Cohesion (geology)

Summary

Cohesion is the component of shear strength of a rock or soil that is independent of interparticle friction. In soils, true cohesion is caused by following:

Electrostatic forces in stiff overconsolidated clays (which may be lost through weathering)

Cementing by Fe2O3, Ca CO3, Na Cl, etc.

There can also be apparent cohesion. This is caused by:

Negative capillary pressure (which is lost upon wetting)

Pore pressure response during undrained loading (which is lost through time)

Root cohesion (which may be lost through logging or fire of the contributing plants, or through solution)

Typical values of cohesion Cohesion (alternatively called the cohesive strength) is typically measured on the basis of Mohr–Coulomb theory. Some values for rocks and some common soils are listed in the table below. Apparent cohesion of soil During critical state flow of soil, the undrained cohesion results from effective stress and critical state friction, not chemical bonds b

Official source

https://en.wikipedia.org/wiki/Cohesion_(geology)

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Calcium aluminate cements (CAC) are often used in combination with calcium sulfate and Portland cement for special applications where rapid setting, rapid drying and shrinkage compensation are required. A growing fraction of the clinker, which has the highest CO2 intensity, is being replaced by supplementary cementitious materials (SCM). These SCM are usually from industrial by-products such as slag, fly ashes or can simply be raw materials such as limestone. This project aims at replacing the Portland cement and fraction of the CAC in ternary binders with such SCM. Indeed, the understanding of their interactions, both physical and chemical, with calcium aluminate cements and calcium sulfate is fundamental for the development of innovative products. This thesis focuses first on the hydration of systems with and without SCM, slag or limestone, from the first hours up to two years of hydration. Second, this study focuses on the expansion of these systems and on the mechanisms underlying the dimensional stability. The influence of the ratio between calcium sulfate and calcium aluminate on the hydration of the cement, on the reaction of SCM, and on the expansion is highlighted. This study indicated that the calcium sulfate reacts with CAC from the first hours of hydration and, only when sulfate is exhausted, does slag or limestone react with aluminate phases. Moreover, this research brings new insights on the main parameters influencing the dimensional stability of the studied systems and on the mechanisms governing expansion. Pore solution analyses showed that supersaturation with respect to ettringite increases with the calcium sulfate content, which results in an increase of the crystallization pressure. The supersaturation determines the minimum pore size in which crystals can grow. Therefore, with increasing supersaturation a larger pore volume can be accessed by ettringite crystals exerting pressure. This could explain the critical amount of calcium sulfate leading to high expansion. Both the confinement in nanometer size pores and the supersaturation of ettringite are necessary for expansion to occur.

EPFL2014

Expansion mechanisms in calcium aluminate and sulfoaluminate systems with calcium sulfate

Julien Bizzozero, Christophe Gosselin, Karen Scrivener

The long-term expansion of calcium aluminate cement and calcium sulfoaluminate cement in the presence of added gypsum has been studied for samples cured under water. Progressively higher amounts of gypsum were added to the CAC or CSA and it was found that there is a critical amount of gypsum leading to unstable expansion and failure of the samples. The microstructures of systems with gypsum additions just below and above the threshold were similar. Pore solution analyses showed that supersaturation with respect to ettringite increases with the calcium sulfate content, which results in an increase of the crystallization pressure. The supersaturation determines the minimum pore size in which crystals can grow. Therefore with higher supersaturation a larger pore volume is accessible to growing ettringite crystals exerting pressure in the porous skeleton. This could explain the critical amount of gypsum leading to high unstable expansion. (C) 2013 Elsevier Ltd. All rights reserved.

Elsevier2014

Morphology and interfacial strength of nonisothermally fusion bonded hard and soft thermoplastics

Pierre-Etienne Bourban, Rajasundar Chandran, Jan-Anders Månson

Nonisothermally fusion bonded butt joints were prepared by overmolding thermoplastic elastomers (TPEs) onto isotactic polypropylene (iPP) inserts in order to investigate the effect of processing conditions on the bond strength and interfacial microstructure. The mold temperature (Tm) was the most important factor for bond strength, as determined from interfacial mechanical tests. Extensive melting and recrystallization took place at the surface of the iPP insert at high Tm, promoted by migration of plasticizer from the TPE, whereas the original structure of the iPP remained intact at low Tm. Bond strengths of at least 50% of the cohesive strength of the TPE were nevertheless obtained at low Tm, suggesting intimate contact between the TPE melt and the iPP surface to be sufficient to provide useful adhesive bond strengths in these materials. The influence of pressure was less marked than the Tm, high pressures not being necessary to achieve intimate contact for the bonding times of about 5 s used here. However, the combination of a low bonding pressure with a high Tm typically led to poor quality bonds in thick specimens owing to uncompensated shrinkage during solidification, and voiding at the interface and in the melt zone of the iPP insert.

2018

EPFL2014