3rd International Conference on the Application of Superabsorbent Polymers (SAP) and Other New Admixtures Towards Smart Concrete

With the application of isothermal calorimetry technique, the speed of the hydration reactions can be evaluated in a simplified and efficient way, over time, by the heat evolution curves. This technique can become a suitable tool to understand the process of absorption and desorption of the SAPs in the cement matrix, a fundamental parameter that determines the efficiency of the polymers as mitigating agents of autogenous shrinkage and their behavior during the fresh state. It is also possible to identify the water retention or early release during the fluid period of the pastes. This methodology also permits a better understanding of the participation of the water incorporated by the polymer, in the kinetics of hydration of the cement, over time. It was possible to observe that the presence of the SAP slightly alters the reaction kinetics of the cement as it reduces its rate of acceleration of the curve of the second calorimetric peak. The higher the SAP content, the lower the acceleration rate of the curves. The addition of the SAP generated a lightly delay of the second peak time of the pastes and a deceleration of the kinetics reaction of the cement, as compared to the reference mixture. This behavior seems to be related to the desorption kinetics of the polymers. It was also possible to conclude that the higher the SAP amount, the higher the total quantity of accumulated heat in the end of 3 days of test, that is, a higher volume of hydration reactions.

Currently, superabsorbent polymers (SAP) constitute a promising class of chemical admixtures for concrete. However, since their sorption capability and kinetics can differ significantly depending on chemical composition and grading, when choosing a SAP for a particular application it should be pre-tested with respect to its sorption capacity before actual implementation in concrete. Due to the influence that the test liquid ionic composition has on SAP sorption behaviour, they should be tested not only for free water sorption but also for sorption of solutions with chemical compositions as close as possible of those of concrete or mortar pore solutions; furthermore, this should be complemented with the evaluation of sorption behaviour in an environment similar to the one the SAP will experience when in a mortar or concrete. This article focuses two free sorption assessment testing methods, teabag and filtration methods, and two methods that indirectly provide the SAP sorption capacity, slump flow consistence method, using a cementitious mortar, and a new method, plunger penetration consistence method, using a cement paste. It was found that a good agreement exists between the results obtained with the two indirect methods, and that both the direct and indirect methods produced similar sorption trends. Hence, from the results obtained so far, it appears that the plunger penetration method could be a useful complement to the teabag and filtration methods, by providing a simple and quick way of estimating SAP sorption capacity in an environment more similar to that found by the SAP in concrete.

Polyacrylamide (PAM) superabsorbent polymer microspheres were synthesized for use as internal curing agents in low water-to-cement ratio mixtures. An increase in PAM swelling capacity in pore solutions was achieved by reducing the crosslinking density during synthesis: 5.1 g/g swelling was observed for 2 wt.% crosslinker and 20.6 g/g for 0.5 wt.% crosslinker. Capillary micromechanical experiments showed that a reduction in crosslinking density decreased the PAM elastic modulus from 1100 ± 310 kPa to 110 ± 15 kPa. Yet all PAM maintained mechanical integrity when mixed with cement and cured under a compressive load. SEM analysis showed hydrated product formation within cement voids left behind by dehydrated PAM. PAM with reduced crosslinking densities resulted in a 26 ± 7.3% increase in hydrated product formation within these voids. Cement paste density and compressive strength was not compromised by the addition of PAM.

Superabsorbent Polymers (SAP) have been recently subject of investigation as smart admixtures for cement-based materials. The properties of these polymers enable their use for internal curing, increasing freeze/thaw resistance, boosting autogenous self-healing and providing a crack self-sealing effect in cementitious composites. Except for the earliest application, the functioning of these beneficial effects invloves the absorption by the polymers of ingress water in the hardened cementitious matrix and later release, as well as their capacity to complete multiple absorption/desorption cycles. In this work, the absorption of water in mortar with superabsorbent polymers is monitored during the first 60 min of absorption through micro-CT. The experimental series included the presence of cracks. The registration and differentiation of sub-minute (18 s) scans enabled the individuation of bulk water content distribution in the mortar with a resolution of 55 μm. The swollen volume of SAP could also be quantified and studied in time. The results point out that although embedded SAP absorb water from the matrix, this absorption is slow and reduced with respect to water absorption during mixing for the used SAP. Same effect is observed for SAP in the cracks.

