Expansion and Strength Evolution Mechanism of Granite-Based Geopolymer Based on the Reactivity of Calcium Expansive Agents

Granite has been regarded as one of the most abundant rocks in the world and it has been deployed in many applications. Norway possesses about 2.3 billion cubic meters of granite across its landscape [1]. Granite has been deployed in the construction for concrete making as a mostly replacement material for sand. The deployment of granite tends to improve the compressive strength of different concrete mixes [2,3]. Moreover, the replacement with granite also improves the flexural strength of the concrete mixes [3,4]. The potential utilization of granite in the formulation of geopolymers (GP) has also been investigated. One should note that granite is a rock containing minerals with varying chemical composition.

Tchadjié et al. [5] synthesized a granite-based geopolymer with compressive strength ranging between 6.25 MPa and 40.5 MPa by improving the reactivity using the alkali fusion method at 700 ℃. Their granite had a SiO₂/Al2O₃ ratio of 3.46, a CaO/SiO₂ of 0.027, and an iron oxide content of 8.7 wt%. Similar studies have been conducted by different researchers [6,7], with 7 days and 28 days ranging between 5.64 to 19.76 MPa and 8.45 to 23.35 MPa, respectively. Alkali fusion method (650 ℃) was applied in these works to activate granite. The used granite had a SiO₂/Al₂O₃ ratio of 1.4, while its CaO content was below 0.5 wt%.

Geopolymers have been regarded as an alternative to American Petroleum Institute (API) cement as a result of their chemical and physical properties and low CO₂ emission compared to that of API cement. Much attention has been given to a viable, sustainable, and environmentally friendly group of geopolymers made from waste granite from mining fields in Norway. Different researchers have studied these granite-based geopolymers (GBGPs) for different applications in well cementing. Alvi et al. [8] studied the effect of Al₂O₃ and MWCNT-OH nanomaterials on the mechanical properties of GBGPs. Agista et al. [9] synthesized the granite-based geopolymers for shallow gas zone application. Chamssine et al. [10,11] developed retarders for GBGPs to meet the consistency requirement.

Furthermore, other critical aspects of these geopolymers have been investigated, such as the impact of mud contamination on their chemical and mechanical properties, fluid-state properties, and bonding properties [12–14]. Omran et al. [15] advanced the synthesis of granite-based geopolymers in the form of one-part geopolymers. The development of one-part geopolymers was done to reduce the risk associated with handling high pH solutions and minimizing logistic issues. These studies and others have shown and proven that GBGPs could be an alternative for applications in wellbores.

The hydraulic sealability of zonal isolation material is one of the key elements to maintaining well integrity during the life cycle of wellbores. One of the factors that causes the loss of cement sheath integrity is the shrinkage of zonal isolation material, creating pathways for the undesirable flow of fluids. Geopolymers can undergo shrinkage due to the produced water during the geopolymerization reaction. Olvera et al. [16] estimated that shrinkage of a fly ash-based geopolymer ranges from 0.9% of the sample to 4.3% within the temperature range of 25 °C to 50 °C. Ridha et al. [17] also observed that a Class C fly ash geopolymer experienced an average shrinkage of 0.644%. However, the volumetric changes and their impact on the bonding properties of GBGPs have not been fully explored. A study conducted by Kamali et al. [18] reveals that the sealability of a commercially available expansive cement is two times or more higher than that of GBGPs. The lower hydraulic sealability of GBGP implies that an expansive agent is needed to mitigate the shrinkage in the granite-based geopolymers. Unfortunately, there is limited knowledge in exploring expansive agents to improve the performance of geopolymers as well as their underlying mechanisms. In addition to this, the ASTM D4644-16 and other studies [19–21] highlight the fact that CaO and MgO can have different reactivities due to the calcination temperature of the raw materials deployed in their production. Ghofrani and Plack [22] reveal that different reactivities of slaked CaO and MgO in Portland cement systems result in different compressive strength and shear bond expansions. In the case of geopolymers, the study on expansive agents is limited, and it lacks full comprehension of how the different reactivities of the expansive agents affect the expansion, the hydraulic bond strength, and the compressive strength of the geopolymers. One should note that the geopolymerization reaction is completely different from the hydration of ordinary Portland cement (OPC), and geopolymers are sensitive to cations with high polarizing ability. Therefore, the addition of such cations may result in the coagulation of amorphous silicates, resulting in flash gelation and subsequently resulting in an unpractical mix design.

