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Article

Self-Compacting Earth-Based Composites: Mixture Design and Multi-Performance Characterisation

by
Ana Mafalda Matos
* and
Humberto Varum
CONSTRUCT, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
*
Author to whom correspondence should be addressed.
Buildings 2022, 12(5), 612; https://doi.org/10.3390/buildings12050612
Submission received: 7 April 2022 / Revised: 30 April 2022 / Accepted: 2 May 2022 / Published: 6 May 2022
(This article belongs to the Section Building Materials, and Repair & Renovation)
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Abstract

:
A new generation of self-compacting earth-based composites (SCEC) for vertical elements with fully exposed earth, in which the main feature is the elimination of compaction, is proposed in the current work. The first objective was to develop earth-based mixture compositions with locally available raw materials and achieve self-compactability. A Portuguese soil, typically employed in rammed earth construction, was stabilised using cementitious materials and a superplasticiser to ensure flowability. The mixture design studies allowed to achieve flowable fresh composites. Results on the hardened state showed that SCEC presents high compressive strengths (5–14 MPa). The electrical resistivity SCEC increased with time and achieved at least 5600 Ωm at 28 days, while mass loss stabilised after 14 days in a controlled environment room. Additional tests were performed on selected optimal SCEC mixtures: drying shrinkage and water capillary absorption, with promising results. Overall, this work provided initial boundaries for the mixture design and assessment of SCEC and opened the door for further studies on new and novel earth materials construction techniques.

Graphical Abstract

1. Introduction

Construction has a key role in the socio-economic development of a country or region. It is responsible for promoting a sustainable built environment, greener and more inclusive societies, and improving the quality of life of populations, economic growth and job creation. The predicted increase in world population (by 2050, it is expected to increase by more than 2000 million people [1]) and the need for housing and infrastructures will further highlight the construction sector’s role. However, higher demand for non-renewable materials and waste generation, greenhouse gas emissions, and energy needs are also expected to fulfil those needs. Therefore, construction professionals, engineers, architects, quantity surveyors, construction managers, town planners, land surveyors, etc., need to promote sustainable construction development [2]. In fact, the World Green Building Council identified that building materials can meet at least 9 of the 17 Sustainable Developments Goals of Agenda 2030 [3]. Thus, there is growing interest in using natural materials, particularly locally available materials with minimal process.
Throughout the world, earth (soils) is one of the oldest and the most common natural building material [4], mainly employed in warm and arid climate zones [5]. It is estimated that more than two billion people live in earthen dwellings, distributed in 150 countries [6,7], although with significant regional differences. In some regions, the earth is the dominant construction resource, such as in Peru, where about 40% of existing buildings are earth-made. In industrialised countries, such as European regions, the number of buildings constructed with earth-based techniques is low [8,9].
Earth as a building material offers important advantages, namely: (i) economic since it is a worldwide available material, without the need for transport or calcination and with easy extraction, transformation and low processing costs; (ii) ecological, with low environmental impact due to low incorporated energy and low CO2, non-toxic, reusable and recyclable [10,11,12,13]; (iii) social (healthy and comfort) since the earth elements, particularly with some thickness, have high thermal inertia, allowing RH control, due to their porous structure and clay content, and providing a well-known quality for interior comfort for both acoustic and hygro-thermal aspects [14].
Although earth seems an excellent solution at environmental, thermally efficient, healthful, aesthetic and cultural levels, the conventional model of this type of construction declined in the second half of the XXth century, namely with the growth of the Portland cement industry and industrial construction techniques. Given the current demand for healthy and ecological buildings in harmony with nature, some developed countries have surrendered to the benefits of earth as a building material, and earth buildings are popping up recently in Europe [15], such as Austria, Switzerland, Germany, Italy and Portugal. Nonetheless, vernacular processes are slow, time-consuming and require a large workforce, which is inappropriate in modern Western economies [16].
A specific characteristic of vernacular earth construction is the use of local material extracted directly from the excavation site or nearby. The design and the manufacture of earth as a building material have been based on local traditional know-how, generally orally passed from one generation to another. The earth construction technique is mainly manual, and the local builders deal with the local soils’ properties without scientific analysis or tools. The on-site construction processes are arduous and lead to high labour costs and long times. [17]. Besides, the variable final product properties associated with the natural variability of raw materials and artisanal construction, influenced by climatic conditions and the skill of the artisans, are also barriers for actual construction stakeholders. Thus, making earth construction a regular building practice is a significant challenge. Hence, there is a need to reinvent the experience regarding the suitability of soils and revisit their potential to meet today’s environmental challenges and housing needs. The scientific community needs to strengthen confidence in earth construction, adapt it to current technologies and know-how, and unlock earth’s potential as building material. Hall and Swaney [18] introduced the concept of “modern earth buildings”. In “modern earth buildings”, the earth construction process must benefit from the high-tech advancement during the XXth and XXIst centuries, seeking eco-efficient solutions integrating the geographical and cultural specificities of local context, the soil properties, and the building’s structural-functional requirements [18].
The transfer of knowledge of concrete technology to earth can transform and upgrade earth construction and provide competitiveness and a wider acceptance and application by construction stakeholders. Recently, some research efforts have been made to fluidify earth material and cast it using the same techniques employed in concrete materials, namely for rammed earth construction. Different authors presented different designations such as concrete earth, poured earth, cast earth or flowable earth [19,20,21,22,23,24,25,26]. This approach is possible because clay presents some similarities with cementitious materials in terms of colloidal interactions and adhesion forces [27]; however, those are weaker among clay particles [28] due to the absence of hydraulic reaction [26]. Some researchers employed plasticiser/superplasticisers commonly used in cement-based materials (such as polycarboxylate ethers) to produce castable earth materials. Usually, those earth materials are stabilised with hydraulic binders, such as lime (10–15 wt% regarding the mass of soil), Portland cement (up to 5–8 wt% regarding the mass of soil), calcium sulfoaluminate cement or blended mixtures [9,20,29,30,31] to allow a more fluid fresh earth composite and to provide a faster setting, improving mechanical and durability performance.
On the other hand, one method to counteract the disadvantages of traditional in situ processing is earth-based components’ industrial prefabrication. The components can be produced close to the construction sites using local raw materials, such as excavations. This system can provide a major advance in the construction process and a lower final cost for each prefabricated unit. Additionally, production in a controlled environment is possible in any season of the year, without weather conditions. Even though there are few examples of such kind of applications and few companies (such as Rammed Earth Enterprise© in Australia, Sirewall© in the USA, Rammed Earth Artisan Ltd.© in Canada and Earth Structures Europe Ltd. in the UK), the production process still relies on hard and time-consuming compacting processes of earth mixtures with low water content to achieve a highly compact and dense matrix; besides, it depends mainly on the team’s technical skills, know-how and experience.
The current work aims to develop a new generation of self-compacting earth-based composites (SCEC), which feature is the no need of compacting, for the precast elements. The prefabricated SCEC elements can be mass-produced and eliminate the most demanding and most time-consuming stage, compaction, exponentially increasing productivity and constant and controlled quality. The architectural flexibility of the earth construction is improved by considering the fluidity of SCEC.
Thus, the first step was the mixture design of earth-based materials, employing locally available raw materials and achieving self-compatibility. The earth-based mixture should present: (i) high fluidity, self-compacting, without segregation; (ii) placed in the mould, it should support its own weight after 24 h; (iii) up to 10% of cement regarding soil mass. A soil from the south of Portugal was studied, typically used in rammed earth construction. A blend of cementitious materials and a polycarboxylate ether superplasticiser were employed to stabilise the soil and achieve flowable fresh composites. The earth mixtures were characterised in terms of flowability, electrical resistivity, mass loss and mechanical strength. Afterwards, the optimal mixtures were selected, and shrinkage and water capillary absorption tests were performed.

