A Percolation Model for Water Sorption in Porous Cementitious Materials

Abstract

The highly disordered pore system of cement paste is described and numerically modelled as fractal structure. Water sorption experiments on cement paste with different porosity and numerical percolation experiments with Monte Carlo simulations are performed and have shown to be self similar, thus the percolation theory with the percolation threshold pc describes the kinetics of water sorption at all porosities consistently. With the same percolation model (connectivity of pores) the deviation from the classical power law for water sorption at long times and low porosities, the drastic reduction of the effective diffusion coefficient and the sharp increase in resisitivity with lower porosity and lower relative humidity can be explained. IntroductionThis microstructure of cement paste consists of solid and porous phases, and it is the spatial and topological arrangement of these phases that is ultimately responsible for the strength and transport properties of the paste. One key topological attribute is the connectivity or percolation of phases within this microstructure (figure 1): a percolating phase forms a three dimensional spanning cluster, while a phase that has not percolated consists of isolated clusters [1]. The durability or service life of cement-based materials is also directly influenced by phase percolation because the transport properties of such materials are determined to a large degree by the amount and connectivity of the capillary porosity within the microstructure. For water sorption in cement paste and for diffusion of degradative species, such as chloride ions, both the connectivity of the pore space and of the porous hydration products are critical: as the capillary porosity becomes disconnected at low w/c ratios or long hydration times, transport rates decrease markedly and become controlled by the gel porosity of the highly-connected calcium silicate hydrate phase [2]. The kinetics of water sorption of porous cement based materials as cement paste, mortar or concrete are generally described by the classical theory with the capillary or potential model resulting in the well known square root dependance of water uptake with time [3]. A lot of literature results, however, showed that this square root law does not fit the experimental data [4] at longer times and / or at low porosity. To explain these deviations, a new model based on the percolation theory has been proposed [5]. The structure of cement paste is described with a 3-d simple cubic lattice, the sites of the lattice being occupied with a probability p

Cements and Cement Paste

“…Permeability is high (Figure 1.6) and transport processes like, e. g., capillary suction of (chloride-containing) water can take place rapidly. The percolation theory has been used to design numerical experiments and applied to transport processes in cement paste and mortars [9]. As a result, water and chlorides will penetrate only a short distance into concrete.…”

El papel del catalizador usado de craqueo catalítico (fcc) como material puzolánico en el proceso de corrosión de armaduras de hormigón

“…Esta influencia de la estructura (geometría) en las propiedades de transporte se puede describir con la teoría de la percolación [8]: por debajo de una porosidad crítica, p c (umbral de percolación), el sistema de poros capilares no está interconectado (sólo existen clusters finitos); por encima de p c el sistema de poros capilares es continuo (clusters infinitos). La teoría de la percolación se ha usado para diseñar experimentos numéricos y se ha aplicado a procesos de transporte en pastas y morteros de cemento [9]. …”