At low temperatures, lime (CaO) has a higher dissolution rate than MgO. CaO has been used over the past century in Portland cement systems for different purposes. Some notable functions of CaO in the OPC include improving the mechanical strength, controlling setting time, and expansive effects [23–26]. Similar studies have been done on the usage of CaO in geopolymers and alkali-activated slag systems (AAS). Kim et al. [27] synthesized an AAS using CaO as an alternative activator. It was revealed that the usage of CaO enhances mechanical strength through the production of CS (Calcium Silicate Hydrate)-like phases. Klimenko et al. [28] also conducted a similar study where the introduction of 15% CaO increases the compressive strength by 2.5 folds. The trend from the previous studies justifies the enhancement of the strength by the introduction of CaO [29].

Volumetric changes are the crucial factors influencing the bonding properties of geopolymers in general. Geopolymers are known to shrink during geopolymerization, which can compromise their bonding to either the casing or the formation, as mentioned earlier. For granite-based systems, the shrinkage has been estimated to be between 0.04% and 0.7% [30,31]. Previous literature [31–33] has shown that expansive agents can improve the volumetric changes, which also affect hydraulic sealability. Our previous work [32] attempted to use CaO expansive agents to improve the performance of granite-based geopolymers for low-temperature applications. Also, the impact of CaO, which was heat-treated at temperatures of 900 °C, 1200 °C, and 1400 °C, was evaluated to understand the impact of the reactivity of the bonding performance of the granite-based geopolymers. In this study, it was revealed that the linear expansion of geopolymers in the presence of CaO expansive agents is proportional to the reactivity. The lower the reactivity, the higher the linear expansion. However, this large expansion witnessed in the case of a lower reactive CaO could be detrimental to mechanical integrity as a result of the overexpansion, thus causing leakage through the matrix of the geopolymer. Even though our previous studies have explored the impact of heat-treated CaO in granite-based geopolymers [32] on the bonding performance, the current literature lacks an understanding of the underlying mechanisms related to the expansion and strength evolution in granite-based geopolymers. Also, the kinetics and the behavior of the CaO agent in the liquid phase of a geopolymer have not been fully explored. Furthermore, an in-depth study of the mechanism related to the differences in compressive strength also needs more understanding.

This study explores the possible underlying mechanisms related to the impact of the CaO expansive agent in terms of the expansion and the strength evolution. In addition, this helps in bridging the understanding between the reactivity of the CaO expansive agents and the performance of the rock-based geopolymer.

The primary precursor, consisting of granite powder, was sourced from Velde in Norway, Ground Granulated Blast Furnace Slag (GGBFS) was obtained from Swecem-Merit in Sweden, and microsilica from Elkem AS Norway. Various inorganic salts, serving as additives, including potassium, zinc, calcium oxide, and sodium salts, were procured from VWR International Company. Halliburton Norway provided silica flour, KOH was sourced from Solberg Industri AS, and the rheology modifier was supplied by Gelest Inc.

The production of CaO is normally done via calcination of CaCO₃ across the temperature ranges from 900 °C to 1500 °C. Previous studies have shown that these preparation temperatures tend to affect the reactivity, leading to three groups of CaO based on their reactivities. The temperature for a highly reactive CaO is normally between 900 °C to 1000 °C, medium reactive is between 1100 °C to 1300 °C, and low reactive ranges from 1300 °C and above. In this study, the CaO was heat treated to control the reactivity and classify them into these groups. Thus, heat-treating CaO was selected as a means to produce CaO of different reactivities. An equal amount of the calcium oxide was placed in an oven for 3 h, and it was allowed to be cooled down. The temperatures were selected above the decomposition of carbonates, from 900 °C followed by 300 °C increment. However, due to limitations, the final temperature for the heat treatment was selected at 1400 °C. They were labeled as follows: HT-900 for the calcium produced at 900 °C, HT-1200 for 1200 °C, and HT-1400 for 1400 °C. These different heat-treated calcium oxides were then characterized to evaluate their reactivities on their heat evolution curve, Brunauer–Emmett–Teller (BET)-specific surface area, and scanning electron microscopy (SEM) images of the particle's surfaces.