2. Materials and Methods

2.1. Raw Materials Characterisation

2.1.1. Binder Fraction

The binder fraction aimed to stabilise the soil was prepared with a blended cement type CEM I 42.5R, limestone filler (LF) and metakaolin (MTK). The main chemical and physical properties of cementitious materials are summarised in Table 1. Besides, Figure 1 shows cement, LF and MTK particles observed by scanning electron microscopy (SEM). As can be seen, MTK consists mainly of angular and plate-like shape particles, and LF consists of blocky shaped particles. Cement particles are angular and present a wide size variety. A polycarboxylate ethers (PCE) type superplasticiser was used with a specific gravity of 1070 kg/m3 and 26.5% solid content to ensure self-compacting ability.

2.1.2. Soil

The main constituent of SCEC was soil from the south of Portugal, usually employed on rammed earth construction. Since some coarse particles and organic matter were found, the raw soil was dried until constant mass at 50 °C and sieved at 1 mm. The passed fraction was considered for further study (see Figure 2). The particle size distribution (PDS) of soil is depicted in Figure 3, and it was obtained by sieve analysis and sedimentometry according to ASTM D 6913. As can be seen, the soil presents 80% sand, 8% silty and 12% clay. The PSD reveals the clay content, an essential parameter, since clay provides cohesion to the earth mixture, acting even as the only binder in unstabilised earth construction [31,32,33]. From the literature survey, the optimum clay content is considered between 8% and 14% by mass, the percentage of clay+silt should be between 20% and 35%, and the percentage of sand between 50% and 75% [32]. The soil density was measured by the pycnometer test method (ASTM D 854) and was 2620 kg/m3. The Atterberg limits had no result.

2.2. Experimental Programme

The experimental programme is summarised in Table 2, including standards/procedures, testing ages, curing regime, number and geometry of specimens. The experimental study was divided into two stages: the first stage aimed to reach a self-compacting mixture by an iterative approach, which was defined as presenting a flow diameter higher than 250 mm (procedure described in Section 2.2.2) without segregation or blocking observation. Afterwards, all mixes produced were characterised by electrical resistivity, mass loss and mechanical strength. The second stage consisted in selecting the optimal mixture considering self-compactability behaviour, easy demoulding after 24 h, reduced mass loss (as shrinkage indicator), high resistivity (as durability indicator) and mechanical behaviour. Based on those requirements, two optimal mixtures were selected and further studied in terms of shrinkage and water capillary absorption. The following sections describe the mixture composition study (Section 2.2.1) and the testing procedures employed in the current research in more detail (Section 2.2.2 to Section 2.2.6).

2.2.1. Mixtures Design and Specimens Manufacture

A comprehensive experimental study was carried out on eight earth-based composites incorporating different proportions of constituent raw materials, as shown in Table 3. Besides mixture proportions, Table 3 also presents the main ratios of the mixture design in the last five lines, which are:
  • b/soil—binder to soil weight ratio;
  • c/soil—cement to soil weight ratio;
  • sp/fines (%)—superplasticiser to fines weight ratio (fines were considered: the clay content of the soil, cement, limestone and metakaolin);
  • w/c—water to cement weight ratio;
  • w/soil—water to soil weight ratio.
The mix design study was based on a literature survey (described in Section 1) as well as the knowledge of the authors on self-compacting cement-based materials [34,35]. Therefore, an iterative study of SCEC mixture design follows, considering: (i) cement/soil weight ratio (c/soil) should not exceed 10%; (ii) flow diameter should be at least 250 mm as suggested on EFNARC recommendations [36]; (iii) demouldable at 24 h. Mixture proportions of SCEC are presented in Table 3.
Earth-based mixtures were produced in 1.40 L volume batches, using a mixer in complying with EN 196-1, and the rotation speed was constant during all procedures (140 rpm). The mixing process followed four steps: (i) adding soil, cement, LF and MTK, 80% of water and mixing for 150 s; (ii) stopping to scrape material, adhering to the mixing bowl and mixing for 150 s; (iii) introducing the rest of the water plus 75% of Sp and mix for 150 s; (iv) adding the rest of Sp and mix for 150 s. Immediately after mixing, fresh tests were performed to check the self-compacting behaviour (described in Section 2.2.2).
Prismatic specimens (40 × 40 × 160 mm3) were cast, without any vibration, for mass loss, electrical resistivity and mechanical strength assessment. Stainless steel mesh was involved in fresh specimens to allow resistivity measurement (Section 2.2.3). The specimens remained covered with a plastic sheet and demoulded after 24 h. Thereafter, the samples were maintained in a controlled environment room (T = 20 ± 2 °C and HR = 50 ± 5%) up to testing.
After an integrative analysis of fresh behaviour, electrical resistivity, mass loss and mechanical strength, optimal mixtures were selected (Section 3): SCEC 5 and SCEC 7 (bolt cells in Table 3).

2.2.2. Fresh State Tests

Immediately after production, the (mini) slump flow was assessed, and employed as a benchmark for checking the deformability of fresh earth-based composites. The test followed the procedures of EFNARC recommendations widely applicable to self-compactable cement-based composites. The procedure consists, in brief, in filing the mini cone (see Figure 4a) with the fresh composite and then taking the cone off and observing the flowing capacity in a plane surface. When fresh composite stops moving, two perpendicular diameters are measured, and the average value is considered the flow diameter (Dflow) in mm (see Figure 4b).
Additionally, the V-funnel test was performed to assess viscosity and passing ability using the mini funnel usually employed for self-compactable cement-based mortars (see Figure 5). In brief, after production, the fresh earth-based composite was poured into the funnel. Then, the trap door was open, allowing the fresh composite to flow out under gravity and recording the time for the discharge to complete (the flow time). This is when light is seen from above through the funnel. Then, specimens were moulded and demoulded after 24 h.

2.2.3. Electrical Resistivity

Electrical resistivity was measured on prismatic specimens (40 × 40 × 160 mm3) produced for mechanical strength test at 28 days. The mass loss was controlled by weighting the specimens before resistivity measurements. As mentioned before, specimens were kept under a controlled environment (T = 20 ± 2 °C and HR = 50 ± 5%) and removed just for mass loss and electrical resistivity test assessment at 1, 2, 7, 14, 21 and 28 days.
The two electrode method was followed. Thus, stainless steel mesh was allocated during casting (see Figure 6) and used as electrodes at each testing time. A potential difference from an alternating current was applied across each sample through the signal generator, which performed a sinusoidal wave, with a peak voltage of ±10 V and frequency of 100 Hz, following the recommendations described in [37], and a digital multimeter (Keithley) measures the resulting current. The electrical resistivity against time is reported as an average of results on three identical specimens. The electric resistivity ( ρ ), Ω m , is computed by Equation (1):
ρ = V A I L
where V , voltage (Volts); I , current (A); L , length (m); and A (m2), the cross area of the test specimen through which current passes.

2.2.4. Mechanical Strength

The mechanical performance of SCEC was based on EN 196-1, which is one of the most common procedures employed on cement materials. However, a slight adaptation was made. Instead of load control testing as suggested in EN 196-1, the test was performed on displacement control through a 300 kN Instron testing machine, which was 0.01 mm/s and 0.02 mm/s for the flexural and compressive test, respectively.
Three prismatic specimens, 40 × 40 × 160 mm3, were cast for each mixture and each testing age, 7 and 28 days. After demoulding, the specimens were cured in a controlled environment (T = 20 ± 2 °C and HR = 50 ± 5%) until testing. The specimens employed for 28 days of mechanical strength assessment were additionally used to measure electrical resistivity at 1, 2, 7, 14, 21 and 28 days. At the age of 28 days, subsequently to the resistivity measurements, the mechanical strength was evaluated.

2.2.5. Drying Shrinkage

The drying shrinkage of optimal earth-based mixtures, SCEC 5 and SCEC 7, was evaluated following ASTM C 596. Three prismatic specimens were cast for each mixture (25 × 25 × 285 mm3) with inserted steel studs partially embedded at each end, see Figure 7. The first measurement was taken after demoulding, at 72 h, and considered a “zero” reading. Then, the specimens were stored in a controlled environment room at T = 20 ± 2 °C and HR = 50 ± 5%. Measurements were repeated 7, 14, 28, 35, 42 and 56 days in a vertical plunger and using a digital indicator with a precision of 0.001 mm. After each length change measurement, the specimens were also weighted. The linear drying shrinkage deformation and mass loss against time are described as an average of results on three identical bar specimens at each age.

