Factors affecting the slump and strength development of geopolymer concrete

Highlights

• Significant practical knowledge beneficial for geopolymer production.

• A practical approach to determine the volume-to-mass conversion of NaOH solution.

• Recommendations for wax-based mould agents for successful de-moulding.

• Increasing GGBS/FA ratio decreased slump but increased compressive strength.

• Curing at 10 °C retarded compressive strength development; over 30 MPa was achieved.

Abstract

Cement production is estimated to be responsible for 5–8% of global total carbon dioxide (CO₂) emissions. Geopolymer concrete (GC) is claimed to release up to 45% less CO₂ for a comparable concrete, but is more difficult to manufacture. This study investigated the effect of factors other than mix design on the slump and strength development of GC produced from low-calcium fly ash (FA) and up to 50% ground granulated blastfurnace slag (GGBS) replacement; these were: curing methods and temperatures (at 10, 20 and 75 °C); FA fineness; superplasticiser type; water content; and GGBS/FA ratio. Methods are also presented for the volume-to-mass conversion of sodium hydroxide at a specific molarity and ambient temperature, and an effective combination of wax-based mixtures as mould agents to overcome the inherent manufacturing mould release difficulties. Steam-curing improved the compressive strength of the FA-based GC over oven drying by up to 20%, as did increasing the FA fineness (although this became negligible with 50% GGBS replacement). Both naphthalene and polycarboxylate superplasticizers improved the slump of GC (from 110 to 210 mm) without significantly reducing the compressive strength (less than 5 MPa). Water content of GC had a great effect on the slump, but less so on the compressive strength. Increasing the GGBS content gradually decreased the slump but rapidly increased the strength, regardless of the curing temperatures of 20 or 75 °C. The GC with the minimum of 20% GGBS replacement achieved 33 MPa after 28 days curing at 10 °C. Air-dry curing provided a greater strength development of FA-based GC than water curing, though the opposite was observed for the 50/50 GGBS/FA GC. Consideration of these factors can significantly ease the manufacture of GC, enhancing its potential application in real structures, and consequently helping reduce global (CO₂) emissions.

Introduction

Portland cement (PC), the primary binder to produce concrete, has become the most man-made material consumed worldwide with a total global production of 4.6 Gt/year [1]. In addition, cement production is estimated to be responsible for 5–8% of global total carbon dioxide (CO₂) emissions [2], [3]. To try and address this issue, many approaches have been considered e.g. reducing CO₂ emissions from PC production by burning waste raw materials, increasing efficiency of the manufacturing process, and developing CO₂ capture and storage [4]; incorporating a higher percentage of mineral admixtures such as fly ash (FA) and ground granulated blastfurnace slag (GGBS) in blended PC [5], [6]; and adopting alternative cement-like binders [7], [8].

Alkali-activated materials (AAMs) are a promising alternative to PC. They are synthesised from the reaction product of an amorphous aluminosilicate-rich powder (binder) activated by an alkali metal source [9], which can create concrete with up to 45% less CO₂ than a typical PC-based counterpart [10], [11]. The binder can come from natural sources such as metakaolin (MK) and pozzolan, or industrial by-products such as GGBS and FA, whilst the alkaline activators include alkali hydroxides, silicates and carbonates [12]. It has been shown that alkali-activated concrete can have comparable mechanical properties to PC concrete, as well as improvements in resistance to fire and aggressive chemical attack [13], [14].

Depending on the source of aluminosilicate, AAMs can be categorised into two key gel systems [9], [15]. If geopolymers [16], [17], [18], the activation products of low-calcium binder sources including FA and/or MK are dominated by amorphous three-dimensional alkaline aluminosilicate hydrate (N-A-S-H) gels [19], the high-calcium sources such as alkali activated GGBS have the reaction products dominated by alkali charge-balanced aluminium-substituted calcium silicate hydrate (C-A-S-H) gels, which are similar to the C-S-H gels observed in PC [20], [21]. In between these systems, the coexistence of N-A-S-(H) and C-A-S-H gels have been observed by alkaline activation of FA/GGBS [22], [23] where the dominant product is strongly influenced by the FA/GGBS ratio. This study used the term geopolymer concrete (GC) because it focused on the slump and strength development of low calcium FA-based systems with up to 50% GGBS replacement, part of a larger research programme investigating the performance of the GC initially cured at a range of temperatures from 10 to 75 °C, to be published elsewhere [24], [25].

In order to develop an optimal mix design for GC, the main factors which affect the mechanical strength development must be established. Many studies on GC have concentrated on identifying optimal mix proportions based on the aggregate content, alkaline activator-to-binder ratio, molarity of sodium hydroxide, sodium silicate-to-sodium hydroxide ratio, superplasticizer-to-binder ratio, and water-to-solid ratio, and in particular the influence of curing temperature and duration [26], [27], [28], [29], [30]. Several guidelines have also been published [31], [32], [33], [34]. However, other factors, regardless of the mix proportions, such as initial curing method by steam or oven [26], binder particle size [35], [36], [37], superplasticizer type [26], [38], [39], [40], [41], [42], [43], [44], [45], [46], and in-air or -water curing [47] also have significant effects on the workability or strength development of GCs. These lack systematic investigation and were therefore investigated in this study.

More specifically, this study examined seven factors that influence the performance of GC (listed in the first columns of Table 2 in Section 2.2) including slump and strength development, as follows:

• Initial curing conditions: Oven vs Steam;

• Finest of FA: type N vs type S;

• Types of superplasticiser: two naphthalenes (Conplast Sp430 and Daracem 215) vs two polycarboxylates (Viscocrete 10 and Glenium 51);

• Water/solid ratios (with Alkaline/binder ratios of 0.3 ad 0.4);

• FA replacement with up to 50% GGBS (with GGBS/binder ratios of 0; 0.1; 0.2; 0.3; 0.4; and 0.5), initially cured at 20 and 75 °C;

• Initial curing at ambient temperatures of 10 (for at least 7 days) and 20 °C;

• In-air or -water curing conditions until being tested after the initial curing at 20 or 75 °C.

Practical mould release agents were also trialled and a volume-to-mass conversion method was developed to identify the mass of solid sodium hydroxide dissolved into water at a specified molarity and ambient temperature to produce an 1-kg sodium hydroxide solution. The strength development of GGBS/FA GC cured at as low as 10 °C, has never been reported, but is important for the application of GC in temperate climates such as Europe.