Introduction

World population is growing and so are cities. Projections of current trends estimate that urbanites will comprise 68% of world population by 2050 [24]. Both processes – population growth and urbanization – create a growing need for rapid construction of all building types. However, the construction industry is already responsible globally for significant portions of energy consumption, CO2 emissions and waste products [17]. As such, it is directly responsible for resources usage, environmental degradation, and public health. The way we build, not least the materials we use, impact energy consumption it two ways. The extraction of raw materials, their processing into construction materials and components, transporting these, the construction process itself, and finally building demolition, account for at least 10% of the overall energy used in industrialized countries, a percentage known as Embodied Energy (EE) [14, 15]. Another 40% account for the energy used primarily for heating and cooling buildings, as well as lighting and ventilating them, and providing for other energy-related needs (pumps, elevators, and other electromechanical systems) and are known as Operational Energy (OE) [4,5,6, 16, 19]. However, as building design and construction become more energy-aware and environment-responsive, the relative parts of EE and OE change constantly—insulating materials enable energy conservation, thus a lower OE account, but may potentially need much more EE in their production compared to conventional, energy wasting construction [20].

Thus, a wholistic approach is needed if the construction industry is to limit its environmental impact in all respects and if its products are to provide decent indoor conditions at a minimal energy investment.

This research attempts to tackle the issue wholistically:

  1. 1

    using waste products for the production of construction materials

  2. 2

    which have a low EE and high carbon sequestration, or low Embodied Carbon (EC)

  3. 3

    and promote the construction of building envelopes with a high insulating value, lowering OE and OC.

Cement and concrete are currently the most ubiquitous building materials worldwide, with global cement-based concrete production surpassing 30 billion tons per year. Cement has a high EE since its production involves a kiln temperature of 1450 °C, this in addition to the energy involved in quarrying, crushing and other stages of pulverizing rock. Thus, it also accounts for significant CO2 emissions, with a rough estimate of 1:1 ton of CO2 emitted in the atmosphere for each ton of cement produced. Although concrete has significant advantages as a structural material, its thermal properties are poor and thus it is a poor wall infill material, though it is used as such as well. Alternative infill materials, such as Autoclaved Aerated Concrete (AAC) blocks, have a thermal conductivity lower than that of concrete and its derivatives, e.g., Hollow Concrete Blocks (HCB). As such, they enable the production of buildings with a lower OE, yet still have a high EE and EC due to their energy intensive production processes (involving high temperature and vapor pressure autoclave) and ingredients, not least lime and alumina powder. Nevertheless, AAC has become very popular and is extensively used for a number of reasons.

As of recent, low EE and EC products have been introduced as AAC substitutes, among them Lime Hemp Concrete (LHC) otherwise known as Hempcrete (e.g., Bevan and Wooley [1]; Zampori et al. [25]; Hirst et al. [13], Ip and Miller [18]; Florentin et al. [7]). This has been a significant step forward since hemp shives used in the production of LHC are a waste product of the hemp agro-industry focusing on the plant’s fibres, accounting only for 30% of the plant. Hemp shives are very lightweight with a high air content which gives them insulating properties. However, the other component of LHC, lime, is still an energy intensive material, with EE and EC values very similar to those of cement (~ 5 MJ/kg EE and ~ 0.8 kg CO2/kg EC respectively) [12]. Some of the more recent research has adhered to this lime-hemp mixture searching for other, technically more complicated production methods to improve its properties, e.g., an aggregated density which may result in one single product with both indoor thermal mass and external insulation [22]. However, the potential of using lime substitutes has not been thoroughly investigated. Such substitutes may, for example, include clay which is abundant worldwide and can be used as an unfired binder. Furthermore, mining and quarrying of other minerals may well have clay as a waste byproduct.

Aims and Targets

The purpose of this research has been studying the potential substitution – in part or whole—of lime in the hempcrete mixture with clay from different sources and of different composition, this without compromising the mechanical and thermal properties of the mixture and its products. The success of such a process was assumed to include minimizing EE and EC as well as OE and OC of both the material and the end product, a building, over its average lifespan, assumed to be 50 years. Thus, waste can be shown to be a useful resource, thus promoting thinking, planning and production towards a circular and sustainable economy.

Tools and Methods

The first stage of this research program focused on the analysis of different waste byproducts originating from quarrying activities, e.g., clay, limestone, dolomite, basalt. Chemical and phase compositions of the alternative binders were determined by X-Ray Fluorescence (XRF), X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) with Energy Dispersive Spectroscopy (EDS). Particle Size Distribution (PSD) was determined, and particle morphology was characterized by SEM. Unfired binders were mixed as lime substitute in different percentages. Subsequent tests also included Thermogravimetric Analysis (TGA) to identify possible transformations during hydration and carbonation. Following this, five powders were chosen (see Table 1), and mixtures of alternative binder and water were prepared with hemp, at a ratio of 2:1 (binder:hemp), yielding a density of 330 kg/m3. This density is optimal for walls, as it possesses low thermal conductivity combined with reasonable compressive strength and relatively low infill weight allowing to optimize the structural elements. Compressive strength of the cured specimens was tested in an INSTRON 5982. The next stages included the characterization of mechanical and thermal properties of different LHC mixture sample blocks. Results showed that two of the chosen clays were more suitable for the production of LHC, and that replacing lime with alternative unfired binders affected LHC thermal properties negligibly, whereas using only alternative unfired binder instead of lime containing mixture improved compressive strength; yet such blocks were not stable when immersed in water. The samples with 50% lime and 50% unfired binder showed increased compressive strength up to 134% higher than only lime based LHC. Thermal conductivity of the different mixtures ranged between 0.055–0.07 W/m°K. These results refer to LHC samples produced with a density of 330 kg/m3 and compressive strength of ~ 0.8–1.2 MPa and are fully compatible with AAC blocks of different producers available on the market today. An extensive presentation and discussion of this research stage may be found in Haik et al. [8].

Table 1 Alternative binders description
Full size table

The second stage aimed at investigating the actual compatibility with and possible exchangeability of conventional building materials with LHC. Towards this purpose four test cells with different LHC mixtures (50:50 Kalgir:lime, 50:50 Mamshit:lime, 100% Mashit – no lime, 100% lime, no alternative binder) were built, alongside three identical cells built with conventional building materials – AAC, HCB and Expanded Polystyrene (EPS). All test cells (100/100/60 cm) were built on solid concrete bases, with identical EPS slopped roofs with operable apertures (south facing 40/37.5 cm, north facing 25/20 cm) allowing solar heating in winter days and cross ventilative cooling during summer nights. Table 2 shows the properties of the materials used.

Table 2 Thermal properties and density of the tested materials
Full size table

The test cells were constructed at the Blaustein Institutes for Desert Research (BIDR) Campus of the Ben-Gurion University of the Negev (BGU), situated on the Negev Highlands, an arid desert approximately 475 m above Mean Sea Level (MSL). Monitoring of indoor and outdoor air temperature, relative humidity, solar radiation, wind velocity and direction included hot, cold and transition seasons. Monitoring of each cell was done by three thermocouples – centre of indoor space, centre of external southern wall surface, centre of internal southern wall surface – and one HOBO UX100-03 temperature and relative humidity logger, placed at the centre of each cell, alongside the respective thermocouple. Parallel to monitoring test cell indoor conditions were simulated in EnergyPlus [3], in order to allow theoretical upscaling onto a full-size (100m2) single family detached house under the same climatic conditions. Outdoor data were collected from the BIDR campus meteorological station.

Results showed that replacing lime with unfired binders hardly affected the behaviour of cells in terms of temperature and humidity. Best summer results were measured in the LHC cells (lowest temperature peaks) compared to those of the cells made with conventional materials, be they light, heavy or medium weight. Winter results showed a slightly better performance (highest night minima) in the AAC cell, though LHC was better than both HCB and EPS. Humidity measurements highlighted the LHC advantages in terms of “breathability”, i.e., gradual absorption and release of humidity in tandem with outdoor conditions. Here again, AAC had a marginal advantage in summer. Both summer and winter measurements showed LHC cell advantages in moderating extreme values, lowering diurnal fluctuation, and creating a time lag, compared to the cells built with conventional materials (Fig. 1). Similarly, the material’s ability to absorb and release indoors water vapours in tandem with outdoor relative humidity fluctuations (Fig. 2) promotes a much more stable and healthier indoor environment, not least in an arid area which experiences significant relative humidity fluctuations often ranging between less than 20% at noon and reaching even 100% late at night and before down, as a function of wide temperature fluctuations.

Fig. 1
figure 1

24 h data of outdoor and indoor air temperature during hot (left) and cold (right) season monitoring of representative LHC cell (50% unfired binder Mamshit) and conventional building materials [9]

Full size image
Fig. 2
figure 2

24 h data of outdoor and indoor relative humidity during hot (left) and cold (right) season monitoring of representative LHC cell (50% unfired binder Mamshit) and conventional building materials [9]

Full size image

An extensive discussion of the experimental setup, monitoring and results analysis of this stage may be found in Haik et al. [9].

Monitoring and simulation results were in good agreement. This allowed to proceed to the next stage aimed at verifying the advantages of LHC as a full-size house building material, promoting improved indoor climate with significant OE savings both for heating and cooling.