Predominant SAPs used for internal curing are anionic gels, such as polyacrylamide, polyacrylic acid, and polyacrylate. They contain or can be hydrolyzed to form carboxylate functional groups. This allows them to retain water up to a few hundred times their dry weight, especially in a high pH environment. SAPs’ swelling behavior is desirable, but other characteristics of SAPs are important for their application as internal curing agents. These anionic SAPs are not only sensitive to the pH environment but also the types of ions. SAPs show less swelling in a calcium-rich solution than in a sodium-rich solution at the same pH. This has direct implications for internal curing because the cement pore solution is rich in various kinds of cations.

In this study, two types of SAPs are developed. One is a neutral SAP in which its swelling ability is independent of its ionic environment, and the other is a semi-anionic SAP, which contains an anionic moiety (polyacrylamide). As expected, the SAPs with anionic moieties show a higher degree of swelling in all environments. The cement pastes dosed with the semi-anionic SAP show a lower calcium concentration in the pore solution. These cement pastes show a lower initial rate of hydration, observed from isothermal calorimetry. The results suggest that the lower calcium content in the pore solution due to calcium absorption into the semi-anionic SAPs may interfere with the initial cement hydration.

The absorption capacity of superabsorbent polymers (SAP) in a real cement environment is a need-to-know property as a new admixture for concrete. In this study, fundamental stereology techniques were applied to cross sections of cement pastes in which solution-polymerized SAP particles were embedded. Its absorption capacity was estimated from the area fraction and particle size distribution of the SAP in the 2D planes. Further, the representative spacing between the SAP particles was also evaluated using point process statistics. The absorption capacity estimated was greater than that obtained by the tea-bag method using a cement filtrate. The SAP seems to absorb mixing water quickly in the initial short time during mixing. The number density of the SAP particles in the cross sections was also greater than the estimation calculated from the absorption capacity and the particle size distribution of initial dry SAP. This fact suggests that the SAP particles broke away during mixing. The centroids of SAP particles were distributed as to form a regular pattern. A procedure to evaluate the median distance from a given location to the surface of the nearest SAP particle was proposed by combining the mean diameter of swollen SAP profiles and the point process G- and F-functions. The distances between SAP particles were found at most a few mm for the mass fraction of 0.58% against cement.

Russian standardized method for an estimation of concrete frost resistance is characterized by number of cycles of freezing and thawing of specimens under standard test conditions without essential strength decrease. The frost resistance tests were carried out in accordance with GOST 10060-2012 and GOST 26134-2016. Two SAP-types were selected for study. One basic and two accelerated freezing-and-thawing test methods were used. Cubic concrete specimens which were subjected to freezing-and-thawing tests had dimensions of 100 × 100 × 100 mm. Frost-resistance factor К_fr of concretes, i.e. the ratio of strength of key samples after a given number of alternate freezing and thawing cycles to the strength of control samples, was calculated for each sample after test cycles. Control value was estimated as 0.95. For tests in salt water, К_fr was even over 1 sometimes that indicates to the available reserve of durability. In contrast, the frost resistance of some samples does not exceed 200 cycles depending on the type of SAP. A different picture was observed when testing concrete in fresh water. In this case, the frost resistance of concrete with SAP was always higher than that of the control samples. It seems that SAP reduces the strength of concrete at the age of 28 days compared to the check sample without SAP by 10 … 13%; in the process of testing, concrete with SAP is gaining strength, resulting in an К_fr increase. Thus, the procedure prescribed by the Russian standard should be clarified in respect to testing of concretes with SAP.

