Influence of crystallizing type chemical admixture on precast micro concretes: a statistical analysis and holistic engineering overview

1. INTRODUCTION

Concrete structures are exposed to several durability problems associated with water content penetration through their pores and capillary networks (11. Al-Rashed R, Jabari RM. 2020. Dual-crystallization waterproofing technology for topical treatment of concrete. Case Studies. Constr. Mater. 13:e00408. https://doi.org/10.1016/j.cscm.2020.e00408.
) and the consequent reduction of their service life. In general, in order to prevent this from happening, solutions have been developed for existing and new concrete structures.

For existing concrete structures, researchers have developed solutions such as different products, chemical bases and commercial systems to apply on concrete surfaces as waterproofing barrier systems. These products and systems include emulsions, acrylic membranes, asphalt blankets, polymeric mortars (22. Yazigi W. 2003. The technique of building. in Portuguese: A técnica de edificar. 5th edition, São Paulo: PINI.
), liquid-applied films or sheets made of acrylic resins, bitumen, styrene-butadiene-styrene copolymer, ethylene propylene diene monomer (EPDM), acrylics, polyethylene; silane, siloxane (33. Feng Z, Wang F, Xie T, Ou J, Xue M, Li W. 2019. Integral hydrophobic concrete without using silane. Constr. Build. Mater. 227:116678. https://doi.org/10.1016/j.conbuildmat.2019.116678.
) and silicone (44. Li J, Cao J, Ren Q, Ding Y, Zhu H, Xiong C, Chen R. 2021. Effect of nano-silica and silicone oil paraffin emulsion composite waterproofing agent on the water resistance of flue gas desulfurization gypsum. Constr. Build. Mater. 287:123055. https://doi.org/10.1016/j.conbuildmat.2021.123055.
) and, more recently, crystallizing hydrophobic minerals (55. Rahman MM, Chamberlain DA. 2016. Application of crystallizing hydrophobic mineral and curing agent to fresh concrete. Constr. Build. Mater. 127:945-949. https://doi.org/10.1016/j.conbuildmat.2016.08.150.
), dual-crystallization (DCE) (66. Al-Kheetan MJ, Rahman MM, Chamberlain DA. 2018. Development of hydrophobic concrete by adding dual-crystalline admixture at mixing stage. Struct. Concr. 19(5):1504-1511. https://doi.org/10.1002/suco.201700254.
, 11. Al-Rashed R, Jabari RM. 2020. Dual-crystallization waterproofing technology for topical treatment of concrete. Case Studies. Constr. Mater. 13:e00408. https://doi.org/10.1016/j.cscm.2020.e00408.
) and multi-crystallization technology (77. Al-Rashed R, Al-Jabari M. 2021. Concrete protection by combined hygroscopic and hydrophilic crystallization waterproofing applied to fresh concrete. Case Studies in Constr. Mater. 15:e00635. https://doi.org/10.1016/j.cscm.2021.e00635.
, 88. Al-Rashed R, Al-Jabari M. 2021. Multi-crystallization enhancer for concrete waterproofing by pore blocking. Constr. Build. Mater. 272:121668. https://doi.org/10.1016/j.conbuildmat.2020.121668.
) with hygroscopic and hydrophilic characteristics. On the other hand, in new concrete structures, solutions involve a concrete mix design using supplementary cementitious materials such as metakaolin, silica fume, nano-silica, fly ash (99. Cascudo O, Peres P, Carasek H, Castro A, Lopes A. 2021. Evaluation of the pore solution of concretes with mineral additions subjected to 14 years of natural carbonation. Cement Concrete Comp. 115:103858. https://doi.org/10.1016/j.cemconcomp.2020.103858.
, 1010. Oliveira AM, Cascudo O. 2018. Effect of mineral additions incorporated in concrete on thermodynamic and kinetic parameters of chloride-induced reinforcement corrosion. Constr. Build. Mater. 192:467-477. https://doi.org/10.1016/j.conbuildmat.2018.10.100.
), nano metakaolin (1111. Zhan PM, He ZH, Ma ZM, Liang CF, Zhang XX, Abreham AA, Shi JY. 2020. Utilization of nano-metakaolin in concrete: A review. J. Build. Eng. 30:101259. https://doi.org/10.1016/j.jobe.2020.101259.
), rice husk ash and waste glass (1212. Younes MM, Abdel-Rahman HA, Khattab MM. 2018. Utilization of rice husk ash and waste glass in the production of ternary blended cement mortar composites. J. Build. Eng. 20:42-50. https://doi.org/10.1016/j.jobe.2018.07.001.
) or super absorbent polymers (SAPs) (1313. Park B, Choi YC. 2018. Self-healing capability of cementitious materials with crystalline admixtures and super absorbent polymers (SAPs). Constr. Build. Mater. 189:1054-1066. https://doi.org/10.1016/j.conbuildmat.2018.09.061.
, 1414. Agostinho LB, Pereira AC, da Silva EF, Toledo Filho RD. 2021. Rheological study of Portland cement pastes modified with superabsorbent polymer and nanosilica. J. Build. Eng. 34:102024. https://doi.org/10.1016/j.jobe.2020.102024.
). Also, they could involve the use of chemical admixtures (CAs) such as shrinkage-reducing admixtures (1515. Mohammed TU, Hamada H. 2003. Durability of concrete made with different water-reducing chemical admixtures in tidal environment. ACI Mater. J. 100(3):194-202.
, 1616. Zhan PM, He ZH. 2019. Application of shrinkage reducing admixture in concrete: A review. Constr. Build. Mater. 201:676-690. https://doi.org/10.1016/j.conbuildmat.2018.12.209.
) and crystallizing or crystalline chemical admixtures (1717. Oliveira AS, Gomes ODFM, Ferrara L, Fairbairn EDMR, Toledo Filho RD. 2021. An overview of a twofold effect of crystalline admixtures in cement-based materials: From permeability-reducers to self-healing stimulators. J. Build. Eng. 41:102400. https://doi.org/10.1016/j.jobe.2021.102400.
), which can also be useful to prevent moisture damage and to decrease the permeability and the capillary absorption of hardened concrete (1818. Martin JFM. 2005. Concrete admixtures (in Portuguese: Aditivos para Concreto). In: G.C. Isaia. Editor. Concrete: Teaching, Research and Achievements (in Portuguese: Concreto: Ensino, Pesquisa e Realizações. 1. São Paulo: IBRACON, 2v.
).

