Portland and Belite Cement Hydration Acceleration by C-S-H Seeds with Variable w/ c Ratios

Abstract

The acceleration of very early age cement hydration by C-S-H seeding is getting attention from scholars and field applications because the enhanced early age features do not compromise later age performances. This acceleration could be beneficial for several low-CO₂ cements as a general drawback is usually the low very early age mechanical strengths. However, the mechanistic understanding of this acceleration in commercial cements is not complete. Reported here is a contribution to this understanding from the study of the effects of C-S-H gel seeding in one Portland cement and two belite cements at two widely studied water-cement ratios, 0.50 and 0.40. Two commercially available C-S-H nano-seed-based admixtures, i.e., Master X-Seed 130 and Master X-Seed STE-53, were investigated. A multi-technique approach was adopted by employing calorimetry, thermal analysis, powder diffraction (data analysed by the Rietveld method), mercury intrusion porosimetry, and mechanical strength determination. For instance, the compressive strength at 1 day for the PC (w/c = 0.50) sample increased from 15 MPa for the unseeded mortar to 24 and 22 MPs for the mortars seeded with the XS130 and STE53, respectively. The evolution of the amorphous contents was determined by adding an internal standard before recording the powder patterns. In summary, alite and belite phase hydrations, from the crystalline phase content evolutions, are not significantly accelerated by C-S-H seedings at the studied ages of 1 and 28 d for these cements. Conversely, the hydration rates of tetracalcium alumino-ferrate and tricalcium aluminate were significantly enhanced. It is noted that the degrees of reaction of C₄AF for the PC paste (w/c = 0.40) were 10, 30, and 40% at 1, 7, and 28 days. After C-S-H seeding, the values increased to 20, 45, and 60%, respectively. This resulted in larger ettringite contents at very early ages but not at 28 days. At 28 days of hydration, larger amounts of carbonate-containing AFm-type phases were determined. Finally, and importantly, the admixtures yielded larger amounts of amorphous components in the pastes at later hydration ages. This is justified, in part, by the higher content of amorphous iron siliceous hydrogarnet from the enhanced C₄AF reactivity.

Keywords: C-S-H nano-seeding; CO2 footprint; Rietveld analysis; accelerators; ettringite.

Figure 1

Mechanical strength results for the eighteen studied mortars at 1, 7, and 28 days of hydration. Compressive strengths (left panels: (a,c)). Flexural strengths (right panels: (b,d)).

Figure 2

Calorimetries for the eighteen studied pastes, showing the outcomes of the employed accelerator admixtures. The heat flow curves, left panels, are referenced to 100 g of cement and only the first 48 h is shown for clarity. The total heat released, right panels, during the first seven days of hydration. (a,b) PC pastes, (c,d) BC-Buz pastes, (e,f) Non-activated BC pastes. The insets report the heats developed at 1 and 7 days by the studied pastes.

Figure 3

The fifty-four thermal analysis traces for the pastes after arresting the hydration and holding 30 min at 40 °C. The curves for the series have been vertically displaced for clarity. (a) PC-42.5 w/c = 0.50; (b) PC-42.5 w/c = 0.40; (c) BC-Buz w/c = 0.50; (d) BC-Buz w/c = 0.40; (e) BC-n.a. w/c = 0.50; (f) BC-n.a. w/c = 0.40. The Portlandite contents (wt%) given refer to the neat pastes, i.e., containing the free water, for direct comparison with the RQPA results. The total weight losses (wt%) are also displayed to the right of the traces. The insets give the free water contents, determined as described in the experimental section.

Figure 4

Phase development for PC-42.5 pastes based on the RQPA results: (top) w/c = 0.50, (bottom) w/c = 0.40. The total amorphous content determined by the internal standard methodology has been divided into three components: calculated free water, C-S-H content, and ACn, as described in the Supplementary Information; these amounts are obtained taking into account the chemical hydration reactions. ACn accounts for any other amorphous phases, i.e., iron siliceous hydrogarnet, AFm-type phases, and the amorphous content within the unreacted clinker phases. The blue vertical lines highlight the amount of ACn at 28 days in the two pastes without admixtures. The seeded pastes have significantly higher amounts of amorphous phases; see the blue lines. Minors-C stands for all minor phases in the pristine cement.

Figure 5

Phase development for BC-Buz (top) w/c = 0.50, (bottom) w/c = 0.40, with all details as in Figure 4. Minors-H stands for any minor hydrated phase not accounted for in the explicitly given ones.

Figure 6

Phase development for BC-n.a. (top) w/c = 0.50, (bottom) w/c = 0.40, with all details as in Figure 5.

Figure 7

Mercury intrusion porosimetry cumulative porosity curves for the eighteen studied pastes at 1, 7, and 28 days of hydration. Left panels (a,c,e) display data for the w/c = 0.50 pastes. Right panels (b,d,f) show data for the w/c = 0.40 pastes.