Concrete of strengths classes ≥ C55/67 referred to as high strength or high-performance concrete (HSC/HPC) are noted to be generally of low water/binder (W/B), made from binary or ternary cements with silica fume (SF) being a necessary constituent, and often requiring internal curing. Non-availability and high cost of SF in most sub-Saharan Africa like Nigeria however makes HSC/HPC production in this region very difficult and hence the continued search for alternative supplementary cementitious materials (SCM) with good performance properties as constituents of ternary/binary cements in HPC. This study thereby examines the strength properties of metastable calcined clay (MCC) based HPC cured internally with superabsorbent polymer (SAP) 0.2–0.3% (by weight of binder (b_wob)). HPC mixtures of varied MCC and Rice husk ash (RHA) contents containing two SAP grain sizes labelled (SP₁ ˂ 300 µm and SP₂ ˂ 600 µm) were cast in 100 mm cubes and cured for varying ages (7, 14, 28 and 56 days) before testing. The hardened specimens were subjected to compressive strength and water absorption tests at the varied curing ages for the performance assessment of the binder types and SAP grain sizes in HPC with age. This study revealed the possibility of achieving Class 1 HPC (50–75 N/mm²) utilizing industry manufactured calcined clay and locally produced RHA in Nigeria. The compressive strength of HPCs increased as the curing age increases for both SCM type, SAP contents and grain sizes. RHA based HPCs however showed better strength performance at the early ages than the MCC based. SAP addition in MCC based HPCs led to slight decrease in compressive strength as the SAP contents increased while the RHA based HPCs on the other hand, revealed slight increase in compressive strength with increase in SAP contents.

Utilisation of superabsorbent polymers (SAP) and pre-soaked lightweight aggregates (LWA) as internal curing (IC) agents for the mitigation of autogenous shrinkage and micro-cracking of high strength/high-performance concrete (HSC/HPC) have been well researched and documented in literature. Rice husk ash (RHA) on the other hand has been adjudged to be of good pozzolanic activity and a possible alternative to silica fume (SF) in low water/binder (W/B) concrete production. An experimental comparative study was conducted in the current work to assess the effectiveness of the two known IC-agents on rice husk ash (RHA) based HPC. HPC mixtures of \( f_{c,cube28} = 60 \) MPa minimum target strength produced and internally cured with 0.3% content of SAP by weight of binder (b_wob) and varied content of pre-soaked pumice (5 to 10% in steps of 2.5%) by weight of coarse aggregate (b_wocg) were cast using 100 mm cubes samples. Thereafter, the samples were cured for 7, 14, 28 and 56 days by water immersion before subjecting them to compressive strength test. The results showed 0.2% b_wob SAP HPC (SHPC₁) to be the best performed internally cured HPC at the early ages with similar long-term strength values as 5 and 7.5% b_wocg saturated pumiced HPC (PHPC_1&2). The study thereby recommends SAP content of 0.2% b_wob and saturated pumice content up to 7.5% b_wocg for use as IC-agent in HPC.

Since the beginning of the 20th century, scientists and cement composite technologists are working on developing various types of new structural multi-component cement composites. Several obstacles prevent more widespread use of these newly developed cement composites in construction. One of the main problems is insufficient information about the long-term properties, which are essential in ensuring the safe and long exploitation of structures. The purpose of this research is to determine the long-term properties of several new cement composites: ultra-high strength cement composite with PVA fiber “cocktail” (2% by volume), with micro silica and nano silica additive; ultra-high strength cement composite with 1% montmorillonite mineral nano-size particles; reference composition. Test specimens were prepared and subjected to constant compressive load in permanent room temperature and level of moisture. There were properties such as compressive strength, modulus of elasticity, shrinkage as well as uniaxial creep deformations investigated in the laboratory. Afterward parameters of long-term properties were determined. The obtained results showed that after approximately 90 days of loading the creep coefficient values of new cement composites were 0,5–3; specific creep values were 30–55 microstrain/MPa; creep modulus was 2–90 GPa. The experimental study proves that new elaborated mixes can be successfully used in the production of concrete, thus potentially decreasing the use of cement, which would lead to the reduction of carbon dioxide released into the atmosphere.