In particular, the crystallizing or crystalline chemical admixtures, also called crystalline catalyst in the literature (1919. Takagi EM. 2013. Self-healing concrete with Brazilian blast furnace slag cements activated by crystalline catalyst (in Portuguese: Concretos auto cicatrizantes com cimentos brasileiros de escória de alto-forno ativados por catalisador cristalino). Dissertation - Instituto Tecnológico de Aeronáutica (ITA). São José dos Campos.
), can be classified as PRAH (permeability-reducing admixture in hydrostatic conditions), admixtures that increase the resistance to the penetration of water under pressure (2020. American Concrete Institute (ACI). 2010. ACI 212.3R-10: Report for chemical admixtures for concrete. United States.
). These crystalline admixtures have a hydrophilic nature, and their active components react with calcium hydroxide to form crystalline products that fill cracks and disconnect pores in the concrete only in the presence of sufficient moisture (2121. Sisomphon K, Copuroglu O, Koenders EAB. 2012. Self-healing of surface cracks in mortars with expansive additive and crystalline additive. Cem. Concr. Compos. 34(4):566–574. https://doi.org/10.1016/j.cemconcomp.2012.01.005.
). As a result, crystalline products can be formed, depending on the size of the pores and the pH concentration of the pore solution (1919. Takagi EM. 2013. Self-healing concrete with Brazilian blast furnace slag cements activated by crystalline catalyst (in Portuguese: Concretos auto cicatrizantes com cimentos brasileiros de escória de alto-forno ativados por catalisador cristalino). Dissertation - Instituto Tecnológico de Aeronáutica (ITA). São José dos Campos.
, 2222. Takagi EM, Lima MG, Helene P, Medeiros-Junior RA. 2018. Towards the concrete of tomorrow: engineered self-healing concrete with blast furnace slag cement activated by crystalline additive - PRAH 4G (in Portuguese: Rumo ao concreto do amanhã: concreto auto cicatrizante engenheirado com cimento de escória de alto forno ativado por aditivo cristalino - PRAH 4G). Revista Estrutura da Associação Brasileira de Engenharia e Consultoria Estrutural. 5(2):39-45. Retrieved from https://www.phd.eng.br/wp-content/uploads/2018/04/Rev_Estrutural_N_5.pdf.
), resulting in a reduction of water content penetration through the pores.

Furthermore, while the crystalline admixture, consisting of Portland cement, quartz sand and active chemicals, has its compositions kept confidential by manufacturers (2222. Takagi EM, Lima MG, Helene P, Medeiros-Junior RA. 2018. Towards the concrete of tomorrow: engineered self-healing concrete with blast furnace slag cement activated by crystalline additive - PRAH 4G (in Portuguese: Rumo ao concreto do amanhã: concreto auto cicatrizante engenheirado com cimento de escória de alto forno ativado por aditivo cristalino - PRAH 4G). Revista Estrutura da Associação Brasileira de Engenharia e Consultoria Estrutural. 5(2):39-45. Retrieved from https://www.phd.eng.br/wp-content/uploads/2018/04/Rev_Estrutural_N_5.pdf.
, 2323. Pazderca J, Hájková E. 2016. Crystalline admixtures and their effect on selected properties of concrete. Acta Polytechnica. 56(4):306–311, https://doi.org/10.14311/AP.2016.56.0306.
), it can be defined generically as a composition similar to cement (2424. Weng TL, Cheng A. 2014. Influence of curing environment on concrete with crystalline admixture. Monatsh. Chem. 145:195-200. https://doi.org/10.1007/s00706-013-0965-z.
) with a greater number of finer particles (2525. Hassani ME, Vessalas K, Sirivivatnanon V, Baweja D. 2017. Influence of permeability-reducing admixtures on water penetration in concrete. ACI Mater. J. 114(6):911-922. https://doi.org/10.14359/51701002.
). This type of admixture has been studied in concrete, including self-healing or autogenous crystallization of concrete, with good results (26-2826. De Rooij MR, Schlangen E, Joseph C. 2013. Self-healing phenomena in cement-based materials. Draft of the RILEM State-of-the-Art Report. Retrieved from https://link.springer.com/content/pdf/bfm%3A978-94-007-6624-2%2F1.
27. Van Tittelboom, K, De Belie, N. 2013. Self-healing in cementitious materials—A review. Materials. 6(6):2182-2217. https://doi.org/10.3390/ma6062182.
28. Ferrara L, Krelani V, Carsana M. 2014. A “fracture testing” based approach to assess crack healing of concrete with and without crystalline admixtures. Constr. Build. Mater. 68:535-551. https://doi.org/10.1016/j.conbuildmat.2014.07.008.
).

