Towards ternary binders involving limestone additions — A review

Abstract

The review summarises literature to examine the transition from portland limestone cement system to composite ternary binder systems involving limestone. Interest in limestone addition as an ideal component in multicomponent binder systems has surged as evident from the large volume of literature published in the recent past. A ternary blended system, with co-substitution of limestone, has the potential to complement the reaction of the supplementary cementitious materials (SCMs). The direct addition of limestone powder helps to attain higher substitution levels of portland cement clinker, improve early age properties, and supplement SCM's reactivity. However, the dilution of hydrates could hamper the long-term benefits. In this review, the interaction of fine limestone is classified and elaborated under two broad umbrellas: physical and chemical interactions. The physical interactions can manifest in three ways, namely, filler action, shearing action and improved packing, which alters reaction rate and extent at early ages. The chemical interactions also modify the reaction kinetics and phase assemblage due to nucleation of C-S-H on calcite surfaces, preservation of the ettringite phase and formation of carboaluminates. Two different forms of carboaluminate hydrates — hemicarboaluminate and mono-carboaluminate can be present in the hydrate matrix depending on the balance between carbonate ions and aluminates in the pore solution. Several factors such as replacement level, particle size, choice of SCM, its reactivity and reactive aluminates content, sulphate levels, curing temperature, and duration of curing can control the carboaluminate formation, reaction degree of SCMs and chemical interaction from limestone additions.

A combination of physical and chemical effects makes fine limestone a potential material for co-substitution with aluminosilicate based SCMs, mainly fly ash, slag, and calcined clay. In this review, the factors affecting limestone-SCM composites are summarised based on a detailed literature survey. The effects of SCM-limestone cement composites on hydration kinetics, reaction chemistry, the reactivity of SCMs, the stability of hydrated phases, and contribution to the physical structure development and macroscopic properties by evaluating hydration and mechanical properties are discussed. The importance of AFm (Al₂O₃–Fe₂O₃-mono) phases in various deterioration mechanisms in concrete and their influence on performance characteristics in different exposure environment is critically appraised.

Introduction

Many strategies to reduce the 5–8% anthropological CO₂ emissions associated with portland clinker production have been put forward. Some of the strategies include — improving energy efficiency of kilns and pre-calciners, alternative fuels, alternative raw materials, new alternative clinkers, improving grinding efficiency, carbon capture and storage, and substituting clinker with supplementary cementitious materials (denoted as SCM hereafter) [1,2]. Among the listed technological solutions, clinker substitution has a potential to accommodate about 37% contribution to the reduction in total CO₂ emissions to realize a sustainable transition to 2 °C emission control scenario by 2050 [3]. Cement consumption is projected to increase 2 to 4-fold in several growing economies. Notably, cement consumption in India will quadruple in the next few decades. The estimates suggest the consumption in India will grow to about 1300–1500 million tonnes by 2050 [3,4]. The clinkerization process has a severe impact on raw material reserves, which are depleting at a rapid rate [5]. One of the major solutions towards sustainability in cement-based materials is substituting clinker by a combination of alternative cement substitutes [6,7].

The environmental concerns related to clinker production and availability of potential substitute material has powered the use of SCM over the last few decades and acknowledged as an effective means of optimizing the clinker ratio through the use of blended cement. A variety of SCMs are available from industrial by-products and thermal treatment of natural resources [8]. The quest to attain energy-efficient cement has led to the inclusion of several SCMs in conjunction, with varying quality and quantity, to improve substitution levels of portland clinker. Based on the research progress made on cement substitutes, an ambitious target of reducing the average global clinker factor from 0.78 to 0.60 has been set for 2050 [3,9,10]. This necessitates advances in the development of sustainable alternatives across the different supply chains of cementitious systems from cement production to application in concrete systems. In a study on different sustainable technologies, cement replacement with calcined clay-limestone was among the most impactful technologies with about 27% reduction in clinker demand and emission control, and the next being reduction of the amount of cement in concrete which can account for 10% reduction in emission [11].

Cement Sustainability Initiative (CSI) has predicted that limestone may constitute about 18% in cementitious materials by 2050 from the current contribution level of 8% [3]. Increase in allowable limits of limestone by several standardisation bodies indicates the growing trends of limestone additions as a reliable solution for composite binders. For the projected cement demand, the SCM requirement is about 500 million tonnes in India alone (assuming clinker factor of 0.6 as target by 2050). A combined substitution of SCMs is likely to accommodate a wide range of SCMs with various levels of quality and reactivity potential, thereby improving the utility of various SCM sources in a binder system. The low emission factor of limestone, nearly 0.008 kg of CO₂ per kg of limestone, makes it a sustainable choice to reduce emission in cementitious materials by moderately and judiciously increasing filler substitution level [1,12,13]. Studies have shown that a ternary binder with a combination of a clinker, pozzolan and limestone could lower the global warming potential (GWP) to 60%, while portland pozzolan cement (with up to 35% pozzolan or fly ash) reduces GWP to 80% in comparison to plain portland cement [14]. In current practice, most international standards permit the use of 5–30% limestone as performance enhancer or in portland limestone cement (PLC) formulations by blending with ordinary portland cement (OPC). Limestone addition has been a subject of debate in the research community with respect to its use as a filler material or a reactive component. Recently, there has been a considerable shift in approach to increase the amount of limestone powder as a cement substitute to explore its chemical potential and reactivity. Also, the cement industry could benefit from an increase in filler content; specifically, in lower grade OPC (33 and 43 grade cement as per cement standard classification available in Indian standard).

Limestone is one of the traditionally used raw materials for manufacturing of OPC. Hence it is readily available in ample quantities to cement manufacturers and can be used to replace a part of the cement at the manufacturing facility itself. No calcination energy, good grindability, reduced transportation, and utilisation of lower grade limestone are some reasons for the growing interest in support of limestone being used as a cement substitute. However, increasing replacement of a partial reactive material as cement constituent would necessitate a substantial amount of research in understanding the impact of limestone additions on the performance [15]. The reactivity of limestone can be improved by co-substitution with an aluminosilicate based SCM to some extent. A recent review by John et al. (2018) on filler usage in cementitious materials has listed the possibilities of lower grade limestone and high Mg-limestone, and summarised the detailed research needs on the use of limestone, dolomites, and other impure mineral forms containing calcite as a component in binders [16]. The increasing interest in such composite cements can be seen by a measure of the increasing global literature on limestone-based composite cements, as presented in Fig. 1. Fig. 1 also shows the distribution of literature based on different thematic areas, country of origin and journals.

In this paper, the existing literature is categorised under relevant performance characteristics with a critical analysis of the interaction mechanisms, theories and contradictions present in the subject area. The paper attempts to summarise the review under the following major questions:

• What is the role of limestone as a cement substitute in composite binders?

• What are the factors governing hydration characteristics of composite binders involving limestone?

• What is the role of limestone on the mechanical performance of composite binders?

• What is the influence of limestone addition on microstructure characteristics of cement composites?

• How does limestone addition in composite binder influence durability behaviour?

In view on the growing interest on ternary binder systems due to sustainability perspectives, the review attempts to capture the evolution of limestone addition from PLC to ternary binders. A comprehensive classification of governing factors that should be considered in binders containing limestone additions with respect to hydration chemistry, microstructural development, mechanical, and durability characteristics performance is presented. Finally, the key gaps in literature to be addressed in research and scaling up utilisation of ternary binder are provided. The need for systematic durability characterization of composite cement s, and the lack of long-term performance studies is also put forward with relevant hypotheses.