Despite their growing popularity in construction, cement-based materials containing ground granulated blast-furnace slag (GGBS) may suffer from various deteriorative actions, including development of early cracks caused by shrinkage processes. Moreover, the degree of GGBS reaction (at later stages) can be limited by lack of space already filled by early products of Portland cement (PC) hydration. In attempt to reduce these negative effects, Superabsorbent polymers (SAP) can be used as internal curing agents and facilitate hydration processes. This paper intends to evaluate and correlate shrinkage behaviour and microstructural features of SAP-GGBS mortars during the first 90 days. Six sets of mortars (with 0% and 50% GGBS) modified by two SAPs were analysed. Autogenous, plastic and drying shrinkages were tested, and microstructural characteristics were analysed by the MIP technique. The experimental results showed significant effect of SAPs on shrinkage reduction in GGBS matrices. The paper argues that deposition of later GGBS hydration products in pores below 20 nm, triggered by SAP water, leads to a relative expansion of SAP-PC-GGBS mortars. In this context, the role of SAPs in GGBS-PC matrices is twofold: firstly, a provision of additional water for hydration and, secondly, a provision of the required space for later products formation.

Shrinkage in concrete structures has been the focus of many studies. Lately a lot of attention has been given to autogenous shrinkage. Although it may not be prominent in ordinary concrete structures, in systems with very low water-to-cement/binder ratio (ultra-high performance concrete for example) it can become a serious issue associated with the cracking of the structure at early age. This type of shrinkage develops due to a reduction in the internal relative humidity of the material and it is also associated to the development of capillary pressure in the pore system due to receding menisci. A big challenge in studying autogenous shrinkage is determining the “time-zero”. Given a lack of consensus in literature, this study aimed to investigate the influence of different estimations of time-zero: the final setting time determined by both an electronic Vicat apparatus and ultrasonic measurements; the “knee-point” in the shrinkage curve; and the capillary pressure build-up. Cement pastes with and without superabsorbent polymers (SAPs) were produced with Portland cement CEM III-B 42.5 N and superplasticizer (Glenium 51, 35% conc.). SAPs have proven to be quite effective in the mitigation of autogenous shrinkage as they can act as water reservoirs for the system. Among all methods, the capillary pressure was very suitable for all mixtures. For those containing SAPs no difference was found in picking the time-zero with any method. For the one without SAPs and lower w/c the choice of time-zero based on the setting time led to a different magnitude of autogenous shrinkage deformation in comparison to the other methods, which could be interpreted as an underestimation of the autogenous shrinkage deformation.

The Carlson-type strain gages are the most suitable for the measurement of autogenous shrinkage. However, your high cost is usually a limiting factor for your employment. A more economical alternative would be the use of Self-Temperature-Compensation Gages to be embedded in concrete. The main objective of this work was to verify if Self Temperature-Compensation Gages can be used instead of the Carlson type for the determination of the autogenous shrinkage in high performance concrete. It was carried out measurements of the autogenous shrinkage, for the same concrete mix, using the two types of extensometers. The difference in the means of the autogenous shrinkage results between these two types of extensometers was typically less than 10 × 10⁻⁶ m/m, showing that the Self-Temperature-Compensation Gage is an alternative to replace the Carlson type strain gage for the measurement of autogenous shrinkage of concrete.

Two superabsorbent polymer (SAP) samples were studied with respect to their sorption kinetics in freshly prepared cement pastes that were exposed to evaporation. SAP 1 was a self-releasing type material when tested in extracted cement pore solution, whereas SAP 2 was a retentive type. These polymers had proven efficient to mitigate plastic shrinkage and its related cracking propensity, however, to different degrees. The observed difference in the performance was elucidated using neutron radiography imaging. Cement pastes with water-to-cement ratios of 0.25 and 0.50 were investigated which were exposed to two different climates: the one mimicking ordinary central European climate (20 °C, intermediate relative humidity) and the other one representing hot and dry weather conditions (40 °C, very low relative humidity).

