Carbonates are stable in hardened cementitious materials because of alkaline environment of the materials. They can even help to durability some extent by filling pores of the materials. However, acids react with carbonates and generally form salt, water and carbon dioxide. Moreover, carbonates help to carbonation mechanism and dissolve hydration products of cement. By dissolving of cement hydration products (CH) corrosion risk occurs because of reduction of pH and impairing of passive layer on reinforcement.

There are potentially two aspects here; carbonates produced via carbonation of the alkaline matrix, and the role of carbonates in altering the microstructure (and therefore durability) of cement.

Dealing with carbonation first of all, the reaction between the alkaline cement matrix and atmospheric carbon dioxide leads to the gradual formation of calcium carbonate, primarily from the portlandite (calcium hydroxide) in the cement, but also from the C-S-H (calcium silicate hydrate). The formation of calcium carbonate is a dual edged sword. Carbonation of portlandite leads to pore filling, thus improving durability and strength, but it also reduces the pH of the pore solution, so leaves reinforced concrete more susceptible to reinforcement corrosion. In composite cements, i.e. those containing supplementary cementitious materials, such as PFA or slag, the reduced portlandite contents can mean that these systems are less resilient to carbonation (although they have a finer pore structure which mitigates this). [changes to the pore structure upon carbonation can be more complex than this, but I'm trying to keep things succinct].

Secondly, certainly in Europe, small quantities of calcium carbonate are now added to many cements during production. At levels of up to ~5% (given a similar typical sulphate content) the calcium carbonate reacts with the tricalcium aluminate in the cement to produce calcium monocarboaluminate. By consuming some of the tricalcium aluminate during the early stages of hydration, the normal conversion of ettringite to calcium monosulphoaluminate is prevented. This can help to improve resistance to sulphate attack, whereby any calcium monosulphoaluminate reacts with ingressing sulphates to re-form ettringite.

There is considerable research underway at the moment to determine the role of calcium carbonate at higher levels of replacement, particularly in composite cements containing slag or PFA. My former student, now colleague, Sam Adu-Amankwah did his PhD on such systems. I've linked to one of his papers below and to his Researchgate profile below.

https://www.researchgate.net/profile/Sam_Adu-Amankwah/contributions

Article Influence of limestone on the hydration of ternary slag cement

Dealing with carbonation is dealing also with ageing of cement paste. Indeed ageing induces higher uptake of humidity and then of  carbonation of the cement paste. This implies higher amounts of portlandite and carbonate calcium and then a decreae in CSH quantities which induces modification of the microstructure of an aged specimen compared to a fresh one. In general the study of the evolution of this microstructure is based on the observation of CSH phases portlandite and calcium carbonate present in the samples. In the litterature quantification of these indices is monitored using  several measurement techniques such as scanning electron microscopy, spectroscopy thermal differential and thermogravimetric analysis TGA/DTG and X-ray analysis. More details concerning the two last techniques are available in the link's paper below, showing the carbonation impact, comparing a fresh cement paste with a 2 years old specimen  :

Article The effect of ageing and heat treatment on microstructure ev...

Addition of calcium  carbonate  accelerates the hydration  process. Hence crystal growth is rapid but long term strength is reduced. Durability is reduced due to porosity of carbonate. 

Dear Sir,

pl check it.

http://www.sciencedirect.com/science/article/pii/S1110016816000375