The third and final stage of this program involved the upscaling and simulation of a full-size single-family house (100m2, 10.0/10.0/2.9 m) based on the previous stage results, allowing estimation of OE savings stemming from the use of LHC, compared with conventional building materials similar to those used in the comparison test cells. Thickness of walls and roofs of the simulated house model was assumed to be 0.2 m. South facing windows were designed to be of a total area of 13.5m2, or 14.6% of the floor area, to comply with the 14–16% Window to Floor Ratio (WFR) stipulated by the Standard of Israel SI 5281 Sustainable Building Part 2 [23] for the relevant climatic zone (C—Mountain Zone) to allow for direct solar gains in winter. The same standard sets a ratio of 7–8% north facing fenestration to floor area to provide sufficient summer night ventilation for thermal comfort and structural cooling. A door was located in the eastern façade. The floor was detailed as concrete, similar to that of the test cells. Windows were detailed as double glazed 3–13-3 mm. Summer simulations included closed and externally fully shaded windows during the day (08:00–19:00), and fully open afterwards to allow cross ventilation estimated (based on measurements) at 10 ACH. In winter, south facing windows were fully exposed to solar radiation during the day, and fully insulated at night with EPS panels (as were the test cells in reality and simulations). Two different scenarios were simulated: a fully passive house, and an HVAC-supported house, the thermostat of which was set at 26 °C in summer and 18 °C in winter. Weather data were real ones measured at the BIDR meteorological station, used for the simulations of test cells in the previous stage. This was defined as preferable to the Typical Meteorological Year (TMY) file used by EnergyPlus, since weather peculiarities (extended hot season, unusual hot and cold spell fluctuations in transition seasons, etc.) may have a detrimental effect on the thermal behaviour of a building and its subsequent energy needs.

The passive building performed satisfactorily, maintaining indoors ~ 24 °C while the windows are closed, approximately 8 °C lower than the ambient (32 °C). Indoor temperature peaks at ~ 26 °C very close to the window opening time, when ventilation air movement can compensate for the higher air temperature. Indoor maxima were the lowest in the LHC building compared to all other buildings (AAC, HCB, EPS), while indoor minima were mostly similar to the outdoor minima, being higher only than the indoor minima in the HCB building. In winter, with solar gains and night window insulation, LCH and AAC temperatures are located between the EPS (highest) and the HCB (lowest), with a marginal advantage for LHC. Indoor maxima and minima in the LHC building maintain roughly an advantage of 9–11 °C compared to the outdoor ones, reaching ~ 21 °C during the day, and maintaining at least 15 °C during the night.

The HVAC supported building’s thermostat is set at 26 °C in the summer and 18 °C in winter. Following the same operation pattern for windows and shading, energy consumption was the lowest for LHC both in winter and summer, reaching an annual consumption of ~ 7kWh/m2/year, surpassing the highest standards for nearly Zero Energy Buildings (nZEB) [2]. A detailed presentation of this stage of the research program may be found in Haik et al. [10] (Fig. 3).

Fig. 3
figure 3

Annual energy requirements for heating and cooling of buildings made of LHC, AAC, HBC, EPS [10]

Full size image

Discussion and Conclusions

Summarizing the different stages of this research program, it may be said with a high degree of certainty that LHC may be brought to a significantly lower EE and EC level by incorporating in it unfired binders to substitute lime. This does not affect its thermal properties while improving its compressive strength. The thermal performance of test LHC cells monitored in extreme desert climatic conditions was shown to be better than that of identical test cells built with conventional building materials (AAC, HCB, EPS). A parallel simulation of these cells alongside their monitoring showed good agreement between actual and simulated results. This allowed a theoretical upscaling from test cells to full-size house, which was then simulated for different building materials, both in passive and HVAC supported operation. In both cases, the thermal performance of LHC constructed building was shown to be better than that of the other buildings built with conventional materials. However, when simulating the building as being HVAC supported, it was shown that LHC brought it to a level higher than that stipulated by EU nZEB standards. Further investigation showed that, based on ISO 14040, Life Cycle Assessment (LCA) of the specific LHC mixtures and the structures built with them save up to 90% of the total energy (EE and OE) consumption and CO2 emissions [11], as shown in Table 3. Considering the fact some of the unfired binders were quarries waste, and hemp itself is an agro-industry waste byproduct, it can be concluded that Lime Hemp Concrete proves once again that “One man's trash is another man's treasure”, thus closing another cycle.

Table 3 Comparing total energy requirements and total CO2 emissions for a prototypical building’s life span (50 years), for all LHC mixtures, as compared to conventional materials
Full size table