It was found that SAP 1 released its absorbed aqueous solution right from the beginning, nearly independent of w/c and climatic conditions. This corresponded to its individual ab- and desorption behaviour in extracted pore solution. Contrarily, the inherently retentive SAP 2 desorbed water upon demand that arouse in the paste. While the cracking propensity of the pastes generally largely decreased in the presence of SAP, SAP 2 was found to be more efficient than SAP 1 in this aspect, which can be traced back to the temporally delayed supply of water by SAP 2 upon demand. Obviously, it did not matter much by which physico-chemical mechanism the water was extracted from the SAP particles, i.e. low w/c or hot and dry climate.

At early ages, plastic shrinkage can arise when a cement paste is subjected to harsh drying conditions during hardening. Furthermore, when using a low water-to-binder ratio, the cementitious material may show autogenous shrinkage during setting. Super Absorbent Polymers (SAPs) are a promising admixture to mitigate shrinkage in cement pastes. By introducing internal curing by means of the stored mixing water in the SAPs, the plastic shrinkage can be partially mitigated, next to the mitigation of autogenous shrinkage during setting of the cement paste. The kinetics of water release by the SAPs towards the cementitious matrix are an important factor. To effectively and non-destructively monitor the effects induced by the SAPs during the plastic and hardening period as a function of time, Nuclear Magnetic Resonance (NMR) was applied. Using NMR, a clear distinction could be made in terms of the free water signal and the entrained water signal for SAP particles. The SAPs are able to protect the cement paste internally from the harsh ambient drying conditions by sustaining the internal relative humidity. The plastic settlement was reduced and there was less plastic shrinkage measured. By mitigating shrinkage, shrinkage cracking can be partially prevented. However, upon acting mechanical stresses, the cementitious material may crack nevertheless. SAPs are also interesting to first seal a crack due to their swelling capacity and to heal cracks afterwards by promoting autogenous healing. This healing was also monitored by NMR as a function of healing cycles and the amount of healing products formed were estimated based on the water signals obtained by NMR. Part of the water going into the crack was used to trigger further hydration of unhydrated cement particles. Healing in wet/dry cycles was stimulated by means of SAPs and healing at high relative humidity conditions only occurred in samples containing SAPs.

Chemical admixtures constitute indispensable ingredients for the production of modern advanced concrete. In developed countries, at least 80% of the concrete produced contains one or several admixtures. They include plasticizers, superplasticizers, retarders, accelerators, stabilizers, defoamers, foamers and shrinkage reducers, to name the most important classes. With their help it is possible to optimize the properties of fresh and hardened concrete in such way as to adapt better to local climate and processing conditions and to enhance the mechanical properties and durability. Furthermore, highly sophisticated products such as ultra-high strength concrete (UHPC) or self-levelling and self-compacting concrete (SCC) became possible only with the invention of specific high performance admixtures.

This article gives an overview of major classes of chemical admixtures (e.g. PCE superplasticizers, C-S-H-PCE nanocomposites, stabilizers for SCC, shrinkage-reducing agents) and their current status of development. The main technologies will be described and their role in the formulation of modern advanced concrete will be highlighted. Finally, an outlook on potential developments in the future (e.g. improved curing agents, admixtures which enhance the ductility of concrete) will be provided.

Polymers that help tailoring rheological properties during the casting process have become inevitable constituents for all kinds of high-performance concrete technologies. Due to lacking industries, these typically crude-oil based admixtures are not readily available in many parts of the world, which limits the implementation of more sustainable high-performance construction technologies in these regions. Alternative polymers, which often demand for less processing, can be derived from local plant-based resources.

The paper provides experimental data of flow tests of cement pastes with polysaccharides from Triumfetta pendrata A. Rich, acacia gum and cassava without and in the presence of polycarboxylate ether superplasticizer. The flow tests are amended by observations of the zeta potentials and the hydrodynamic diameters in the presence of and without calcium ions in the dispersion medium. The results show that in the presence of and without calcium ions all polysaccharides provide negative zeta potentials, yet, they affect flowability and thixotropy in different ways. Cassava starch, acacia gum, and the gum of Triumfetta pendrata A. Rich qualified well for robustness improvement, strong stiffening, and additive manufacturing, respectively. The reason for the different effects can be found in their average sizes and size distribution.

