IV.34 Mortars from Roman Cement and their Properties

Transcription

1 IV.34 Mortars from Roman Cement and their Properties Adéla Peterová 1, Kateřina Doubravová 1, Vladimír Machovič 1 and Josef Jiroušek 2 1 Institute of chemical technology Prague, Czech Republic 2 Rabat, Czech Republic Abstract A new fast-setting cement, called Roman cement, appeared recently in the market. To identify the possibilities of using it in the field of cultural heritage, it is important to know as much as possible about its properties. In the Czech Republic, one of the possible uses is as jointing and plastering materials for spongilite walls (spongilite is a highly heterogeneous material with an anisotropic character, this sedimentary rock consists of calcite and different forms of SiO 2 ), as well as a repair material for spongilite architectural details. The first part of our research, presented in this work, studies effects of the addition of various proportions of lime hydrate on the properties of fresh and hardened mortar from Roman cement. First of all, properties of fresh mortar such as water/binder ratio, speed of setting, volume stability and volume density were measured on the different mixes prepared. After 28 days of setting, the hardened mortars were characterized. Volume density, open porosity and pore size distribution were all determined. Then, the samples were subjected to testing: compressive and bending strength and resistance to salt crystallization. In order to characterize the components in the Roman cement, XRD, XRF, infrared spectroscopy and Raman spectrometry were used. The future work will focus on comparing the properties of Roman cement mortars to those of spongilite, in order to determine a mortar based on Roman cement suitable for the use in spongilite restoration work. 1 Introduction Roman cement is a natural high-hydraulic binder. It is manufactured from marlstones and from not very high-purity limestones naturally enriched with a clay component. Roman cement differs from the conventional Portland cement particularly by its lower firing temperature, which is C [1] and is lower 1151

2 than the sintering temperature. Roman cement possesses an ochre colour, which is the deeper the higher the cement hydration degree. This makes for shading of the hardened surface. At sites where the surface is wetted, the binder continues to hydrate after setting. Typical of Roman cement is a very fast start of the setting process, from 7 to 20 minutes [2]. Its workability period depends on temperature and on the amount of setting retarder added [3]. Hydrated lime (lime putty) is an air binder obtained by slaking calcium oxide (lime) obtained by calcination of limestone. The setting process for mortars from hydrated lime consists of 2 phases: (1) drying of the colloidal gel of the lime binder and (2) carbonation in suitable conditions, i.e. in the presence of atmospheric carbon dioxide and some amount of water in the mortar. The mortar drying mechanism includes evaporation of water and absorption of the mix water from the mortar by the underlying material. The setting time is long as compared to other building materials and depends on the resulting layer thickness. The setting time is long due to the low carbon dioxide concentration, and only starts after a partial evaporation of water from large pores. Carbon dioxide dissolves in water and reacts with the lime binder giving rise to calcium carbonate [4, 5]. This work studies effects of the addition of various proportions of lime hydrate on the properties of fresh and hardened mortar from Roman cement. Combination of the two binders results in a higher mortar strength than that achieved with hydrated lime alone; in addition, the strength is attained sooner and the workability time of Roman cements is longer [3]. 2 Experimental 2.1 Materials Hydrated lime (CL90-S Vápenka Čertovy schody a.s., Czech Republic) and Roman cement (Prompt cement, Vicat, France) served as the binders, and pure silica sands (AQUA obnova staveb s.r.o., Czech Republic) were used as the filler (Fig. 1). Citric acid (Tempo setting retarder, Vicat, France) was added in order to slow down the mortar setting process. 100 throughs [wt. %] aperture size of sieve [mm] 10 Fig. 1 Sand particles size distribution (1:2:1, fraction : : mm). 1152

