Special Cements Questions and Answers

Answer: A

The first two are correct. The third is wrong in almost every case, since substituted materials react more slowly than clinker and early strength is the usual casualty. Later strength often exceeds that of the plain cement, however.

Answer: B

Sorel, magnesium phosphate and geopolymer binders contain no Portland clinker at all and set by entirely different chemistry. Grouping them under one heading reflects their unusual use rather than any common composition.

Answer: C

Correct answer: handling the caustic activator safely on site

Geopolymers are activated by strong alkali - sodium hydroxide or silicate solution - which burns skin and eyes and demands protective equipment and training that ordinary site practice does not include. The chemistry works; the practicality of the activator is what has held the material back.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. A one-part system supplies the activator as a dry solid blended with the precursor, so the material is mixed with water like ordinary cement. That removes the caustic liquid from the site entirely, which is the obstacle a two-part system presents.

Answer: A

A structure is expected to last for many decades, and no accelerated test yet substitutes fully for the record of a material that has been in service that long. Caution here is a considered position rather than mere conservatism.

Answer: B

Correct answer: False

Accelerated tests compress years into weeks by raising temperature or concentration, which can change the mechanism as well as the rate - so a material may pass a test and fail in service, or the reverse. They are essential for screening and for comparison, and they do not substitute for structures that have stood for thirty years.

Answer: C

Correct answer: resistance to sulphate and chloride attack

A high slag content leaves less calcium hydroxide and gives a much denser, less permeable paste, so sulphate and chloride ingress are greatly slowed. That suits marine and buried work, and it is bought at the cost of slow early strength and a longer curing requirement.

Answer: D

They gain strength slowly at early ages and are selected instead for durability and low heat evolution. Choosing one where rapid formwork striking is needed would be a straightforward mistake. Each cement is chosen against the property the job actually needs.

Answer: A

The other three are all Portland cements with something added or with the clinker composition adjusted. A geopolymer sets by alkali activation and dispenses with clinker entirely. That is precisely why its emissions can be so much lower.

Answer: B

Twenty divided by eight is two and a half hours. Calculations of this kind decide whether a rapid cement is worth its price on a particular job, since the closure cost is usually the dominant figure. Material price alone rarely settles the choice for such work.