Due to the promising results, a flow chart for local value chains is derived on the example of yet unused cassava wastes, which can be converted in parallel into a chemical admixture, energy, and supplementary cementitious material.

The influence of different supplementary cementitious materials (SCM) and superplasticisers on the rheological properties of concrete was investigated to identify potential compatibility issues. Superplasticisers and SCM often have unexpected interaction with certain cementitious compounds, resulting in concrete that is difficult to place in the fresh state due to poor rheological properties. Various mixes were designed containing different superplasticisers and/or SCM in different quantities. Slump, slump flow and concrete rheometer tests were conducted to determine the yield stress, plastic viscosity and thixotropic behaviour of the concrete. Obtained results showed that the specific Sulphonate Naphthalene Formaldehyde (SNF) and Polycarboxylic Ethers (PCE) superplasticisers used, reduced the yield stress, thixotropic behaviour and plastic viscosity of concrete. Modified Acrylic Polymer (ACR) superplasticiser showed a similar effect except for the plastic viscosity which increased at higher dosages. The addition of fly ash and slag to concrete containing superplasticiser had little effect on the rheology and showed similar results as mixes only containing superplasticiser. The use of superplasticiser in conjunction with silica fumes caused a decrease in yield stress and thixotropic behaviour while plastic viscosity increased. The use of superplasticiser in conjunction with higher than normal dosages of gypsum also caused a decrease in yield stress and thixotropic behaviour but had negligible effect on plastic viscosity. It was also found that the use of PCE superplasticiser in conjunction with gypsum, used to control the set of concrete, can cause potential slump loss issues.

Africa needs a new approach for the use of admixtures that provide specific modification properties to concrete in addition to being compatible with African climatic conditions. For example, when concrete is mixed at elevated temperatures, there is quick loss of workability due to high evaporation of mixing water and the tendency is to add more water to the mix. Thus, there is need to develop admixtures that are natural (do not have to undergo derivation processes), readily available, cheap and environmentally-friendly. Gum Arabic (GA) is a sticky natural fluid which oozes from the Acacia tree when an insertion is made and contains natural resin which has arabin. GA comes from two species of Acacia tree, i.e. Acacia Senegal and Acacia Seyal. Gum Acacia Karroo (GAK) which is readily available in these hot areas was used as an admixture for mortar and concrete. GAK comes from Acacia Karroo Haynes which grows mainly in the Southern countries of Africa (Zimbabwe, Mozambique, Zambia and Angola) while Gum Arabic from Acacia Senegal or Seyal comes from countries in Northern Africa (Sudan, Chad, Nigeria). Preliminary results showed that mixes containing GAK have improved compressive strengths and chloride penetration resistance compared to the mixes without GAK, when temperature was increased from 23 °C to 40 °C at the age of 56 days. This suggests that GAK can be used at high temperatures as an admixture to improve these properties of concrete.

In the past, industrial waste and by-products have successfully been used to improve the properties of concrete. Used engine oil is a waste product which is burdensome to discard of and, due to frequent replacement, is produced in high quantities in the construction industry. The utilisation of used engine oil in concrete has shown potential as an admixture by reducing slump and increasing air-content. The main disadvantage is a reduction in long term compressive strength. This study investigates used engine oil (UEO) and used hydraulic oil (UHO) as admixtures to concrete, focusing on its effect on the rheological properties. Slump, air-content, compressive strength and rheometer tests are conducted for concrete containing different dosages of UEO and UHO. Adding low dosages of UEO and UHO have no noteworthy effect on the compressive strength, although increasing air-content and altering the rheological properties significantly. UEO and, to a lesser extent UHO, reduced the energy required to initiate flow (static yield stress) as well as decreased the plastic viscosity. Adding UEO has a similar effect on the static yield stress and plastic viscosity as increasing water content or substituting cement with a proportion of fly-ash. In conclusion, UEO shows potential as an air-entrainer or water-reducing admixture.

