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TECHNICAL PAPER

of 50-95 MPa [10-15] . The behavior of SCM-blended systems Ca + SiO 4 + Al(OH) + H 2 O → C-A-S-H gel (5)
4−
4–
2+
drove these developments, are governed by two primary
chemical mechanisms, including pozzolanic reactions and Unlike pure Portland systems, slag contains appreciable
latent hydraulic reactions, both of which directly influence the quantities of MgO and Al 2 O 3 , leading to the formation of
phase composition and microstructure of the hardened matrix, secondary hydrates such as hydrotalcite-like phases and
[25-27]
collectively marking the transition from early 1900s conventional modified calcium aluminosilicate hydrate ,
concretes to modern HPC [16-18] .
2+
–
C-A-S-H + Mg + OH → Hydrotalcite
(Mg-Al layered double hydroxide) (6)
2. MECHANISM ACTION WITH THE
These products enhance chemical binding of chlorides and
INCLUSION OF SECONDARY CEMENTITIOUS
improve resistance to sulfate attack and carbonation. The slower
MATERIALS (SCMS)
dissolution rate of slag results in delayed heat evolution and
Pozzolanic SCMs such as Class F fly ash and silica fume contain later strength gain, influencing the thermal and kinetic profile of
amorphous SiO 2 and Al 2 O 3 , which do not hydrate independently hydration [27-28] .
but react with calcium hydroxide (CH) released during cement
hydration [17, 19] . The primary reaction involving silicate (silica Silica fume, a highly reactive supplementary cementitious
fume, rice husk ash etc.) and alumino-silicate rich (metakaolin, material containing more than 90-98 % amorphous SiO 2 , plays a
fly ash) pozzolans can be expressed as [20-22] , significant role in modifying cement hydration chemistry. Due to
its ultra-fine particle size (~0.1 μm) and extremely high specific
SiO 2 (amorphous) + CH + H 2 O → C-S-H gel surface area, silica fume does not hydrate independently, but
(calcium silicate hydrates) (3) rapidly reacts with calcium hydroxide (CH) produced during the
[3]
hydration of C 3 S and C 2 S . This pozzolanic reaction consumes
SiO 2 -Al 2 O 3 (amorphous) + CH + H 2 O → C-A-S-H gel CH and forms additional calcium silicate hydrate (C-S-H),
(calcium aluminosilicate hydrates) (4)
represented as [3, 28] ,
These reactions consume CH, reducing the pH-sensitive free
SiO 2 (amorphous) + 2CH + H 2 O → C-S-H
lime content by converting Ca(OH) 2 into additional C-S-H, (low Ca/Si ratio) (7)
thereby lowering the amount of unbound lime susceptible to
leaching, carbonation, and expansive reactions [19, 21] . However, The newly formed C-S-H has a lower Ca/Si ratio and a more
this occurs at a slower rate because pozzolans initially act as compact structure than the primary C-S-H from clinker hydration,
inert fillers until the pore solution pH rises above ≈12 due resulting in a chemically denser matrix. The reduction of CH
to the dissolution of alkali hydroxides (NaOH and KOH) and not only lowers free lime content, decreasing susceptibility to
calcium hydroxide (CH) released during early hydration. These carbonation and sulfate attack, but also stabilizes the internal
–
hydroxides dissociate into OH ions, rapidly elevating the chemistry by maintaining high alkalinity without excessive
pore solution alkalinity before the pozzolanic reactions begin crystalline CH [28-29] .
[23]
to gradually consume CH . As a result, the thermal-kinetic
profile of early hydration is generally minimized due to reduced In addition to pozzolanic reactivity, silica fume influences cement
[29]
heat evolution when both silicate and aluminosilicate-rich chemistry through nucleation and micro-filler effects . It’s
pozzolans are incorporated [18-19, 21] . The physical effects, including extremely fine spherical particles provide abundant nucleation
refinement of capillary pore structure and the corresponding sites for C-S-H growth, accelerating early hydration kinetics
reduction in permeability, develop predominantly at later ages and refining the microstructure. The physical filling of gel
as secondary C-S-H and C-A-S-H progressively fill voids and and capillary pores significantly reduces permeability and
strengthen the microstructure [22-24] . interconnectivity of the pore network. This dual mechanism,
chemical CH consumption and physical densification, leads
In contrast, latent hydraulic SCMs such as GGBS possess to enhanced compressive strength, reduced diffusivity, and
inherent hydraulic potential but require activation, typically improved resistance to chloride intrusion. However, silica fume
through the highly alkaline pore solution (pH > 12.5) and increases water demand and can elevate autogenous shrinkage
the availability of Ca ions released during early hydration due to rapid internal moisture consumption, its effectiveness
2+
of ordinary Portland cement (OPC) [3, 25] . Once dissolved, the depends on optimized dosage (typically 5-10 % by weight of
4–
reactive glassy phases of slag undergo ion dissolution of SiO 4 cement) and proper dispersion within the matrix [28-30] .
–
and Al(OH) 4 species, and subsequently react in a manner
analogous to clinker minerals to form the primary hydration Although SCMs significantly enhance long-term durability
product, calcium silicate aluminate hydrate, a more cross-linked through these chemical transformations, by lowering CH
and chemically stable hydrate [17, 25] , concentration, refining pore connectivity, and reducing transport

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