1. INTRODUCTION
⌅OPC production is related to large-scale extraction of non-renewable raw materials, has high CO2 emission values and the energy consumption in the production process is high (11. Pacheco-Torgal F, Cabeza LF, Labrincha J, De Magalhães AG. 2014. Eco-efficient construction and building materials: life cycle assessment (LCA), eco-labelling and case studies. Woodhead Publishing, Cambridge., 22. Ortiz O, Castells F, Sonnemann G. 2009. Sustainability in the construction industry: A review of recent developments based on LCA. Constr. Build. Mater. 23(1):28-39. https://doi.org/10.1016/j.conbuildmat.2007.11.012). The operation and production technologies of cement plants worldwide are gradually being modified to eliminate negative environmental impacts (33. Schneider M, Romer M, Tschudin M, Bolio H. 2011. Sustainable cement production—present and future. Cem. Concr. Res. 41(7): 642-650. https://doi.org/10.1016/j.cemconres.2011.03.019, 44. Salas DA, Ramírez AD, Rodríguez CR, Petroche DM, Boero AJ, Duque-Rivera R. 2016. Environmental impacts, life cycle assessment and potential improvement measures for cement production: a literature review. J. Clean. Prod. 113:114-122. https://doi.org/10.1016/j.jclepro.2015.11.078, 55. Georgiades M, Shah IH, Steubing B, Cheeseman C, Myers RJ. 2023. Prospective life cycle assessment of European cement production.Resour. Conserv. Recycl. 194:106998. https://doi.org/10.1016/j.resconrec.2023.106998). Along with the trend of innovation in OPC production, current research in this area is focused on the development of new hydraulic binders with low impact categories (66. Monteiro PJM, Miller SA, Horvath A. 2017. Towards sustainable concrete. Nat. Mater. 16(7):698-699. https://doi.org/10.1038/nmat4930,77. Aïtcin PC, Mindess S. 2011. Sustainability of concrete. CRC Press, London.,88. Palomo A, Monteiro P, Martauz P, Bilek V, Fernández-Jiménez A. 2019. Hybrid binders: A journey from the past to a sustainable future (opus caementicium futurum). Cem. Concr. Res. 124:105829. https://doi.org/10.1016/j.cemconres.2019.105829,99. Staněk T, Sulovský P. 2015. Active low-energy belite cement. Cem. Concr. Res. 68:203-210. https://doi.org/10.1016/j.cemconres.2014.11.004). Historical building structures are also inspiring in this regard (1010. Yaseen IAB, Al-Amoush H, Al-Farajat M, Mayyas A. 2013. Petrography and mineralogy of Roman mortars from buildings of the ancient city of Jerash, Jordan. Constr. Build. Mater. 38:465-471. https://doi.org/10.1016/j.conbuildmat.2012.08.022, 1111. Ushizima D, Xu K, Monteiro PJM. 2020. Materials data science for microstructural characterization of archaeological concrete. MRS Adv. 5(7):305-318. https://doi.org/10.1557/adv.2020.131, 1212. Delatte NJ. 2001. Lessons from Roman cement and concrete. J. Prof. Issues Eng. Educ. Pract. 127(3):109-115. https://doi.org/10.1061/(ASCE)1052-3928(2001)127:3(109), 1313. Elsen J, Jackson MD, Ruiz-Agudo E. 2022. Historic concrete science: opus caementicium to “natural cements”. Elements: An International Magazine of Mineralogy, Geochemistry, and Petrology, 18(5):301-307. https://doi.org/10.2138/gselements.18.5.301). As it turns out, historical hydraulic mortars resist various degradation factors and achieve high durability in the long term (1414. Hewlett P, Liska M. 2019. Lea’s chemistry of cement and concrete, fifth ed. Butterworth-Heinemann, Oxford., 1515. Jackson MD, Oleson JP, Moon J, Zhang Y, Chen H, Gudmundsson MT. 2018. Extreme durability in ancient Roman concretes. Am. Ceram. Soc. Bull. 97(5):22-28. https://ceramics.org/wp-content/uploads/2021/05/Extreme-durability-in-ancient-Roman-concretes.pdf). Currently, worldwide, there is interest in studying these historic materials and technologies for at least three reasons:
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Development of specific materials for the restoration of cultural heritage (1616. Andrejkovičová S, Maljaee H, Rocha D, Rocha F, Soares MR, Velosa A. 2022. Mortars for conservation of late 19th and early 20th century buildings—combination of natural cements with air lime. Materials, 15(10):3704. https://doi.org/10.3390/ma15103704)