The slurry was mixed in accordance with the API standard, API 10B-2 [34], in which the solid phase is introduced into the liquid phase during the first 15 s at 4000 rpm and followed by high energy mixing for 35 s of stirring at a rate of 12,000 rpm. The mix design is described in Table 1. The retarders and the strength-enhancing agent were dissolved in the water and added to the 4 M KOH solution prior to adding the solid phase with or without the expansive agent. Calcium oxide was utilized as an expansive agent within the composition. The selection of 8 g of CaO was determined with primary consideration for its compatibility with the geopolymer mixture. It is worth noting that a higher calcium content could induce rapid slurry setting through the occurrence of precipitation phenomena. Also, it is worth noting that the API 10B-2 was developed for basically testing cement slurry (OPC-based systems). It can also help to provide valuable information about the behavior of geopolymers. The geopolymerization process is very different from that of the hydration process of OPC (ordinary Portland cement). While minor deviations might occur due to differences in reaction mechanisms, the API 10B-2 standard remains useful in evaluating critical parameters such as strength development and linear expansion in non-OPC systems like the geopolymers.

The isocalorimetry was performed in an eight-channel TAM Air Isocalorimeter to evaluate the reactivity of the different heat-treated CaO by monitoring the heat evolution profile. The test was conducted after waiting for the stabilization of 30 min, which is usually for the collection of the baseline values for the TAM Air isocalorimeter before conducting the isocalorimetry of the heat-treated CaO and water mixture. Powdered heat-treated CaO samples were weighed into a plastic ampoule, and the liquid (water) was placed into the in situ mixer liquid holders using a water-to-solid ratio of 8. When the plastic ampoule attached to a homogenizer is set into the device, the liquid is introduced to the solid gradually, and the homogenizer is turned on for about 1 min to homogenize the solution. The heat of hydration is recorded for 3 h. In the case of evaluating the impact of HT-CaO in the geopolymer mix, the test was not conducted in situ. The different CaO were used to prepare different geopolymer mixes, as shown in Table 1. An amount of 6 g of the slurry is then weighed into a plastic ampoule and placed into the TAM air after waiting for a stabilization period of 30 min, and the data are collected for 7 days at 25 °C. To further understand the behavior of the CaO expansive, an additional isocalorimetry study was conducted.

An adequate amount of the slurry is prepared, as stated in Sec. 2.3. The sample is then transferred into the API ring test device as prescribed by API [35]. An automated caliper was built in-house and fixed at the two contact ball rings on the API ring test device to monitor the changes for 7 days (the automated caliper is not prescribed in the API standard), as shown in Fig. 1. The attached automated caliper is a custom-made caliper to assist in monitoring the expansion with time. The caliper has a limitation of only reading the outward (expansion) and not the inward reading. This tool was deployed in helping to monitor the changes in outward movement to evaluate the potency when an expansive agent is introduced as compared to the neat recipe without any expansive agent. The data are then normalized to offset errors between the device and the conventional caliper of the annular ring setup.

The slurry was made into cylindrical molds of dimensions of 50 mm and 100 mm in height. The samples were cured under conditions of 25 °C and 34.5 bar for 1 day, 7 days, and 28 days. The cured samples were then crushed in accordance with API-recommended practice, API TR17 [36], with a slender ratio of 1:2. The loading rate for the test was set at 10 kN/min.

Crushed samples from the compressive strength test were taken and soaked in acetone for 48 h to stop/delay the geopolymerization reaction. Afterward, the samples were then dried in an oven at 45 °C and then crushed into a powder for the thermogravimetric analysis (TGA) test. The test was conducted in the range of 25–1000 °C at a rate of 10 °C/min and also a gas rate of 10 cm³/min. The TGA tests were conducted for 7-day samples on the basis of having more insights about the microstructure at these curing periods.

The crushed samples were ground into a powdered form after soaking them in acetone for 24 h and then were dried in an oven at 45 °C. The samples were then vacuumed for 24 h to remove further moisture. Prior to inserting the sample into the X-ray powder diffraction (XRD) device, the samples were further grounded by hand to ensure the high fineness of the particles. The test was then conducted in a Bruker D8 Advance Diffractometer. The experiment method was a 2ϴ range of 5–92 deg with a 1-deg/min step and 0.010-deg increment. Rietveld refinements of the scans were performed using TOPAS for additional information regarding the effect of the different HT-CaO. The structure files for the refinements were taken from Bruker's Inorganic Structure Database.

Scanning electron microscopy was used to study the morphology of the different CaO used in this study. The CaO samples were placed and thin tape and coated with copper to prevent surface charge issues when taking the images. The sample was then inserted into the SEM from Zeiss Supra 35VP.

