Chapter 3 Cement

3.2.3 Technical Properties of Portland Cement Fineness Setting Time Soundness of the Portland Cement Strength Other properties

Soundness of the Portland Cement Definition Reasons for poor dimensional stability Evaluating dimensional stability

The process of reducing and averaging volume of the cement in the process of cement hardening. Poor dimensional stability Uneven volume change happened in the process of cement hardening, such as cracking, blending The opposite of soundness of the Portland cement causing the decrease of quality Soundness of the Portland Cement

Three reasons –too much disassociated f- CaO –too much disassociated f-MgO –too much gypsum Three reasons too much disassociated f- CaO too much disassociated f-MgO too much gypsum Reasons for Poor Dimensional Stability

The hydrating speed of the overburnt f-CaO or f- MgO is slow, f-CaO or f-MgO begins to hydrate after cement hardening and causes the hardened cement to expand and crack. When added too much, gypsum continues to react with calcium aluminate hydrate to form calcium sulfoaluminate hydrate, whose volume increase 1.5 times as big as gypsum and causes the hardened cement paste to crack. At this time, sulfoaluminate hydrate is called cement bacillus. Reasons for Poor Dimensional Stability

Boiling Method - accelerating test There are two methods to evaluating dimensional stability: –Disc method Evaluating dimensional stability of boiled cement disc by observation –Le chatelier soundness method Measuring expansion of hardened cement after boiling Evaluating Dimensional Stability

水泥安定性的测定饼法

Fig Boiling testing instrument Fig Poor dimensional stability Fig Le chatelier soundness testing instrument Evaluating Dimensional Stability

Restrict The accelerating test method can only test the over-use of f-CaO f-MgO and gypsum can not be tested by the above test So they must be controlled in the producing process to avoid over-use Standard defines –MgO ≯ 5% ， gypsumSO 3 ≯ 3.5%