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Development of environmentally acceptable modern building materials
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Carbon sequestration
Natural cement, also known as Roman Cement in Europe, was a popular building material in the 19th century and early 20th century in Europe and America. Raw material with a higher proportion of clay was used to produce NC and the calcination temperature was significantly lower than that of modern OPCs (1717. Francis AJ. 1977. The cement industry 1796–1914: a history. David & Charles, Newton Abbot., 1818. Weber J, Gadermayr N, Bayer K, Hughes D, Kozlowski R, Stillhammerova M, Ullrich D, Vyskocilova R. 2007. Roman cement mortars in Europe’s architectural heritage of the 19th century. J. ASTM Int. 4(8):1-15. https://doi.org/10.1520/JAI100667, 1919. Hughes DC, Sugden DB, Jaglin D, Mucha D. 2008. Calcination of Roman cement: A pilot study using cement-stones from Whitby. Constr. Build. Mater. 22:1446-1455. https://doi.org/10.1016/j.conbuildmat.2007.04.003, 2020. Eckel EC. 1922. Cements, limes and plasters, second ed. John Wiley & Sons, New York.).
The calcination temperature of the raw material in current commercially produced NCs (2121. Hugnes D, Weber J, Starinieri V, Pinter F, Gosselin C, Feldman S, Pesce C. 2019. European natural cements – their key technical properties under standardised conditions, in: Alvarez JI, Fernandez JM, Navarro I, Duran A, Sirera R (Eds.). Proceedings PRO130: 5th Historic Mortars Conference. RILEM Publications SARL, Pamplona, pp. 833–847.) depends on its composition and is approximately in the range of 800-1200°C. The specific composition of the raw material, calcination temperature, calcination time, mode of cooling and other factors influence the final phase composition of the NC (2222. Gosselin C, Scrivener KL, Feldman SB, Schwarz W. 2012. The Hydration of Modern Roman Cements Used for Current Architectural Conservation, in: J. Válek, J. Hughes, C. Groot, (Eds), Historic Mortars. RILEM Bookseries, Springer, Dordrecht, pp. 297-308. https://doi.org/10.1007/978-94-007-4635-0_23, 2323. Tišlová R. 2009. Hydration of Natural Cements, Tribun EU, Brno., 2424. Hughes DC, Daglin D, Kozlowski R, Mucha D. 2009. Roman cements—Belite cements calcined at low temperature, Cem. Concr. Res. 39(2):77-89. https://doi.org/10.1016/j.cemconres.2008.11.010). For this reason, the structure and properties of these binders are highly variable. This fact is a certain disadvantage in the practical application of NC in mortars and concretes and in relation to current standardization.
The main hydraulic components of NC are crystalline belites (dicalcium silicates - C2S) and amorphous aluminates. The stable belites in cements calcined at lower temperatures are largely of the α’C2S type. At higher calcination temperatures, βC2S is stabilized. The proportion of belitic phases can be up to 60 % (2323. Tišlová R. 2009. Hydration of Natural Cements, Tribun EU, Brno., 2525. Chatterjee AK. 1996. High belite cements—Present status and future technological options: Part I. Cem. Concr. Res. 26(8):1213-1225. https://doi.org/10.1016/0008-8846(96)00099-3, 2626. Popescu DC, Muntean M, Sharp JH. 2003. Industrial trial production of low energy belite cement. Cem. Concr. Compos. 25(7):689-693. https://doi.org/10.1016/S0958-9465(02)00097-5).
In terms of practical applications of NC in mortars and concretes, the strategic challenge is to retard the start of the mortar setting while maintaining their acceptable consistency and workability time. At the same time, it is necessary to achieve the designed flexural tensile strength and compressive strength parameters of the hardened mortar or concrete.
Natural cement is characterized by rapid setting, usually within 7-10 minutes of adding water. This is very inconvenient from the point of view of applying this material to construction.
Various methods are currently used to retard the start of setting of fresh mix (also referred to as retardation). The most widely used method is the application of retarding additives such as: citric acid, sodium citrate, potassium citrate, sodium gluconate, gypsum, lime, etc.