3 2.2 Binder properties Binder mixtures were prepared using variable hydrated lime-to-roman cement weight ratios, i.e. from 0:10 to 10:0, with an addition of 0.6 wt.% of the setting retarder with respect to the Roman cement fraction. The setting times and volume stability were determined for the mixtures. Volume stability is characterized by the quantity Δ l = l 2 l 1 (1) where l 1 is the sleeve feet distance before boil [cm] l 2 is the sleeve feet distance after boil [cm]. 2.3 Mortar and test specimen preparation Mixtures were prepared in the 1:3 binder-to-filler ratio. The binder composition encompassed the weight ratio span from 0:10 to 10:0, with an addition of 0.6 wt.% of the setting retarder with respect to the Roman cement fraction. After addition of water, the mixture was stirred with an electric stirrer at a slow speed for 3.5 minutes. Cubes (4 4 4 cm), beams ( cm), and disks (6.7 1 cm) were prepared from each mixture. The samples were wetted periodically for 28 days from the mortar preparation. Subsequently, carbonation and hydration were discontinued by drying the samples to constant weight at 60 C. The samples so prepared were used to examine their physico-chemical properties (following 24 h acclimation to room temperature and RH). The mortars were labelled xc + yv where x and y were the weight fractions of Roman cement (C) and hydrated lime (V). 2.4 Properties of the hardened mortars The mortar carbonation depth was measured by using an acid-base indicator (0.5% phenolphthalein in ethanol) spread over a fresh specimen section. Compressive strength was determined on 10 cubes (see above) of each mortar, bending strength was determined on 10 beams of each mortar. For water absorption capacity experiments, the test specimens, dried to constant weight, were placed in a container, and water was added to one-half of their height. During the first hour, water was added to fill three-quarters of the specimen height, and in another hour, additional water was added to submerge the specimens completely. In this condition the specimens were allowed to stand for 48 hours. Subsequently, the specimens were placed in a desiccator and evacuated with a water pump. The specimens were weighed, both in the 48 hours and following evacuation, i.e. hydrostatically and in air. The weight data provided 1153

4 water absorption under atmospheric pressure in 48 hours and after evacuation, as well as the open porosity and bulk density of the hardened mortars. Mortar pore size was determined by mercury intrusion porosimetry. The standard test method described in EN Natural stone test methods. Determination of resistance to salt crystallization was modified to determine resistance to salt crystallization. Two specimens of each mortar were submerged in a 14% Na 2 SO 4 solution for 2 hours. Then the specimens were placed in a drier at 60 C and a high initial relative humidity. The specimens were dried to constant weight and subjected to the next cycle. Water vapour permeability was determined as described in EN Methods of test for mortar for masonry. Determination of water vapour permeability of hardened rendering and plastering mortars (saturated KNO 3 solution maintains RH at 93% and LiCl maintains RH at 12% (20 C)). The internal structure of the binders in the mortars was examined by scanning electron microscopy, the phase composition of the hardened mortars was characterized by X-ray diffraction. The hydration process in the mortar prepared from Roman cement was examined by measuring changes in the intensities of the characteristic peaks in the Raman spectrum. 3 Results and discussion In the binding mixture setting speed measurements, only the mixture containing 9 wt. part of lime hydrate set appreciably more slowly than the remaining mixtures (Fig. 2). The result is basically similar to the slow drying process of pure lime mortar. Although setting at a rate similar to that of the remaining mixtures, the 2C+8V mixture exhibited markedly inhomogeneous properties. distance from the ground [mm] time [min] Fig. 2 Time of binders setting. 0V : 10C 1V : 9C 2V : 8C 3V : 7C 4V : 6C 5V : 5C 6V : 4C 7V : 3C 8V : 2C 9V : 1C 10V : 0C In the hardened mortars, the presence of hydrated lime affected both the bending strength and the compressive strength (Fig. 3). The 5C+5V mixture 1154

