper, the most appropriate REV size for numerical modeling for mass transport in hydrating cement paste is investigated. Numerous series (264) of virtual three-dimensional (3D) microstructures with different porosities and pore morphologies were generated using Hymostruc, a numerical simulation model for cement hydration and microstructure development. The influence of the initial particle size distribution, hydration evolution, numerical resolution and type of transport boundary conditions (periodic versus non-periodic) was investigated. The effective diffusivity was obtained by using a 3D finite difference scheme. The connectivity, dead-end porosity, tortuosity and constrictivity of the capillary pore network was evaluated. Based on a statistical chi-square analysis, it was concluded that the REV size largely depends on the complexity of the pore morphology, which in its turn, primarily depends on the degree of cement hydration, i.e. the porosity of the simulated microstructure, the employed numerical resolution as well as on the initial particle size distribution of the unhydrated cement grains. Furthermore, the results proved that employing periodic boundary conditions can effectively decrease the variability of the calculated effective properties and, thus, lower the size of an REV.

No abstract

In order to gain a deeper understanding of the rheological development of hydrating ordinary Portland cement (OPC) pastes at initial state, and to better understand their underlying processes, quantitative X-ray diffraction (XRD) analysis and rheological measurements were conducted and their results combined. The time-dependent relation between phase development and flow behavior of cement paste was investigated at two different temperatures (20 and 30 °C), over a period of two hours. Regarding the phase development during hydration, ettringite precipitation was identified as the dominant reaction in the first two hours. For both temperatures, the increasing ettringite content turned out to correlate very well with the loss of workability of the reacting cement paste. An exponential relationship between ettringite growth and flow behavior was observed that could be explained by applying the Krieger-Dougherty equation, which describes the influence of solid fraction on the viscosity of a suspension.

No abstract