In general, the main results obtained by some of these researches were that the crystalline admixture reduced the concrete permeability to water (2323. Pazderca J, Hájková E. 2016. Crystalline admixtures and their effect on selected properties of concrete. Acta Polytechnica. 56(4):306–311, https://doi.org/10.14311/AP.2016.56.0306.
), especially with a more evident effect for a higher w/c ratio and depending on the type of binder (2525. Hassani ME, Vessalas K, Sirivivatnanon V, Baweja D. 2017. Influence of permeability-reducing admixtures on water penetration in concrete. ACI Mater. J. 114(6):911-922. https://doi.org/10.14359/51701002.
). Also, it observed that it was a tendency to decrease the capillarity absorption (2929. Nascimento RS, Pereira BCG, Joffily IAL. 2017. The effects of different crystallizing additives on the properties of hardened concrete: capillary water absorption and compressive strength (in Portuguese: Os efeitos de diferentes aditivos cristalizantes nas propriedades do concreto no estado endurecido: absorção de água por capilaridade e resistência à compressão). In: 59th Brazilian Concrete Congress, Rio Grande do Sul. Retrieved from https://www.researchgate.net/publication/326819517_os_efeitos_de_diferentes_aditivos_cristalizantes_nas_propriedades_do_concreto_no_estado_fresco_e_endurecido.
), the total absorption of water (by immersion) and the void index at each wetting and drying cycle (3030. Petrucci RSP, Hastenpflug D. 2017. Influence of the crystallizing additive on the porosity of Portland cement concrete (in Portuguese: Influência do aditivo cristalizante na porosidade do concertos de Cement Portland). In: 59th Brasiliano Concrete Congress, Rio Grande do Sul.
). At the same time, the formation of needle-shaped crystalline compounds (2424. Weng TL, Cheng A. 2014. Influence of curing environment on concrete with crystalline admixture. Monatsh. Chem. 145:195-200. https://doi.org/10.1007/s00706-013-0965-z.
), an increase in resistance to chloride penetration (3131. Helene P, Guignone G, Vieira G, Roncetti L, Moroni F. 2018. Evaluation of colorido penetra íon and service life of self-healing concrete activated by a crystalline admixture (in Portuguese: Avaliação da penetração de cloretos e da vida útil de concretos autocicatrizantes ativados por aditivo cristalino). Revista IBRACON de Estruturas e Materiais. 11(3):544-563. https://doi.org/10.1590/S1983-41952018000300007.
) and a self-healing ability (32-3632. Li D, Chen B, Chen X, Fu B, Wei H, Xiang X. 2020. Synergetic effect of superabsorbent polymer (SAP. and crystalline admixture (CA. on mortar macro-crack healing. Constr. Build. Mater. 247:118521. https://doi.org/10.1016/j.conbuildmat.2020.118521.
33. Cuenca E, Tejedor A, Ferrara L. 2018. A methodology to assess crack-sealing effectiveness of crystalline admixtures under repeated cracking-healing cycles. Constr. Build. Mater. 179:619-632. https://doi.org/10.1016/j.conbuildmat.2018.05.261.
34. Cuenca E, Messene A, Ferrara L. 2021. Synergy between crystalline admixtures and nano-constituents in enhancing autogenous healing capacity of cementitious composites under cracking and healing cycles in aggressive waters. Constr. Build. Mater. 266:121447. https://doi.org/10.1016/j.conbuildmat.2020.121447.
35. Roig-Flores M, Moscato S, Serna P, Ferrara L. 2015. Self-healing capability of concrete with crystalline admixtures in different environments. Constr. Build. Mater. 86:1-11. https://doi.org/10.1016/j.conbuildmat.2015.03.091.
36. Vieira DV, Damasio AS. 2017. Self-healing concrete using Brazilian Portland pozzolan cements, activated by a crystalline admixture (in Portuguese: Concertos auto vicariate com uso de cimentos Portland brasileiros de pozolana, ativados por aditivo cristalino. In: 59th Brazilian Concrete Congress, Rio Grande do Sul.
) were noted. However, some research has also shown that the use of the crystalline admixture did not have great effects. For example, for high-performance concrete (HPC) with fibers, the addition of this admixture did not significantly change the mechanical properties and water permeability when subjected to constant loading (3737. Escoffres P, Desmettre C, Charron JP. 2018. Effect of a crystalline admixture on the self-healing capability of high-performance fiber reinforced concretes in service conditions. Constr. Build. Mater. 173:763–774. https://doi.org/10.1016/j.conbuildmat.2018.04.003.
). In this HPC, the main product found in the cracks was aragonite, a crystalline form of calcium carbonate, probably formed due to the presence of magnesium in the admixture.

Another study (3535. Roig-Flores M, Moscato S, Serna P, Ferrara L. 2015. Self-healing capability of concrete with crystalline admixtures in different environments. Constr. Build. Mater. 86:1-11. https://doi.org/10.1016/j.conbuildmat.2015.03.091.
) tested four different types of exposure (water immersion, water contact, a humidity chamber, and air exposure in laboratory conditions) for the self-healing of concretes with and without crystalline admixture for 42 days. It was concluded that, among the types of exposure tested, water immersion was the best condition to promote self-healing and that the presence of water was essential for the healing phenomenon in concrete with and without crystalline admixtures.

In some circumstances, the self-healing ability of the concretes cannot be confirmed (3737. Escoffres P, Desmettre C, Charron JP. 2018. Effect of a crystalline admixture on the self-healing capability of high-performance fiber reinforced concretes in service conditions. Constr. Build. Mater. 173:763–774. https://doi.org/10.1016/j.conbuildmat.2018.04.003.). In this case, it was found that under air curing, the admixture did not make any difference and under wet curing, there was a small, but insignificant improvement, confirming the need for water for its actuation. On the other hand, another study (3838. Ourives CN, Bilesky PC, Yokoyama CM. 2009. Performance evaluation of concrete capillary crystallization waterproofing systems (in Portuguese: Avaliação do desempenho dos sistemas de impermeabilização por cristalização capilar do concreto). Revista IBRACON.) demonstrated a good self-healing effect of the crystalline admixture after intentional cracking and 7 weeks of water flow under constant pressure (1.5 MPa). As a result, the water percolation reduced significantly (99%) and stabilized at a low value. Thus, there is still not a full consensus on the beneficial effects of crystallizing admixtures in concrete, or related to the effect of using this type of admixture in concrete with a low water binder ratio. However, concrete with a low water/binder ratio, usually molded by compaction, vibration or compression, is widely used in Brazil for the manufacturing of precast concrete, such as blocks, pipes, and columns, among others. This type of concrete can demonstrate good cohesion, low porosity and low permeability (3939. Marchioni ML. 2012. Development of techniques for the characterization of dry concrete used in the manufacture of concrete pieces for interlocking paving (in Portuguese: Desenvolvimento de técnicas para a caracterização do concreto seco utilizado na fabricação de peças de concreto para pavimentação intertravada). Dissertation - Escola Politécnica da Universidade de São Paulo, São Paulo. Retrieved from https://www.teses.usp.br/teses/disponiveis/3/3146/tde-18072013-150832/en.php.
).

Due to the need for water to act on the crystallization admixture, its performance in concrete with a lower water/binder ratio can be decreased in the early ages. However, with the supply of external water, after the cement has hydrated and reached its maturity, there is the possibility that this admixture will reduce the permeability in precast concrete for which the maximum value of water absorption is 8% (4040. Associação Brasileira de Normas Técnicas (ABNT). Brazilian Technical Standards Association. 2014. NBR 6136: Simple hollow concrete blocks for masonry: requirements (in Portuguese: blocos vazados de concreto simples para alvenaria: requisitos), Rio de Janeiro.
). This paper moves forward in this direction and, in this perspective, addresses a knowledge gap by investigating the effect of the crystalline admixture in precast micro concrete (concrete with small aggregates). Thus, the aim is to compare the performance of precast micro concrete, in terms of its compression strength and water absorption, analyzing the water contents (7% and 11% of water over the total dried mass) and the crystalline admixture in two types of environmental conditions. A statistical analysis was conducted for better analysis of the results.

