A multi-technique investigation of the nanoporosity of cement paste

Abstract

The nanometer-scale structure of cement paste, which is dominated by the colloidal-scale porosity within the C-S-H gel phase, has a controlling effect on concrete properties but is difficult to study due to its delicate structure and lack of long-range order. Here we present results from three experimental techniques that are particularly suited to analyzing disordered nanoporous materials: small-angle neutron scattering (SANS), weight and length changes during equilibrium drying, and nanoindentation. Particular attention is paid to differences between pastes of different ages and cured at different temperatures. The SANS and equilibrium drying results indicate that hydration of cement paste at 20°C forms a low-density (LD) C-S-H gel structure with a range of gel pore sizes and a relatively low packing fraction of solid particles. This fine structure may persist indefinitely under saturated conditions. However, if the paste is dried or is cured at elevated temperatures (60°C or greater) the structure collapses toward a denser (less porous) and more stable configuration with fewer large gel pores, resulting in a greater amount of capillary porosity. Nanoindentation measurements of pastes cured at different temperatures demonstrate in all cases the existence of two C-S-H structures with different characteristic values of the indentation modulus. The average value of the modulus of the LD C-S-H is the same for all pastes tested to date, and a micromechanical analysis indicates that this value corresponds to the denser and more stable configuration of LD C-S-H. The experimental results presented here are interpreted in terms of a previously proposed quantitative "colloid" model of C-S-H gel, resulting in an improved understanding of the microstructural changes associated with drying and heat curing.

Uncertainty propagation of a multiscale poromechanics-hydration model for poroelastic properties of cement paste at early-age

“…The porosities φ LD and φ HD are respectively set to 37.3 and 24.7% with respect to the estimations of Jennings [31] and Ulm et al [1]. Because the density of early-age calcium-silicate-hydrates is much less certain than the density of latter hydration product [70], the porosity of LD C-S-H is considered as a random input parameter while φ HD is kept deterministic. A lognormal PDF is considered for φ LD with a coefficient of variation of 10%.…”

The Colloid/Nanogranular Nature of Cement Paste and Properties

“…Drying alters the structure of pores [8] and therefore by implication of packing arrangement of the particles. This is almost self-evident from the observed variability of surface area as measured by nitrogen.…”

“…The rearrangement of nanobricks has the advantage of providing a mechanism for viscous flow and poses new questions that may be investigated experimentally. Aging due to time, increased temperature, load and irreversible drying shrinkage are due to rearrangement of packing of the bricks often with the effect of reducing the volume of pores in the 5 -12 nm range [8].…”

Characterization of chemical and physical parameters of post copper slag

“…Independently of their chemical activity, such additives should modify the physical and mechanical properties of the paste, mortars and concretes. It is known that existing sub micron pores in the hydrated cement paste are filled with the gel fraction of C-S-H hydrated calcium silicates and this material can be modified with the use of reactive and nonreactive nanoparticles, efficiently increasing the share of the gel fraction [5,6]. Post copper slag has been recently proposed as a filling material for concrete, and as a binding additive for cement paste production [7][8][9][10][11] as well as for obtaining composites with higher resistance to different destructing agents [8].…”