Two polycarboxylate ether (PCE) superplasticizers with different molar masses and functionalities were studied in relation to their strain and time-dependant rheological properties. The effect of dosage on cement paste fluidity and structural-build up was of particular interest. Through size exclusion chromatography (SEC) the relative molar mass and polydispersity index (PDI) was determined. Superplasticizer A (SP-A) was found to be three times the molar mass of superplasticizer B (SP-B). The dosage limitations were then determined through a spread flow test. The SP-B, with a lower molar mass, and a backbone functionality of a methacrylate - ester, began to flow at higher dosages and with relatively small changes in dosage, a large impact on the fluidity was notable. Dynamic oscillatory rheology was used to determine structural build-up, as a non-destructive method. The structural build-up of cement paste is a time-dependent phenomenon therefore a time sweep was done. A constant strain and angular frequency, within the linear viscoelastic regime (LVE), was used. The LVE was determined through a series of oscillatory strain sweeps for cement pastes with and without SP-A and B. Thereafter the rate of rigidification (G_ridge) and percolation time (t_perc) as a function of hydration time was investigated.

The interaction between superplasticisers and cement in concrete is complex and can result in unpredictable and unwanted concrete behaviour. It is known that the positively charged tricalcium aluminate (C₃A) component of the cement does not only react with the sulphate (gypsum) present in ordinary Portland cement, but also absorbs the superplasticiser. However, the exact interaction is still not fully understood. This study aims to identify compatibility issues between superplasticisers when exposed to gypsum, fine sand dust and fly ash using the Marsh cone test. The results showed that especially gypsum, which is used in the production process of cement, influence the flow time of the concrete and affects the interaction between the C₃A component and the superplasticiser. The more gypsum added, the more superplasticiser is needed to have the same effect. This indicates that the ratio between C₃A, gypsum and superplasticiser can result in compatibility issues. Particle size and shape, as in the case of the fly ash, was found to play a role in the effectiveness of the superplasticiser. When fly ash is used to increase the flowability of a concrete mix, the superplasticiser does not result in the same significant improvement in flowability than without fly ash.

Autogenous shrinkage is a phenomenon that affects the High Performance Concrete (HPC), mainly due to its refined microstructure, the high cement content, the low w/c ratio and the presence of mineral additions. This is closely associated to the hydration process, not depending on any external interference. The internal curing with Superabsorbent Polymer (SAP) is reported as the most effective mitigating strategy, whereas that it provides the necessary water for hydration, preventing the appearance of tensile stresses that may generate autogenous shrinkage and consequently the cracking of structural elements. The use of SAP increases the porosity of the concrete, mainly because the unconnected voids leaved inside the material and the additional water added for SAP absorption, which would reduce mechanical strength. In this work was used the Nano-silica (NS) particles to compensate this effect. Nine micro concretes with different amounts of SAP and NS were produced. For this work, the mechanical strength was evaluated and the autogenous shrinkage was determined from Time Zero (T₀) until 28 days. The results indicate that SAP was efficient for the mitigation of autogenous shrinkage (reduction of 84% for the 0.3% content of SAP), while the addition of NS increases the mechanical properties (there was an improvement in the compressive strength of about 10% for the 2% content of NS). The concrete containing SAP and NS were very promising, since the beneficial action of SAP in mitigation of autogenous shrinkage was not impaired by the presence of NS, while the NS maintained the mechanical strength values of the mixtures containing both SAP and NS additions approximated of the values of the reference.

This study demonstrates the efficacy of biochar in a dual role of internal curing agent as well as a viable candidate for cement replacement in ultra-high performance concrete (UHPC). Bio-char (BC), a product of pyrolysis of mixed wood saw dust, was prepared by pyrolytic conversion at 500 °C. Biochar produced was manually grinded into micron-sized particles. It was then pre-soaked for 24 h to achieve saturation, which was then mixed to replace 2%, 5% and 8% of cement by wt. in UHPC. Isothermal calorimetry (ITC) tests showed that the presence of BC improved the hydration in BC-UHPC mix compared to reference. This is further confirmed from the bound-water measurements using thermo-gravimetric analysis (TGA), where a substantial improvement in BC-UHPC mix compared to reference was observed. Scanning electron microscope (SEM) images revealed that pores of BC serve as nucleation sites for hydration products. The drop in compressive strength in BC-UHPC mix were limited to 10% of the reference mix, and comparable strength was achieved at 5% replacement level. Overall, the results indicate that bio-char from wood waste can be a potential mineral admixture in UHPC, which might be effective to reduce cement demand and create novel avenue for waste valorisation.