5 possesses compressive strength twice as high as a pure lime or less than one-half the strength of pure Roman cement mortar. As regards bending strength, as little as 4 parts of hydrated lime reduced the resultant strength to the level of pure lime mortar. Those results are supported by SEM photographs (Fig. 4), clearly displaying two different homogeneously mixed structures, viz needle of AFm phase, globular particles of C-S-H phase and calcite and portlandite crystals from the hardened lime mortar. This is in agreement with the XRD results (Table 1). strength [MPa] 8,0 7,0 6,0 5,0 4,0 3,0 2,0 1,0 0,0 compressive strenght bending strenght 10V 1C + 9V 2C + 8V 3C + 7V 4C + 6V 5C + 5V 6C + 4V 7C + 3V 8C + 2V 9C + 1V 10C mortars Fig. 3 Graph of compressive strength and bending strength. Fig. 4 SEM images of mortars, from the left: 10C, 5C+5V, 1C+9V. Table 1 Composition of some mortars from XRD in relative percentage weight. Formula Name Roman cement 1C+9V 5C+5V 10C CaCO 3 calcite Ca(OH) 2 portlandite SiO 2 quartz CaCO 3 aragonite Ca 2 SiO 4 bellite Ca 2 (FeAl) 2 O 5 brownmillerite Porosity, water absorption and vapour permeability measurements clearly demonstrate the effect of hydrated lime on Roman cement mortar (Table 2). Although exhibiting lower water absorption and porosity and hence, water vapour permeability, mortars with an appreciable fraction of Roman cement do not differ markedly from pure lime mortar. This is also demonstrated by resistance to salt 1155

6 crystallization (Table 3), where no unambiguous trend in dependence on the hydrated lime fraction is observed. The pore size distribution in the mortars is also similar, only, perhaps, mortars with a predominant fraction of Roman cement possess rather fine pores as compared to the pure lime mortar. Table 2 Porosity, water absorption and vapour permeability. Mortar Porosity (mercury porosimetry) [%] Open porosity [%] Water absorption [%] Vapour permeability [g.h -1 ] 10V C+9V C+8V C+7V C+6V C+5V C+4V C+3V C+2V C+1V C Table 3 Determination of resistance to salt crystallization, cycles of destruction samples. Cycle Mortar which was destroyed 1. 2C+8V 2. 3C+7V, 6C+4V 3. 3C+7V, 7C+3V, 10C 4. 2C+8V, 4C+6V, 4C+6V, 5C+5V, 5C+5V, 6C+4V, 8C+2V, 8C+2V, 9C+1V, 10V, 10V 6. 1C+9V, 10C 7. 1C+9V 15. didn t destroy 7C+3V, 9C+1V The Raman spectra show how the bellite (dicalcium silicate) and C-S-H gel peak intensities vary during the Roman cement hydration process (Fig. 5). This process of hydration can be expressed by its predominant chemical reaction: C 2 S + H 2 O C-S-H + Ca(OH) 2. As a result of this process, we can observe that bellite s relative peak intensity decreases while C-S-H gel s relative peak intensity increases. 1156

7 Fig. 5 Raman spectra of the 10C mortar hydration. 4 Conclusion Mixed mortars prepared from Roman cement with a fraction of hydrated lime behave similarly to mortars prepared from pure Roman cement in terms of setting. This process is only slowed down if the fraction of hydrated lime predominates over that of Roman cement. As expected [3], the strength of hardened mortars grows with increasing proportion of Roman cement. An appreciable increase as compared to hydrated lime mortar, however, only occurs if the fraction of Roman cement is the same as or higher than that of hydrated lime. 5 Acknowledgement This study was part of research programme MSM References 1. The Louis Vicat technical center, Materials and microstructures laboratory Special Binders Section (2007) Technical specifications, Prompt natural cement The Roman cement of Grenoble. Accessed 8 September Vicat company (2010) Co je románský cement? Accessed 4 April Peterová A (2010) Vlastnosti malt z románského cementu. VŠCHT, Praha 4. Kotlík P (1999) Stavební materiály historických objektů. VŠCHT, Praha 5. Michoinová D (2007) Studium historických postupů přípravy vápenných malt pro péči o architektonický památkový fond. VUT, Brno 1157