As expected, citric acid inhibits the formation of C-A-H (calcium aluminate hydrates), AFm phases (hexagonal calcium ferro-aluminate hydrates); ettringite formation has been shown to be affected. The observation is consistent with the assumption that citrate ions act as a chelating agent due to their preferential coordination at Ca2+, Al3+ or Fe3+ centres. This reaction produces very stable chelate complexes of iron (II or III) hydroxocitrates with aluminium, which affect the nucleation and subsequent crystal growth of AFm or AFt (trigonal calcium ferro aluminate hydrates) phases (2727. Schwarz W, Sujata K, Jennings HM, Gerdes A, Sadouki H, Wittmann FH. 1997. Chemically modified hydration of Portland cement and its implication for the kinetics of the hydration of ordinary Portland cement. In: Second International RILEM Symposium on Hydration and Setting. RILEM Publications SARL, Paris. p. 235–253. https://www.rilem.net/publication/publication/18?id_papier=1615
28. Ozkul MH. 2000. Utilization of citro-and desulphogypsum as set retarders in Portland cement. Cem. Concr. Res. 30(11):1755-1758. https://doi.org/10.1016/S0008-8846(00)00409-9
29. Cody AM, Lee H, Cody RD, Spry PG. 2004. The effects of chemical environment on the nucleation, growth, and stability of ettringite [Ca3Al(OH)6]2(SO4)3.26H2O]. Cem. Coc. Res 34(5):869-881. https://doi.org/10.1016/j.cemconres.2003.10.023-3030. Rai S, Chaturvedi S, Singh NB. 2004. Examination of Portland cement paste hydrated in the presence of malic acid, Cem. Noc. Res. 34(3):455-462. https://doi.org/10.1016/j.cemconres.2003.08.024).
However, some retarders can adversely affect the strength of hardened mortar (2121. Hugnes D, Weber J, Starinieri V, Pinter F, Gosselin C, Feldman S, Pesce C. 2019. European natural cements – their key technical properties under standardised conditions, in: Alvarez JI, Fernandez JM, Navarro I, Duran A, Sirera R (Eds.). Proceedings PRO130: 5th Historic Mortars Conference. RILEM Publications SARL, Pamplona, pp. 833–847., 3131. Heikal M, El-Didamony H, Kishar EA, Negeada MA. 2015. Hydration characteristics of prompt cement in the presence citric acid as retarder. Ceram.–Silik. 59(1):17-23. https://www.ceramics-silikaty.cz/2015/pdf/2015_01_017.pdf, 3232. Daňková J, Valentová A, Koudelková P, Špaldová M. 2024. Possibilities of retarding the setting of PROMPT natural cement by dosing lignosulfonates, in: Journal of Physics: Conference Series (Vol. 2792, No. 1, p. 012002). IOP Publishing. https://doi.org/10.1088/1742-6596/2792/1/012002). Conversely, some retarders and retardation techniques can increase the long-term strength of hardened mortar, although in a specific way (2424. Hughes DC, Daglin D, Kozlowski R, Mucha D. 2009. Roman cements—Belite cements calcined at low temperature, Cem. Concr. Res. 39(2):77-89. https://doi.org/10.1016/j.cemconres.2008.11.010, 3333. Starinieri V, Hughes DC, Wilk D. 2013. Influence of the combination of Roman cement and lime as the binder phase in render mortars for restoration. Constr. Build. Mater. 44:192-199. https://doi.org/10.1016/j.conbuildmat.2013.03.012). The suitability of using a particular chemical retarder and/or retardation technique for all types of NC cannot be conclusively confirmed.
Among the historical retardation technologies for NC-based mortars, the pre-hydration method is described. The pre-hydration method has been experimentally investigated by Starinieri et al (3333. Starinieri V, Hughes DC, Wilk D. 2013. Influence of the combination of Roman cement and lime as the binder phase in render mortars for restoration. Constr. Build. Mater. 44:192-199. https://doi.org/10.1016/j.conbuildmat.2013.03.012, 3434. Starinieri V, Hughes DC, Gosselin C, Wilk D, Bayer K. 2013. Pre-hydration as a technique for the retardation of Roman cement mortars. Cem. Concr. Res. 46:1-13. https://doi.org/10.1016/j.cemconres.2013.01.004). The authors of this study were able to show that for NC made from raw materials originating from Gartenau, Austria, the start of the mortar setting can be extended to several hours by the pre-hydration process. However, there are a number of factors that influence both the retardation time and the final properties of the hardened mortar. In this study, the authors state that the pre-hydration method is not suitable for all NCs. For example, the natural cement PROMPT (Vicat, France) did not respond convincingly to this method (3434. Starinieri V, Hughes DC, Gosselin C, Wilk D, Bayer K. 2013. Pre-hydration as a technique for the retardation of Roman cement mortars. Cem. Concr. Res. 46:1-13. https://doi.org/10.1016/j.cemconres.2013.01.004).
