Effectiveness of a calcium silicate hydrate – Polycarboxylate ether (C-S-H–PCE) nanocomposite on early strength development of fly ash cement

Highlights

• Synthetic C-S-H–PCE nanocomposite prepared by co-precipitation from Na₂SiO₃/Ca(NO₃)₂.

• C-S-H–PCE enhances early strength (6–24 h) of mortar and concrete of fly ash cement.

• No any adverse effect on final strength.

• C-S-H–PCE accelerates silicate hydration

• C-S-H–PCE also stimulates the pozzolanic reaction of fly ash.

Abstract

C-S-H–PCE nanocomposites are known seeding materials for Portland cement. Here, the effectiveness of a C-S-H–PCE admixture on early strength development of a fly ash blended cement (35 wt% fly ash) was investigated. It was found that addition of C-S-H–PCE drastically increases the compressive strength of mortar and concrete, particularly at 6–24 h of hydration. XRD and calorimetric measurements confirm that the hydration of C₃S/C₂S in cement is greatly accelerated. Most interestingly, however, the C-S-H–PCE nanocomposite also stimulates the pozzolanic reaction of fly ash, thus explaining the exceptionally high early strength values. Accordingly, C-S-H–PCE might also be useful in other CEM II cements holding pozzolanic materials.

Introduction

CO₂ emissions resulting from cement production are owed to the decarbonation of CaCO₃, the calcination and the milling process. About 900 kg of CO₂ are released for every ton of clinker produced, thus producing approximately 5% of global anthropogenic carbon dioxide emissions [1], [2]. Clinker substitution with supplementary cementitious materials (SCMs) such as fly ash, blast furnace slag, limestone etc. such as in blended cements (CEM II/III) allows to reduce CO₂ emission from cement production, making these cements more environmentally friendly. However, the drawback of blended cements is their slow development of early strength owed to the slow pozzolanic reaction of SCMs.

Generally, calcium based salts such as calcium chloride, nitrate or formate are used as accelerators to increase the rate of hydration and to boost the early strength of Portland cement [3]. In fly ash blended cements, several studies reported an increase of early strength through addition of accelerating admixtures such as sodium sulfate (Na₂SO₄) [4], [5] or alkanolamines (e.g. triethanolamine, TEA; diethanolamine, DEA; tri-isopropanolamine, TIPA etc.) [6], [7]. However, those admixtures sometimes led to a decreased final strength.

Recently, several organic-inorganic or pure inorganic nanocomposites have been introduced for the purpose of improving the properties of concrete. Typical examples include polycarboxylate/graphene oxide nanocomposites [8], polycarboxylate/SiO₂ core shell particles [9], TiO₂-coated nano-SiO₂ [10] and SiO₂/TiO₂ nanocomposites [11]. The main effects sought from them were higher compressive and/or tensile strength and anti-bacterial action.

Calcium silicate hydrate (xCaO ⋅ ySiO₂ ⋅ zH₂O, C-S-H) is well known as the primary hydration product of Portland cement. It acts as the binding phase and is accountable for the strength development and durability of hardened cement. Recently, synthetic C-S-H particles have been used as an accelerator to enhance early cement hydration [12], [13], [14], [15], [16]. However, they produce only a minor accelerating effect owed to their relatively large size, possibly due to agglomeration and/or OSTWALD ripening. To avoid these effects, the size of the C-S-H particles can be controlled to nanoscale by the addition of polymeric dispersants such as polycarboxylates (PCEs) [17], [18], [19]. Such particularly small C-S-H particles achieved by specific PCE molecules exhibit a huge surface area which produces an extremely strong seeding effect for the hydration of C₃S/C₂S [20], [21], leading to significantly higher early strength of Portland cement [22], [23], [24], [25]. Consequently, such admixtures based on C-S-H–PCE nanocomposites allow increased production rates e.g. in the manufacturing of precast or prestressed concrete. The improvement of early strength is even more desirable for concrete products made from blended cements. However, so far no report on the effectiveness of C-S-H–PCE nanocomposites on the strength development of a fly ash blended cement has been presented.

In this paper, the effectiveness of a C-S-H–PCE nanocomposite on the development of early strength (6–24 h) of mortar and concrete produced from a fly ash blended cement is investigated. Additionally, the impact of the C-S-H–PCE admixture on the workability of mortar and concrete in terms of superplasticizer consumption to achieve a specific fluidity was addressed. Finally, the working mechanism of the C-S-H–PCE seeding admixture in the blended cement was studied via in-situ X-ray diffraction (XRD) measurements and isothermal heat flow calorimetry.