Core Raw Material—Cement - The Issues and Discussion of Modern Concrete Science - page 39

3.3 Effect of Modern Cement Production Process
on Quality of Concrete

Continuous advances in production techniques of modern cement have made
unprecedented effects on concrete science. Although it has built foundation for
concrete to develop at the direction of high strength and performance, the adverse
effects on various performance of concrete, especially durability, are also tremen-
dous, and even bring great challenge to old concrete theory.
This chapter will mainly discuss the above issue for the purpose of providing
caution and reference for cement production and application.

1. Gypsum

As an agent for adjusting setting time of cement, gypsum is employed when
grinding. However, gypsum with low quality has no adjusting effect on cement
setting time, but easily results in false setting and crack instead.

2. Grinding aid

Grinding aid is employed during cement grinding in order to overcome statics
among cement particles and increase grinding ef ciency as well as reducing cost.

Nevertheless, during engineering practice in recent years, the author had found that
grinding aid will enlarge concrete shrinkage, which lowers the compatibility of
additives and intensi

es possibility of generating cracks.

3. Close-circuit grinding and high-ef ciency separator

ciency separator are newly developed tech-
niques. Hydration rate of coarser cement particles is slow, especially particles above
60

Close-circuit grinding and high-ef

m. As they rarely have contribution on 28-day strength of cement, technician in
cement factory regards they are a kind of material wasting simply. Modern cement
technology has changed open-circuit grinding to closed-circuit grinding and added
high-ef

μ

ciency technique, for the purpose of screening out and regrinding the
particles above 60 m so as to boost 28-day strength of cement. As a result,

μ

particles above 60 m in cement are very few. However, in our country, minimal

μ

particle size of fine aggregate in concrete is 75 m; this approximately leads to a

μ

lack of particles between 60 and 75 m in concrete, which will bring adverse effect

on impaction of concrete as well as cement.
In fact, temporarily unhydrated coarse particles in cement have great in

μ

uence
on a number of concrete performances. They can not only decrease shrinkage and
cracking, but also enhance self-healing ability and durability of concrete. Therefore,
close-circuit grinding and high-ef ciency separator techniques have many negative

effects on engineering quality of concrete.

4. Overdosage of admixture

Increasing dosage of admixture and decreasing clinker content in cement as
much as possible is the main approach of lowering cost and boosting pro t for