Study of natural hydraulic lime-based mortars prepared with masonry waste powder as aggregate and diatomite/fly ash as mineral admixtures
Introduction
Ancient cultural relics and historical record revealed that lime with pozzolanic mineral addition was used to prepare hydraulic mortars in ancient civilization (Callebaut et al., 2001, Velosa et al., 2007). Air lime was gradually replaced by hydraulic lime after the discovery of hydraulic binder at 18th century. At the beginning of 20th, Portland cement (PC) became the dominant construction material because of its merits such as faster setting rate and higher mechanical properties (Sabbioni et al., 2001), which led to the shrinkage of hydraulic lime application. Recent years, it was well known that PC mortars presented several problems associated with physicochemical and mechanical properties when they were used to restore cultural relics, and it generated huge damage to the relics (El-Turki et al., 2010a, El-Turki et al., 2010b). NHL has moderate mechanical strength and it can maintain matrix appropriate gas and water permeability. Relevant research demonstrated that the use of NHL will not introduce soluble salt (Kalaitzaki et al., 2005) and it will greatly reduce the pollution of the environment compared with the use of cement (Grist et al., 2015). Application of NHL is experiencing a resurgence because of its superiority in the field of restoration of old relics (El-Turki et al., 2010a, El-Turki et al., 2010b).
With the increase in demand for NHL, the consumption of energy and resources, as well as the generation of carbon dioxide (CO2) and harmful dust, will rise during its production (Tsai et al., 2014). Exploiting alternative fuels and adopting pozzolan as partial cement replacement were both effective approach to enhance sustainability of concrete construction (Gäbel and Tillman, 2005, Pimraksa et al., 2013, Alexandre et al., 2014, Bras et al., 2010, Lucia et al., 2012, Amin et al., 2015). The use of fly ash (Ravina, 1998, Mahdi et al., 2015, Dinakar et al., 2013) and perlite (Yu et al., 2003, Lanzón and García -Ruiz, 2008) as partial substitution of cement enhanced mechanical strength of concrete. Diatomite incorporation also improved compressive and flexural strength of concrete mortars (Ergün, 2011, Fragoulis et al., 2005). Zhao et al. (2015) found that the introduction of fly ash and ground granulated blast furnace slag to the concrete mortars increased initial slump flow, lowered mechanical properties at the early curing period until 90 days. Gritsada Sua-iam and Makul (2014) pointed that compressive strength of concrete mortars decreased with fly ash content and Degirmenci and Yilmaz (2009) concluded that the use of diatomite decreased mechanical strength of concrete mortars. Thus, a certain pozzolan was not suitable for all cementious system, therefore, it is important to design and develop an appropriate formula through experiment.
NHL, mainly including NHL-based grout (Bras and Henriques, 2012, Xu et al., 2014, Xu et al., 2015, Baltazar et al., 2013, Baltazar et al., 2014) and NHL-based mortars (Lanas et al., 2004, Grist et al., 2013, Xu et al., 2014, Xu et al., 2015), was not extensively studied until recent decades. Especially, there were only a few researches on the use of pozzolan as partial replacement of NHL. Grilo et al., 2014a, Grilo et al., 2014b concluded that the addition of metakaolin enhanced strength of NHL-based mortars, and the strength underwent a decrease during later curing period. Iucolano et al. (2013) and Liguori et al. (2015) employed glass fiber for the reinforcement of NHL-based mortars, at the same time NHL was replaced by zeolite at proper proportion. They found that mortars made with chopped glass fiber showed lower plastic shrinkage and durability of mortars was improved, the zeolite addition further enhanced mechanical properties of these mortars.
Diatomite is a common pozzolan and it has been used as replacement in mortars and grouts. Besides, there is relatively abundant resources and accessibility for diatomite (Posi et al., 2013). Fly ash is an industrial by-product, which derives from coal combustion residues with good crystallinity. It is estimated that 50 million tons of fly ash are generated each year in China and it will cause severe pollution without proper utilization. This work selected diatomite and fly ash as partial replacement of NHL to prepare mortars.
It is estimated that 200 million tons of construction waste are generated each year, which accounts for about 40% of urban total waste in China. Masonry (brick and stone) waste that come from modern industry, architecture and scientific research are hazardous to environment. It will also cause environment pollution when natural aggregates are used in the preparation of hydraulic mortars, for that large amount of CO2 will be released during the exploitation of these natural aggregates (Miguel et al., 2015). Therefore, the use of WMP as aggregate in mortar is of great significance from the ecological point of view. So far researchers (Topçu et al., 2009, Hebhoub et al., 2011, Enrique et al., 2015, Uygunoğlu et al., 2014, André et al., 2014) had employed waste marble as aggregate in concrete and had demonstrated the feasibility of waste marble as aggregate to prepare concrete.
Most of the ancient architectural sites are in the open air environment. Mortars are susceptible to deterioration due to acid rain, and acid may also come from groundwater. It had frequently caused the structural damage and incomplete of architecture because of the long exposure to corrosive acid environment and seriously affect stability of the architecture (Fattuhi and Hughes, 1988). Natural weathering processes, such as freeze and thaw, rising damp and salt attack are also key durability issues that affect the mortars' life cycle performance. Sabbioni et al. (2002) found that there were non-water-soluble sulfates (ettringite) in Venice Arsenal structures when hydraulic mortars were used with sulfates material in it, and the formation of ettringite caused severe expansion and cracking of Venice Arsenal structures. So these issues associated with aggressive environments should be considered when hydraulic mortars were used to restore or reinforce the architecture relics.
The aim of this work is to prepare mortars suitable for masonry architecture restoration. Diatomite and fly ash were utilized to replace partial NHL, and MWP was used as the aggregate to prepare mortars. Feasibility of the use of diatomite and fly ash in NHL-based mortars was evaluated. Physical and mechanical properties of mortars with different formulations were investigated. The possible mechanism of the property improvement was also proposed and discussed.
Section snippets
NHL
The main binder NHL2 was supplied by Chaux De Saint-Astier company, France. The chemical composition of NHL2 was detected by X-ray fluorescence analysis (XRF, Philips PW1010 XRF) spectrometer and the results were shown in Table 1. The mineralogical compositions of NHL2 were obtained by using Rigaku D/max 2200 X-ray diffractometer. XRD results showed that the main components in NHL2 include dicalcium silicate (C2S), portlandite, calcite and small amount of quartz.
Mineral admixtures
Diatomite and fly ash were the
Main physical properties of mortars
Table 3 shows flow values of mortars in fresh state, and apparent density/water absorption of mortars in hardened state. Flow values of mortars with diatomite incorporation decreased, and they decreased with increasing diatomite content. This is due to the porous structure of diatomite which could store certain amount of water. Diatomite has stronger water absorption property than NHL2, the increased amount of absorbed water would reduce mortars' flow value under the same W/B condition (Hebhoub
Conclusions
Mortars containing diatomite and fly ash were successfully prepared, and they had excellent workability with flow values above 185 mm which could ensure successful construction. The presence of diatomite and fly ash reduced density of mortars, and effectively reduced the total amount of raw materials, especially the amount of NHL, to prepare same volume mortars. Lightweight mortars are also advantageous for reducing weight of architecture and enhancing bearing capacity and seismic capacity of
Acknowledgments
The financial support from national “Twelfth Five-Year” Plan for Science & Technology Support No. 2013BAK01B00 and the technical support from Chinese Academy of Cultural Heritage are highly appreciated by all authors.
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