Page 6 - ICJ Jan 2026

TECHNICAL PAPER

properties, high replacement levels of slow-reacting pozzolans (≡Si-O-Si≡) bridges on the nano-silica surface, generating highly
– [36]
such as Class F fly ash often delay setting and early strength reactive silanolate groups (≡Si-O ) . These silanolate sites then
development [19, 31-33] . This occurs because pozzolanic reactions undergo hydrolysis to form additional silanol (≡Si-OH) groups
depend on the availability of CH and proceed more slowly than while regenerating OH , making the reaction autocatalytic under
–
the primary hydration of C 3 S and C 2 S, resulting in early-age high-pH conditions [36-38] . This alkaline depolymerization increases
behavior dominated mainly by physical filler effects rather than surface solubility and produces dissolved monomeric silicate
chemical binding . In contrast, silica fume, due to its extremely species, enabling rapid participation in hydration chemistry
[31]
high amorphous SiO 2 content and very fine particle size, rather than behaving as a passive filler.
provides abundant nucleation sites and exhibits rapid pozzolanic
reactivity, thereby enhancing early-age strength compared to These reactions increase the concentration of monomeric
other SCMs. Despite its extremely high specific surface area, silicate ions, predominantly represented in cementitious pore
silica fume does not hydrate independently because it lacks solutions as H 3 SiO 4 , which immediately participate in early
–
intrinsic hydraulic reactivity; instead, it reacts only pozzolanically pozzolanic reaction with calcium hydroxide (CH) ,
[37]
by consuming CH released from cement hydration, rather than
[28]
2+
–
forming hydration products on its own . These limitations H 3 SiO 4 + Ca + (1-x) H 2 O → C-S-H x (10)
in reactivity and activation windows paved the way for the (low Ca/Si, highly polymerized)
development of next-generation nanoscale silica, where the
same chemical principles are amplified, enabling immediate It is noted that some schematic representations, such as those
–
dissolution, accelerated pozzolanic conversion, and nucleation- in Figure 1, illustrate dissolved silicate species as H 2 SiO 4 ²
driven C-S-H formation [19, 31, 34] . for simplification; however, under typical pore solution pH
conditions (≈12.5-13.5), thermodynamic modelling and pore-
–
3. MECHANISMS WITH THE INCLUSION solution analyses confirm H 3 SiO 4 as the dominant monomeric
species [3, 25, 37] . This clarification aligns with the chemical
OF NANO-SILICA IN CEMENTITIOUS
mechanism without altering the conceptual interpretation
COMPOSITES
of nucleation-driven C-S-H formation presented by Singh
A super pozzolanic material such as nano-silica (n-SiO 2 ) contains et. al., (2013) .
[35]
the ≈99 % amorphous silica phase with particle sizes in the
range of 10-100 nm, resulting in an exceptionally high surface- Unlike conventional pozzolanic systems, this reaction occurs
area-to-volume ratio and elevated surface energy [31, 33, 35] . This within hours, rapidly consuming CH and suppressing portlandite
dramatically enhances both its dissolution rate and chemical crystallization [31,35] . This shifts the gel chemistry towards lower
[31]
reactivity compared to micro-silica . The involvement of Ca/Si ratio (≈1.2-1.5 for silica-fume-derived C-S-H), and the
nano-silica (n-SiO 2 ) in cement chemistry is governed by its addition of nano-silica, a super pozzolanic material further
exceptionally high surface energy and reactive silanol (≡Si-OH) reduces this ratio by forming even more silica-rich C-S-H
functional groups, which fundamentally alter the dissolution and (≈0.8-1.2). This increase in silicate content enhances chain
[36]
precipitation pathways of hydration products . Unlike silica polymerization from dimeric to Q²-Q³ structures, producing a
fume, nano-silica undergoes rapid surface depolymerization in highly cross-linked C-S-H network. In addition to secondary gel
the highly alkaline pore solution (pH ≥ 12), releasing reactive formation, super pozzolanic nano-silica modifies the primary
silicate species into the system through hydroxylation [36-37] , hydration chemistry of clinker minerals . Its high surface area
[39]
provides energetically favourable nucleation sites, reducing the
–
≡Si–O–Si≡ + OH → 2 ≡Si–O (8)
activation energy for C 3 S hydration,
–
≡Si–O + H 2 O → ≡Si–OH + OH – (9)
C 3 S + H 2 O → C-S-H + CH
In this process, the attacking OH ions break the siloxane (nucleation-accelerated pathway) (11)
–
2+
Cement + H 0 + nano SiO Ca + H SiO 4 2– + H SiO 4 2– + OH –
2
2
2
2
Froms C S From nano-silica
J
C-S-H C-S-H Ca(OH) 2
(additional)

Figure 1: Nucleation reaction, formation of extra C-S-H seeds [35]

62 THE INDIAN CONCRETE JOURNAL | JANUARY 2026