The connection between the study conditions and differences corresponding to the microstructural changes induced in the concretes (including changes in pore systems and their properties) will be produced in terms of technical, technological and engineering arguments on the different behaviors and performances of the studied micro concrete and their predicted durability. Besides that, general and substantial changes in the studied micro concretes are discussed, in relation to both the microstructure and the pore system, as well as in relation to mechanical changes, based on statistical analysis and an engineering context. The effect of the chemical admixture is intimately associated with the mechanical, micro structure and porosity (fundamental aspects of durability), and this part constitutes the main novelty of this paper.

2. MATERIALS AND METHODS

Precast micro concrete was produced with unitary binder: aggregates proportions of 1:3.0, by mass. The binder: filler: micro filler: artificial sand proportions were 1:0.24:0.16:2.6, respectively. This mix composition was adopted and recommended by a local company that manufactures precast concrete.

The crystalline chemical admixture was used in a content of 2% in relation to the cement mass (as the maximum content indicated by the manufacturer) as a partial replacement for the micro filler. Besides that, the water contents (percentages of water over the total dry mass) were determined according to the literature (4141. Marques DO. 2017. Modular microconcrete blocks incorporating waste and synthetic polypropylene fibers (in Portuguese: Blocos modulares de microconcreto com incorporação de resíduos e fibras sintéticas de polipropileno). Undergraduate thesis. Escola de Engenharia Civil da Universidade Federal de Goiás. Goiânia.
), in which the optimum water content was 9% (determined in a compaction test with normal energy). Thus, the values adopted in this research were 11% and 7% over the total dry mass of mixture, corresponding to the water: cement ratio of 0.44 and 0.28, respectively. These water content values allowed the handling of the precast concrete after molding without damage and with good cohesion.

2.1. Materials used

 ⌅

Brazilian Portland cement type CP II F-40 was used (composed of 11‒25% in mass of limestone filler and 75‒89% of clinker, with a small amount of calcium sulfate), whose mechanical strength requirement is a minimum of 40 MPa of compressive strength at the age of 28 days (similar to the European CEM II/A-L), together with micro filler, filler, fine aggregate, crystalline chemical admixture and water. The chemical and physical properties of the cement used are given in Table 1.

TABLE 1.  Physical characteristics and chemical composition of Brazilian Portland cement type CP II F-40, similar to European CEM II/A-L.

Property		Test method	Equivalent European test	Result	Requirement according to Brazilian Standard NBR 16,697 (4242. ABNT. 2018. NBR 16697: Portland cement – Requirements (in Portuguese: Cimento Portland - Requisitos). Rio de Janeiro. )
Specific mass (kg / dm³)		NBR 16605 (4343. ABNT. 2017. NBR 16605: Portland cement and other powdered materials - Determination of specific mass (in Portuguese: Cimento Portland e outros materiais em pó - Determinação da massa específica). Rio de Janeiro. )	UNE 80103 (4444. UNE - Spanish Standard. 2013. UNE 80103 Test methods of cements. Physical analysis. Actual density determination. )	3.1	-
Standard paste consistency (%)		NBR 16606 (4545. ABNT. 2018. NBR 16606: Portland cement – Determination of normal consistency paste (in Portuguese: Cimento Portland — Determinação da pasta de consistência normal). Rio de Janeiro. )	EN 196-3 (4646. EN - European Standard. 2016. - EN 196-3: Methods of testing cement – Part 3: Determination of setting times and soundness. )	29.1	-
Chemical composition (%)	Loss on Ignition (%)	NBR NM 18 (4747. ABNT. 2012. NBR NM 18: Portland cement – Chemical analysis - Determination of ignition loss (in Portuguese: Cimento Portland - Análise química - Determinação de perda ao fogo). Rio de Janeiro. )	EN 196-2 (4848. EN - European Standard. 2013. EN 196-2: Methods of testing cement – Part 2: Chemical analysis of cement. )	5.0	≤ 12.5
	Insoluble residue	NBR NM 15 (4949. ABNT. 2012. NBR NM 15: Portland cement – Chemical analysis - Determination of insoluble residue (in Portuguese: Cimento Portland - Análise química - Determinação de resíduo insolúvel). Rio de Janeiro. )	-	1.3	≤ 7.5
	MgO	NBR NM 11-2 (5050. ABNT. 2012. NBR NM 11-2: Portland cement – Chemical analysis - Determination of major oxides for complexometry - Part 2: ABNT method (in Portuguese: Cimento Portland - Análise química - Determinação de óxidos principais por complexometria - Parte 2: Método ABNT). Rio de Janeiro. )	EN 196-2 (4848. EN - European Standard. 2013. EN 196-2: Methods of testing cement – Part 2: Chemical analysis of cement. )	2.5	-
	SO3	NBR NM 16 (5151. ABNT. 2012. NBR NM 16: Portland cement – Chemical analysis - Determination of sulfuric anhydride (in Portuguese: Cimento Portland - Análise química - Determinação de anidrido sulfúrico). Rio de Janeiro. )		2.8	≤ 4.5
Fineness	Fineness index by means of 75 μm sieve (nº 200)	NBR 11579 (5252. ABNT. 2012. NBR 11579: Portland cement – Determination of the fineness index by means of 75 μm sieve (nº 200. In Portuguese: Cimento Portland — Determinação do índice de finura por meio da peneira 75 μm (nº 200). Rio de Janeiro. )	EN 196-6 (5555. EN - European Standard. 2018. EN 196-6: Methods of testing cement – Part 6: Determination of fineness. )	0.3	≤ 10.0
	Fineness index by means of aerodynamic sieving	NBR 12826 (5353. ABNT. 2014. NBR 12826: Portland cement and other powdered materials - Determination of fineness index by means of aerodynamic sieving (in Portuguese: Cimento Portland e outros materiais em pó — Determinação do índice de finura por meio de peneirador aerodinâmico). Rio de Janeiro. )		1.9	-
	Specific surface (m2/kg)	NBR 16372 (5454. ABNT. 2015. NBR 16372: Portland cement and other powdered materials - Determination of fineness by the air permeability method (Blaine method. in Portuguese: Cimento Portland e outros materiais em pó - Determinação da finura pelo método de permeabilidade ao ar (método de Blaine)). Rio de Janeiro. )		4,516.0	-
Setting times (minutes)	Initial	NBR 16607 (5656. ABNT. 2018. NBR 16607: Portland cement – Determination of setting times (in Portuguese: Cimento Portland — Determinação dos tempos de pega). Rio de Janeiro. )	EN 196-3 (4646. EN - European Standard. 2016. - EN 196-3: Methods of testing cement – Part 3: Determination of setting times and soundness. )	158.0	≥ 60
	Final			205.0	≤ 600
Compressive strength (MPa)	7 days	NBR 7215 (5757. ABNT. 2019. NBR 7215: Portland cement – Determination of compressive strength of cylindrical specimens (in Portuguese: Cimento Portland - Determinação da resistência à compressão de corpos de prova cilíndricos). Rio de Janeiro. )	EN 196-1 (5858. EN - European Standard. 2016. EN 196-1: Methods of testing cement – Part 1: Determination of strength. )	28.0	≥ 25.0
	28 days			40.0	≥ 40.0
	90 days			41.0	-

