Recent Advances in C-S-H Nucleation Seeding for Improving Cement Performances

Abstract

Reducing cement CO₂ footprint is a societal need. This is being achieved mainly by replacing an increasing amount of Portland clinker by supplementary cementitious materials. However, this comes at a price: lower mechanical strengths at early ages due to slow pozzolanic reaction(s). This is being addressed by using accelerator admixtures. In this context, calcium silicate hydrate nucleation seeding seems to have a promising future, as it can accelerate cement and pozzolanic reactions at early ages, optimising their microstructures, without compromising late strength and durability performances. In fact, these features could even be improved. Moreover, other uses are low temperature concreting, precasting, shotconcrete, etc. Here, we focus on reviewing recent reports on calcium silicate hydrate seeding using commercially available admixtures. Current knowledge on the consequences of nucleation seeding on hydration reactions and on early and late mechanical strengths is discussed. It is noted that other features, in addition to the classic alite hydration acceleration, are covered here including the enhanced ettringite precipitation and the very efficient porosity refinement, which take place in the seeded binders. Finally, because the seeded binders seem to be denser, durability properties could also be enhanced although this remains to be properly established.

Keywords: C-S-H nanoseeds; accelerators; admixtures; ettringite; microstructure.

Figure 1

Schematic representation of the role of C-S-H nucleation seeding on the hydration of PC. The green arrows highlight the movement of the C-S-H growth away from the dissolving clinker particle volumes (inner C-S-H) towards the capillary porosity regions (outer C-S-H) caused by the nucleation seeding. The red arrows note the concentration gradients in ionic species which are exacerbated by C-S-H nucleation seeding. Reproduced from reference [25] with permission from Elsevier.

Figure 2

Calorimetric curves for a paste of CEM I 32.5 R substituted by 20 wt% of FA, w/b = 0.30 (green trace, 20FA0NA), and the same paste with a 4.0 wt% addition of XS100, amount referring to the admixture (blue trace, 20FA4NA). For the meaning of the lines and arrows, see the text. The subscript ‘s’ refers to the signatures in the seeded pastes. Adapted from reference [67] with permission.

Figure 3

Calorimetric traces for pastes based on CEM I 52.5 R substituted by 50 wt% of GGBFS, w/b = 0.40. The trace for the reference paste (unseeded) is blue. The seeded pastes, 3 wt% of C-S-H stabilised nanoparticles, have increasing contents of anhydrite (grey traces). Symbols (lines and arrows) as in Figure 2. Adapted from reference [80] with permission from Elsevier.

Figure 4

Calorimetric traces for pastes of CEM I 42.5 R with w/c = 0.50 and 0.40, seeded with 2 wt% of different Master X-Seed admixtures. Data for a paste with 0.05 wt% of TIPA is also given as a second reference. (a) Heat flow traces, w/c = 0.50, (b) cumulative heat data, w/c = 0.50, (c) heat flow, w/c = 0.40, and (d) cumulative heat, w/c = 0.50. Symbols (lines and arrows) as in Figure 2. Replotted from data of the authors of this work originally reported in references [71,78].

Figure 5

Rietveld quantitative phase analysis results for CEM I 42.5 R pastes, w/c = 0.40, seeded with 2 wt% of different Master X-Seed admixtures and 0.05 wt% of TPA. (a) C₃S and CH contents with time, (b) C₃A and C₄AF contents with time, and (c) gypsum and ettringite contents with time. Replotted from data of the authors of this work originally reported in reference [71].

Figure 6

(Top) Calorimetric study for BRC pastes, w/c = 0.40, seeded with 2 wt% of different Master X-Seed admixtures and 0.05 wt% of TIPA. (a) Heat flow traces. (b) Cumulative heat values. (Bottom) UPV study for the same materials but processed as mortars. (c) Acceleration values, derivatives of the velocities. (d) Velocities through the mortars. Replotted from data of the authors of this work originally reported in reference [71].

Figure 7

MIP data for a low-carbon cement blend (5 wt% PC 52.5 and 95 wt% GGBFS) with and without C-S-H seeding (HyCon^® S 7042 F) at 1 d and 28 days of hydration. Lower overall porosities and pore threshold entry sizes are clearly observed after seeding. Reproduced from reference [80] with permission from Elsevier.

Figure 8

Schematic representation of the role of C-S-H nucleation seeding (dark green tiny particles) on the hydration of Portland cements (brown and grey particles) as currently understood by the authors. The different components are labelled in the right part. In addition to the previously known main features: (i) enhanced secondary nucleation in the pore solution space of outer C-S-H gel; and (ii) increase in the amount of (slightly lower density) outer C-S-H gel at the expenses of (slightly higher density) inner C-S-H gel; some additional properties should be noted: (iii) enhancement of calcium sulphate dissolution rate at early ages; (iv) slightly faster calcium aluminate dissolution rate(s) at early ages; (v) slightly faster ettringite crystallisation rate; (vi) smaller threshold pore size at later hydration ages; and (vii) in many cases, smaller overall porosities at later hydration ages. Note 1: the size of inner C-S-H gel at 1 day is not to scale but enlarged for better visualization. Note 2: the dissolution rates of the calcium aluminate phase (and therefore the crystallisation rate of ettringite) is further (synergistically) enhanced by the simultaneous addition of alkanolamines. This could further enhance alite hydration at early ages as C₄AF intergrown with alite slows down alite hydration. Note 3: the faster dissolution rate of aluminates is very important for activating Al-containing SCMs, i.e., for instance in LC³ binders.