2.2.6. Water Capillary Absorption

The water capillary absorption was assessed following the Recommendation RILEM TC 116-PCD [38]. For the test, four cylindrical specimens (diameter 50 mm and height 100 mm) were cast for each optimal mixture, SCEC 5 and SCEC 7. After 28 days of controlled environment curing (T = 20 ± 2 °C and HR = 50 ± 5%), specimens were dried at 40 ± 1 °C until they reached constant mass. After cooling in the testing room, water capillary absorption was measured through a shallow water bath (see Figure 8). The water level was adjusted automatically, and a constant depth of approximately 3 mm was kept during all tests. From the start of testing, water intake by capillary suction was measured by weighting the specimens at 0 (immediately before allocating the specimens in the water bath), 10, 20, 30, 40, 50, 60, 120, 180 and 240 min using a scale with 0.01 g precision.
The sorptivity coefficient S is the indicator obtained from the procedure described. Using linear regression analysis, S is determined from the slope of the best fitting line, relating the weight readings and the square root of the elapsed time (240 min). Equation (2) computes the equation in which S 0   is the water absorbed initially by pores in contact with water (kg/m2) ,   S is the water absorption by unit area of the specimen’s surface from the moment the sample was dipped in water (kg/m2), t is the elapsed time (h) [39]. The sorptivity is reported as an average of results on four identical specimens.
i = S 0 + S * t 0.5

3. Results and Discussion

3.1. Self-Compactability

After production, flow and V-funnel tests were performed to check the mixtures’ deformability, viscosity and passing ability and confirm self-compactability (or not). The flow diameters of fresh earth-based mixtures are depicted in Figure 9. The obtained values for flow diameter and t-funnel are presented in Figure 10 and listed in Appendix A. The mixtures SCEC 1 and SCEC 2 were not accessed regarding T-funnel due to the occurrence of blocking in the V-funnel.
As can be perceived from Figure 10, results show a general trend of increasing slump-flow diameter with a reduction in flow time. The experimental plan covered a range of Dflow from 197 to 288 mm; however, T-funnel ranges were small, from 2.5 to 5.7 s. Dflow measured seems to be adequate for self-compacting since those intervals include target values of Dflow of 250–260 mm, as suggested by Okamura [40]. Concerning T-funnel, Okamura [34] suggests a target value of 10 s for self-compactable based mortars. As best to the authors’ knowledge, studies on self-compactable earth-based composite are still scarce [20,23,24,41,42] and, in most cases, may not be considered self-compacting due to vibration/compacting needs. Besides, no similar tests of flow diameter and t-funnel were found in the literature survey. Thus, T-funnel target values for earth composite may differ from cement-based materials. Further studies are needed. However, it must be pointed out that no segregation was observed.
The first trial (SCEC 1) started with sp/fines of 2.05%, which was verified not to be sufficient to reach self-compacting ability (see Figure 9a). Afterwards, superplasticiser, water content and supplementary cementitious materials were adjusted by an iterative process. As can be perceived by comparing SCEC 3 and SCEC 4, LF played an important role in stabilising and on the flowability of fresh earth-based composites. It follows a brief explanation. A binary blended with cement+MTK (SCEC 4, Dflow = 195 mm, see Figure 9c) with more superplasticiser loses flowability compared to SCEC 3 (Dflow = 200 mm). Therefore, a ternary blended cement was produced with a small increment on the superplasticiser (SCEC 5). With SCEC 5, self-compactability was established with a Dflow = 255 mm (see Figure 9d). Then, additional trials were performed with a binary blend of cement and LF, since LF is easier available in Portugal and other European Countries. Both SCEC 6 and 7 reach self-compactability with Dflow of 288 mm and 253 mm, respectively (see Figure 9e,f, respectively). At last, trial SCEC 8 reduced the cement content to 7.5% regarding soil mass and a flow diameter loss occurred (Dflow = 239 mm, see Figure 9g). The flow diameters’ circular and uniform shape indicates no segregation occurrence (Figure 9).
The relationship between sp/fines content and Dflow and T-funnel is presented in Figure 10 and Figure 11, respectively, in which selected optimal mixtures are included in a red circle. Like cement-based composites, the superplasticiser effect is more pronounced on the composites’ flow diameter than the T-funnel test. The sp/fines dosage between 2.0–3.5% did not significantly affect flowability; afterwards, it involved and originated flowable composite with no need for compacting.
The PCE superplasticiser has a key role in reaching self-compactability and seems to be compatible with the sandy soil employed in the current work, which corroborates with previous research [20,25,41].
After 24 h, the moulded specimens were demoulded. Mixtures SCEC 6 and SCEC 8 were found to present delay on the setting, with a pronounced fresher state regarding the remaining mixtures. Therefore, they were demoulded very carefully.

3.2. Electrical Resistivity

Figure 12 presents the electrical resistivity evolution up to 28 days, and, in addition, the mass loss observed on those specimens is shown in Figure 13 (and listed in Appendix A). As expected, the electrical resistivity evolves with time. In the case of cement-based materials, it is well known that the evolvement of electrical resistivity with time is due to the development of the hydration reactions in the cementitious matrix providing a denser microstructure and the connectable pores volume decrease [43]. Electrical resistivity results at 28 days ranged from 5255 to 7683 Ω m, indicating a very compact matrix. According to Polder [37], it is expected that a ten-year dense-aggregate concrete incorporating 5% silica fume, submerged at 20 °C, presents a resistivity from 300 to 1000 Ω m [37].
The electrical resistivity evolution rate was generally slight at first ages, and the significant evolution occurred between 7 and 14 days, except for SCEC 1, SCEC 2 and SCEC 3. Those earth-based composites presented higher water contents, which may delay the cement hydration process. In addition, they contain a lower MTK amount. On the other hand, SCEC 4 still has a high electrical resistivity evolution rate at 28 days, typical observed on very high fine pozzolanic materials [43]. Pozzolanic reaction keeps evolving with the available cement hydration products. After 14 days, in general, resistivity still has considerable evolution up to 28 days, but in most mixtures with a lower rate. Besides, an increasing pattern was still presented at the end of testing (28 days).
According to previous research, in cement-stabilised soils, the hydration compounds such as calcium silicate hydrate and calcium aluminate hydrate gels from the hydraulic and pozzolanic reactions fill the pore and result in a denser microstructure. Besides, the free water space and porosity decrease, and tortuosity increase for electric current. Consequently, electrical resistivity increases more significantly [44,45,46]
The mass loss was more prominent in the first 7 days; then, it started to reduce to a slower rate up to 14 days, and at this age, it became approximately constant (see Figure 13).