The filler and micro filler used were waste from crushed granitic rocks. Figure 1 shows the granulometric distribution curves of these two materials. D50 particles of 26 μm and 62 μm were observed for the micro filler and filler, respectively. The granite sand was used as fine aggregate (Table 2 and Figure 2). It was observed that the sand used was almost entirely within the “usable zone”, according to Brazilian Standard NBR 7211: 2009 (6363. ABNT. 2009. NBR 7211: Aggregates for concrete – Specification (in Portuguese: Agregados para concreto – Especificação). Rio de Janeiro.
). The crystalline admixture utilized was a crystallizing powder admixture. This was a gray powder with a pH value of 12 ± 1.0, equivalent sodium oxide less than or equal to 3%, and a volume density of approximately 750 kg/m³. The recommended content is between 1% and 2% of the weight of the binder. Besides that, this crystalline chemical admixture is usually composed of a mixture of cements, amino alcohols, and other binding materials (6464. SIKA. Product datasheet – Sika® WT-200P, (2017). Retrieved from https://gcc.sika.com/en/documents-resources/pds.html.
).

FIGURE 1.  Filler and micro filler particle size distribution curve.

TABLE 2.  Characteristics of the granite sand (fine aggregate) used.

Property	Brazilian Standard	Equivalent European test	Result
Maximum diameter (mm)	NBR NM 248 (5959. ABNT. 2003. NBR NM 248: Aggregates - Determination of particle size composition (in Portuguese: Agregados – Determinação da composição granulométrica). Rio de Janeiro. )	EN 933-1 (6060. EN - European Standard 2012. EN 933-1: Tests for geometrical properties of aggregates - Part 1: Determination of particle size distribution - Sieving method. )	4.75
Fineness module			3.16
Specific mass (kg / dm³)	NBR NM 52 (6161. ABNT. 2009. NBR NM 52: Aggregate - Determination of specific mass and apparent specific mass (in Portuguese: Agregado miúdo - Determinação da massa específica e massa específica aparente). Rio de Janeiro. )	BS 812-2 (6262. BS – British Standard. 1995. BS 812-2: Testing aggregates - Part 2: Methods for determination of density. )	2.65
Unit mass (kg /dm³)			1.60

FIGURE 2.  Grain size distribution curve of the granite sand.

3. RESULTS AND DISCUSSION

3.1. Compression strength

 ⌅

The averages of the results obtained for density and compressive strength are shown in Table 3. Figure 3 presents the average and the standard deviation of compressive strength at the age of 28 days (orange color) and 154 days with sealed (blue color) and saturated (grey color) micro concrete specimens, with (A) and without (N) crystalline chemical admixtures for 7% and 11% of water content.

TABLE 3.  Density (g/cm³) and compressive strength (MPa) at the ages of 28 days and 154 days in the two different conditions of exposure (sealed and saturated).

Type of concrete/ water content	Age test	Exposure condition	Density (g/cm³)*	Compressive strength (MPa)*	CV**
Without crystalline admixture - 11% water content (N11)	28	Sealed	2.17	32.7	10
	154	Sealed	2.15	32.0	17
	154	Saturated	2.20	31.6	13
With crystalline admixture - 11% water content (A11)	28	Sealed	2.19	28,9	6
	154	Sealed	2.15	30.0	23
	154	Saturated	2.20	29.8	18
Without crystalline admixture - 7% water content (N7)	28	Sealed	2.04	19.4	13
	154	Sealed	2.01	17.7	10
	154	Saturated	2.14	21.9	7
With crystalline admixture - 7% water content (A7)	28	Sealed	2.05	21.6	10
	154	Sealed	2.05	19.8	6
	154	Saturated	2.15	23.3	4

*Average value. **CV - Coefficient of Variation (%)

FIGURE 3.  Compressive strength results for the micro concretes studied: average results and standard deviation.

From Table 3 and Figure 3, it is observed that the densities of the concretes with 11% water content were considerably higher than those with 7%, as expected. Comparing the densities with and without crystalline admixtures for the same water content, the values were the same or the differences were negligible. This same observation was validated for the densities of the micro concretes at 28 and 154 days with and without crystalline admixture and in the water content studied.

In relation to the compressive strength, the micro concretes with 11% water content had higher values of compression than the micro concretes with 7% water content at all ages and conditions studied (about 33%). Thus, the greater amount of water in the micro concrete with 11% water content caused both greater compaction of the mixtures and greater hydration of the cement. As a result, the compressive strength increased. The effect of the age of these micro concretes is negligible or small.

Analyzing the micro concretes with 11% of water content, there was a tendency to a reduced compressive strength when the crystalline admixture was used. This trend occurred for both ages: 28 days (reduction of 12%) and 154 days, in both conditions of exposure (about 6%). As for the micro concretes with 7% water content, there was an inverse behavior, which meant an increase in compressive strength when the admixture was added (from 6% to 12%, depending on the age and exposure condition). This tendency of behavior with the two water content levels of the micro concretes can be related to the interaction between the filler effect and the influence of the greater surface area of the fines included in the mixtures with crystalline admixture. However, a more accurate analysis can be done through consideration of the statistical results. Table 4 shows the analysis of variance (ANOVA) of the compressive strength results.

