Cements with a high limestone content – Mechanical properties, durability and ecological characteristics of the concrete

Highlights

• Cements with limestone content up to 50 wt.-% can be used by an adopted concrete technology and reduction of w/c ratio to about 0.35.

• Limestone seems not to be totally inert component. The contribution of limestone to the compressive strength is also remarkable when higher amount of Portland cement clinker is replaced with limestone.

• The production of concretes made of limestone-rich cements exhibited roughly 25% less CO₂ emission, but needs approximately the same energy demand.

Abstract

This paper deals with the performance of concretes made of cements containing high levels of limestone between 35 and 65 wt.-%. The Article mainly focuses on cements with 50 wt.-% limestone. Several experiments regarding the fresh and hardened concrete properties were carried out. Chloride penetration, freeze-thaw resistance, carbonation resistance and long-term deformation behavior were analyzed.

The results show that concretes with cements containing up to 50 wt.-% limestone and a water/cement-ratio of 0.35 may have sufficient properties for practical application if a stringent supervision is ensured. Furthermore, these concretes can exhibit mechanical and durability properties comparable to concretes according to EN 206-1 and the German national application document DIN 1045-2 made of EN 197-1 cements. Besides, the results revealed that these properties depend highly on the limestone characteristics. Life cycle assessment analysis revealed that a cut-off up to 25% in global warming potential of concretes made with such cements is achievable in comparison with German average cement with the same performance.

Introduction

Concrete is known as the most widely used building material of our time. The major environmental impact of concrete comes from the CO₂ emissions during cement production, which is altogether responsible for more than five percent of global anthropogenic CO₂ release. In 2013 more than 4 billion tons of cement were produced and mainly used in the concrete industry [1]. The CO₂ emissions are mainly related to the decalcination of the limestone, the fuel and the electricity consumption [2]. It is visible that the reduction of the Portland cement clinker content in cement will reduce the environmental impact of concrete.

The Portland cement clinker content in cement can be reduced by using supplementary cementitious materials (like ground granulated blast furnace slag (GGBFS) as a latent hydraulic component and fly ash according to DIN EN 197-1 as a pozzolanic component). In addition to the reduction of the environmental impact [3] the mechanical and durability properties can be improved.

Cements with GGBFS are allowed in practice for many decades, e. g. in Germany or the Netherlands. Due to the technical, environmental and the economic benefits, Portland composite cements (CEM II) were developed and increasingly used since early 1990’s especially in Europe (see Fig. 1). In Portland composite cements the clinker is efficiently used together with other main constituents like GGBFS, fly ash, silica fume and limestone to certain ratios. EN 197-1 limits the amount of main constituents besides Portland cement clinker in CEM II cements to maximum value of 35 wt.-%. However, within the last years the production of CEM II cements has reached a constant level, for example in Germany due to the locally limited availability of GGBFS and fly ash (see Fig. 1).

As shown in Fig. 2, the total amount of produced GGBFS and fly ash are nearly completely used by the cement and concrete industry in Germany. This means that an increased use of these constituents in the cement and concrete industry is not probably possible in many countries.

For decades, Portland limestone cement CEM II-LL is used widely in Europe [7] in accordance with the European cement standard EN 197-1. For instance, in year 2012 near 27% of total produced CEM II in Europe were Portland limestone cements (see Fig. 3). The use of cements with a limestone content up to 15 wt.-% is now allowed also in the US and Canada since 2013 and 2008, respectively [8], [9]. In Europe, the cement type CEM II/B-LL can even contain limestone up to 35 wt.-%. According to EN 197-1, the total organic carbon (TOC) of LL limestone is limited to 0.2 wt.-%, and the content of clay minerals (obtained from methylene blue test) in limestone must not exceed 1.2 wt.-%. The CaCO₃ content of the limestone is not allowed to be lower than 75 wt.-%. However, the use of such cements in concrete is restricted on national levels for severe environmental exposures.

Several efforts by many researchers worldwide have been carried out to develop cements which are not based on Portland cement clinker anymore [10], [11], [12], [13], [14], [15]. Many of these solutions could have considerable environmental benefits. However, for the time being either their technical suitability or their economic value and the availability of the constituents are often not verified. Therefore, the efficient use of Portland cement clinker in cement with an increased content of e.g. limestone is worth to be further investigated and developed.

Mechanical performance and durability of concrete made of Portland limestone cement with up to 20 wt.-% limestone are already investigated in depth, e.g. [16], [17], [18], [19], [20], [21]. Although limestone is usually considered as an inert component (e.g. [16]), it may influence the microstructure positively and improve the mechanical properties as well as the durability of concrete [16], [22], [23], [24], [25]. Voglis et al. [22] observed that presence of limestone in Portland limestone cement paste increases the early strength due to formation of tricalcium aluminum carbonate hydrates (3CaOAl₂O₃·CaCO₃·11H₂O). Stark et al. [26] reported that addition of 6 wt.-% of limestone influences the reaction products of C₃S, C₃A and C₄AF which affects the early strength of cement up to four days. Lothenbach et al. indicated that addition of finely ground calcite can accelerate the hydration slightly by providing more surfaces for nucleation and hydration products [24]. A similar conclusion was also made by Proske et al. when using very fine limestone fillers with a Blaine specific surface area of about 16,000 cm²/g [27]. As a result of the aforementioned alterations of the microstructure, a certain amount of limestone may contribute to the development of the compressive strength [28], [29], [30], [31].