3.3. Mechanical Performance

Figure 14 depicts the flexural (three-point bending) and uniaxial compressive strength of SCECs mixtures at 7 and 28 old days (see Appendix A). Compressive strength ranged between 5.8–12.8 MPa and 6.4–13.2 MPa at 7 and 28 days, respectively. The compressive strength gain between 7 and 28 days was slight and corresponded to a maximum of 21% gain for SCEC 4 and a minimum of 3% for SCEC 6. SCEC 4 corresponds to an earth-based composite stabilised with a binary blend of cement and MTK (see Table 3). This significant evolution of compressive strength from 7 to 28 days can be attributed to the high pozzolanic activity of MTK [47,48], also unveiled in the electrical resistivity test (SCEC 4 presented the highest electrical resistivity at 28 days which was 7683 Ωm, see Section 3.3). On the other hand, SCEC 6 corresponds to an earth-based composite stabilised with a binary blend of cement and LF (see Table 3). LF is commonly available and used in the concrete industry in European countries due to technical, economic and environmental advantages; however, LF pozzolanic activity is not significant, and its action is primarily physical. As such, for higher dosages (more than 10% of cement weight) alone as Portland cement, replacement may result in higher porosity and weaker strength [49,50]. This also corroborates resistivity tests (Section 3.3), in which SCEC 6 results in the second-lowest result (5572 Ωm). The maximum compressive strength was achieved by mixture SCEC 5 and was 12.8 MPa and 13.2 MPa, for 7 and 28 days, respectively.
The scientific community has scrutinised the compressive strength of raw earth-based or cement-stabilised earth materials [33,51,52]. Compressive strength was mainly assessed on small cylindrical specimens with a diameter equal to twice the height and cubic specimens with different dimensions [33]. The manufacturing, compaction, density, moisture content and testing procedure also vary, due to the lack of international standardisation, thus not allowing a proper comparison among the available results in the literature [49]. However, some key facts can be pointed out as follows. The compressive strength of raw earth materials ranges from 1 MPa to 2.5 MPa [33].
On the other hand, for cement-stabilised earth materials, it can be drawn that high cement contents, around 10%, resulting in compressive strength over 5 MPa, even reaching 10 MPa in some cases [52]. Besides, there is a linear relationship between the cement content and compressive strength for certain soils and testing conditions in the studies in which more than two cement contents were tested [52]. This increase is mainly due to improved contact between particles by an improved granular skeleton and hydraulic (and pozzolanic) reactions, thus increasing its mechanical properties [11].
As can be seen, the earth mixture developed in the current work revealed very promising compressive strength with a maximum of 10% of cement regarding soil mass, with no compacting process needed. The SCEC developed seems to present a good granular skeleton, compact, which provides higher mechanical and resistivity than the previous studies.
The tensile strength is a relevant parameter in analysing the rammed earth construction technique namely in seismic risk areas. Research on the tensile behaviour of earth-based materials has been assessed through different test methods, such as Brazilian tests or pull-off tests. Generally, the tensile strength of the unstabilised earth material can be considered to be approximately 10% of compressive strength, which corresponds to tensile strength between 0.10 MPa and 0.35 MPa [33].
Some authors have attempted to improve earth materials’ tensile strength by stabilisation with hydraulic binders, SCM and several types of fibre reinforcement (straw, palm, coir, jute, barley, fibreglass, plastic fibres). In those studies, tensile strength reaches from 0.25 to 1.16 MPa. The improvement of tensile strength in stabilised earth materials also leads to an increase in the ratio between tensile and compressive strength to 0.10 and 0.21 [52].
The SCEC developed in the current work revealed mechanical properties above the ones found in the literature, in which flexural strength reached 2.4 to 2.7 MPa without fibre reinforcement. Additionally, the tensile and compressive strength ratio ranged between 0.28 and 0.42 at 28 days.

3.4. Optimal Mixture Selection and Additional Characterisation

As aforementioned in Section 2.2.1, the SCEC mixture design performance requirements were: (i) cement/soil weight ratio (c/soil) should not exceed 10%; (ii) flow diameter should be at least 250 mm as suggested in EFNARC recommendations [36]; (iii) demouldable at 24 h.
In the first stage, the developed mixtures were analysed in terms of fresh state behaviour and determined the range of mixture parameters that satisfy the self-compactability requirements, i.e., to determine the mixtures which exhibit a mini-slump flow diameter of 250 mm and no segregation occurrence. The mixtures considered self-compacting corresponded to SCEC 5 to SCEC 8, as discussed in Section 3.1. Then, the demouldable after 24 was evaluated, and, as referred to in Section 3.1, SCEC 6 and SCEC 8 seemed to experience a setting delay regarding the remaining mixtures. This might be explained by the lower cement content in SCEC 8 (7.5% regarding soil mass). Concerning SCEC 6, the setting delay may be attributed to lower reactive cementitious content (no MTK added) while keeping the same water amount (comparing with SCEC 5, for instance), which provided fewer hydration products and, consequently, a lower bounded matrix.
Afterwards, the hydration process was evaluated by electrical resistivity and mass loss measurements. As perceived from Section 3.2, in general, earth-based mixtures lost 10% of the mass in the same environmental conditions (T = 20° ± 2 and HR = 50% ± 5) after 28 days. No significant differences among produced earth-based mixtures were found, as can be seen in Figure 13b and Appendix A. In terms of electrical evolution, earth mixtures generally present a similar evolvement pattern, with few exceptions discussed in Section 3.2.
The mechanical strength was above the literature survey on stabilised earth materials. Among mixtures reaching self-compacting ability (SCEC 5 to SCEC 8), SCEC 5 presented the higher performance, followed by SCEC 6 and SCEC 5. As expected, SCEC 8 showed lower strength because of lower cement content compared to the remaining earth-based composites.
Based on the aforementioned data and discussion, the authors selected earth-based mixtures SCEC 5 and SCEC 7 for further study regarding drying shrinkage behaviour and sorptivity. It follows some discussion on such characterisation.