TABLE 4.  Analysis of variance (ANOVA) of compressive strength results.

Description	SQ	GL	MQ	Fcal	Ftab	p-value	Result
Model	1,375.84	11	125.08	9.660	2.07	0.0000	Significant
Error (residue)	466.2	36	12.95	-	-		-
Total	1,842.04	47	-	-	-		-
Water content	1,199.72	1	1,199.72	92.64	4.11	0.0000	Significant
Crystalline admixture	2.18	1	2.18	0.17	4.11	0.6841	Insignificant
Conditions of exposure (sealed or saturated)	24.66	1	24.66	1.90	4.11	0.1761	Insignificant
Age	0.2	1	0.2	0.015	4.11	0.9020	Insignificant
Crystalline admixture x age	1.93	1	1.93	0.15	4.11	0.7018	Insignificant
Crystalline admixture x conditions of exposure	0.14	1	0.14	0.011	4.11	0.9175	Insignificant
Water content x crystalline admixture	64.28	1	64.28	4.96	4.11	0.0322	Significant
Water content x age	0	1	0	0	4.11	0.9860	Insignificant
Water content x conditions of exposure	34.73	1	34.73	2.68	4.11	0.1102	Insignificant
Water content crystalline admixture x age	3.8	1	3.8	0.29	4.11	0.5913	Insignificant
Water content x conditions of exposure (crystalline admixture x age)	0.47	1	0.47	0.036	4.11	0.8503	Insignificant
Error (residue)	466.2	36	12.95	-	-		-

Coefficient of determination (R²_mod) 0.75Correlation coefficient (R_mod) 0.86

Table 4 shows that the model adopted was significant, with a coefficient of determination of the model (R²) of 0.75. This means that 75% of the total variation of the data is explained by the model. The analysis of variances showed that the only statistically significant individual effect was that of water content at a 95% confidence level. This expresses that this single variable influences the compressive strength of the micro concretes, with the formation of two statistical groups of water contents: one with 11% and other with 7%. The crystalline admixture used did not influence the compressive strength results. Additionally, only the double interaction between the water content and the admixture was significant, with the water content having more influence.

In the literature, there is no agreement relating to the effect of the crystalline admixture on the compressive strength of micro concrete, considering that there was no pre-cracking in self-healing studies. Pazderca and Hájková (2323. Pazderca J, Hájková E. 2016. Crystalline admixtures and their effect on selected properties of concrete. Acta Polytechnica. 56(4):306–311, https://doi.org/10.14311/AP.2016.56.0306.
) evaluated the influence of two different crystalline admixtures on the compressive strength at 28 days and concluded that the result was practically the same as for the concrete without admixture. By contrast, in the research developed by Hassani et al. (2525. Hassani ME, Vessalas K, Sirivivatnanon V, Baweja D. 2017. Influence of permeability-reducing admixtures on water penetration in concrete. ACI Mater. J. 114(6):911-922. https://doi.org/10.14359/51701002.
), the crystalline admixture used significantly influenced the compressive strength at the age of 28 days, with rates of increase of 11% and 21% and a rate of decrease of 24%, depending on the type of cement used. The cement type had a greater influence than the type of admixture studied. The concrete with admixture studied by Reddy and Ravitheja (6868. Reddy TCS, Ravitheja A. 2019. Macro mechanical properties of self-healing concrete with crystalline admixture under different environments. Ain Shams Eng. J. 10(1):23-32. https://doi.org/10.1016/j.asej.2018.01.005.
) achieved an increase of 11% compared to the reference concrete, which was attributed to the filling of the voids by C-S-H and calcite (one of the morphologies of calcium carbonate). In another study, Al-Rashed and Al-Jabari (88. Al-Rashed R, Al-Jabari M. 2021. Multi-crystallization enhancer for concrete waterproofing by pore blocking. Constr. Build. Mater. 272:121668. https://doi.org/10.1016/j.conbuildmat.2020.121668.
) observed improvements in the compressive strength of different concretes with multi-crystallization, mainly at the ages of 28 and 56 days.

Regarding the effect of exposure at different ages, Weng and Cheng (2424. Weng TL, Cheng A. 2014. Influence of curing environment on concrete with crystalline admixture. Monatsh. Chem. 145:195-200. https://doi.org/10.1007/s00706-013-0965-z.
) tested different levels of crystalline admixtures (0, 3, 5 and 7%). They observed that the compressive strength of the concretes studied after water exposure was considerably higher than those after air curing exposure at the ages of 7, 28 and 56 days. The increase of crystalline admixture content showed an increase of compressive strength. For example, with 3% of this admixture, there were increases of 14%, 20% and 22% at the ages of 7, 28 and 56 days, respectively, in comparison to those without the admixture.

3.2. Water absorption and void index

 ⌅

The water absorption and void index results are summarized in Table 5. Dixon’s test was performed and no data were excluded. As mentioned before, this test was performed only for the age of 154 days.

TABLE 5.  Summary of water absorption (%) and void index (%) average results.

Type of concrete/ water content	Exposure condition	Water absorption (%)	CV* (%)	Void index (%)	CV* (%)
Without crystalline admixture - 11% water content (N11)	Sealed	8.5	3.3	17.6	3.2
	Saturated	8.0	1.2	16.7	0.9
With crystalline admixture - 11% water content (A11)	Sealed	8.4	1.2	17.6	1.0
	Saturated	8.1	0.9	16.8	0.7
Without crystalline admixture - 7% water content (N7)	Sealed	13.1	1.8	26.0	1.4
	Saturated	12.6	1.7	25.0	1.3
With crystalline admixture - 7% water content (A7)	Sealed	12.4	2.1	25.0	1.3
	Saturated	12.3	1.3	24.6	1.0

*CV - Coefficient of Variation

Similar to the compressive strength, the effect of the water content of the micro concretes was clear in this test. By increasing the water content from 7% to 11%, the average value of the total water absorption decreased by 56% and the void index by 47%.

Figures 4 and 5 show the average and standard deviation results of the water absorption and void index of all the studied micro concrete mixes.

FIGURE 4.  Water absorption (%) - average and standard deviation values of the micro concretes studied.

FIGURE 5.  Void index (%) - average and standard deviation values of the micro concretes studied.