The reactivity of various heat-treated calcium oxides (CaO) was characterized through isocalorimetry data and their respective specific surface area measurements. This evaluation aimed to monitor the exothermic reaction resulting from the production of calcium hydroxide (Ca(OH)₂) from the reaction of the CaO with water. The differences in the degree of reactivity among these samples led to distinct patterns in the heat flow. The heat flowchart of different CaO reactions with water is presented in Fig. 2. Two distinct stages characterize the heat flow curve shown in Fig. 2, that is, the acceleration period (time taken to reach the maximum peak of the curve) and the deceleration period. From the plot, it can be assumed that the maximum reaction occurred when the highest peak was attained, just as in the case of cement, the final setting time is related to the maximum peak [37]. The specific surface area of the various samples was estimated. These results are shown in Table 2.

As shown in Fig. 2 and Table 2, HT-900 tends to have a higher reactivity as compared to HT-1200 and HT-1400. This trend could be possibly attributed to the availability of a higher number of reactive spots on the surfaces of HT-900. These reactive spots on the surfaces of the HT-900 are readily available to react quickly upon coming into contact with water. In the case of HT-1200 and HT-1400, the calcination temperature tends to cause a sintering effect, which blocks the ready access to the reactive spots. A similar phenomenon was witnessed in a similar study by Commandre et al. [38]. In addition, Moropoulou et al. [20] revealed that lower calcination temperatures tend to give a higher specific surface area, which is also an indication of higher reactivity of CaO. A similar trend was witnessed in our study. Gomado et al. [32] believe that increasing the heat-treatment temperature increases the sintering effect on the surfaces of CaO, thereby controlling the reactivity of the CaO.

The heat release and cumulative heat for 7 days were monitored to verify the influence of the different calcinated CaO in a geopolymer system. The results are presented in Fig. 4. The initial dip on the normalized heat flow curve is related to the initial dissolution and also lowering of the sample into the isocalorimeter, whereas the main highest peak is related to the geopolymerization process. The slowly reactive sample, HT-1400 had a higher main peak as compared to its other counterparts. The geopolymerization reaction is characterized by dissolution, speciation equilibrium, gelation, reorganization, polymerization, and hardening [39]. The geopolymerization process kicks off with the dissolution of the aluminosilicates to generate complex systems of ions such as OH⁻, Ca²⁺, [SiO4]⁴⁻, [Al(OH)4]⁻, and other species [15,40,41]. This complex mixture of ions tends to proceed to a speciation equilibrium where gels are formed. At this stage, Si–Al oligomers are formed in addition to high concentrations of Ca²⁺ [42]. These species continue to rearrange and reorganize themselves to connect to form a gel network. Previous literature has evaluated the impact of calcium on the geopolymerization reaction [43,44]. From previous literature [44,45], the presence of calcium is readily available to react quickly with the silicate species to form a gel. The formed gel consumes water thereby increasing the pH and thus providing more OH⁻ ions to increase the aluminate concentration through enhanced dissolution of the precursors [44–48]. The reaction rate at this point is dependent upon the Ca²⁺ content relating to the precursors, activators, and additives present in the medium [42].

From Fig. 3, the normalized heat flow curve shows that the max peak of geopolymerization is related to the reactivity of the heat-treated CaO. Surprisingly, the low reactive CaO tends to have a higher max heat peak of geopolymerization followed by medium reactive and then high reactive CaO. The underlying mechanism for the observed trend is unknown. As a result of that, the behavior of the heat-treated CaO is explored in the liquid phase (where the liquid phase comprises the KOH plus the admixture dissolved in water). The result is presented in Fig. 4.

From Fig. 4, it is evident that the heat treatment indeed affected the release and the interaction of the Ca²⁺ ions with the liquid phase of the geopolymers as a result of the blocking of the reactive spot on the surfaces of the CaO as a result of the sintering effect which delays the reaction rate of CaO (that is the release of Ca²⁺ to further react with different species) as shown in Fig. 2. A similar trend was seen between Figs. 2 and 4. The maximum heat peak increases with increasing reactivity of the CaO. From Fig. 4, it can be deduced from the plot that both HT-900 and HT-1200 had an intense and quick reaction upon coming into contact with the liquid phase of the geopolymer. This reaction lasted for around 8 h. However, HT-1400 had a distinct peak as compared to the others. Two distinct peaks were witnessed. The first peak hints at the initial interaction upon coming into contact with the liquid phase, and the second peak hints at the controlled interaction of Ca²⁺ with the liquid phase of the geopolymer due to the sintering effect, as explained earlier. This effect could be the reason why the lower reactive, HT-1400, gives rise to a higher maximum heat peak in the geopolymer mix (as shown in Fig. 3).