In mechanized shield tunneling, the annular gap between the tunnel structure and the surrounding soil needs to be filled with an adequate grouting mortar to ensure a rapid and safe bedding of the segment rings and to minimize settlements on the surface above the tunnel lining. After mounting of the segment rings and filling of the annular gap, a rapid solidification of the used grout must prevent possible displacements or a floating of the tunnel. In the case of nearly impermeable soils, two-component grouts are necessary, which develop an adequate strength and stiffness in a short period of time by the use of powerful activators like water glass (component B). In addition to the commonly activated cementitious materials, it is feasible to ensure an immediate and sufficient bedding by physical effects. Therefore, the use of superabsorbent polymers (SAP) as component B has been investigated. Experimental studies have been carried out in a systematic way in order to determine the type of the SAP, which leads to a sufficient absorption rate in the alkaline pore water of a cementitious grout. After identification of a suitable “alkali-stable” polymer, tests were carried out in order to examine the necessary amount of SAPs in a slightly modified one-component grout (component A) to cause a sufficient solidification of the whole system within a short period of time. Next to this, the short and long term strength development like shear strength or compressive strength of the combined system (component A and B) were determined. Considering the state of the art of the structural design of the grouting technology on a tunnel boring machine and the generally used liquid activators, a permanent pre-suspension of the SAP was tested and also the strength development of the activated system was examined.

Increased shrinkage is often noted as a concern for alkali activated materials. In this study, two slag-fly ash paste and mortar mixtures with slag:fly ash ratios of 30:70 and 50:50 activated using 4M sodium hydroxide are formulated. The effects of two dosages of a commercial superabsorbent polymer (SAP) on the reaction heat, strength gain, autogenous shrinkage, drying shrinkage, and mass loss behavior are presented here. The SAP increases the heat of reaction of the alkali activated pastes, however, this increase is less than 5% at 7 days. The SAP slightly decreases the compressive strength of the alkali activated mortars, and this decrease is generally less than 10% at 1, 7, and 28 days. The SAP significantly reduces the ultimate autogenous shrinkage (by more than 50%) and reduces the drying shrinkage (by 15–30%) of the mortars. Mixtures with SAP have autogenous shrinkage between 50–300 με and drying shrinkage between 600–700 με. When SAP is used, the mass loss in the mortars increases, however, the slope of the mass loss-drying shrinkage curve decreases. Shrinkage mitigation in the studied mixtures increases as the SAP dosage increases. Further studies on this system, and on other binders, activator combinations, and SAP types are currently ongoing.

The overall objective of this study was to develop an optimised UHPFRC matrix based on the modified Andreasen and Andersen optimum particle packing model by using available South African materials. The focus of this study was to determine the optimum combined fibre and superplasticiser content for UHPC by using a response surface design. The UHPFRC was appropriately designed, produced and tested. The flowability, density and mechanical properties of the designed UHPFRC were measured and analysed. It is clear from the results, that both the fibre and superplasticiser content play a significant role in the flowability of the fresh concrete. The addition of fibres significantly improved both the compressive and indirect tensile strength of the UHPFRC. However, the addition of superplasticiser slightly decreased both the compressive and indirect tensile strength of the UHPFRC. Both the fibre and superplasticiser content had an insignificant effect on the modulus of elasticity. The results show that the superplasticiser content can be increased if a more workable mix is required without decreasing the strength significantly. The study demonstrate that it is possible to efficiently produce a dense and workable UHPFRC with relatively low binder amount and low fibre content. This can result in more cost-effective UHPFRC, thus improving the practical application thereof.