The durability of concrete with Portland limestone cement with up to 20 wt.-% limestone against freeze-thaw attack with and without de-icing salts was amply evaluated by several researchers [7], [17], [32], [33]. Most of them found that concrete specimens with Portland limestone cement (CEM II-L and LL) can exhibit more or less the same resistance against freeze-thaw attack in comparison with samples composed with Portland cement. Sprung and Siebel [16] showed a relatively low freeze-thaw resistance of concrete specimens with Portland limestone cement, if the limestone does not meet the requirements of EN 197-1. Schmidt [17] found as well that the influence of the limestone quality on the durability is remarkable.

The carbonation resistance of concrete containing cement with limestone up to 20 wt.-% was reported to be similar to those with Portland cement [34], [35]. Schmidt [17] reported that concretes with Portland limestone cement show a slightly higher carbonation depth compared to concretes with Portland cement but they exhibit better resistance against carbonation in comparison with concrete using GGBFS- and fly ash-cements. Barker and Matthews [36] concluded that irrespective for concrete with Portland limestone cement, the carbonation resistance is directly related to the compressive strength. Such behavior was also reported by Hainer [29] and Dhir et al. [37] for cements with up to 50 wt.-% limestone. In similar studies, it was observed that considering the same compressive strength, slightly lower resistance was observed for concrete samples made of Portland limestone cement in comparison to those with Portland cement [27], [37]. Studies showed that the chloride ion permeability of concretes made of Portland limestone cements is highly dependent on their w/c-ratios [18], [37], [38]. Increase of limestone content up to 45 wt.-% worsens the chloride ion diffusion resistance for w/c-ratios between 0.45 and 0.65 [38]. Tsivilis [39] concluded that the Portland limestone cement with 20 wt.-% limestone can exhibit a competitive resistance against permeability of chloride ion compared to samples with Portland cement.

The influence of limestone content up to 20 wt.-% and higher on the long-term deformation is discussed in [37], [38]. In some studies, different conclusions were drawn for creep and shrinkage. Espion [40] observed that shrinkage of ternary cements with limestone content up to 30 wt.-% is similar to that of CEM I and could be precisely predicted by available prediction models. In contrast to the shrinkage, he reported that the creep coefficient of such cements is higher than values of CEM I and the predicted values are mostly underestimated. Comparable creep and shrinkage values were reported for concretes with high amounts of GGBFS and limestone as concrete additions [41], [42], [43].

The particle size distribution and the fineness of limestone play a key role in the fresh and hardened concrete properties. Parts of the limestone should be ground finer than clinker to serve as nucleation sites and therefore enhance the progress of hydration [44]. Other parts of the limestone should be ground coarser than clinker to provide a broader particle size distribution and therefore a better packing density and a lower water demand.

These works show that concretes made of cements with high limestone contents (up to 50 wt.-% or higher) under conventional concrete technology conditions (e.g. low packing density and high w/c-ratios >0.45) cannot fulfil the requirements to be used in practical purposes, especially due to durability issues. Modifications of conventional concrete technology are essential when the application of concretes made of cements with high limestone contents (up to 50 wt.-%) is aimed.

As was mentioned above, the performance of concrete made of Portland limestone cement with a limestone content above 20–35 wt.-% and common water/cementratios is reported to be critical [45]. A desirable durability of the concrete cannot be achieved unless the water/cement-ratio is reduced. Principles for the development of such concrete with reduced clinker and water content are described in [27]. This procedure suggests the use of high performance superplasticizer. Furthermore, the optimization of the packing density is advantageous. This allows the reduction of the water demand and therefore simultaneously the minimization of the Portland cement clinker content in the cement [27], [46], [47].

A sufficient workability of the concrete must be provided. Neto and Campiteli [48] observed that the addition of limestone up to 15 wt.-% increases the viscosity but reduces the flow yield stress. The reduction of the w/c-ratio and the total water (which is necessary for concretes with very high limestone content) is limited by the requirements on the rheological properties. If the water content is reduced, the plastic viscosity of the concrete will generally increase. Proske et al. [49] concluded that the modification in particle size distribution of both cement and concrete can reduce the plastic viscosity to an acceptable extent, even for concretes with low water and high limestone powder content.

The aim of the present study is to develop and analyze Portland limestone cements with high limestone content of up to 65 wt.-% for the production of sustainable concrete structures in both laboratory and plant scales. In this article, mainly results of cements with 50 wt.-% limestone are shown. To evaluate the performance of these concretes, the fresh and hardened properties were compared to those of reference concretes. To achieve sufficient strength and durability, the w/c—ratio had to be adjusted. Extensive experiments were conducted to answer questions regarding the fresh and hardened concrete properties as well as long-term characteristics of concretes made of such limestonerich cements.