The linear expansion profile was tracked for the neat geopolymer and the geopolymers with different heat-treated calcium oxides. From Fig. 5, it can be seen that the introduction of CaO into the geopolymer mix causes an increase in the linear expansion for the curing period of 7 days. However, there was a difference in the linear expansion behavior. N-GP, HT-900, and HT-1200 had an expansion that plateaued around the first day whereas the linear expansion of HT-1400 rose steadily for roughly 3 days before the linear expansion plateaued. The final linear expansion values were 0.09%, 0.21%, 0.31%, and 1.06% for N-GP, HT-900, HT-1200, and HT-1400, respectively. This observation highlights the relationships between the reactivity of the CaO and the linear expansion as observed in previous literature [22,32,49]. From Figs. 2 and 5, HT-900 reacts very quickly. Its reaction is likely to occur in the fluid state where there are no supportive structures to hold the accompanied crystal growth, which causes expansion. In the case of HT-1400, the slower reactivity rate ensures that the reaction of CaO occurs at a stage where there are enough supportive structures for the accompanied crystal growth for the expansion; see the linear expansion rate for all the mixes in the first 24 h. Figure 5 also hints that there might be a relationship between the normalized heat generation profile of the heat-treated CaO alone in the liquid phase and the linear expansion. From Fig. 6, it can be seen that the HT-1400 had a longer reaction in the liquid phase, which corroborates with linear expansion, which also proceeded for more than 24 h.

The XRD diffractograms of the geopolymer mix with the different CaOs were analyzed to predict the phases related to the linear expansion witnessed in Fig. 5. The diffractogram of N-GP was used as the reference in this case. From Figs. 6 and 7, it is evident that the reactivity of the calcium oxide has an impact on the intensity and the proportions of the phases. Generally, the principal minerals detected in all the mixes are quartz, clinochlore, microcline, albite, oligoclase, and Hardysonite. In addition to these, there were other minerals present such as chamosite, biotite, and hydrotalcite. The reactivity of the CaO tends to affect the peaks observed in the range of 20–30 deg 2Ө. This is an indication that the reactivities affect the geopolymerization process, as previously shown in Fig. 3.

From the phase proportions, phases like quartz, albite, hardystonite, and hydrotalcite were seen to be decreasing with the reduction in the reactivity of the CaO, whereas microcline, oligoclase, and clinochlore were increasing with the reduction in the reactivity of the CaO. However, it is worth noting that the portlandite phase was not detected on XRD patterns, even though, in cement, portlandite is a very renowned expansive crystal.

From the above results, the effect of the reactivity of the CaO has been explored in understanding the probable mechanism of expansion in the granite-based geopolymers. Looking at Figs. 3 and 4, it can be deduced that the different heat-treated CaOs with different reactivities alter the geopolymerization process. The different heat-treated CaO samples have different interactions in the geopolymers, which led to the difference in the normalized heat flow graph in Fig. 3. These differences led to the difference in the result effects, as shown in Fig. 5, with HT-1400 given the highest linear expansion, followed by HT-1200 and HT-900, respectively. Looking at Fig. 7, it can be deduced that the alteration witnessed in both the heat flow profile led to the difference in the linear expansion and the mineralogy detected from the XRD test. From the phase proportions in Fig. 7, it can be inferred that the different reactivities led to different proportions of the minerals detected from our XRD study. The reduction in the reactivity favors the formation of minerals such as microcline, clinochlore, and oligoclase. This increment in these proportions can be linked to the linear expansion evolution witnessed in Fig. 5. It can also be considered that the reactivity of the CaO in the geopolymerization process regulates the formation of minerals, which thereby affects the linear expansion reading.

