9-10 SETTING AND HARDENING OF PORTLAND CEMENT

9-10 SETTING AND HARDENING OF PORTLAND CEMENT

Setting and hardening of hydraulic cements are the result of hydration reactions occurring between the cement compounds and water. When the cement is mixed with water to a paste, hydration reaction begins, resulting in the formation of gel and crystalline products. These are capable of binding the inert particles of the aggregate into

a coherent mass. Setting is defined as the stiffening of the originally plastic mass of cement and water to such a consistency that no significant indentation of the mass is obtained when it is subjected to certain standardized pressures. Hardening follows setting and is the result of further hydration processes advancing gradually into the interior of the particle core. The strength developed by cement depends on the amount of gel formed and the degree of crystallization.

Hydration Reactions The course of hydration reactions is illustrated by the following chemical equations:

First, hydrates are formed from the corresponding anhydrous products that passed into the solution. The hydrates have lower solubility than their corresponding anhydrous First, hydrates are formed from the corresponding anhydrous products that passed into the solution. The hydrates have lower solubility than their corresponding anhydrous

variable composition 3CaO.Al 2 O 3 .xCaSO 4 .yH 2 O, where x = 1 to 3 and y = 10.6 to 32.6. This reaction presents a high concentration of alumina in the solution, thus retarding the initial set of the cement. In the presence of iron oxide the amount of C3A in the cement is reduce because the corresponding amount of alumina combines with the iron oxide to form tetracalcium aluminoferrite. As this latter hydrates more slowly than tricalcium aluminate, less gypsum is required. Tetracalcium alumino ferrite combines with water to form crystals of tricalcium aluminate and a gel that is probably hydrated monocalcium ferrite.

Both dicalcium and tricalcium silicate hydrate at a slower rate than tricalcium aluminate and yield an amorphous mass (gel) of dicalcium silicate. Tricalcium silicate also releases excess lime as calcium hydroxide, which precipitates out of the saturated solution as crystals. This is believed to account for the high rate of hardening and early high strength of cement. Any water in excess of that which entered into the chemical reactions will fill the capillaries because the vapor pressure in the capillaries is less than that of the water in bulk. The capillary-held water tends to diffuse slowly into the inner cores of the cement particles, causing hydration. This results in a slow but continuous expansion of the hardened cement when totally immersed in water. This expansion is only of the order of a 0.1% increase in length per annum, but it must be allowed for in laying large masses of concrete. Heat of hydration may be immaterial in many cases, but it cannot be easily dissipated in certain engineering structures involving large masses of concrete. This may cause the temperature to rise by as much as 50°C (122°F), resulting in the possible cracking of the structure on cooling and the lowering of the strength and quality of the concrete. On the other hand, heat of hydration can be beneficial in cold-weather concreting.

Structure of Cement Paste. The hardened Portland cement paste consists of the calcium—silicate hydrate (C—S—H) products which, like other gels, contain a network of capillary pores and gel pores (Fig. 9-6). The total porosity of the paste is about 30% to 40% by volume, having a very wide pore-size distribution ranging from 10 to 0.002 m in diameter. This imparts to the hardened paste an extremely large surface area of 200 to

400 m 2 /g. The gel porosity, consisting of very small pores, is about 26%; the remaining porosity is due to a capillary network. The latter can be regarded as the remnants of the

water-filled space of the initial fluid paste that is gradually filled with hydration products. The total porosity of the paste is an important factor in determining the strength and durability of the cement paste.

Fineness. Fineness of cement greatly affects the setting time and the strength of the hardened cement because the chemical activity of a solid is directly proportional to its surface area, which greatly increases with the increased fineness of particles. As hydration proceeds from the outside to the inner core of the particle, the smaller the particle, the greater the probability that nearly the whole core will be converted to gel and crystals. For coarser particles a considerable portion of the inner part will not be available for hydration. Consequently, the finer cement will develop more gel per unit weight than the coarser cement of the same composition. This accounts for a more rapid hardening and a greater strength of the finer cement as compared with that of the coarser one. On the other hand, too fine a cement tends to give considerable shrinkage on setting, and a compromise must be sought to obtain the optimum properties.

Extremely high strength Portland cement pastes can be produced using specially ground cement with the assistance of surfactant grinding aids to make surface areas ranging from

0.6 to 0.9 m2Ig. When mixed with water and plasticizing agents, the hardened pastes show very low porosity and high compressive strength of 196 MPa (28.4 ksi). This is about twice the strength of the cement paste produced by conventional methods.

Very high strengths are also obtained by hot pressing conventional cement pastes under pressures of 196 to 392 MPa (28.4 to 56.8 ksi) at 150°C (302°F). A nearly zero porosity is obtained for the hardened cement paste that shows typical strength: 490 MPa (71 ksi) (compression), 44 MPa (6.4 ksi) (tensile) and 83 MPa (12 ksi) (shear). These values are about four times greater than the strength values for cement pastes produced by conventional methods. These techniques are still at the experimental stage, but they clearly indicate the potential possibilities of making the Portland cement concrete much stronger than that produced today.

When the hardened cement is exposed to dry air, it shrinks because of the loss of capillary water but then expands on rewetting in moist air. This causes reversible shrinking and expansion of the hardened cement on drying and wetting, respectively.

FIGURE 9-6 Structure of cement paste showing elongated crystals, X 3000.