Admixture

Question 1

Admixture

Answer 1

; an ingredient other than water, agg.,hydraulic cement and fiber reinforcement
:it is added to batch immediately before or during mixing

Question 2

Mineral Admixtures

Answer 2

; Fly ash (C,F) - by products of coal 
; Silica fume
; Meta kaolin
; slag
; calcined shale

Question 3

superplasticizer (chem admix)

Answer 3

; Break electro charged inter particle attraction ( should check again)

Question 4

cementitious activity (with water)

Answer 4

; hydraulic cement reaction (with water)

Question 5

main reasons of using SCM

Answer 5

economic
environmental
work ability
durability

Question 6

OPC&raquo_space; SLG&raquo_space; Flyash (C(not calcium but memorize), high lime)&raquo_space; Flyash(F, low lime)&raquo_space; Slicafume

Answer 6

high cementitious (calcium + silica)&raquo_space;> Highly pozzolanic (lower calcium + higher silica)

Question 7

Lengthens set time of fly as on concrete

Answer 7

; it means less heat and strength development

depends on situation. it’s not always beneficial. what if cold weather

Question 8

Flyash

Answer 8

; commonly used to reduce heat
; non combustible residue of powdered coal react with react with CH to produce CSH
; slower reaction rate -> strength develops slowly
improve durability by reducing permeability

Question 9

Limitation of high content of silica-fume

Answer 9

; only little calcium hydroxide is going to react

Question 10

Size of silica fume grains and workability

Answer 10

; Large surface area (small particles) = randomly charged this tiny particles have even stronger attractions btw inter particle
; To break out the interconnection, Need admixtures (superplasticizer)

Question 11

Benefits of Silicafume

Answer 11

\+ Superpozzolanic (reactivity, high strength)
\+ Shape of particles (round, workable)
\+ densify (small size, particle packing)
\+ high chemical resistance
\+ low permeability

Question 12

pozzolanic reaction (or activity)

Answer 12

; extra silica takes/ reacts calcium hydroxide that has been produced by hydration btw cement and water

;and produces more C-S-H (helps strength, reducing permeability, durability)

; reaction rate is affected
calcium hydroxide could make salt and create voids in the system.

Question 13

why is extra silica important

Answer 13

; ??? because of pozzolanic reaction

Question 14

Higher content of C-S-H and lower content of CH

Answer 14

+ stronger b/c less porous
+ less porous means more durable
+ more resistant to leaching b/c more C-H contents ???
+ more durable

Question 15

benefits of shape of fly ash(FA)

Answer 15

+ FA is spherical so better workability, better packing efficiency, reduced voids

+ Gained workability provides a chance to use less water so less porosity, less permeability, and better durability

Question 16

fly ash, slower reaction rate

Answer 16

; react with CH and produce additional C-S-H

; slower the strength gain rate

; slower development of strength

+ improve durability by reducing permeability

Question 17

silica fume

Answer 17

; byproduct of the silicon metal, very small grains

; high silica content (95%)

; super pozzolan

Question 18

why we can get rid of fly ash and make full of silica fumes in mixtures

Answer 18

; maximizing reaction in the system is limited to calcium hydroxide reaction, silica fume react instead

Question 19

Ca(OH)2 = CH

Answer 19

; if concrete is exposed to water, calcium hydroxide could make salt and create void

Question 20

effects of fly ash in set time

Answer 20

+ lengthening set time can control a heat in hot weather conditions.

; Reduce early age strength but higher strength in long term age

• In cold weather, not helping

Question 21

Air Entraining Agents

Answer 21

; Introduce air bubbles on purpose

; voids are not inter connected = not increasing permeability

; entrained air helps reducing freeze and thaw cycles

Question 22

Air Entraining Agents

Answer 22

; Introduce air bubbles on purpose( void spacing, size, and amount), causing water to foam and to produce spherical air bubbles in the cement paste

; voids are not inter connected = not increasing permeability

+ entrained air helps reducing freeze and thaw cycles, acting as pressure release vessels because bubbles are well distributed

; air bubbles are not visible

Question 23

Entrapped Air

Answer 23

• entrapped air is created during improper mixing, consolidating and placement

Question 24

effects of entrained airs

Answer 24

+ slightly improve workability because bubbles reduce internal friction, acting like fine elastic agg.)

+ lower segregation and bleeding b/c improving workability

+ improve cohesiveness

+ reduce freeze-thaw

• reduce strength

Question 25

Flocculation

Answer 25

; Flocculation is one of the reasons that cement paste shows thixotropy. This term refers to a phenomenon where the viscosity of fresh concrete decreases when it is under constant shear, and then recovers when shearing is removed.

Question 26

(super)Plasticizer

Answer 26

; neutralize all the particles, now negative charges and no attraction. Repel particles each other
+ water soluble polymers designed to enhance workability or to reduce water requirements

+ lowering cement content, lower amount of agg.

+ reduce $ and heat

Question 27

Flocculation process

Answer 27

; opposing electronically charged particles b/c of water can exert electrostatic attractions, causing particles to flocculate and trap water

Question 28

Retarders

Answer 28

; extend the time that concrete remains workable

; allow time for finishing on site

; massive structure ( produces lots heat) to slow heat of hydration = controlling heat and reaction rate

Question 29

Accelerators

Answer 29

; used in cold weathers

; Higher early stage of strength (early stripping forms)

; used for repair and leak for emergencies