The workability time of the mortar can be extended by increasing the value of the w/c ratio. However, for NC-based mortars, the increase in workability time by this method is not very significant. The value of the w/c ratio is also related to the effect on the structure and final functional properties of the hardened mortar (3535. Wilk D, Bratasz L, Kozłowski R. 2013. Shrinkage cracking in Roman cement pastes and mortars. Cem. Concr. Res. 53:168-175. https://doi.org/10.1016/j.cemconres.2013.03.012, 3636. Klisińska – Kopacz A, Tišlová R. 2013. The effect of composition of roman cement repair mortars on their salt crystallization resistance and adhesion, Procedia Eng. 57:565-571. https://doi.org/10.1016/j.proeng.2013.04.072).
Another historical mortar retardation technology is remixing (hereafter remixing = repeated and/or prolonged mechanical mixing of fresh concrete and mortar). This practical experience has been mostly orally transmitted since the 19th century. Previously, when the fresh mortar mixture began to set on site, the plasterers would remix it several times, thus ‘reviving’ the mortar. This experience in sample preparation is also described by Starinieri et al (3434. Starinieri V, Hughes DC, Gosselin C, Wilk D, Bayer K. 2013. Pre-hydration as a technique for the retardation of Roman cement mortars. Cem. Concr. Res. 46:1-13. https://doi.org/10.1016/j.cemconres.2013.01.004) but they do not investigate or discuss this phenomenon further.
The effect of remixing is described in older literature (3737. Keil F, Gille F. 1938. Unterbrochene und längere Verarbeitung von Beton (Intermittent and prolonged processing of concrete). Zementverlag, Berlin.
38. Bechyně S. 1954. Technologie betonu – svazek 1 (Technology of concrete – volume 1). STNL, Praha.-3939. Jethi NG. 1971. Effect of repeated revibration and steam curing on the strength of concrete. The University of Manitoba, master thesis. http://hdl.handle.net/1993/15956). Bechyně (3838. Bechyně S. 1954. Technologie betonu – svazek 1 (Technology of concrete – volume 1). STNL, Praha.) states that the processing of fresh concrete (or mortar with hydraulic binder) is not necessarily completed before the start of the mortar setting. The concrete mix can be processed if it is “live” (if the mix remains ductile). Bechyně further states that mechanical intervention in the setting mix, although brutal and requiring special machinery and adequate formwork design, is very effective. After curing, the concrete remixed in this way shows high densities and significantly higher strength values (3838. Bechyně S. 1954. Technologie betonu – svazek 1 (Technology of concrete – volume 1). STNL, Praha.).
In recent years, there has been a growing interest among researchers in verifying and explaining the effects of remixing and/or modifying the mixing process in fresh concrete, cement mortars and pastes (4040. Bashandy AA. 2012. The feasibility of remixing concrete, Eng. Res. J. 10:135.
41. Kassim MM. 2012. Effects of revibration on early age retarded concrete, in: Wilde WP, Brebbia CA, Hernández S (Eds.). High-Performance Structures and Materials VI. WIT Press, Southampton, pp. 85–93.
42. Han D, Ferron RD. 2015. Effect of mixing method on microstructure and rheology of cement paste. Constr. Build. Mater. 93:278-288. https://doi.org/10.1016/j.conbuildmat.2015.05.124
43. Sereewatthanawut I, Panwisawas C, Ngamkhanong C, Prasittisopin L. 2023. Effects of extended mixing processes on fresh, hardened and durable properties of cement systems incorporating fly ash. Sci Rep 13:6091. https://doi.org/10.1038/s41598-023-33312-x-4444. Trejo D, Prasittisopin L. 2016. Effects of mixing variables on early-age characteristics of portland cement systems. J. Mater. Civ. 28(10):04016094. https://doi.org/10.1061/(ASCE)MT.1943-5533.0001610). This comes especially in the context of the development of new, more environmentally attractive construction materials and technologies.
