Fast-setting cements

CONTROL OF PORTLAND CEMENT SETTING BY THE USE OF CHEMICAL ADMIXTURES 

Some anions exhibit a distinct accelerating effect on Portland cement hydration. They include halides, nitrate, nitrite, formiate, thiosulfate, and thiocyanate. Their activity depends not only on the dosage used, but also on the identity of the associated cation. 

Generally, divalent and tiivalent cations enhance the effectiveness of these anions more than monovalent ions do. This category of additives affects mainly the hydration of the alite phase, thus accelerating the development of early strength. The setting time is also shortened, but less effectively than with admixtures that increase the pH of the hquid phase. 

FAST-SETTING GYPSUM-FREE PORTLAND CEMENT 

Rapid setting may also take place in Portland cements that contain calciitm sulfate, if they simultaneously contain elevated amounts of K2O. In this case setting is due to the formation of syngienite [K2Ca(S04)2.H20] in the paste. 

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E.G. Sherif, F.A. Via, L.B. Post, and A.D.F. Toy, Improved fast-setting cements from ammonium phosphate fertilizer solution, European Patent No. EP0203485, 1986. 

A.D. Wilson and J.W. Nicholson, Acid—Base Cements (Cambridge University Press, Cambridge, 1993). F.G. Sherif and E.S. Michaels, Fast-setting cements from liquid waste phosphorous pentoxide containing... 

F.G. Sherif and E.S. Michaels, Fast-setting cements from solid phosphorous pentoxide containing materials,... 

The result was fast-setting cement with a high compressive strength of 55 MPa (8000 psi) in one day and an ultimate tensile strength of —2.1 MPa (300 psi). Applications of this cement are discussed in Chapter 18. 

Jetset. A fast-setting cement developed by the Portland Cement Association. Reported to harden in 20 minutes after pouring. Accelerating agent has not been disclosed. 

It is known, that the monoclinic tricalcium aluminate with alkalis in solid solution reacts with water slowly (see Chap. 2). Therefore this phase can be used as a source of aluminate ions in reaction of ettringite formation. The fast-setting cements are known which contain 35 % NCj A, apart from the high belite content of 57% or calcium orthosilicate phases belite and alite (C2S—38%, 038—21%) [1]. 

Large aluminate (C ) crystals Extremely slow cooling fast-setting cement (Eby, 1985)... 

FAST-SETTING CEMENTS CONTAINING THE PHASES CijA. CAFj... 

If OPC+CAC mixes ate used as fast-setting cements, compositions in which Portland cement is the main constituent ate preferred. Ratios of 75-90% of Portland and 10-25% of calcium aluminate cement may be considered typical. 

Fast-setting cements 267 (sodium tetraborate, Na2B4O7.10H2O). 

Other fast-setting cements not mentioned so far include alkali-activated slag cement (see section 8.5), alkali-activated fly ash binder (see section 9.1), geopolymeric cement (see section 15.7.1), zinc phosphate cement (see section 12.4), magnesium oxychloride and oxysulfate cement (see section 15.1), and alkali silicate binder(see section 15.3). 

To accelerate the process of hardening it has been suggested that carbon dioxide should be injected into the spreadable mix of wood, cement, and water, or that ammonimn, sodium or potassium carbonate should be added (Simatupang et al, 1995). Under these conditions the cement sets in a very short time, owing to the formation of calcimn carbonate. Effective acceleration of the hardening process may also be achieved by the use of a fast-setting cement produced by combining Portland cement with hmited amounts of calcium aluminate cement (see section 10.10.1). 

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