3.4.1. Drying shrinkage

Figure 15 presents the evolution of total shrinkage deformation and mass loss for optimal mixtures, SCEC 5 and SCEC 7, up to 56 days (see also Appendix A). As shown in Figure 15a, SCEC specimens shrinkage occurs in the first ages, namely up to 14 days, corroborating with mass loss measurements performed on the same specimens and previous results from Section 3.2. The shrinkage rate is high during the first 14 days. Then, the rate decreases gradually in the function of time and stabilises after 21 days. Linear drying shrinkage measurements started at the age of 72 h after casting, thus are mainly due to drying shrinkage, which is closely related to water loss to the surrounding external environment. After 56 days, the shrinkage was 0.235% and 0.238% for SCEC 5 and SCEC 7, respectively, corresponding to 2353 and 2375 µm/m or 0.0023 and 0.0024 strain. The shrinkage and mass loss pattern are in accordance with previous findings on earth-based materials [46,53,54,55].
Previous research revealed that the combination of cement and sand in stabilised soil reduces the shrinkage slightly better than when only cement is added [56]. Kouta et al. [55] obtained shrinkage values around 7000 µm/m on an artificial soil composed of 30% bentonite clay and 70% sand, stabilised with 3% natural hydraulic lime and 8% of cement CEM I 52.5N. Drying shrinkage test performed by Kariyawasam and Jayasinghe [53] on wall prototypes of 3 different soils, sandy, clay and gravely, revealed that gravelly soil showed much better results with a shrinkage strain in the range of 0.0017. In contrast, sandy laterite has shown a 0.003 strain. Woyciechowski et al. [54] studied earth-based mixtures containing 70% of aggregate and 30% of loam stabilised with Portland cement CEM I 42.5R (9% of the dry mass of the soil). The highest shrinkage—0.35%—was recorded in specimens with soil without any gravel aggregate. The shrinkage value, in this case, was approximately two times regarding specimens produced with a similar mix composition and stored under the same conditions but containing 2–4 mm gravel fractions.

3.4.2. Sorptivity

Figure 16 depicts the cumulative capillary water absorbed (per m2 of inflow area) obtained for four samples for each optimal earth composites, SCEC 5 and SCEC 7, versus the square-root of elapsed hours of immersion in water during the first 4 h of measurement. The average sorptivity obtained was 2.15 ± 0.15 kg/(m2 h0.5) and 2.16 ± 0.19 kg/(m2 h0.5), for earth-based composites SCEC 5 and SCEC 7, respectively (see Appendix A). In addition, the total water absorbed after 4 h in contact with water was 3.84% and 4.10% for earth-based composites SCEC 5 and SCEC 7, respectively, considering the initial mass of the specimens.
Sorptivity is one of the key parameters for assessing the durability of cementitious materials, indicating how easily moisture, which may contain detrimental agents (such as chlorides), can penetrate by capillary absorption through the concrete cover porosity. The RILEM technical committee for performance-based specifications and control of concrete durability (RILEM TC 230-PSC) [57] proposed a classification for concrete quality considering water sorptivity at 28 days, as presented in Table 4. As can be perceived and using concrete materials as reference, sorptivity for optimal SCEC of the current study is classified as very good.
Usually, a high water affinity is a drawback of earth materials, which may influence the mechanical properties and durability [11,51,58]. The soil stabilisation with cementitious materials has been the most common solution to address that drawback. Cementitious materials are an effective and economical way to improve the earth (soils) engineering properties. It was already observed that the capillary absorption of earth-based mixes decreases significantly with increasing cement content. This is attributed to the formation of cementation compounds resulting from the cement, and eventually SCM, hydration, filling the voids of the soil and thus obstructing the water flow [52,59,60]. Additionally, a higher sand percentage in earth-based mixtures was associated with decreased capillary water absorption. This is mainly because sand has a lower affinity than clay; thus, total absorbed water is smaller [11].
In addition to a high compacting energy, the soil PSD modification or protection of earth materials from water sources can also be employed to improve earth-based materials’ properties. In some traditional earth construction techniques, high-energy compaction in manufacturing is used to enhance the contact between particles, increase the material density and reduce the size and number of pores. Besides improving mechanical properties, high-energy compaction makes earth elements less susceptible to the water effects [51].
The current work shows that SCEC produced with sandy soil with the feature of no compaction need to provide very low sorptivity comparable to very good concrete. This result is in accordance with the high resistivity measurements obtained (Section 3.2). Sorptivity results seem very promising, particularly considering the desire for building with fully exposed earth walls.