This paper publishes the results of a case study where a specific historical mortar remixing technology based on PROMPT natural rapid-setting cement (Vicat, France) was experimentally verified. The remixing effect was investigated and compared with a setting retardation technology using citric acid (TEMPO product). Citric acid is recommended by the manufacturer of PROMPT natural cement for setting retardation (4545. Vicat, Prompt Technical Document. https://www.freedomcement.com/sites/default/files/Vicat-Prompt_TDS.pdf (accessed September 24, 2025).).
2. MATERIALS AND METHODS
⌅2.1. Mortar samples and materials for sample production
⌅For the purpose of this case study, 3 sets of mortar samples were produced (Table 1). For each set, 12 samples were produced. The samples were 40 x 40 x 160 mm in size. The mortar samples labelled RR were reference samples, the R samples were affected by citric acid (supplied by VICAT under the label TEMPO), the A samples were prepared by a special procedure prolonged mixing (Chapter 2.2).
The binder was the commercial product PROMPT Natural Cement (hereafter PROMPT). The filler in the mortars was quartz (standard) sand, the sand grain size was 0.08-2 mm. The w/c ratio of the mortars was high w/c = 1. The mortar ingredients were proportioned by weight. Drinking water from the water supply system was used as mixing water.
| Sample | PROMPT : Water : Sand | Citric acid (TEMPO) |
|---|---|---|
| RR/7,14,28,56 | 1:1:1 | - |
| R/7,14,28,56 | 1:1:1 | 0.6% by weight PROMPT |
| A/7,14,28,56 | 1:1:1 | - |
Natural Cement PROMPT
⌅The properties of this commercial product are specified in the technical documentation (4545. Vicat, Prompt Technical Document. https://www.freedomcement.com/sites/default/files/Vicat-Prompt_TDS.pdf (accessed September 24, 2025)., 4646. The Louis VICAT Technical Center: Materials and Microstructures Laboratory - Special Binders Section, Technical Specifications. http://www.romanportland.net/files/doc/cahier_technique_cr_cnp_eng.pdf (accessed September 12, 2025).), further information, e.g. chemical and mineralogical composition with loss of ignition at temperature 975 °C (Tables 2 and 3), is available in publication (4747. CORDIS, project ROCARE (Roman Cements for Architectural Restoration to New High Standards). https://cordis.europa.eu/project/id/226898/results (accessed September 8, 2025)).
The chemical composition of PROMPT has been verified for the purpose of sample production by X-ray fluorescence spectroscopy (XRF), see Table 4. The differences in the composition of PROMPT (cf. Table 2 and Table 4) are slight.
| LOI | SiO2 | Al2O3 | Fe2O3 | CaO | MgO | SO3 | K2O | Na2O |
|---|---|---|---|---|---|---|---|---|
| 9.28 % | 18.09 % | 7.24 % | 3.2 % | 53.07 % | 3.84 % | 3.24 % | 1.16 % | 0.28 % |
| C3S | C2S | C3A | C4AF | C12A7 | C4A3S | Periclase | Free lime | Calcite | Sulfates | Others, incl. amorphous phases |
|---|---|---|---|---|---|---|---|---|---|---|
| 5-15% | 40-60% | 6±2% | 9±2% | 3±1% | 3±1% | 4±1% | 2±2% | 10-15% | 3±1% | 10-15% |
| LOI | SiO2 | Al2O3 | Fe2O3 | CaO | MgO | SO3 | K2O | Na2O | P2O5 | MnO | TiO2 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 8.14% | 18% | 7.68% | 3.2% | 55.1% | 3.69% | 2.38% | 0.97% | 0.14 % | 0.072 % | 0.065% | 0.28% |
2.2. Sample production
⌅The samples were produced in the laboratory. A specific procedure different from the standard was used to produce the mortar samples (4848. EN 1015-11. 2019. Methods of test for mortar for masonry – Part 11: Determination of flexural and compressive strength of hardened mortar. CEN, Brussels.). A standardized programmable laboratory mixing HOBART device fulfilling the specific requirements of the standard EN 196-1 (4949. EN 196-1. 2016. Methods of testing cement – Part 1: Determination of strength. CEN, Brussels.) was used to prepare the mortars.
