MgO AND EXPANSION IN CEMENT

Cement soundness due to free lime can be checked by cement paste expansion in boiling water with the Le-chatlier test.

The potential delayed expansion caused by the hydration of CaO and MgO in hydraulic cements is tested by an Autoclave test. (It should be noted that a High C₃A and low C₄AF content in cement, in addition to the known additive effects of CaO and MgO may also contribute to the total autoclave expansion.)

The causes of the cement expansion due to periclase (Crystalline MgO) are also identified.

It is found that MgO recrystallized as dense, compact periclase crystals at rotary kiln. Its harmful expansion seemed to be inhibited by chilling of clinker so that MgO was retained in the solution in the glass form. If Clinker was cooled slowly, more periclase crystallized out of the melt and larger crystals were formed. Rapid Chilling clinker causes the MgO in the solution in the glass form and reduces the autoclave expansion.

Coarser ground cements have always exhibited a greater amount autoclave expansion, all other parameters remain same. There is a decrease in Autoclave expansion in cement if fineness is increased.

No autoclave expansion was found for smaller periclase crystals which were predominately located in the interstitial phases. The clinker cooled by more rapid cooling process results in smaller periclase crystals. Rapid cooling clinker decreases the size of periclase crystals and ultimately decreases the autoclave expansion.

Long-term Storage and hydration of cement decreases the autoclave expansion.

The free MgO will decrease as Fe₂O₃ increases in the raw mixture. C₄AF is an excellent stabilizer for MgO and capable of transformation of considerable amount of MgO into a non- expanding compound. It is suggested that lowering the alumina modulus and thereby lowering the C₃A and increasing the C₄AF contents, could readily counteract the autoclave expansion influence of MgO, but it was conceded that rapid cooling of clinker also has very beneficial effect on the stabilization of MgO.