International audienceA semiempirical model is proposed to predict the evolution of chemical shrinkage and Ca(OH)(2) content of cement paste at early age of hydration. The model is based on chemical equations and cement compound hydration rates. Chemical shrinkage and Ca(OH)(2) amount are computed using the stoichiometric results of the hydration reactions considered in the model and the density of hydration products and reactants. The model validation is conducted by comparison between computed and experimental results achieved on ordinary cement pastes with different water-to-cement (w/c) ratios (0.25, 0.30, 0.35 and 0.40) cured at 10, 20, 30, 40 and 50 degreesC, respectively. Hydration degree and Ca(OH)(2) content are determined using the thermogravimetric analysis (TGA) and chemical shrinkage evolution using a gravimetric method. The comparison reveals a good consistency between modelled and experimental data at early age of hydration

International audienceThis paper presents the results of an experimental study concerning the incorporation of polyurethane (PUR) foam wastes into cementitious mixtures in order to produce lightweight concrete. A semi-empirical method is first proposed to predict the density of fresh PUR foam-based concrete mixtures. Seven concrete mixtures containing various PUR foam volume fractions (from 13.1% to 33.7%), and two reference concrete mixtures (without PUR foam) were prepared and characterized. In particular, their thermal and mechanical properties were determined. This permitted to quantify the influence of the PUR foam volume fraction on these parameters. Some specimens were maintained under water during 28 days, while the others were dried in air. The PUR-foam concrete thermal conductivity and compressive strength are, respectively, 2–7 times and 2–17 times lower than those of the reference mixture, depending on the volume fraction of PUR foam and on the curing conditions. Besides, the use of PUR foam in concrete implies a strong increase in the drying shrinkage and in the mass loss during the first seven days. These results can be related to the high porosity and the weak compressive strength of alveolar polyurethane

No abstract

No abstract

This article presents a study on the influence of limestone filler and granular inclusions on the chemical shrinkage of cementitious matrices at very early age (624 h). Measurements of chemical shrinkage and hydration degree are carried out on cement pastes and mortars. During this study, two cement types (CEM 1 and CEM 2), two water-to-cement ratios (W/C = 0.30 and 0.40) and three substitution rates of cement by limestone filler (LF/C = 0; 0.25 and 0.67) are used. The effects of aggregate shape (glass beads and natural sand), aggregate-to-cement mass ratio (A/C = 0.5 and 1) and particle size distribution (D = 1 and 2 mm) on the chemical shrinkage and the hydration rate are quantified. The results obtained show that limestone filler causes an acceleration of both Le Chatelier's contraction and hydration process since the very first hours of hydration. In addition, the chemical shrinkage amplitude is not significantly influenced by the presence of aggregates. Finally, the presence of limestone filler and granular inclusions does not cause significant modification of the quasi-linear relation observed at early age between the chemical shrinkage and the hydration degree of the cementitious matrices.

This paper presents a contribution to the modeling of the chemical shrinkage of the slag-blended cement paste (binder) at early age. Assuming that the chemical shrinkage is a direct result of hydration, the hydration modeling of slag-blended cement was studied by considering the interaction between the hydrations of blast furnace slag (BFS) and ordinary Portland cement. The reaction of BFS in the presence of calcium hydroxide CH (Portlandite) produced from the hydration of the cement was investigated. The kinetic hydration of cement was developed, and the volume phases in the cementitious material during the hydration process were calculated. The chemical shrinkage, which is the negative volume balance between the reactants and the products formed, is then calculated. In parallel with this numerical modeling, an experimental study was conducted to investigate the effect of slag's addition (0%, 30%, 50% and 80%) on the heat of hydration and chemical shrinkage at early age (maturation up to 7 days). The proposed hydration model incorporates the effect of following variables; the chemical composition of the binder, the fineness, the water to binder ratio (w/b), the curing time and the temperature.

This study examines the mechanical properties and the durability parameters of lightweight aggregate concretes (LWAC) incorporating rigid polyurethane (PUR) foam waste as coarse aggregates (8/20 mm). The influence of both the increasing incorporation of PUR foam waste and the presence of superplasticizer on the workability, bulk density, mass loss, drying shrinkage, compressive strength, dynamic modulus of elasticity, total porosity, gas permeability and chloride diffusion coefficient of the different concretes, has been investigated and analyzed. The results showed that the use of PUR foam waste enabled to reduce by 29-36% the dry density of concrete compared to that of the normal weight concrete (made without foam waste). The reduction of density was due to the increase of total porosity in the lightweight concretes, which also induced higher gas permeability and chloride diffusion coefficient. These negative effects on durability of concrete were lowered by improving the characteristics of the cementitious matrix. The mechanical properties of the LWAC ranged between 8 and 16 MPa for the compressive strength and between 10 and 15 GPa for the dynamic modulus of elasticity; the concrete mixture with the higher performances almost satisfied the mechanical and density criteria of structural lightweight concrete. These results consolidate the idea of the use of PUR foam waste for the manufacture of lightweight aggregate concretes.

A micro-macro experimental study has been performed, from the end of mixing up to 2 years, on a set of plain cement pastes prepared with the same type I ordinary Portland cement (OPC) and various water-to-cement ratios (W/C), and cured at various constant temperatures. In this part I of the paper, volumetric autogenous shrinkage has been analysed in relation to various parameters characterizing the hydration process: chemical shrinkage, degree of hydration of the cement, Ca(OH) 2 content and Vicat setting times, within the early-age period (V24 h). The effects of the curing temperature (ranging from 10 up to 50 8C) have in particular been investigated. Its effects recorded on both the rate and the magnitude of volumetric autogenous shrinkage vs. time have pointed out the irrelevance of the usual maturity concept to describe such phenomenon within the whole early-age period. An improved maturity concept has hence been proposed. It is based on separating the early-age period in different phases and on using chemical shrinkage data for the calculation of the apparent activation energy applied to the prediction of autogenous deformations occurring after the setting period. Furthermore, micro-macro relationships have been pointed out, illustrating in particular the determining role of Ca(OH) 2 .