OPC/CAC

Blends of ordinary Portland cement (OPC) and calcium aluminate cement (CAC) exhibit a very fast setting over a wide range of OPC/CAC ratios. Figure 17.1 shows a typical example of such behavior, but the exact setting time will also depend strongly on the... 

The reasons for the fast setting of OPC+CAC blends are not fully understood. It is assumed that in blends high in Portland cement the rapid setting is due to the formation of ettringite in a reaction between the CA phase of the calcium aluminate cement, calcium sulfate present in the Portland cement, and calcium hydroxide formed in the hydration of the tricalcium silicate phase present in Portland clinker ... 

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. 

The hydration process in OPC+CAC blends may also be accelerated by adding small amoimts of a lithium salt, such as Li2C03, to the mix. The hthium salt, which accelerates the hydration of the monocalcium aluminate phase very effeetively, is also highly effective in mixes in which the calcium aluminate cement is the minor component. 

The development of OPC-CAC based expansive cements can be traced back to the 1920s.P l Fast set control was a problem that limited... 

Electrochemical methods appear to have distinct advantages in the study of cement hydration. Methods involving potential measurement (including pH, zeta potential, and selected ion potential), conductivity measurement, and a.c. impedance measurement provide useful information related to both ion concentration of pore solution and microstructural change in hydrating cement paste. The early hydration and setting behavior of OPC-CAC and OPC-Hydrated-CAC paste systems can be determined using these techniques. 

It appears that the impedance behavior of the OPC-CAC paste system is determined by the following microstructural events 1" ... 

Compressive strength results for three pastes (OPC/CAC ratios 92.5/7.5 80/20 20/80) prepared at w/c ratio = 0.40 are presented in Table 1 [43] jjjg early strength development of the 92.5/7.5 and 20/80 pastes is very similar to that for pure OPC and CAC pastes. The 80/20 paste develops strength earlier than the other two pastes, and it appears to have unique strength development behavior. 

Many factors affect the strength development of OPC/CAC systems. In the OPC/CAC binaiy cement pastes, ettringite can form in the first few minutes of the hydration mainly through the following reactions ... 

Table 1. Compressive Strength Values of the Three OPC/CAC Pastes...

The calcium aluminate cement (CAC) was developed as a solution to the sulphates attack in OPC, and was patented in France in 1908. The CACs are cements consisting predominantly of hydraulic calcium aluminates mainly monocalcium aluminate, CA, but also contains minor amounts of Ci24y, C 2 and C AF. 

P=portlandite E=ettringite S=stratlingite MS=monosulfate H=high M=medium L=low Note In combinations - the strengths were compared with that of a paste made from pure OPC in combinations 5-6 with the strength of a pure CAC-A paste. 

Figure 17.1 Effect of the ratio CAC/OPC on the setting time of the resulting cement paste.

In the later stages the hydration progresses similarly to that of pure Portland cement in OPC-rich mixes and also to that of pure calcium aluminate cement in CAC-rich mixes. In mixes of intermediate composition stratlingite (gehlenite hydrate, C2SAHg) may also be formed. 

The early strength of the binary paste systems is attributed to the above reactions. The OPC normally provides the sulfate and the CAC is... 

The observation of a Ca(OH)2 peak at 48 hours by x-ray diffraction analysis suggests that the slow strength development in the 80/20 paste may be due to a delay in the hydration of C3S. The observation that C3S remains unhydrated until the conversion is completed suggests an incompatibility between the OPC and CAC hydration products. 

Table 3. Compressive Strength of 80% OPC-20% CAC Pastes Containing Lithiiun Salts...

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