As concerns the type of exposure conditions, it was observed that the saturated condition promoted a tendency of reducing both the total water absorption and the void index in the mixes studied. However, this trend was greater for N11 (5.4% for both the water absorption and void index). In the A11 micro concrete with the crystalline admixture, these reductions were 4.1% and 4.6%, respectively. For the mixture with 7% water content, the greatest reduction trend was for the mix without admixture (N7), with 3.9% for both indexes. For the micro concrete with admixture (A7), this trend was much lower, with values of 0.9% and 1.5%, respectively.

Related to the effect of the crystalline admixture, for the 11% water content, it was observed that both the water absorption and the void index showed no considerable reduction. However, for the water content of 7%, this admixture caused a tendency to reduce the total water absorption and the void index in both cases (sealed and saturated samples). This behavior was consistent with an increase in compressive strength obtained, although, in that case, such an increase could not be confirmed by the statistical analysis performed.

Tables 6 and 7 present the ANOVA results for the absorption and void index, which were analyzed together because they were obtained from same test.

TABLE 6.  ANOVA water absorption results of micro concretes.

Description	SQ	GL	MQ	Fcal	Ftab	p-value	Result
Model	0.011526	7	0.0016	445.06	2.66	0.0000	Significant
Error (residue)	0.000059	16	0.000004				-
Total	0.011585	23
Water content	0.01137	1	0.01137	3,073.13	4.49	0.0000	Significant
Crystalline admixture	0.000029	1	0.000029	7.93	4.49	0.0124	Significant
Conditions of exposure (sealed or saturated)	0.000075	1	0.000075	20.38	4.49	0.0003	Significant
Water content x crystalline admixture	0.000038	1	0.000038	10.31	4.49	0.0055	Significant
Water content x conditions of exposure	0.000001	1	0.000001	0.33	4.49	0.5727	Insignificant
Crystalline admixture x exposure conditions	0.000009	1	0.000009	2.55	4.49	0.1300	Insignificant
Water content x crystalline admixture x exposure conditions	0.000003	1	0.000003	0.80	4.49	0.3844	Insignificant
Error (residue)	0.000059	16	0.000004

Coefficient of determination (R²_mod) 1.00 and Correlation coefficient (R_mod) 1.00

TABLE 7.  Void index ANOVA results.

	SQ	GL	MQ	Fcal	Ftab	p-value	Result
Model	0.039084	7	0.005583	550.82	2.66	0.0000	Significant
Error (residue)	0.000162	16	0.00001
Total	0.039246	23
Water content	0.038523	1	0.038523	3,800.40	4.49	0.0000	Significant
Crystalline admixture	0.000068	1	0.000068	6.70	4.49	0.0200	Significant
Conditions of exposure (sealed or saturated)	0.000374	1	0.000374	36.90	4.49	0.0000	Significant
Water content x crystalline admixture	0.000082	1	0.000082	8.10	4.49	0.0118	Significant
Water content x conditions of exposure	0.000005	1	0.000005	0.50	4.49	0.5000	Insignificant
Crystalline admixture x conditions of exposure	0.000023	1	0.000023	2.30	4.49	0.1488	Insignificant
Water content x crystalline admixture x conditions of exposure	0.000009	1	0.000009	0.90	4.49	0.3554	Insignificant
Error (residue)	0.000162	16	0.00001

The ANOVA results evaluated the influence of three independent variables on the absorption and void index: water content, crystalline admixture and exposure conditions. All three factors were significant, and the interaction between the water content and crystalline admixture was equal in the compressive strength. When comparing the Fcal values, considering that Ftab has the same value, it was observed that the most significant variable, much more than the others, was the water content, followed by the exposure conditions, the interaction between the water content and crystalline admixture, and finally the crystalline admixture.

By the grouping of means, using the Duncan test, the mixes were divided into four distinct groups, shown in Table 8. Across the groups, the water content and conditions of exposure were more predominant than the other variables analyzed. The crystalline admixture, although significant, did not interfere much in the division of groups (see Table 8).

TABLE 8.  Grouping of means by Duncan’s test.

Type of concrete/ water content	Exposure conditions	Water absorption average value (%)	Average void index (%)	Groups
				1	2	3	4
N11	Saturated	8.0	16.7		x
A11	Saturated	8.1	16.8		x
N11	Sealed	8.5	17.6			x
A11	Sealed	8.4	17.6			x
N7	Saturated	12.6	25.0	x
A7	Saturated	12.3	24.6	x
A7	Sealed	12.4	25.0	x
N7	Sealed	13.1	26.0				x

Different from the results obtained in this research, a study by Petrucci and Hastenpflug (3030. Petrucci RSP, Hastenpflug D. 2017. Influence of the crystallizing additive on the porosity of Portland cement concrete (in Portuguese: Influência do aditivo cristalizante na porosidade do concertos de Cement Portland). In: 59th Brasiliano Concrete Congress, Rio Grande do Sul.
) indicated that 0.8% content of crystalline admixture reduced the total absorption (by immersion) and the void index at each wetting and drying cycle (four in total), from a 3.1% water absorption rate to 0.3% for conventional concrete. In comparison with the reference concrete (without admixture), the reduction was 90.5% for the water absorption rate and 90.2% for the void index.

In the research developed by Hassani et al. (2525. Hassani ME, Vessalas K, Sirivivatnanon V, Baweja D. 2017. Influence of permeability-reducing admixtures on water penetration in concrete. ACI Mater. J. 114(6):911-922. https://doi.org/10.14359/51701002.
), also with conventional concrete, the crystalline admixture used did not influence significantly the void ratio (ANOVA results), calculated by the ASTM C642-13 standard, presenting a maximum reduction of 10% for a given type of concrete.

3.3. SEM analyses

 ⌅

To connect the results obtained with the microstructure of the micro concretes evaluated, the SEM images are shown in Figures 6 and 7. It was thus possible to observe the influence of the water content and the type of exposure condition on the appearance of the micro concretes.

FIGURE 6.  SEM images without the crystalline admixture, with amplification of 500x, showing influence of water content and exposure conditions: (a) saturated with 7% water content; (b) saturated with 11%; (c) sealed with 7% water content and (d) sealed with 11%.

FIGURE 7.  SEM images with the crystalline admixture with amplification of 100x, showing influence of water content and exposure conditions: (a) saturated with 7% water content; (b) saturated with 11%; (c) sealed with 7% water content and (d) sealed with 11%.