Initially, for all samples, the dry ingredients were mixed by hand. After a period of 10 minutes, water was added, in the case of the R samples, water with dissolved TEMPO retarder. The time of water addition was recorded as t = 0, the mixing device was started.
The RR and R samples were mixed at low speed according to the standard EN 196-1 for 3 minutes and then the mortar samples were placed in the moulds (4848. EN 1015-11. 2019. Methods of test for mortar for masonry – Part 11: Determination of flexural and compressive strength of hardened mortar. CEN, Brussels.). Compaction of the mortars in the moulds was done by tapping to avoid segregation of the ingredients. Despite this, segregation was observed in the R samples.
The A samples were mixed using a procedure that was suggested based on previous laboratory experiments and the authors’ observations. The mortar was “revived” a total of 3 times during mixing – “remixed” using the following procedure:
The time of water addition was recorded as t = 0, mixing at a slow speed for 2 minutes.
At time t = 2 minutes, mixing was stopped, the walls of the vessel were wiped with a spatula.
At time t = 4 minutes, mixing was continued at slow speed for 1 minute, then the mixing was stopped.
At time t = 7 minutes, solidification of the mixture was observed.
At time t = 8 minutes, mixing was started at a slow speed for 1 minute and then stopped.
A change in the consistency of the mixture was observed.
At time t = 12 minutes, mixing was continued at a slow speed for 1 minute and then mixing was stopped.
The mortar was placed in the moulds.
Other information about sample production:
The RR samples set within 7 minutes after the addition of water. All samples were stored in an environment according to standard (4848. EN 1015-11. 2019. Methods of test for mortar for masonry – Part 11: Determination of flexural and compressive strength of hardened mortar. CEN, Brussels.). The A samples were still formable at 7 days in the moulds. After 14 days, samples A and R were soft and unsuitable for demoulding. The samples were removed from the moulds after a period of 28 days. The samples in the moulds and samples after removal from the moulds were conditioned in a controlled environment of an air conditioning unit at a temperature of 20 ± 1°C and a relative humidity greatet than 90 %.
2.3. Properties of fresh mortars
⌅The following properties were determined on the fresh mortar samples:
determination of the normal consistency of fresh mortars (5050. EN 1015-3. 1999. Methods of test for mortar for masonry – Part 3: Determination of consistence of fresh mortar (by flow table). CEN, Brussels.),
determination of the bulk density of fresh mortars (5151. EN 1015-6. 1998. Methods of test for mortar for masonry – Part 6: Determination of bulk density of fresh mortar. CEN, Brussels.),
determination of the air content of fresh mortars (5252. EN 1015-7. 1998. Methods of test for mortar for masonry – Part 7: Determination of air content of fresh mortar. CEN, Brussels.),
the workability of fresh mortars was determined according to EN 1015-9 (5353. EN 1015-9. 1999. Methods of test for mortar for masonry – Part 9: Determination of workable life and correction time of fresh mortar. CEN, Brussels.) by both methods.
2.4. Analysis of the development of the heat of hydration
⌅Isothermal conductivity calorimetry test was performed on pastes (without sand, see Table 1) in a TAM Air (TA Instruments), for 50 days. From approximately day 46 onwards, the calorimetric curves of the paste samples did not change (Figure 1). Curves are shown for normalized heat flux values (mW/g), which is the conversion of heat flux to sample mass.
2.5. Strength of hardened mortars
⌅The bending and compressive strength was determined on mortar samples after 28, 56 and 90 days after mixing. The samples had standardized dimensions according to EN 196-1 (4949. EN 196-1. 2016. Methods of testing cement – Part 1: Determination of strength. CEN, Brussels.) with width 40 mm, height 40 mm and length 160 mm. Testing was performed on equipment FORM+TEST 10kN for determining the strength properties of hardened mortars by loading speed 50N/s. The samples were loaded to failure with loading scheme according to EN 196-1 (4949. EN 196-1. 2016. Methods of testing cement – Part 1: Determination of strength. CEN, Brussels.).
2.6. SEM analysis
⌅Scanning electron microscopy (SEM) analysis were performed for the morphology evaluation on a JEOL JSM-7610Fplus. Samples were coated by a very thin layer of Pt (30 nm) to make sample surface conductive and prevent charge accumulation.
3. RESULTS AND DISCUSSION
⌅3.1. The properties of fresh mortars
⌅The test results are given in the tables (Tables 5, 6, 7, 8). As expected, the reference mortar (RR samples) set within 7 minutes of the addition of water. Therefore, the normative methods used were not appropriate and did not produce the expected results. In comparison to the reference mortar, both retardation methods (samples R and A) achieved the target and both mortars had a significantly longer fresh mortar workability time. Samples R with TEMPO (citric acid) additive showed a liquid consistency with a clear segregation of the ingredients and the workability time was higher than 180 minutes (test was terminated after 180 minutes). The remixed fresh mortar (samples A) had an ideal plastic consistency and a workability time of >120 minutes. The air content was twice as high in mortar A compared to mortar R. However, the bulk density was almost identical. These results may have been influenced by the segregation of the components in mortar R.
| Sample | Flow value |
|---|---|
| RR | Note 1 |
| R | Note 2 |
| A | 180 mm |
Note 1: Not measurable by the method, setting occurred within 7 minutes from t=0; does not allow the test to be performed
Note 2: Liquid mixture with clear segregation of components does not allow the test to be performed
| Sample | Bulk density ρm (kg/m3) |
|---|---|
| RR | Note 3 |
| R | 1 690 |
| A | 1 680 |
Note 3: Not measurable by method, setting occurred within 7 minutes of t=0 (addition of water); does not allow the test to be performed.
| Sample | Air content of fresh mortar (%) |
|---|---|
| RR | Note 3 |
| R | 2.2 |
| A | 4.5 |
Note 3: Not measurable by method, setting occurred within 7 minutes of t=0 (addition of water); does not allow the test to be performed.
| Sample | Method A (min) (4545. Vicat, Prompt Technical Document. https://www.freedomcement.com/sites/default/files/Vicat-Prompt_TDS.pdf (accessed September 24, 2025).) | Method B (min) (4545. Vicat, Prompt Technical Document. https://www.freedomcement.com/sites/default/files/Vicat-Prompt_TDS.pdf (accessed September 24, 2025).) |
|---|---|---|
| RR | Note 3 | Note 3 |
| R | > 180 | > 180 |
| A | > 120 | > 120 |
Note 3: Not measurable by method, setting occurred within 7 minutes of t=0 (addition of water); does not allow the test to be performed.
3.2. Analysis of the development of the heat of hydration
⌅For the samples without additive (RR, A), an almost identical curve is evident, with the development of hydration heat starting after approximately three days, with a significant peak in values at day 5 of hydration. The normalized heat flux curve of the sample R (TEMPO retarder) has a significantly different shape. The hydration heat evolution is significantly lower compared to samples RR and A, starting after 10 days with a peak at approximately day 18. The calorimetry results (Figure 1) express a different hydration mechanism in mortar R, where the influence of citric acid on the formation of ettringite and early hydration products was evident, as described above in Chapter 1.
3.3. Strength of hardened mortars
⌅The number of mortar samples in each set for determining the bending strength of was 3. Each set for determining the compressive strength of mortars contain 6 samples.
The mean values for the sets are presented in the form of graphs (Figure 2 and 3). The statistical parameters of the sets are presented in Table 9 and 10. According to the assumptions, the highest strength values were achieved for the reference mortar RR. The strength of the mortars R and A increases with the increasing age of the mortar. The statistical evaluation (Table 9 and 10) provide interesting information in view of retarded mortars strength which are more stable than the data for the reference mortar. This fact can be associated to the formation of a hydrated structure of materials, rapid solidification of the mortar RR, etc. and deserves more attention in subsequent research.
| Sample | Standard deviation | Range | ||||
|---|---|---|---|---|---|---|
| 28 days | 56 days | 90 days | 28 days | 56 days | 90 days | |
| RR | 0.13 | 0.23 | 0.27 | 0.017 | 0.055 | 0.073 |
| R | 0.20 | 0.06 | 0.09 | 0.041 | 0.004 | 0.008 |
| A | 0.16 | 0.32 | 0.16 | 0.027 | 0.101 | 0.024 |
| Sample | Standard deviation | Range | ||||
|---|---|---|---|---|---|---|
| 28 days | 56 days | 90 days | 28 days | 56 days | 90 days | |
| RR | 0.30 | 0.65 | 0.52 | 0.091 | 0.425 | 0.266 |
| R | 0.08 | 0.16 | 0.20 | 0.007 | 0.026 | 0.039 |
| A | 0.01 | 0.07 | 0.15 | 0.00017 | 0.005 | 0.021 |
3.4. SEM analysis
⌅The differences in the development of the hydrated structure are evident from the SEM (scanning electron microscopy) pictures.