4. Conclusions

A new concept of earth-based materials with no need for compacting was developed and explored within the current work. A multi-performance characterisation of self-compacting earth-based composites was accomplished based on common tests applied to cement-based composites. Based on the experimental data from the fresh state, electrical resistivity, mass loss and mechanical strength optimal SCEC were selected and further studied in terms of drying shrinkage and water capillary absorption. The main conclusions are drawn as follows:
  • The methodology developed allowed flowable composites, which can have the formwork removed after 24 h;
  • The polycarboxylate ether superplasticiser played an important role with a cementitious blend ensured self-compactability;
  • Specimens presented a very good surface finishing quality;
  • The transfer of physics and rheology principles used in cement-based materials science seems to fit stabilised earth materials;
  • Electrical resistivity increased with time and achieved at least 5600Ωm at 28 days, with a still-growing pattern at this age;
  • Capillary water absorption was equivalent to good quality concrete and seemed very promising, particularly considering the desire to build with fully exposed earth walls;
  • SCEC presents higher compressive strengths compared to the literature survey on stabilised earth-based material.
Even though soil properties vary locally and raw materials characterisation is essential to replicate the current approach, this work provided initial boundaries for the mixture design and assessment of SCEC and showed it is possible to upgrade earth construction using actual technologies and know-how. The self-compacting ability with no need for any vibration contributes in an integrated way to reducing external variables to the final product properties while reducing cost and construction time, which can strengthen the construction stakeholders’ confidence in earth construction. In addition, SCEC technology can be easily adopted by prefabrication companies and can put forward new economic opportunities and the market’s stimuli and promote employment creation.
However, efforts are needed, especially from the technical and scientific community, to create and/or update and/or compile universal earth materials standards in the light of industrial construction methods [16].

Author Contributions

Conceptualisation, A.M.M.; methodology, A.M.M.; formal analysis, A.M.M.; investigation, A.M.M.; resources, A.M.M. and H.V.; data curation, A.M.M.; writing—original draft preparation, A.M.M.; writing—review and editing, A.M.M. and H.V.; visualisation, A.M.M.; supervision, A.M.M. and H.V.; project administration, A.M.M. and H.V.; funding acquisition, A.M.M. and H.V. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by: Base Funding—UIDB/04708/2020 and Programmatic Funding—UIDP/04708/2020 of the CONSTRUCT—Instituto de I&D em Estruturas e Construções—funded by national funds through the FCT/MCTES (PIDDAC); and by FCT—Fundação para a Ciência e a Tecnologia through the Scientific Employment Stimulus 2021.01765.CEECIND (individual call).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors would like to thank LMC for laboratory technical support, and collaboration and materials supply by Secil, Omya, Sika, and Betão e Taipa.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

b/soilbinder to soil weight ratio
c/soilcement to soil weight ratio
DflowSpread flow diameter (mm)
hhours
LFLimestone filler
LOILoss on ignition (%)
MTKMetakaolin
PCPortland cement
PCEPolycarboxylate ethers
PSDParticle size distribution
SSorptivity coefficient (kg/(m2 h0.5))
SCMSupplementary cementitious materials
SCECSelf-compactable earth-based composites
SEMScanning electron microscopy
SpSuperplasticiser
RHRelative Humidity (%)
sp/fines (%)Superplasticiser to fines weight ratio (where fines where considered: clay content of the soil, cement, limestone and metakaolin)
TTemperature (ºC)
T-funnelFunnel time flow (s)
w/bwater to binder weight ratio
w/cwater to cement weight ratio
w/soilwater to soil weight ratio

Appendix A

Table
Table A1. Experimental programme test results summary.
Table A1. Experimental programme test results summary.
Mix IDDflow (mm)T-Funnel (s)Electrical Resistivity (Ωm), 28dMass Loss (%),28dFlexural Strength (MPa), 7dFlexural Strength (MPa), 28dCompressive Strength (MPa), 7dCompressive Strength (MPa), 28dShrinkage (%), 56dSorptivity (kg/(m2 h0.5))
SCEC 1135*5255−9.83%2.383.067.338.83
SCEC 2193*6862−10.59%2.983.328.339.25
SCEC 32004.06963−10.03%2.953.438.9710.93
SCEC 41955.77683−9.54%2.863.008.3510.61
SCEC 52654.36638−9.78%3.624.2212.7613.21−0.2352.05
SCEC 62882.55571−10.22%3.443.749.599.86
SCEC 72533.85997−9.42%3.373.569.4810.28−0.2382.16
SCEC 82394.17118−10.29%2.492.696.365.76
Notes: * Impossible to measure T-funnel, as explained in Section 3.1.