Analyzing Figure 6 showing the micro concretes without admixture, it is noted that the micro concrete specimens with 7% water content (N7) were more heterogeneous and porous than the mixtures with 11% (N11), including the presence of a crack (in the saturated condition). Considering the exposure conditions (saturated or sealed), the appearance was similar: the concrete surfaces had the same aspects. This corroborates the compressive strength and water absorption results, which indicated a better performance of the micro concretes with 11% water content and no difference between the types of exposure conditions.

Considering the water content, the same behavior seen in Figure 6 was observed in Figure 7, especially for the sealed condition. Comparing the exposure type, for 7% water content with the crystalline admixture the saturated condition promoted a more compact and homogeneous cement matrix, corroborating the small improvement in the compressive strength and water absorption results. This could indicate that water is necessary for adequate performance of the crystalline admixture.

To analyze the effect of the crystalline admixture, Figure 8 shows some SEM images of mixtures with 7% water content in the saturated and sealed conditions.

FIGURE 8.  SEM images with 7% water content, with amplification of 500x, showing influence of admixture and exposure conditions: (a) saturated with the crystalline admixture; (b) saturated without the admixture; (c) sealed with the admixture and (d) sealed without.

Figure 8 presents cementitious matrices with no great differences between them. The presence of the crystalline admixture seems not to significantly alter the appearance of these micro concrete samples, but has a tendency to produce slightly more porous micro concretes, especially for the saturated condition. The use of wet curing (saturated) produced, apparently, less homogeneous materials. But, as seen in the macro properties presented before, the exposure condition showed no significant differences in the results. For 11% water content, related trends were observed with similar micro concretes studied, analyzing the same aspects.

4. GLOBAL DISCUSSION

For conventional concretes, the effect of low water/cement ratios on the durability of the concrete is known in the literature, especially in relation to porosity and permeability and from the point of view of the transport and entry mechanisms of aggressive agents and moisture (water content) (99. Cascudo O, Peres P, Carasek H, Castro A, Lopes A. 2021. Evaluation of the pore solution of concretes with mineral additions subjected to 14 years of natural carbonation. Cement Concrete Comp. 115:103858. https://doi.org/10.1016/j.cemconcomp.2020.103858.
, 1010. Oliveira AM, Cascudo O. 2018. Effect of mineral additions incorporated in concrete on thermodynamic and kinetic parameters of chloride-induced reinforcement corrosion. Constr. Build. Mater. 192:467-477. https://doi.org/10.1016/j.conbuildmat.2018.10.100.
).

However, when referring to very low w/c ratios, there seems to be a minimum limit for the hydration of cementitious compounds (6969. Mehta PK, Monteiro PJM. 2014. Concrete: microstructure, properties, and materials. McGraw-Hill Education.
). Thus, concretes with a 0.44 w/c ratio (11% water content) showed better performance in the measured properties than the concretes with w/c 0.28 (7% water content). These results were true, evident and significant in all the properties evaluated, including from a statistical point of view, even with the degree of significance (Fcal in relation to Ftab and the p-value). The water content, taken in isolation, was the most significant variable and, no less important, this variable interacted with the presence of the admixture (taking the variables two by two) and was also significant in all the other properties evaluated.

The effect of the crystalline admixture seems to have been the same for the two water content levels studied, as shown by the properties and analyzed microstructure. This may have been a consequence of the low w/c ratios studied, where the action of the crystalline admixture did not stand out, and this was one of the objectives of the study. This does not mean that for higher water content this effect cannot be more evident and that other types of crystalline admixture cannot be studied in drier precast concretes. The study by Roig-Flores et al. (3535. Roig-Flores M, Moscato S, Serna P, Ferrara L. 2015. Self-healing capability of concrete with crystalline admixtures in different environments. Constr. Build. Mater. 86:1-11. https://doi.org/10.1016/j.conbuildmat.2015.03.091.
) showed positive effects of the use of a crystalline admixture in concrete with a water/cement ratio greater than 0.45. And the research by Al-Rashed and Al-Jabari (88. Al-Rashed R, Al-Jabari M. 2021. Multi-crystallization enhancer for concrete waterproofing by pore blocking. Constr. Build. Mater. 272:121668. https://doi.org/10.1016/j.conbuildmat.2020.121668.
) presented a tendency to an improved self-healing capacity with ratios of 0.37 to 0.54. There are also records of insignificant or even the same effects. In a study by Hassani et al. (2525. Hassani ME, Vessalas K, Sirivivatnanon V, Baweja D. 2017. Influence of permeability-reducing admixtures on water penetration in concrete. ACI Mater. J. 114(6):911-922. https://doi.org/10.14359/51701002.
) these admixtures had less influence on the water penetration in concrete than the water/binder ratio (0.40 and 0.60) and the type of cement. And in the study by Escoffres, Desmettre and Charron (3737. Escoffres P, Desmettre C, Charron JP. 2018. Effect of a crystalline admixture on the self-healing capability of high-performance fiber reinforced concretes in service conditions. Constr. Build. Mater. 173:763–774. https://doi.org/10.1016/j.conbuildmat.2018.04.003.
) on high-performance fiber reinforced concretes with a water/binder ratio of 0.43, the use of these admixtures did not change significantly the mechanical and water permeability properties when submitted to monotonic tensile loading.

Additionally, the exposure conditions expressed significant results among themselves for the properties of the void index and water absorption, with lower values for higher water content. In terms of significance, this is the second most important property considered in this study; that is, Fcal is the second largest in terms of Ftab (or p-value).

As concerns the age in the compressive strength tests, this was Insignificant when taken alone, or with double or triple interaction. This means that the results at 28 days and 154 days were statistically equal to the results of the compressive strength obtained for the studied concretes. From a practical point of view, in this case, the result at 28 days showed a faster response in the investigated property and in the analyzed dataset, and could distinguish the concretes in different compressive strength classes - NBR 8953: 2015 (7070. ABNT. 2015. NBR 8953: Concrete for structural use - Density, strength, and consistency classification (in Portuguese: Concreto para fins estruturais - Classificação pela massa específica, por grupos de resistência e consistência). Rio de Janeiro.
).

The relationship between the average results of the compressive strength and water absorption at 154 days is shown in Figure 9.

5. CONCLUSIONS

This study analyzed the influence of crystalline admixture on the compressive strength and water absorption of precast micro concretes. Based on the discussions from a practical and application point of view, it can be concluded that: