Cements and 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. 

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. 

The low tensile strength of cement paste is, as we have seen, a result of low fracture toughness (0.3 MPa m ) and a distribution of large inherent flaws. The scale of the flaws can be greatly reduced by four steps ... 

In what way would you expect the setting and hardening reactions in cement paste to change with temperature Indicate the practical significance of your result. 

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). 

The important compounds in Portland cement are dicalcium silicate (CazSi04) 26%, tricalcium silicate (CasSiOj) 51%. tricalcium aluminate (Ca3Al206) 11% and the tetracalcium species Ca4Al2Fe2 Oio (1%). The principal constituent of moistened cement paste is a tobermorite gel which can be represented schematically by the following idealized equations ... 

The setting reaction for the great majority of acid-base cements takes place in water. (The exceptions based on o-phenols are described in Chapter 9.) This reaction does not usually proceed with formation of a precipitate but rather yields a substance which entrains all of the water used to prepare the original cement paste. Water thus acts as both solvent and component in the formation of these cements. It is also one of the reaction products, being formed in the acid-base reaction as the cements set. 

These early views are, perhaps, too simplistic to explain in full the rheological changes that occur in polyelectrolyte cement pastes before and at gelation. There are several physicochemical processes that underlie... 

Ion binding by reduction of repulsive forces also causes the attractive forces between polyions to increase, and the cement paste thickens. This interaction between polyions may be regarded as a kind of bridge formed by multivalent ions located between the polyions. At this stage the cement paste has the characteristic of a lyophilic sol - high viscosity. 

As reaction proceeds, the polymer chain (which is in random coil form) unwinds as the charge on it grows as a result of neutralization and ionization. This contributes to thickening of the cement paste. Cations released become bound to the polymer chain. Countercations can either be bound to a polyanionic chain by general electrostatic forces or be site-bound at specific centres. More than one type of site binding is possible. Complex formation and, if the ligand is bidentate, chelate formation enhance the effect. 

An unfortunate characteristic of early zinc polycarboxylate cements was the early development of elastomeric characteristics- cobwebbing -in the cement pastes as they aged, thus shortening working time (McLean, 1972). Improvements in cement formulation, the addition of stannous fluoride to the oxide powder (Foster Dovey, 1974, 1976) and modifications in the polyacid have eliminated this defect. However, the cements have to be mixed at quite a low powder/liquid ratio, 1 -5 1 0 by mass, when used for luting. 

The properties of these cements - the fluidity of the mix, the working and setting times of the cement paste, and the strength of the set cement - are affected by a number of factors. These include the composition of the powder, the concentration, molecular mass and type of the polyacid, the... 

Increase in concentration of the polyacid increases solution viscosity, quite sharply above 45% by mass (Crisp, Lewis Wilson, 1977). The strength of glass polyalkenoate cements also increases, almost linearly, with polyacid concentration. This is achieved at the cost of produdng overthick cement pastes and loss of working time. 

On mixing the cement paste, the calcium aluminosilicate glass is attacked by hydrogen ions from the poly(alkenoic acid) and decomposes with liberation of metal ions (aluminium and calcium), fluoride (if present) and silicic acid (which later condenses to form a silica gel). 

As the poly(alkenoic acid) ionizes, polymer chains unwind as the negative charge on them increases, and the viscosity of the cement paste increases. The concentration of cations increases until they condense on the polyadd chain. Desolvation occurs and insoluble salts precipitate, first as a sol which then converts to a gel. This represents the initial set. 

Figure 5.11 (Crisp Wilson, 1974b) shows the time-dependent variation of the concentration of soluble ions in setting and hardening cements. Note that the concentrations of aluminium, calcium and fluoride rise to maxima as they are released from the glass. After the maximum is reached the concentration of soluble ions decreases as they are precipitated. Note that this process is much more rapid for calcium than for aluminium and the sharp decline in soluble calcium corresponds to gelation. This indication is supported by information from infrared spectroscopy which showed that gelation (initial set) was caused by the precipitation of calcium polyacrylate. This finding was later confirmed by Nicholson et al. (1988b) who, using Fourier transform infrared spectroscopy (FTIR), found that calcium polyacrylate could be detected in the cement paste within one minute of mixing the cement. There was no evidence for the formation of any aluminium polyacrylate within nine minutes and substantial amounts are not formed for about one hour (Crisp et al, 1974).

The most important characteristic of the magnesium oxide powder used in these cements is its reactivity (Glasson, 1963). Magnesium oxide needs to be calcined to reduce this, otherwise the cement pastes are too reactive to allow for placement. Surface area and crystal size are important and relate to the calcination temperature (Eubank, 1951 Harper, 1967 Sorrell Armstrong, 1976 Matkovic et ai, 1977). The lower reactivity of calcined magnesium oxide relates to a lower surface area and a larger crystallite size. 

The progress of precipitation is revealed by the concentration/time curves for zinc and phosphate, since both these species are present initially in solution. There should be maxima for the soluble aluminium, calcium and fluoride which are extracted from the glass, but because of the early onset of precipitation these are not observed. Precipitation is accompanied by an increase in pH when it reaches 1-8, at which juncture 50% of both zinc and phosphate have been precipitated, the cement paste gels (5 minutes after preparation). 

The rheological characteristics of AB cements are complex. Mostly, the unset cement paste behaves as a plastic or plastoelastic body, rather than as a Newtonian or viscoelastic substance. In other words, it does not flow unless the applied stress exceeds a certain value known as the yield point. Below the yield point a plastoelastic body behaves as an elastic solid and above the yield point it behaves as a viscoelastic one (Andrade, 1947). This makes a mathematical treatment complicated, and although the theories of viscoelasticity are well developed, as are those of an ideal plastic (Bingham body), plastoelasticity has received much less attention. In many AB cements, yield stress appears to be more important than viscosity in determining the stiffness of a paste. 

Consistency, working time, setting time and hardening of an AB cement can be assessed only imperfectly in the laboratory. These properties are important to the clinician but are very difficult to define in terms of laboratory tests. The consistency or workability of a cement paste relates to internal forces of cohesion, represented by the yield stress, rather than to viscosity, since cements behave as plastic bodies and not as Newtonian liquids. The optimum stiffness or consistency required of a cement paste depends upon its application. 

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