**3. Conclusions**

after the 5-day healing period. The damage-healing cycle was repeated and the compression

**Table 3.** Cement strength recoveries after repeated compressive damage and two 5 d healing periods. Average recovery

The data show that at short curing times TSRC may successfully recover its strength after repeated damage in different environments. The recoveries of class G and CaP cement blends

As mentioned above one possible mechanism of strength recovery are FAF reactions contributing to the "healing" process of damaged cements. However, although both CaP cement and TSRC contain FAF strength recoveries of CaP cement are inferior to those of TSRC and in the environment of alkali carbonate even to class G blend. One of the reasons for poor strength recoveries is the brittleness and the strong bond nature (chemical bond) of the CaP blend. Fast early compressive strength development through chemical reactions of sodium phosphate with calcium aluminate makes this cement strong but brittle. So the Young's modulus of the control CaP blend after 1 d curing at 300°C is 1896 MPa, while for TSRC it is 971 MPa. As a result wide cracks and fractures form under the compressive damage, making the repairs problematic. Another possible cause of lower healing performance of CaP cement could be

alkalinity dissolution of fly ash and crystallization of healing phases requires longer times or

Addition of slowly-reacting components, such as MGF, to the blends helps to improve strength recoveries (**Table 4**). MGF react under alkaline conditions contributing sodium, aluminum, and silicon to the pore water of the blends. These ions favor formation of zeolites that

In case of MGF additions strength recoveries are improved to above 100% (higher strength of damaged samples after the healing than the original strength) for TSRC and to 80–99% for

mal selection of pozzolanic materials allows binding cement pieces completely broken off the samples. The bond strength of such re-adhered pieces can be evaluated from the stress–strain curves of compressive strength tests. An example of such curve for a TSRC sample broken into two pieces and then cured at 300°C for 5 days in alkali carbonate environment is shown in **Figure 4**. On the curve the yield point (YP) compressive strength is 17.3 MPa. However, the initial failure (IF) of the sample where the re-adhered piece breaks off can be seen as a left shoulder with the failure point at 10.8 MPa. Such points of IF allow evaluation of the

blend. In addition to the exceptional strength recoveries of TSRC samples opti-

at later times may alter to other more stable zeolites or feldspar minerals [30].

blends. Under conditions of low

**Hypersaline brine**

are not as good with the exception of CaP cement in hypersaline brine.

**Water Alkali carbonate**

Class G/SiO<sup>2</sup> C B B TSRC A A A CaP cement C C A

**0.05 M NaCO3**

rates after 1st and 2nd compressive breaks for 1 d 300°C- cured samples: A 80–99%; B 60–79%; C < 60%.

its lower alkalinity in comparison with TSRC and G/SiO<sup>2</sup>

strength recovery data averaged.

230 Cement Based Materials

**Cement system Curing environment (300°C)**

may not happen at all.

class G/SiO<sup>2</sup>

Two types of calcium-aluminate cement – fly ash F blends, chemical (CaP) and alkali activated (TSRC), noticeably outperform common high-temperature well-cement blends of Portland cement and silica. The factors that contribute to their good performance under conditions of thermal shock, CO<sup>2</sup> -rich- and strong-acid environments are as follows.

Good acid and thermal shock resistance of fly ash F reaction products, formation of stable carbonation phases, including carbonated apatite from apatite in CaP cement and cancrinite from zeolites in both CaP cement and TSRC.

**References**

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Cements for High-Temperature Geothermal Wells http://dx.doi.org/10.5772/intechopen.74108 233

O3 -SiO<sup>2</sup> -H<sup>2</sup>

O (CASH) hydro-

The differences in the performance of these blends come from the minor components, sodium polyphosphate (CaP) and sodium-meta-silicate activator (TSRC). In CaP cement fast chemical reactions of sodium polyphosphate produce calcium phosphate-containing phases at early curing times, contributing to the early strength development and CO<sup>2</sup> -resistance through formation of stable carbonated apatite phase. The reactions of fly ash F are delayed in CaP cement because of low ash reactivity at low pH of the interstitial water of this cement. This, along with the brittle cement nature limits early strength recoveries after cement damage. The low pH is also unfavorable for corrosion protection of carbon steel.

On the other hand, in TSRC samples sodium meta-silicate creates highly alkaline slurries that promote fly ash F reactions at earlier curing times than in the case of CaP. Fly ash F reaction products contribute to strong acid-, thermal shock resistance and high pH favorably changes the environment for corrosion protection of carbon steel. Excellent strength recoveries for damaged or broken TSRC samples are possible thanks to the cement's ductile nature and highly alkaline pH of its interstitial solution promoting fly ash F reactions with formation of new healing phases. However, the relatively high (>100°C) temperatures are necessary for fast early strength development of TSRC, while chemical reactions with the formation of phosphate in CaP cement allow fast early strength development over a wide temperature range.

This different natures of the two discussed calcium-aluminate cements may provide important advantages when specific properties are required under aggressive environments – whether it is fast early compressive strength development or formation of tough composites with regenerative potential.