The compressive strength was measured after 1, 7, and 28 days of curing at 25 °C. From Fig. 8, it is evident that compressive strength is also affected by the reactivity of the various CaO. The compressive strength slightly increases with the introduction of HT-900 and decreases with HT-1200 and HT-1400 for both the 1-day and 7-day measurements. This corroborates with previous studies conducted by Gomado et al. [32]. The introduction of CaO in the geopolymers improved the compressive strength of the geopolymers except for the sample with HT-1400. From Fig. 5, it can be deduced that there was a higher linear expansion with respect to HT-1400. The higher expansion for the lower reactive CaO may tend to destroy the matrix of the geopolymer through excessive crystallization pressure [22]. The excessive crystallization pressure tends to create fractures which compromises the compressive strength. The error bar produced for the uniaxial compressive strength (UCS) measurements can also confirm the heterogeneity in the structure when compared to that of the neat GP mix design. Also, this reveals that there expansion of geopolymers should proceed within the threshold of the matrix integrity. A detailed analysis of the underlying mechanism is explained in the next section.

From Fig. 3, it is evident that the reactivity of the CaO alters and influences the geopolymerization process. These changes lead to different resultant properties and compressive strength, as shown in Fig. 8. From Fig. 3, the HT-900 experienced a shorter induction period as compared to its counterparts. The shorter induction period could be attributed to the high reactivity of the HT-900, giving it the ability to undergo a fast reaction to form more geopolymeric gels. The high reactivity of the HT-900 has more impact on the number of gel pores and the fine capillary pores [that is, pore sizes less than 10 nm for gel pores, fine capillary pores (10–50 nm)] as shown in the pore volume distribution plot in Fig. 9. Thus, HT-900 has a higher compressive strength in all the days studied. Also, it could be deduced that the matrix is able to accommodate the emergence of expansive crystals and, thus, mitigating the detrimental effect of the expansion on the matrix. The medium reactive sample, HT-1200, has a balance between both the linear expansion and the compressive strength. However, it could be seen that HT-1400 had an induction period from Fig. 3, signifying a delayed reaction. In the same Fig. 3, the HT-1400 had the highest maximum heat peak. There exist a correlation between the heat flow profile (Fig. 3), the linear expansion and the compressive strength. It can be deduced that the high maximum heat peak can be attributed to the emergence of expansive crystals, which leads to the increase in the large pore volume distribution (>100 nm) in the matrix, as shown in Fig. 9. The large pore volume size is usually detrimental to the mechanical properties of cementitious materials [42,50–52].

The investigation reveals that treated calcium oxide (CaO) serves as a means of controlling the linear expansion in granite-based geopolymers. From this study:

• Heat treatment of CaO tends to sinter the surfaces of CaO, thereby controlling the reactivity of the CaO.

• The change in reactivity is crucial as it affects the geopolymer's exothermic behavior, with a noted increase in heat release corresponding to a decrease in the reactivity of CaO, thereby affecting the resultant effect on the properties of geopolymer.

• The linear expansion of the geopolymer inversely relates to CaO's reactivity, where slower hydration processes result in greater expansion.

• The reactivity of the CaO controls the phase proportions of the geopolymers, and the phase proportions of some of the minerals form a reflection in the linear expansion.

• The compressive strength of the geopolymers is also impacted by the varying reactivities of CaO, with most cases showing a detrimental effect on strength, except for CaO treated at 1200 °C for 28 days, which exhibited positive outcomes.

• Lastly, the presence of highly reactive CaO leads to a denser pore volume structure compared to those with medium or low reactivity, indicating a direct link between CaO reactivity and the microstructural properties of the geopolymer.

These findings show that the expansion in granite-based geopolymers should proceed in a way that the compressive strength is not compromised, and hence, an optimization is needed in fine-tuning the reactivity of the CaO to suit its applicability as an expansive agent for geopolymers in general. In contrast, the results presented in this research were specifically for granite-based geopolymers, but some of the conclusions can extended to other forms of geopolymers. For instance, the effect of the reactivity of CaO on the geopolymerization and resultant mechanical properties (whether it will be beneficial or detrimental to the bulk matrix properties). However, the unique phase formation and expansion behavior will be more dependent on the kind of geopolymers understudy.

The authors gratefully acknowledge TotalEnergies, AkerBP, ConocoPhillips, and the Research Council of Norway for financially supporting the SafeRock KPN Project (RCN # 319014—New Cementitious Material for Oil Well Cementing Applications—SafeRock) at the University of Stavanger, Norway. The authors also acknowledge the efforts of Dr. Paulo Henrique Silva Santos Moreira for his efforts in assisting in the analysis of our XRD data during his tenure at the University of Stavanger.

There are no conflicts of interest. This article does not include research in which human participants were involved. Informed consent is not applicable. This article does not include any research in which animal participants were involved.

The authors attest that all data for this study are included in the article.