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Figure 1. SEM photo of (a) cement, (b) LF and (c) MTK.
Figure 1. SEM photo of (a) cement, (b) LF and (c) MTK.
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Figure 2. Soil: (a) retained fraction in #1 mm and (b) passed fraction in #1 mm (and used in the current work).
Figure 2. Soil: (a) retained fraction in #1 mm and (b) passed fraction in #1 mm (and used in the current work).
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Figure 3. Particle size distribution of soil (after sieving #1 mm).
Figure 3. Particle size distribution of soil (after sieving #1 mm).
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Figure 4. (a) Mini cone for slump flow test (adapted from EFNARC); (b) flow diameter measurement of a SCEC.
Figure 4. (a) Mini cone for slump flow test (adapted from EFNARC); (b) flow diameter measurement of a SCEC.
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Figure 5. Mini funnel (adapted from EFNARC).
Figure 5. Mini funnel (adapted from EFNARC).
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Figure 6. Electrical resistivity assessment.
Figure 6. Electrical resistivity assessment.
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Figure 7. Dry shrinkage-testing specimen.
Figure 7. Dry shrinkage-testing specimen.
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Figure 8. Water capillary absorption test set-up.
Figure 8. Water capillary absorption test set-up.
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Figure 9. Flow diameter of SCEC. (a) SCEC 1; (b) SCEC 2; (c) SCEC 4; (d) SCEC 5; (e) SCEC 6; (f) SCEC 7; (g) SCEC 8.
Figure 9. Flow diameter of SCEC. (a) SCEC 1; (b) SCEC 2; (c) SCEC 4; (d) SCEC 5; (e) SCEC 6; (f) SCEC 7; (g) SCEC 8.
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Figure 10. Slump flow diameter and T-funnel test results.
Figure 10. Slump flow diameter and T-funnel test results.
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Figure 11. PCE superplasticiser effect on (a) Dflow: (b) T-funnel.
Figure 11. PCE superplasticiser effect on (a) Dflow: (b) T-funnel.
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Figure 12. Electrical resistivity evolution up to 28 days: (a) all mixtures; (b) optimal mixtures.
Figure 12. Electrical resistivity evolution up to 28 days: (a) all mixtures; (b) optimal mixtures.
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Figure 13. Mass loss evolution up to 28 days: (a) all mixtures; (b) optimal mixtures.
Figure 13. Mass loss evolution up to 28 days: (a) all mixtures; (b) optimal mixtures.
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Figure 14. Mechanical strength results at 7 and 28 days: (a) flexure; (b) compressive.
Figure 14. Mechanical strength results at 7 and 28 days: (a) flexure; (b) compressive.
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Figure 15. (a) Shrinkage deformation and (b) mass loss results from 4 to 56 days, for optimal SCEC mixtures in the experimental plan.
Figure 15. (a) Shrinkage deformation and (b) mass loss results from 4 to 56 days, for optimal SCEC mixtures in the experimental plan.
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Figure 16. Experimental results, and corresponding linear regression, of water absorption by capillarity during the first 4 h of contact with water obtained on four replicate samples of: (a) SCEC 5 and (b) SCEC 7.
Figure 16. Experimental results, and corresponding linear regression, of water absorption by capillarity during the first 4 h of contact with water obtained on four replicate samples of: (a) SCEC 5 and (b) SCEC 7.
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Table
Table 1. Main oxide composition, LOI, physical and mechanical properties of cement, LF and MTK.
Table 1. Main oxide composition, LOI, physical and mechanical properties of cement, LF and MTK.
CementLFMTK
Main oxide composition (%)SiO219.80 51.70
CaO61.3299.0
Fe2O33.160.020.46
Na2O0.15
Al2O35.08 43.20
MgO1.82
K2O0.58
SO32.90
LOI (%)2.54 0.75
Physical propertiesDensity (kg/m3)311026802200
Specific surface (m2/kg)4405502950
Initial setting time (min)141
Final setting time (min)201
D10 (μm)1.401.122.02
D50 (μm)10.004.557.12
D90 (μm)25.0015.3621.13
Mechanical propertiesRc,2 (MPa)32.2
Rc,7 (MPa)47.4
Rc,28 (MPa)60.8
Table
Table 2. Experimental programme summary.
Table 2. Experimental programme summary.
PropertyTest Standard/ProcedureCuring RegimeTesting AgeNumber of Samples for Each Age and Each MixSamples Geometry
Mixture compositions study
Slump
(Section 2.2.2)		EFNARC
T-funnel
(Section 2.2.2)		EFNARC
Electrical resistivity and mass loss
(Section 2.2.3)		Two electrode method20 ± 2 °C and HR = 50 ± 5%1,2, 7, 14, 21, 28 days3Prismastic
40 × 40 × 160 mm3
Mechanical strength
(Section 2.2.4)		EN 196-17 and 28 days3Prismastic
40 × 40 × 160 mm3
Optimal mixtures additional characterisation
Drying shrinkage and mass loss
(Section 2.2.5)		ASTM C 59620 ± 2 °C and HR = 50 ± 5%4, 7, 14, 21, 28, 56 days3Prismastic
25 × 25 × 285 mm3
Water absorption by capillarity
(Section 2.2.6)		RILEM TC 116-PCD20 ± 2 °C and HR = 50 ± 5% during 28 days (+14 days at 40 °C, until constant mass)42 days4Cylindrical
h = 75 mm, Ø = 100 mm
Table
Table 3. Mixture proportions of SCECs (selected optimal mixtures in bold).
Table 3. Mixture proportions of SCECs (selected optimal mixtures in bold).
Mix IDSCEC 1SCEC 2SCEC 3SCEC 4SCEC 5SCEC 6SCEC 7SCEC 8
Constituint materialskg/m3
Cement155.00155.00155.00155.00155.00155.00160.00122.25
MTK31.0031.0046.5085.0046.500.000.000.00
LF23.2523.2523.250.0023.2580.0082.5062.00
Soil1550.001550.001550.001550.001550.001550.001600.001630.00
Sp8.0910.0012.0014.0016.0016.0016.0016.00
Water310.00300.00290.00280.00289.00289.00269.00269.00
Main ratios
b/soil0.1350.1350.1450.1550.1450.1520.1520.113
c/soil0.1000.1000.1000.1000.1000.1000.1000.075
sp/fines (%)2.047%2.530%2.921%3.286%3.895%3.800%3.682%4.212%
w/c2.0001.9351.8711.8061.8651.8651.6812.200
w/soil0.2020.1950.1890.1830.1890.1890.1710.168
Table
Table 4. Concrete sorptivity indicator (adapted from [57]).
Table 4. Concrete sorptivity indicator (adapted from [57]).
Sorptivity (kg/(m2 h0,5))Concrete Quality
>15Very poor
10–15Poor
6–10Good
>6Very good
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Matos, A.M.; Varum, H. Self-Compacting Earth-Based Composites: Mixture Design and Multi-Performance Characterisation. Buildings 2022, 12, 612. https://doi.org/10.3390/buildings12050612

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Matos AM, Varum H. Self-Compacting Earth-Based Composites: Mixture Design and Multi-Performance Characterisation. Buildings. 2022; 12(5):612. https://doi.org/10.3390/buildings12050612

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Matos, Ana Mafalda, and Humberto Varum. 2022. "Self-Compacting Earth-Based Composites: Mixture Design and Multi-Performance Characterisation" Buildings 12, no. 5: 612. https://doi.org/10.3390/buildings12050612

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