Cement composites

Composite cements contain mineral additions, which may be inert (such as limestone), or possess pozzolanic, or hydraulic properties (such as pozzolana, fly ash and burnt shale). 

EN 197-1 [9.2] permits 6 to 20 % of limestone to be inter-ground with the clinker in Type II/A-L and 21 to 35 % in Type II/B-L Portland limestone cement. The limestone is required to contain at least 75 % by weight of calcium carbonate, with less than 1.2 % of clay and less than 0.2 % of organic material. It notes that limestone with organic material contents of up to 0.5 % may also be acceptable. 

The expected loss of 28-day strength arising from the presence of an inert component can be offset by finer grinding. For equal 28-day strengths, the 1-day strength of a Portland limestone cement can be greater than that of the Portland cement. 

The effects of ground limestone are partly physical and partly chemical. The limestone acts as a filler between the clinker grains and gives a more dense end-product. 

The use of composite cements offers financial advantages, as the additives are less expensive than clinker. They have been widely used for many years in some countries (eg. France and Spain). With the publication of a European Prestandard [9.2], the use of composite cements is likely to grow rapidly. 

Mineral additions may be broadly categorized as pozzolanic materials or latent hydraulic cements. Neither type reacts significantly with water at ordinary temperatures in the absence of other substances. Pozzolanic materials are high in Si02 and often also in AI2O3, and low in CaO they are sufficiently reactive that mixtures of them with water and CaO produce C-S-H at ordinary temperatures and thereby act as hydraulic cements. If they contain AI2O3, calcium aluminate or aluminate silicate hydrates are also formed. Because they are low in CaO, this component must be supplied in stoichiometric quantity. In a composite cement, it is provided by the Portland cement through decreased formation of CH and decreased Ca/Si 

In this chapter and elsewhere, w/s denotes the ratio wj(c + p) and percentage replacement the quantity I00p/(c + p), where ir, c and p are the masses of water, Portland cement and mineral addition, respectively. 

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Cement clinker Cement-composites Cement copper Cemented carbide... 

Industrial finishing systems are applied to a wide variety of substrates, the majority of which are metallic, but they are also applied to paper, wood, wood composites, cement products and plastics. Often a high quality of decoration is required, as well as protection from a number of hazards, such as knocks, abrasions, bending or forming and contact with non-corrosive liquids. Resistance to the weather may be required. Outdoor finishing systems, and many others, are also required to protect metal against corrosion. 

C2A8H8, known by its mineral name of stratlingite and also as gehlenite hydrate , is well established as a natural mineral, hydration product of i certain types of composite cements and laboratory product. Its crystal data... 

CjAHg is the only stable ternary phase in the CaO-AUOj H,0 system at ordinary temperatures, but neither it nor any other hydrogarnet phase is formed as a major hydration product of typical, modern Portland cements under those conditions. Minor quantities are formed from some composite cements and, in a poorly crystalline state, from Portland cements. Larger quantities were given by some older Portland cements, and are also among the normal hydration products of autoclaved cement-based materials. CjAHg is formed in the conversion reaction of hydrated calcium aluminate cements (Section 10.1). 

Studies on other materials show that MIP determines the width distribution of pore entrances and not of the pores themselves (D34). The intrusion of mercury may also coarsen the pore structure this need only imply that, at the higher pressures employed, some of the foils of the gel are displaced so that some pores are widened and entered while neighbouring ones are closed up. The combined result of these processes would be to produce a distribution narrower than that existing before the intrusion began, and a value for the porosity at maximum pressure that corresponded to a minimum pore width before intrusion of less than 3.5 nm. Experiments in which the mercury was removed and subsequently reintruded have indicated that the structure is usually not altered in the case of Portland cement pastes, though it is in that of pastes of composite cements (F35,D32), but cannot show whether an irreversible change occurred during the first intrusion. 

The suitability of a slag for use in a composite cement depends primarily on its reactivity, though grindability and contents of water and of undesirable components, especially chloride, must also be considered. Reactivity most obviously depends on bulk composition, glass content and fineness of grinding, though these are probably not the only factors and the relations with composition and glass content are complex. 

For any specified drying condition, the calculated water contents arc lower and the porosities higher than those of pure Portland cement pastes, and this appears to be true in varying degrees of composite cements in general. Experimental observations support this conclusion. Non-evapor-able water contents of 2-year-old pastes of w/s ratio 0.5 typically decrease with slag content from around 23% for pure Portland cements to 10 13" for cements with 90% of slag (C42). For the paste to which Table 9.4 refers, the observed non-evaporable water content was 17.7% (H49). Porosities and their relations to physical properties are discussed in Section 9.7. 

Uchikawa (UI7) reviewed the hydration chemistry of pfa and other composite cements. Pfa cements differ from pure Portland cements notably in (i) the hydration rates of the clinker phases, (ii) CH contents, which are lowered both by the dilution of the clinker by pfa and by the pozzolanic reaction, (iii) the compositions of the clinker hydration products and (iv) formation of hydration products from the pfa. The two last aspects cannot be wholly separated. 

Fig. 9.4 Degrees of reaction of low-CaO pfa in pastes of Portland-pfa cements hydrated at l5-25°C. Numbers against data points denote kg of pfa reacted per 100 kg of composite cement at the age and percentage of pfa in the composite cement indicated. The curves of equal quantity of pfa reacting arc based on the more typical of the results. Sources of data Cl (K45,K47) O (T44) (UI9.UI7) ) (C43) (DI2).

Ratios of water/cement/pfa = 0.5 0.72 0.28. pfa glass reacted = 6.0%, CH content = 13%, both referred to the ignited weight of composite cement, pfa HP = in situ product from Pfa. Fe HP = hydrogarnet-type product from ferrite phase. Mg HP = hydrotalcite-type phase, excluding any present in pfa HP. Pfa res. = unreacted non-vitreous material from Pfa. Other components mainly C and PjO,. Other phases mainly insoluble residue, and alkalis present in or adsorbed on products or contained in the pore solution. Discrepancies in totals arise from rounding (T5). 

Composite cements may contain mineral additions other than, or as well as, ones with pozzolanic or latent hydraulic properties. Regourd (R34) reviewed the use of ground limestone, which is widely used in France in proportions of up to 27%. The limestones used consist substantially of calcite, with smaller proportions of quartz or amorphous silica and sometimes of dolomite. They must be low in clay minerals and organic matter because of the effects these have on water demand and setting, respectively. The XRD peaks of the calcite are somewhat broadened, indicating either small crystallite size or disorder or both IR spectra confirm the occurrence of disorder. 

Studies on the pore structures of pastes of composite cements have presented... 

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