Concrete cement paste

C. W. Lent2, in Special Keport90, Structure of Portland Cement Paste and Concrete Highway Research Board, NRC-NAS, Washington, D.C., 1966. 

Cement and Concrete Concrete is an aggregate of inert reinforcing particles in an amorphous matrix of hardened cement paste. Concrete made of portland cement has limited resistance to acids and bases and will fail mechanically following absorption of crystalforming solutions such as brines and various organics. Concretes made of corrosion-resistant cements (such as calcium aluminate) can be selected for specific chemical exposures. 

Today, cement and concrete replace stone in most large structures. But cement, too, is a ceramic a complicated but fascinating one. The understanding of its structure, and how it forms, is better now than it used to be, and has led to the development of special high-strength cement pastes which can compete with polymers and metals in certain applications. 

Concrete is a particulate composite of stone and sand, held together by an adhesive. The adhesive is usually a cement paste (used also as an adhesive to join bricks or stones), but asphalt or even polymers can be used to give special concretes. In this chapter we examine three cement pastes the primitive pozzolana the widespread Portland cement and the newer, and somewhat discredited, high-alumina cement. And we consider the properties of the principal cement-based composite, concrete. The chemistry will be unfamiliar, but it is not difficult. The properties are exactly those expected of a ceramic containing a high density of flaws. 

Fig. 20.5. Concrete is a particulate composite of aggregate (60% by volume) in a matrix of hardened cement paste.

When concrete hardens, the cement paste shrinks. The gravei, of course, is rigid, so that smaii shrinkage cracks are created. It is found that air entrainment (mixing small bubbles of air into the concrete before pouring) helps prevent the cracks spreading. 

Here, V and 1/ are the volume fractions of aggregate and cement paste, and E and E are their moduli. As Fig. 20.6 shows, experimental data for typical concretes fit this equation well. 

A concrete consists of 60% by volume of limestone aggregate plus 40% by volume of cement paste. Estimate the Young s modulus of the concrete, given that E for limestone is 63 GPa and E for cement paste is 25 GPa. 

Numerous multiphase composite materials exhibit more than one characteristic of the various classes, fibrous, laminated, or particulate composite materials, just discussed. For example, reinforced concrete is both particulate (because the concrete is composed of gravel in a cement-paste binder) and fibrous (because of the steel reinforcement). 

N. Banthia and S. Mindess. Water permeability of cement paste. Cement Concrete Res, 19(5) 727-736, September 1989. 

Concrete is a composite material composed of cement paste with interspersed coarse and fine aggregates. Cement paste is a porous material with pore sizes ranging from nanometers to micrometers in size. The large pores are known as capillary pores and the smaller pores are gel pores (i.e., pores within the hydrated cement gel). These pores contain water and within the water are a wide variety of dissolved ions. The most common pore solution ions are OH", K+ and Na+ with minor amounts of S042" and Ca2+. The microstructure of the cement paste is a controlling factor for durable concrete under set environmental exposure conditions. 

Aligizaki, K. du Rooij, M.R. Macdonald, D.D. (2000) Analysis of iron oxides accumulating at the interface between aggregates and cement paste. Cement Concrete Res. 30 1941-1948... 

It is known that some of the properties of fresh concrete can be considered in terms of the rheological properties of the cement paste contained in the concrete. Thus a high water-cement ratio concrete will contain a paste content which is more fluid than that of a low water-cement ratio concrete. 

The consistency or fluidity of the concrete will be a function of the viscosity of the cement paste. 

The graphs given in Fig. 1.38 show the logarithmic relationship between the water-cement ratio and the permeability coefficient of hardened cement paste. Thus concrete with a paste water-cement ratio of 0.4 will be almost impermeable. Water-reducing agents can be used to reduce the water- cement ratio, so ensuring that the permeability is kept to a minimum. 

Typically a concrete of paste water-cement ratio of 0.55 could be reduced to 0.50 resulting in a permeability less than half the original value. 

Volume deformations are largely a function of the nature and quantity of the cement paste in the concrete and it has been shown [113] that studies on... 

In order to understand the way in which superplasticizers affect the properties of concrete, studies have often been made on cement pastes in view of the convenience of such investigations. 

A typical distribution of pore radii in the hardened cement paste of concrete was shown in Fig. 1.40 which indicated that the majority of pores lie in the region of 0.05 and 1.0 pm diameter and it is through these pores that water passes by applied pressure or capillary rise, as shown in Fig. 4.5(a). 

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