Hydrated cement contains significant amounts of small pores in its structure. These pores can contain a pore fluid that is highly alkaline, with pH’s greater than 12.5, due to high levels of hydroxyl (OH-). In addition to the OH- in the fluid there is also high amounts of Ca2+, K+, and Na+ [15]. When there are certain minerals present in the cement matrix the hydroxyl will react with these minerals causing expansion and cracking of the cement. This phenomenon is known as alkali-aggregate reactions (AAR) and is split up into two main categories alkali-carbonate reaction (ACR) and alkali-silica reaction (ASR) [18]. ACR occurs when dolomite is present in the cement structure and is attacked by the pore fluid breaking apart the dolomite and making cracks to allowing the reactions to continue to happen with the reaction products causing expansion [18].
ASR occurs when silica (SiO2) is present in the cement matrix and is attacked by the hydroxyl in the pore space in series of reactions. The first reaction that occurs is when the silicon and oxygen bonds are attacked and the oxygen molecules in the silica are replaced by OH forming silanol bonds [15].
SiO2 + H2O →SiOH
Next these bonds are further attacked by the hydroxyl breaking apart…show more content… If there is a high concentrations of Calcium cations in the system the reaction will form a C-S-H gel and is ok, but if there is not a high enough concertation of calcium cation to control the gel reactions the gels becomes a problematic high-alkali gel will form. This gel is hydrophilic and will absorb water around it and begin to swell. This is the processes that is going on during the phenomenon known as ASR. This swelling becomes especially problematic because the cement structure will begin to crack as the gel expands causing the cement to lose structural strength. An example of this cracking as the gel expands can be seen in Figure