The minerals of calcite group formation are evident for 7-days old reference mortar samples RR. The calcite crystals progressed considerably by 14-days from the mixing with apparent ettringite formations are also evident (Figure 4a, 4b).
After the dormant period, 56-day-old mortar samples showed leaf-like formations within the structure (Figure 4c), which have also been described in the literature as a ‘house-of-cards’ arrangement. The formation of hydration products is apparently slowed down for the citric acid retarded mortar (samples R), and significantly underdeveloped or deformed ettringite crystals are already visible at 14 days (Figure 5a). The formation of a crystalline structure (belite crystal shape, or larnite-like) is evident for R samples aged 56 days (Figure 5b).
The mortar prepared by prolonged mixing process (samples A) shows slower and less distinct portlandite formation at 7 and 14-days old samples (Figure 6a, 6b), but ettringite formations are already present at 7 days (Figure 6a). The presence of “leaf-like formations” is visible at 56 days of age, but portlandite formation still seems to be inhibited by some factor (Figure 6c).
4. CONCLUSIONS
⌅The results of a case study analyse the properties of mortars based on natural cement PROMPT (Vicat, France), in which two selected techniques for retarding the setting of fresh mortar were used.
The rapid onset of setting in this type of mortar is a particularly practical problem. Both selected techniques for retarding setting were effective, and both techniques significantly extended the workability time of fresh mortar. However, each of the tested methods has its advantages and disadvantages.
At present, the retardation method with citric acid (TEMPO product, Vicat France) is preferred. As shown in this study, mortars retarded with citric acid have lower initial strengths. In this mortar, the structure and development of the structure of the hydrating mortar are affected. For this reason, it is necessary to respect the specific application requirements, especially for the initial strength of the mortar, and to take into account the dosage of citric acid already when designing the mortar composition. This study shows that at a higher w/c ratio (here w/c = 1) the fresh mortar mixture is very sensitive to the dosage of citric acid and the components separate (blend). In real application conditions, it is usually difficult to accurately dose a small amount of citric acid (only 0.6% of the cement weight) into the mixture. This is a disadvantage for ensuring the quality of the construction product.
The technology of setting retardation by the modification of the mixing process is based on historical experience. As the results of this study show, the initial strength of the mortar is lower. However, the properties of the fresh mortar offer higher application comfort. There is no separation of components (blending), the mortar has an optimal consistency even at a high w/c ratio.
By modifying the mixing process when preparing fresh mortar based on PROMPT natural cement, the workability time can be significantly extended, but the initial strength of the mortar is significantly lower compared to the reference mortar.
The results of SEM analysis and calorimetry in this study do not prove, but only suggest, that both technologies of setting retardation affect hydration and microstructure formation of mortars in a different way. The effect of citric acid on the formation of hydration products has been described (see Chapter 1). However, the mechanism of hydration in mortar retarded by modification of the mixing process is not yet explained and described in this study. On the other hand, the authors will continue further research with the aim of explaining the hydration process. However, it can be hypothesized that in mortars with prolonged mixing, hydration is influenced by the addition of CO2 to the structure of the mortar.
Abbreviations
⌅C3S tricalcium silicate, or alite
C2S dicalcium silicate, or belite
C3A tricalcium aluminate
C4AF tetracalcium aluminoferrite, or brownmillerite
C12A7 dodecacalcium hepta-aluminate, or mayenite
C4A3S anhydrous calcium sulfoaluminate
Funding Sources
⌅The work was supported by the Student Grant Competition of VSB-TUO. The project registration number is SP2024/024. The work was supported by ERDF/ESF New Composite Materials for Environmental Applications (No. CZ.02.1.01/0.0/0.0/17_048/0007399).
Authorship contribution statement
⌅Jana Daňková: Conceptualization, writing, review, validation and research, data curation.
Pavel Mec: Data curation, investigation, formal analysis.
Roman Gabor: Data curation, investigation.
David Bujdoš: Data curation.
Tereza Majstríková: Writing-original draft.
Adéla Valentová: Writing-original draft.
Jiří Šafrata: Data curation.
Declaration of competing interests
⌅The authors of this article declare that they have no financial, professional or personal conflicts of interest that could have inappropriately influenced this work.